Comparative Anatomy of Vertebrates

Presented by: Daylo A. Abante BS-Biology III

By: Editha De Jesus

IntroductionChapter I

Comparative Vertebrate Anatomy

• Study of vertebrate structure (or morphology) from an evolutionary perspective and the functional aspects of these structures.– As a result, an understanding of phylogenesis is

essential (i.e.; an understanding of the ancestral history of the vertebrate orders).

– Evolutionary developments and relationships can be gathered by looking at embryological, and to a lesser extent fetal, development. Ontogenesis is also very important.

The Phylum Chordata

• The phylum chordata is made up of a group of animals all possessing four common features.– Notochord, a rigid cartilaginous rod defining the

longitudinal axis in the embryo– Dorsal Hollow Nerve Cord, the spinal cord an brain– Postanal Tail– Endostyle, a glandular groove in the floor of the

pharynx.

The Phylum Chordata

• The phylum can be divided into three subphyla.– Urochordata, the tunicates and sea squirts– Cephalochordata, amphioxus– Vertebrata/Craniata, chordates having a vertebral

column of bone or cartilage

General Vertebrate Body Plan

Regional Differentiation

• The typical vertebrate body has three regions– head– trunk– Tail

• The development of the head is a developmental process termed Cranialization.– The head contains the brain, a number, of special

sense organs, often jaws, and gills in fishes for respiration.

– The trunk contains the Coelom, a body cavity that houses the visceral organs.

Regional Differentiation

• The coelom is surrounded by the body wall consisting of:– muscles– vertebrae– ribs

• The trunks of the Gnathostomata , the jawed vertebrates and most numerous vertebrates, will typically have paired pelvic and pectoral appendages (ex; fins, wings, legs).

• Many vertebrates have a Neck. The neck is a narrow structure connecting the head to the trunk.– Necks are found in the tetrapods.

• The tail is postanal in vertebrates meaning that it originates posterior to the anus. The tail is present in all embryonic vertebrates but may be lacking in the adult form.

Bilateral Symmetry

• Bilateral symmetry means that the vertebrate body can be divided into two equal right and left hand portions.

• This allows vertebrate anatomy to be studied by dividing the body into Planes.

Types of Symmetry

Bilateral Symmetry

• Three major planes of the body:– Transverse Plane (aka; horizontal plane, cross section-

aplane running horizontally (left to right) dividing the body into inferior and superior portions.

– Sagittal Plane - a vertical plane dividing the body into left and right portions.

• Midsagittal (or Median Sagittal) Plane - runs along the midline of the body.

• Parasagittal Plane - runs at other than the midline of the body.

– Frontal Plane (aka; Coronal Plane) - a vertical plane which divides the body into anterior and posterior portions.

Segmentation or Metamerism

• It is the serial repetition of structures along the long axis of the body.

• It is evident in the embryo.• It is also present in the adult body to varying

degrees.– Examples: muscle segments of fishes, rectus

abdominis in humans, ribs

Segmentation in Arthropods

Notochord and Vertebral Column

• The notochord is a cartilaginous rod that defines the long axis of the embryo.– the first skeletal feature to appear in the embryo.– located immediately ventral to the developing

nerve cord and superior to the digestive tube.• During development, in most vertebrates,

bony or cartilaginous vertebrae will grow around the notochord

Notochord in Amphioxus

Notochord and Vertebral Column

• The weight bearing portion of the vertebra, the centrum , will surround the notochord.– As to how much of the notochord will be retained

in the adult animal is variable.• Most fishes maintain the notochord in their adult form.• Some amphibians will show a pattern similar to that

seen in fishes.• In reptiles, birds, and mammals the notochord is almost

or completely lost.– In mammals remnants of the notochord will remain as portions

of the intervertebral discs (called the Nucleus Pulposus).– In modern reptiles and birds even this vestige has been lost.

Notochord in Human

Notochord and Vertebral Column

• In Agnathans (jawless vertebrates) the notochord grows and is maintained throughout life. It will even develop extensions called Lateral Neural Cartilages that will be found lateral to the spinal cord.

Notochord and Vertebral Column

• The Vertebral or Neural Arch will extend dorsally from the centra to surround and protect the spinal cord.

• A number of processes will radiate from the arch.

• In some vertebrates a second arch, the Hemal Arch , will extend from the centrum ventrally in caudal vertebrae. The hemal arches surround and protect the caudal artery.

Parts of Neural Arch

Dorsal Hollow Nerve Cord

• The vertebrate nerve cord is dorsally oriented and hollow.– It consists of the brain and spinal cord.– The hollow center of the vertebrate nerve cord is termed the

Neurocoel.• The neurocoel includes the central canal of the spinal cord.

• The nerve cord originates by the process of Neuralation.– Neuralation typically occurs along the longitudinal axis of the

embryo dorsal to the notochord.• A groove, termed the Neural Groove , will form dorsal to the

notochord.• The neural groove will sink into the embryonic body and close off to

form the Neural Tube.

Dorsal Hollow Nerve Cord

• In Agnathans and Neopterygians (the gars, bowfins, and teleosts) neuralation has become slightly modified from the typical vertebrate pattern.– The neural groove does not form.– Instead a Neural Keel forms. The neural keel is a

wedge shaped ectodermal structure dorsal to the notochord. It will separate from the surface ectoderm, form a cavity, and become a typical nerve cord

Dorsal Hollow Nerve Cord

• The nerve cord expands anteriorly to form the brain.

• Cranial and spinal nerves will develop and radiate out from the nerve cord.– These nerves allow for communication between

the CNS and the rest of the body.• This is the job of the PNS.

Dorsal Hollow Nerve Cord in Amphioxus

Pharynx

• The pharynx is a vital portion of the vertebrate embryo.– It shows the relationship between vertebrates and other

chordates.– It produces a number of structures:

• gills in fish• lungs in tetrapods• jaw skeleton and musculature• some endocrine glands• the middle ear in tetrapods• and serves a source of stem immune cells in the human fetus.

– As a result, all vertebrate embryos will show a basic pharyngeal architecture.

Pharynx in Human

Pharynx: Development of Pharyngeal Pouches and Pharyngeal Slits

• Pharyngeal pouches arise as outpocketings of gut endoderm.– These endodermal outpocketings will migrate towards

the surface.• At the same time grooves in the above lying

ectoderm, termed Ectodermal Grooves, will grow towards the developing pouches.

• Eventually the two structures will grow close together and will be separated only by a thin membrane of tissue called the Branchial Plate.

Pharynx: Development of Pharyngeal Pouches and Pharyngeal Slits

• If the branchial plate ruptures, a passageway forms termed the Pharyngeal Slit.– In fishes pharyngeal slits are maintained throughout life

for gills.– In tetrapods pharyngeal slits are only temporary

structures.• Examples:

– Frogs have 6 pharyngeal pouches in the embryonic form.– Four will form the gill slits of the tadpole.– These four slits will close up again during the

metamorphosis into the Adult frog

Frog’s Pharyngeal Slits

Pharynx: Development of Pharyngeal Pouches and Pharyngeal Slits

• Examples:– Chick embryos will have six pouches. Pharyngeal

pouches number 1, 2, and 3 will rupture and then close again.

– In mammals only one or two pharyngeal pouches will rupture. The rupturing pouches tend to be anterior pouches and will close up again.

• Pharyngeal pouch number one becomes the Eustachian Tube.• Pharyngeal pouch number two will eventually house the

palatine tonsils.• Several posterior pharyngeal pouches will give rise to certain

endocrine glands.

Pharynx: The Pharyngeal Arches

• Between adjacent pharyngeal pouches/slits are columns of tissue called the Pharyngeal Arches

• Each pharyngeal arch has four components or the blastemas from which these components will arise.

Pharynx: The Pharyngeal Arches

• Typically these four components are:– Pharyngeal or Visceral Skeleton- supportive skeletal

elements– Branchniomeric Musculature- skeletal muscles to

operate the arch due to its relationship with the gills.– Cranial nerve branches to innervate the arch

• These cranial nerve branches will be both sensory and motor in function.

• The cranial nerves that send branches into the pharyngeal arches are CN 5, 7, 9, & 10.

– Aortic arch to connect the dorsal and ventral aorta.

Pharynx: The Pharyngeal Arches

• The four components will also be found anterior to the first pharyngeal arch and often posterior to the last pharyngeal arch.

• The arches numbered from anterior to posterior.– The anterior most arch is the First Pharyngeal Arch.

• The first pharyngeal arch is typically referred to as the Mandibular Arch since it contains the upper and lower jaws and related structures.

– The second arch is referred to as the Hyoid Arch.– The remaining arches are numbered 3 through 7.

• Sometimes arches 3 through 7 are referred to as Branchial Arches 1 through 5 since they resemble unmodified gill arches.

Tube-Within-a-Tube Body Plan• Vertebrates, and many invertebrates for that

matter, have a tube -within-a- tube body plan.– The outer tube is the body wall.– The inner tube is the digestive tract.– The space between the outer and inner tubes, between

the body wall and digestive tact, is termed the Coelom.• The coelom in fishes, amphibians, and reptiles can be divided

into:– The Pericardial Cavity which houses the heart– The Pleuroperitoneal Cavity which houses the rest of the viscera.

» In tetrapods the lungs are located in this cavity.– The pleuroperitoneal and pericardial cavities are separated by a

fibrous connective tissue partition termed the Transverse Septum in tetrapods

Earthworm

Tube-Within-a-Tube Body Plan

• The coelom of birds and mammals is divided into:– The Thoracic Cavity having Pericardial Cavity for the heart and a

pair of Pleural Cavity for the lungs.– In some there will also be a Mediastinal Cavity

• Features that develop earliest in ontogeny are the phylogenetically oldest features, having been inherited from early common ancestors.

• Also features that develop later are of more recent phylogenetic origin.– An exception is the development of extraembryonic membranes

(ex; amnion) which develop later phylogenetically but arise early ontogenetically due to the survival needs of the embryo.

Caenorhabditis elegans

Other Vertebrate Body Features

Integument

• The integument is composed of the skin and the hypodermis.– The skin is composed of the Epidermis and Dermis.– The epidermis is a superficial layer composed of a

many layered epithelium having a number of• functions.

• It produces a variety of glands serving a variety of purposes.• It produces cornified appendages such as hair, feathers, and

scales.• It provides protection from the external environment and

from pathogens.

Integument

• The dermis is deep to the epidermis and is a connective tissue layer.– In some animals it produces dermal bone.– It will also aid in the production of scales.

Respiratory Structures

• Most vertebrates conduct external respiration by means of extremely well vascularized membranes.

• These membranes will be either located in the pharyngeal pouches (i.e.; gills) or will be derived from the pharyngeal floor (i.e.; lungs).

Digestive System

• The digestive system consists of a digestive tube and a number of accessory organs.– The Digestive Tube/Alimentary Canal/ Gastrointestinal Tract is

a long tube running from the mouth to the cloaca.– It has a number of regional specializations along its length but

the entire tube shares the same basic histology.– These specialized regions, or organs, include the oral cavity,

pharynx, esophagus, stomach, small intestine, and large intestine.

– The Accessory Organs are located outside of the digestive tube and release their products into the tube by means of ducts. They include the liver, pancreas, and gall bladder.

Urogenital System

• The urinary and reproductive systems are often considered as one combined system, the urogenital system, due to their shared origins and structures.– Examples: the kidneys and gonads both develop close

together on the coelomic roof, and both the male reproductive and urinary systems utilize the urethra in mammals.

• The urinary system removes wastes from the body so as to maintainhomeostasis.

• The reproductive system allows for the propagation of the species.

Circulatory System

• Vertebrates have a closed circulatory system whereby blood is pumped by a muscular heart through a series of blood vessels.

Skeletal System

• The vertebrate skeleton is internal (an endoskeleton) and composed of cartilage/bone.– It forms a framework providing for the shape of the

body, the anchoring of muscles for movement, and protection of fragile organs.

• The skeleton can be divided into two portions:– Axial Skeleton is composed of the skull, vertebrae,

and rib cage.– Appendicular Skeleton is composed of the bones of

the limbs and their associated girdles.

Concepts, Premises, and

PioneersChapter II

Patterns and Processes

Homology

• Homology is similarity due to common ancestry.– Initially all structures found in the same location with

the same appurtenance and function were assumed to be homologous.

– Comparative Embryology has shown that not all look–alike structures are homologous and some homologues bear no resemblance to one another.

• Examples: the human stapes bone and the hyomandibular cartilage of sharks,

• The intermaxillary bones of human embryos and the premaxilla of adult apes.

Homology

• One way to test the veracity of suspected homologues is to use comparative embryology.

• If two structures (in two different animals) come from the same embryonic precursor then they are homologs.

• Another test is the structure’s location and the evolutionary relationship of the two animals in question.

The wing of a Bat and Bird

Homoplasy

• The opposite of homology is homoplasy.• Homoplasy is similarity due to any other cause than

common ancestry.– Serial Homology is a term applied to segmental

structures that as a unit are homologous.• Examples: spinal nerves of fishes and mammals.• However, one segment of the entire unit may not be

homologous to the similar portion of a metameric structure in another organism.– Serial homology is concerned with the entire metameric

unit, not the individual components.

Analogy

• Analogy is the term for two structures that have similar function but not necessarily the same origin. It is a coincidental resemblance.

• Examples:– The wing of a fly and a bird are analogous while the wing of a

bird and a bat are homologous.– The horn of a bison and the antler of an elk are analogous.

• Sometimes homologous structures have a similar function and are termed Analogous Homologues.

• Sometimes unrelated ,nonhomologous structures have the same function and are termed Analogous Homoplasies.

Wings of Fly vs. aves vs. mammal

Adaptation and Speciation

Adaptation

• Adaptation is the ability of a species to change due to some influence.– Biological Adaptation is a hereditary modification

of a species structure, or phenotype, to increase the probability of the species survival.

– Since phenotype is typically the result of genotype, adaptations are believed to be a change in the genetic make up of a species due to environmental factors.

Adaptation

• Environmental factors result in genetic mutations that, if beneficial, will be retained by the species.

• Preadaptation is the presence of traits that enable a phenotype to handle new environmental stresses prior to the to the development of those stresses.– Example: Lungs were present in fishes prior to the

movement of vertebrates onto the land. The lungs served to allow some fish to utilize atmospheric oxygen when dissolved oxygen in water was low.

Speciation

• Speciation is the development of two new species from one original species.– Often this involves the separation of one species into two

reproductively isolated populations.– Over time, slight genetic differences will begin to accumulate

and the populations will begin to develop into two genetically distinct species.

• This is termed Genetic Drift.• Speciation is typically the result of geographical isolation coupled

with genetic change but other factors can play a role.

• Example: – Polar bears evolved from brown bears.

Convergent Evolution

• Convergent Evolution is a form of evolution where two or more unrelated organisms acquire similar morphological features to exist in similar environments.– Example: sharks, ichthyosaurs, and dolphins; bats,

birds, and pterodactyls– The two species can exist concurrently or be

separated by millions of years.– Convergent evolution produces similar features in

two species that do not share a common ancestor.

Development

Development

• Development is useful to in the study of Comparative Vertebrate Anatomy because patterns of development provide a further point of comparison among species.– Comparative embryology is used to test the veracity of

homologies.– Heterochrony is a change in the timing of developmental events.

• Example: a delay in metamorphosis in some amphibians due to environmental stresses.

• Heterochrony can result in speciation.

– One way is by causing the retention of juvenile characteristics termed Paedomorphism.

• Example: the axolotyls

Ontogeny and Phylogeny

• Ontogeny/Ontogenesis is the developmental history of an organism. It is the series of morphological events that occur from zygote to neonate, to adult, and even to death.

• Phylogeny is the evolutionary history of a group or Taxon.– Taxon is a related group of organisms

• Example: species, genus, family.

– Phylogeny chronicles the changes that occur over the evolutionary history of a group while ontogeny chronicles the changes that occur during the lifetime of one individual.

Ontogeny and Phylogeny

• There is an old saying:” Ontogeny recapitulates phylogeny.”– This means that as you watch the development of an

individual, especially embryologically, you are also watching the divergence of a species from its common ancestors.

– However this is not quite true. Von Baerʼs Law better describes the relationship.

• Karl Ernst Von Baer was an embryologist who noted that the features common to all members of a taxonomic group develop earlier in ontogeny than do features that distinguish subdivisions within the group.

Ontogeny and Phylogeny

• So features diagnostic of the subphylum appear earlier in embryogenesis than do features that differentiate families within the phylum.

• Today Von Baer’s Law has had a corollary added to it:– Features that develop earliest in ontogeny are

thephylogeneticallyoldest features, having been inherited from early common ancestors..

– Also features that develop later are of more recent phylogenetic origin.

– An exception is the development of extraembryonic membranes (ex; amnion) which develop later phylogenetically but arise early ontogenetically due to the survival needs of the embryo.

Systematics and Taxonomy

Definitions of Classification

• Systematics is the process by which organisms are grouped in classification.

• Taxonomy is the conventions used to apply names to these groups.– Taxonomy is still based on the Linnaean system

established by Carolus Linnaeus; although it has been refined.

– It uses a hierarchical system and binomial nomenclature.

Taxonomy

• Hierarchical system: kingdom->phylum->class->order->family->genus->species– Example of binomial classification; Canis lupus

• Currently systematics group organisms based on their phylogenetic (or historical) relationships.

• In the past classification involved groupings based on physical similarities but that can be misleading.– Today Synamorphies are used.

• Synamorphies are shared derived characteristics.

– A phylogeny represents the current hypothesis as to the relationship between species today.

• These hypotheses are open to testing.

Cladistics

• Cladistics is the methodology used by most systematics today.• Cladistics is based on the following assumptions:

– The hierarchy of relationships is knowledgeable and can be represented in by branching pattern called a Cladogram.

• A cladogram is a useful tool to graphically display the relationships between various organisms.– The cladogram consists of a branching pattern with terminal taxaat

the end of each branch.– Adjacent taxa represent sister taxa.– Where the taxa split is the location of a common ancestor (often

this common ancestor is hypothetical).– The branching pattern indicates relationships between taxa.

Cladistics

• Only the characteristics that distinguish a group are useful in cladistics.– Example: The presence of vertebra is not useful to distinguish fish

from birds.• The distribution of derived characteristics on a branching

diagram determines homoplasy versus homology.– Example: The distribution of derived characteristics on a branching

diagram determines the independent development of similar structures versus the development of similar structures due to common ancestry.

• The distribution of characteristics follows the rules of parsimony: – the solution that is most economical is usually correct.

Organic Evolution and

Evolutionary Selection

Organic evolution

• Organic evolution is based on the concept that organisms on earth have been changing and that those species on earth today are descendants of earlier species.– This concept is partially based on fossil evidence.

• The Evolutionary Theory– Jean Baptiste Lamarck(1744-1829) developed the concept of

the Doctrine of Acquired Characteristics.• It states that structures which are used over successive generations

become stronger and better developed while structures that are not used over successive generations will degenerate and disappear.

• Although it recognizes that species change over time it is not a good mechanism for explaining change because it does not state how those changes occur.

Organic evolution

• The Evolutionary Theory– Charles Darwin (1809-1882) developed the

Theory of Natural Selectionin 1859.• It was superior to Lamarck ‘s idea because it provided a

mechanismto cause the changes.

The Protochordates

Chapter III

Introduction of

Protochordates

Protochordates

• The term Protochordata is used to include to phyla of the chordates.

• These two phyla are:– Urochordata – the sea squirts and other loosely

related groups.– Cephalochordata – represented by amphioxus.

• Both phyla are marine organisms that most likely share a common ancestor with vertebrates.

Protochordates

• Protochordates, vertebrates, and the most likely invertebrate relations the echinoderms and hemichordates can be collectively grouped under the heading Deuterostomata.

• Deuterostomata means “two openings” and refers to an event during embryological development.– During embryogenesis a blastopore develops.– The blastopore develops into the anus and a second opening

will form during development that will become the mouth.– This pattern of development indicates the possibility of a

common ancestor for these disparate groups.

Basal Deuterostomes

Echinodermata

• The echinoderms first arose during the Cambrian Period and include 20 extant classes such as sea stars, sea cucumbers, and sea urchins.– All echinoderms have a calcium carbonate skeleton, are

marine, and most species show radial symmetry.– However bilateral symmetry is a primitive characteristic

of echinoderms.• The relationship between echinoderms and

vertebrates has been demonstrated by embryogenesis.

Hemichordata

• Hemichordates are a group of marine invertebrates commonly called “acorn worms”.

• The relationship between the hemichordates and the vertebrates is widely disputed. The hemichordates are a unique taxon.– Supporters of a close relationship cite the following

items as evidence:• Hemichordates have a structure that resembles a dorsal

hollow nerve cord.• It is a dorsally oriented strand of nerves that occasionally

possesses a lumen.

Hemichordata

• There are two problems with this, however:– The presence of the lumen is highly variable.– Hemichordates also have a ventral nerve cord (as do many

invertebrates).– . Hemichordates lack a true CNS.

• Hemichordates have slits in their pharynx. These slits represent gill slits and may be pharyngeal slits.

• Hemichordates possess a structure called the Stomochord.– The stomochord may be homologous with the notochord.– The relationship between the stomochord and notochord lacks

evidence.– It is a short outpocketing of the foregut into the proboscis of the

acorn worm.

Hemichordata

• Developmental and molecular biological studies indicate a closer relationship exists between hemichordates and echinoderms than between hemichordates and vertebrates.– The larval forms are very similar.– Muscle proteins and other traits are close

between the two.

Urochordata

Urochordata

• Urochordates are a group of filter feeding, marine organisms.– Urochordates typically have a thin, transparent,

nonliving outer tunic.• For this reason they are often also known as the tunicates”.

– Since they possess a notochord in their tail, tunicates are considered to be chordates.

– There are three classes of Urochordates: Ascidians, Larvaceans , and Thaliaceans.

• Two of the three classes retain the larval form throughout life.

• Only the ascidians mature into an adult form. They lose the tail and its notochord.

The Class Ascidians

• Ascidians are tiny (between 0.5mm to 11 mm), short living (hours to days) organisms found either in colonies or existing solitarily.

• The larval form consists of a trunk and a muscular tail.– The tail is locomotory and possesses a notochord.

• Running along the notochord are between 36 and 1,600 uninucleated, striated muscle fibers.

– The larval form does not feed.– As a result the viscera is reduced.– The larval form feeds off of stored calories.

The Class Ascidians• The larval form’s nervous system consists of a dorsal, hollow nerve

cord, several ganglia, and nerves.• It also has a primitive brain composed of a few ganglia with associated

sensory structures. These structures are:– Ocellus, a light sensitive receptor cell– Otolith, a receptor for stereoreception.

• During respiration, water is drawn into the mouth and then passes through the pharynx and over the gills lining the pharyngeal slits.– Then it passes into a chamber surrounding the pharynx called the Atrium .– Oxygen depleted water will exit the atrium via the Atriopore.– When the larval form metamorphosis into the adult form the following

events occur:• Three adhesive papillae form to attach the sessile adult to a substrate.• The notochord is reabsorbed and used as a source of nutrition to supply the energy

required for metamorphosis.

The Class Ascidians

• During respiration, water is drawn into the mouth and then passes through the pharynx and over the gills lining the pharyngeal slits– The viscera changes.

• The mouth becomes and incurrent siphon and the atrioporebecomes and excurrent siphon.This modification allows for filter feeding.

• A ciliated, glandular groove develops on the floor of the pharynx called the Endostyle.The endostyle traps food and passes it on to the esophagus by ciliary beating.

• The ascidian also develops a short esophagus, stomach, and intestine.

• Wastes are dumped into the atrium and released along with deoxygenated water by means of the atriopore.

The Class Larvaceans

• Larvaceans typically occur as free - floating plankton.

• They are found in waters of less than 100m depth and below the level of light penetrance.

• They are tiny being about 8mm in length.• They have a small body and a long tail

supported by a notochord.

The Class Thaliceans

• Thaliceans are free living or colonial organisms.

• They alter between free living and colonial over succeeding generations.

• Thaliceans resemble adult ascidians in appearance.– Like adult ascidians they lack a tail.– They propel themselves by squirting water out of

the atriopore.

Cephalochordates

Cephalochordates• Represented by Amphioxus.• AKA lancelets due to the

appearance of their cylindrical bodies which taper at both ends.

• Small, marine organisms found immediately off of sandy beaches.

• They burrow into the sand leaving only their heads exposed so as to filter feed.

Figure . The Amphioxus

Amphioxus (Anatomical Features)

• Body is consists mostly of a trunk.– The length of the trunk has repeating units of

muscle termed Moyer`s that serve for locomotion.• The myomeres are composed of uninucleated, striated

muscle fibers.• Adjacent myomeres are separated by a thin connective

tissue partition called a Myoseptum/Myocomma

Amphioxus (Anatomical Features)

• It has a semitransparent skin composed of two layers.– There is an epidermis composed of a single layer

of cells and a thin dermis that facilitates cutaneous respiration.

– The integument also includes a thin hypodermis.

Amphioxus (Anatomical Features)• Possesses pharyngeal slits.

– The pharyngeal slits do not open directly to the exterior.

• Instead they open into the Atrium , a fluid - filled cavity that surrounds the pharynx.

– Food cleansed waters are pumped from the pharynx into the atrium and expelled out of the body by a ventral opening called an Atriopore.

– The slits do not serve a respiratory role but instead help to facilitate filter feeding.

– Amphioxus uses the skin for respiration.– The number of pharyngeal slits can number up to 60.– They are reinforced by bars of cartilage termed Pharyngeal

Bars.

Amphioxus (Anatomical Features)

• Amphioxus has a notochord that runs from the rostrum to the tip of the tail.– Unlike the typical notochord, the notochord of

amphioxus consists of sequentially arranged muscular discs.

• These muscular discs have fibers that anchor the discs to a surrounding notochord sheath composed of fibrous connective tissue

• Muscular contractions increase the stiffness of the notochord that may help in digging.

Amphioxus (Anatomical Features)• The nervous system of amphioxus is similar to that

of vertebrates.– It has a CNS consisting of a brain and spinal cord.

• The CNS is covered by a vascular membrane called the Leptomeninx

• The brain of amphioxus has two subdivisions.– Radiating from the spinal cord are spinal nerves.– Amphioxus does possess special senses but they are rudimentary.

• Chemoreceptors are scattered over the surface of the body.They are particularly concentrated in the anterior pharyngeal region and mouth.

• There also are light sensitive pigmented ocelli embedded along the length of the spinal cord in the venterolateral walls.

Amphioxus (Anatomical Features)

• Amphioxus feeds by filtering food out of the water.– Anterior to the mouth is the Vestibule , a

structure designed o collect seawater.• The vestibule is surrounded laterally by the Oral

Hood.• Caudally there is a perpendicular membrane

called the Velum.– The velum is perforated by the mouth.– The mouth opens into the pharynx.

Amphioxus (Anatomical Features)

• Cilia on the pharyngeal bars pull water through the vestibule, through the mouth, and into the pharynx.– Within the vestibule is a mucus covered structure called

the Wheel Organ.• The wheel organ has stubby projections to help trap food

missed by the mouth.

• The opening of the vestibule is lined by Buccal Cirri which serve to partially strain incoming water.– The buccal cirri have chemoreceptor's to monitor

incoming water.– Once food is in the pharynx it will be processed

Amphioxus (Anatomical Features)

• The processing of food requires specialized features of the pharynx:– The pharynx has a ventral invagination lined with

cilia and mucus called the Endostyle or Hypobranchial Groove.

– There is also a dorsal invagination lined by cilia and mucus termed the Epibranchial Groove.

– Connecting these two grooves are Pharyngeal Bands.• Pharyngeal bands cover the pharyngeal bars and are

covered by mucus and cilia.• Mucus secreted by these structures trap food.

Amphioxus (Anatomical Features)

• The mucus and food become sticky mucus strands that are propelled by ciliary beating into the epibranchial groove and then pass caudally into the gut..– In the gut digestive enzymes will be added to the

food strand.– Ciliary beating pushes the food – enzyme-mucus

mixture into theintestine.• Some of the mixture is passed to the cecum for the

introduction of more enzymes.• The intestine terminates at the anus.

Amphioxus (Anatomical Features)

• The coelom is reduced in adult amphioxus.– It becomes laterally compressed as the

larvae elongates and more pharyngeal bars are added.

– As a result, only a trace coelom is found in the adult.

Amphioxus (Anatomical Features)

• Amphioxus circulation differs significantly from that of vertebrates.– It lacks a heart and formed elements– The blood is a colorless serum.

• One similarity to vertebrate circulation is that the amphioxus venous pathways resemble those of embryonic vertebrates.

Amphioxus (Anatomical Features)

• Cephalochordates lack an organized kidney.– Instead individual cellular components

called Cyrtopodocytes filter the blood.– Cyrtopodocytes are intermediary between

the Protonepheridia of invertebrates and the podocytes of vertebrates.

Amphioxus (Anatomical Features)

• In mature amphioxus, the gonads will be prominent and will bulge into the atrium.– The gonads release sperm or eggs into the atrium.

• The gametes are released from the atrium by the atriopore.

– Amphioxus are Dioecious meaning that they develop only ovaries ortestes.

– Amphioxus resembles the larval form of the lamprey called the Ammocoete.

• Initially the ammocoete larvae were actually classified as protochordates.

• One reason is that they stay in the larval state for a long time (between 2 to 6 years).

Ammocoetes VS. Amphioxus

• Ammocoetes have a notochord running from their midbrain into the tail.

• Like amphioxus ammocoetes have a dorsal, hollow nerve cord and a brain.– Development of the nerve cord is the same for both.– One difference is that the ammocoete brain has

three vesicles.– Ammocoetes have discrete special sense organs

although they are rudimentary by the typical vertebrate standard.

The VertebratesChapter IV

The Agnathans

• The vertebrates can be divided up into two super classes based on the presence or absence of jaws.– The jawed vertebrates are called the Gnathostomata.

• b) The vertebrates lacking jaws are called the Agnathostomata.– Agnathans are considered to be the most primitive

vertebrate taxon.– Today only two extant groups exist; lampreys and

hagfish.

The Agnathans

• Although there are only two extant groups today, there are a large number of extinct agnathans called the Ostracoderms.– Ostracoderms were a group of armored fish- like

organisms of the late Cambrian through the early Devonian periods.

– Ostracoderms are the oldest known vertebrates with fossils dating back to the Ordovician period and they are expected to have existed prior to that during the late Cambrian.

The Agnathans :Ostracoderm Anatomy

• Ostracoderms were typically small marine species being between 2 to 3 cm in length.– However some species grew up to 1 m in length.

• They are characterized by lacking jaws, lacking paired fins, and having a body composed of overlapping plates of dermal bone.

• The plates were largest on the head forming a bony shield and smaller, more tile - like bones, on the trunk and tail.

• Ostracoderms had three eyes.– There was a pair of upwardly oriented orbital eyes.– There was a third singular eye called the Pineal Eye.

The Agnathans :Ostracoderm Anatomy

• They had a singular naris (nostril) which opened into a Nasopharyngeal Duct connecting to an olfactory sac.

• They had a small, jawless mouth.• They had rows of external gill slits.• Ostracoderms , along with the dermal bone,

also had an endoskeleton composed of bone and cartilage.

The Agnathans:Extant Agnathans

• The extant agnathans are hagfishes and lampreys.• The two groups are not really closely related and occupy two

separate classes.– Hagfishes belong to the class Mixini.– Lampreys belong to the class Cephalospidomorphi that also includes

the ostracoderms.• Extant agnathans share some common characteristics:

– They have a prominent notochord that forms the axial skeleton throughout the agnathan`s life.

– They lack a bony skeleton, scale, or dermal bony plates.• They lack bony jaws.• They lack a vertebral column.• These missing features may actually be derived characteristics that evolved to

maximize a life as a parasite.

The Agnathans:Extant Agnathans

• Agnathans have fewer semicircular canals than do gnathans (which have three).– Lampreys have two semicircular canals.– Hagfish have only one.– They had a singular naris that opened into a

nasopharyngeal duct connecting to an olfactory sac.

– The mouth is surrounded by a Buccal Funnel.

The Agnathans: The Hagfishes; Class Mixini

• Hagfish are a class of marine, eel - like organisms.• They are bottom dwelling scavengers.• Hagfish have shallow buccal funnel surrounded

by finger – like papillae resembling short tentacles.

• They have vestigial eyes covered by a thin, opaque membrane.– Lampreys have better developed eyes.

• The number of pairs of gill slits varies between species.

The Agnathans: Lampreys

• Lampreys resemble hagfish in appearance being eel - like aquatic organisms.

• Most species are marine but some spend at least part of their lives in freshwater.

• Lampreys have changed very little from the time of the Carboniferous.

• All lampreys are parasitic, feeding off of the blood of fishes.– They have a well developed buccal funnel with tooth - like

structures that anchor the lamprey on to its prey.– Lamprey have a muscular tongue coated with tooth - like

denticlesdesigned to scrap away the flesh of the prey so as to feed on its blood.

The Agnathans: Lampreys

• Like ostracoderms, lamprey have a singular naris that opened into a nasopharyngeal duct connecting to an olfactory sac.– Unlike ostracoderms, in lampreys the olfactory sac

terminates blindly into a Nasohypophyseal Sac.• All lamprey species have seven pairs of gill

slits.

The Vertebrates

Superclasss Gnathostomata

Class Placoderm

• Placoderms are the earliest of the jawed vertebrates existing during the Paleozoic Era and primarily during the Devonian Period.– They are extinct.– They evolved from the ostracoderms.

• Placoderms were named for their bony dermal plates.

• They had paired fins; both pelvic and pectoral.• Placoderms species ranged in size from a few inches

in length to a species that reached 20 ft.

Class Chondrichthyes

• Chondrichthyes is an ancient class and has two extant subclasses; Elasmobranchii and Holocephali.

• Elasmobranches have a cartilaginous skeleton.– The only bony component is found in the teeth and placoid

scales.• Male elasmobranches have structures called Claspers which

serve as intermittent organs.– Elasmobranchs are the sharks and rays.

• They first arose in the late Silurian Period and continue to exist today.• They have exposed gill slits, typically five pairs, and one pair of spiracles.• Holocephalians are the chimera (aka; rat fishes).

– They differ from elasmobranches in: an operculum that covers thegill slits, and the teeth are replaced by bony plates.

Class Teleostomi: Acanthodians

• The acanthodians are a group of fishes named for their hollow spines.– Along with spines, bony plates also cover the

body.– They had an operculum.– The skeleton had both cartilaginous and bony

elements.• They are extinct having died out in the

Carboniferous Period.

Class Teleostomi: Osteichthyes

• The osteichthyes are the ray- finned fishes.– They include both ancient and modern bony fishes.

• The osteichthyes can be divided into the Actinopterygii and Sacropterygii.

• They first arose sometime between the early Devonian and late Silurian.

• The group is named for their bony skeletons (although that is not true for all).– They have membranous fins supported by bony rays.– They have an operculum.– They have scales.

Class Osteichthyes : Subgroup Actinopterygii

• Subclass Chondrostei– This is a primitive group of osteichthyes have few

extant species.– Today represented only by the paddlefish and

sturgeons.– They have an endoskeleton composed mostly of,

but not exclusively of, cartilage.

Class Osteichthyes : Subgroup Actinopterygii

• Subclass Neopterygii– The Gars and Bowfins– This division represents another primitive group of

fishes having few extant species, the gars and bowfins.

– Unlike the chondrosteins, their skeleton is mostly ossified.

– A distinct feature of this div ision is their scales that are Ganoid scales.

Class Osteichthyes : Subgroup Actinopterygii

• Division Teleostei– The teleosts are the modern fishes.– They are the most successful group of vertebrates

today.• The teleosts make up 96% of all extant fish species and

have three suborders.

Class Osteichthyes : Subgroup Sarcopterygia

• This is a group of bony fishes having a fleshy lobe to their paired fins.

• The sarcopterygians gave rise to the tetrapods.

• They have internal nares that open internally into the oropharyngeal cavity.

• They retain a gas filled air sac that has the potential to develop into a lung.

Class Osteichthyes : Subgroup Sarcopterygia

• The sarcopterygians have two orders:– Actinistians, the coelacanths– Rhipidistians, the lungfishes.

• This order includes the suborder Dipnoans which is the group that gave rise to amphibians.

Class Amphibia

• The amphibians are the first tetrapods.• They first appeared in the late Devonian and

began to radiate in the Carboniferous.• The class can be divided into two subclasses:

– Labyrinthodontia were a group of early extinct amphibians.

– Lissamphibiaare the modern, extant amphibians.

Class Amphibia: Subclass Labyrinthodontia

• The labyrinthodonts were the first amphibians.– The oldest known representative was Ichthyostega that

appeared in the late Devonian.• Anatomical Features:

– The labyrinthodonts are a diverse group but all share a similar patternon the apical surfaces of their teeth (which gave the subclass its name).

– They share many features with rhidipstian fishes such as:– dermal scales, a fish - like tail supported by ray, rhidipstian -

like skull and the presence of a lateral line, at least on the skull.

• Today w see lateral lines in the tadpole but it is lost in the adult form of extant groups.

Class Amphibia: Subclass Labyrinthodontia

• One group of labyrinthodonts was the Anthracosaurs.– This group is believed to have given rise to amniotes in

general and to reptiles in particular.– They included the Seymouria that is believed to have laid

in leathery eggs.• Another group of labyrinthodonts were the

Temnospondyls.– Temnospondylian skulls display some features found in

modern amphibians (especially frogs) and is believed to be the ancestor of the Lissamphibia.

Class Amphibia: Subclass Lissamphibia

• These are the modern amphibians. There are three groups.– Order Anura/Salientaare the frogs and toads.

• Anuran fossils date back to the Triassic.

– Order Urodela/Caudataare the salamanders.– Order Apoda/Gymnophiona are the caecilians.

• This is a group of tropical, limbless amphibians.• They can either be aquatic of burrowing.• The apoda may have a different origin than do the rest

of theLissamphibia.

Class Reptilia

• The reptiles are the first Amniotes.– Amniotes include all of the mammals, birds, and reptiles.

• Amniotes are characterized by:– the development of shell - covered eggs

• This trait can be replaced over evolutionary time by becoming live -bearing.

• This trait increased the ability of tetrapods to conquer land.

– possession of extra embryonic membrane such as the amnion.• These membranes allowed reptiles to lay eggs on land and

avoid an aquatic larval stage. They are:– The Amnion is a fluid filled sac that surrounds the embryo.– The Allantois– The Chorion

Class Reptilia

• Along with the development of the amniotic egg, reptiles show other features that increased terrestrial adaptiveness.– They are covered by scales composed of cornified

epidermal scales so as to decrease dehydration.– They developed a neck.– They have better developed limb girdles and claws on their

digits.– They developed the Metanepheric Kidney.– They display the partial or complete division of the heart

into right and left chambers.

Class Reptilia

• Modern and ancestral reptiles are Ectothermic.• However they have given rise to endothermic tetrapods on a

few occasions such as mammals, birds, and possibly some dinosaurs.

• Reptiles developed from a labyrinthodont, most likely an Anthracosaur, during the early Carboniferous.– Based on skull structure, two reptilian lineages arose from

a common ancestor.– Synapsids are the lineage that gave rise to mammals.

Class Reptilia

• The skull possesses one temporal fossa on each side.

• They are believed to have evolved separately from the other reptiles.

• Reptilia(or true reptiles or Sauropsida are all the modern reptiles, the dinosaurs, and gave rise to the birds.

Class Reptilia

• Reptilia can be divided into two subgroups based on skull structure:– Anapsids are reptiles whose skulls lack temporal fossa.

• This is a primitive condition found in basal reptiles.• Today the only extant representatives are the turtles.• Anapsids gave rise to the diapsids.

• Diapsids are all of the other reptiles.– Diapsid skulls have two temporal fossa on each side.– Diapsids are all of the extant reptiles (except for turtles) and many

extinct species. They include:• Squamates; the snakes and lizards.• Archosaurs; the dinosaurs, pterosaurs, and crocodilians

– The diapsid reptiles gave rise to Euryapsid Reptiles that have lost one pair of temporal fossa (only one temporal fossa on each side). Euryapsids were the plesiosaurs and ichthyosaurs.

Class Aves

• Aves are bipedal, endothermic amniotes that are covered with insulating feathers and have evolved for flight (although some have lost this trait).– The origin of birds is still controversial.– The prevailing hypothesis today is that they evolved

from small theropod dinosaurs or share a common ancestor with them.

• Both groups share numerous anatomical features.• Some even classify birds as reptiles of the group

Archosauria today.

Class Aves

• Along with the previous mentioned traits, birds have evolved other traits for flight.– lightweight bones many of which possess air sacs– loss of teeth– modified forelimbs

• There are two generally recognized subclasses:– Archaeornithesare the extinct, ancient birds (ex;

archaeopteryx).

Class Aves

– Neornithes are the modern birds. There are three orders:• Odontognathaeare a group of extinct, toothed

marine birds.– There only two known representatives ichthyrinis and

hesperornis.

• Palaeognathae(aka; ratites) are a group of flightless birds (ex; ostriches and rheas).

• Neognathae(aka; carinates) are all other modern birds.

– Neognathans have a large keel, or carina, for anchoring flightmuscles.

Class Mammalia

• Mammals are endothermic amniotes covered by insulating hair and producing milk for their young.

• Mammals evolved from synapsid reptiles.• Synapsid reptiles diverged from all other reptiles very

early inreptilian history (in the early Carboniferous).– The earliest synapsid reptiles were the Pelycosaurs.– The pelycosaurs evolved into the Theraspids that gave rise to

mammals.– There are a number of features found in modern mammals

that were present in therapsids:• such as 2 occipital condyles, a secondary palate, and heterodontic

teeth.

Class Mammalia• Mammals first appeared in the Triassic period.• Modern mammals still display a synapsid skull.• Other features of mammals include:

– 3 auditory ossicles– a muscular diaphragm separating the thoracic and

abdominopelvic cavities– absence of a cloaca in the adult in all mammals excepting

themonotremes– the vast majority possess sweat glands– presence of heterodontic teeth with adult and juvenile dentitions– The homodontic teeth of cetaceans are a derived characteristic.

Class Mammalia

• Other features of mammals include:– loss of the fourth aortic arch– a single dentary bone on each side of the lower jaw that

willarticulate with the squamosal bone– pinna to gather sound waves– a more specialized larynx– extensive development of the cerebral cortex.

Class Mammalia

• Based on reproductive strategy there are two groups of mammals.– Prototheria (or Monotremes) are egg laying mammals.– Metatheria are mammals that give rise to live young. There

are two groups:• Metatheria(or Marsupials) give birth to almost larval

young that migrate to, and finish development in a pouch.

– They lack a placenta.– Instead a yolk sac serves as the role of the placenta.

• Eutheria (or Placental Mammals) have young that receive nourishment via a placenta while in the womb.

Development of the Vertebrate

Body Chapter V

Vertebrate Eggs

• Vertebrate eggs vary in the amount and location of their yolk.– The amount of yolk present causes vertebrate eggs to be

divided into 3 categories:• Microlecithal Eggsare eggs having very little yolk.

– Microlecithal eggs are found in amphioxus and therianmammals.

• Mesolecithal Eggs are eggs having a moderate amount of yolk.– Mesolecithal eggs are found in freshwater lampreys, actinopterygians,

gars & bowfins, lungfishes, and amphibians.

• Macrolecithal Eggsare eggs having a lot of yolk.– Macrolecithal eggs are found in marine lampreys, teleosts, reptiles,

birds, and monotremes.– Yolk distribution occurs in two patterns in vertebrate eggs:

Vertebrate Eggs

• Isolecithal is the pattern where the yolk is evenly distributed throughout the egg.– The isolecithal pattern tends to be found in eggs having a

microlecithal yolk content.– Isolecithal eggs are found in therian mammals.

• Telolecithal is the pattern where the yolk is unevenly distributed.– This results in the egg being divided into two poles.

• The Vegetal Pole is a large yolk mass.• The Animal Pole contains the embryo.

– The telolecithal pattern is found in eggs having a macrolecithal or a mesolecithal yolk content.

– Telolecithal eggs are found in all nontherian vertebrates ( including amphioxus).

Oviparity and Viviparity

• Oviparous animals lay eggs.– The eggs contain all of the nourishment necessary

for the development of the offspring.– If there is not enough yolk, as is the case with

amphibians, the young will hatch in a larval form.• Viviparous animals give birth to live young.

– There are two patterns of viviparity based on the source of nourishment.

Viviparity

• Ovoviviparous - nourishment comes form yolk as in oviviparous animals.– The egg hatches shortly before birth or very shortly after birth.

• The mother`s body provides protection for the egg but does not provide nourishment or oxygen.

• Found, for example in Squalus sulkeyi.

• Euviviparous – nourishment is supplied by the mother.– Maternal tissues supply nourishment and oxygen and also remove

wastes.• ex; placental mammals

• There are a variety of intermediary forms.– For example, some reptiles nourish the embryos with yolk early in the

pregnancy and then by maternal tissues later in the pregnancy.

Methods of Fertilization

Fertilization

• Fertilization can be either external or internal.– Internal fertilization occurs in species that are viviparous or are

oviparous species that produce hard shelled eggs.• The male will have an organ designed to introduce sperm into the female

reproductive tract called an Intromittent Organ.• Oviparous animals that produce soft shelled eggs will use external

fertilization.

– External fertilization occurs in most fishes and anuran amphibians.• One exception are rodeles and apodans which utilize internal fertilization

even in oviparous species.• Due to the needs of external fertilization, reproduction will take place in an

aquatic environment.• To compensate for the diluting effects of water, a copious amount of sperm

is released over the eggs as they are being laid.

Early Development in Representative Chordates

• From Zygote Stage Through the Blastula Stage– The Zygote is the fertilized egg, the first

embryonic stage.– Once fertilization has occurred the zygote

immediately begins to undergo a special form of cell division termed Cleavage or Segmentation.

– Unlike typical cell division, cleavage lacks a resting period between mitotic events meaning that there is no time to allow for the recovery of cytoplasm.

Early Development in Representative Chordates

– As a result, each time the cell divides, the two daughter cells are smaller than was the mother cell.

– So the eight celled embryonic stage is roughly the same size as is the one celled zygote.

– The embryo will become subdivided into smaller and smaller cells until forming a hollow ball of cells termed the Blastula.

– Each cell of a blastula is termed a Blastomere and the hollow center of the blastula is called the Blastoceol.

Patterns of Blastula Development

• In microlecithal eggs the blastomeres are of approximately equal size.

• In mesolecithal eggs the increased yolk content (compared to microlecithal eggs) interferes with cleavage resulting in blastomeres of unequal size.– Cells at the animal pole will divide faster than will those at the vegetal

pole.– As a result, the cells of the animal pole will divide faster and be smaller

than will those of the vegetal pole.• In macrolecithal eggs there is even a higher yolk content causing

even more interference with cleavage.– As a result, cleavage is restricted to the animal pole.– This results in a group of cells called a Blastoderm resting on a large

aggregate of yolk.

Patterns of Blastula Development

• Therian mammals have microlecithal eggs with very little yolk and do not show the typical vertebrate pattern of animal and vegetal poles.– As a result therian mammals show a unique cleavage

pattern.– Cleavage starts out as in other vertebrates but will show

divergences as it progresses.– So the blastula in therians is given a special term, the

Blastocyte.• Cleavage will result in a group of cells located within the

blastocoel appearing during the 16 cell stage called the Inner Cell Mass.

Patterns of Blastula Development

• By the 32 cell stage the outer layer of the embryo has expanded to form the Trophoblast.– The trophoblast can absorb uterine secretions to

nourish the inner cell mass prior to the establishment of the placenta.

– The trophoblast will develop into the placenta and the chorion.

Gastrulation

Gastrulation

• Gastrulation is a series of cellular movements in the embryo resulting in the formation of the three primordial germ layers.– The three primordial germ layers are: Ectoderm,

Mesoderm , and Endoderm.– During gastrulation the embryo will go from a hollow

ball of cells to a three layered embryonic stage in which bilateral symmetry is established.

– These cellular migrations are called Formative ,or Morphogenetic, Movements and are controlled by chemical agents termed Morphogens.

Gastrulation in Amphioxus, An Animal with a Microlecithal Egg• Beginning with the blastula: a group of blastomeres,

termed Presumptive Endoderm ,invaginates into the blastocoel.

• This process is called Involution.• Involution causes the formation of the Archenteron,

the primitive gut.– This embryonic stage is called the Gastrula.– The entrance into the archenteron is termed the

Blastopore.– Cells from the blastopore migrate into the gastrula to

begin the formation of the notochord.

Gastrulation in Amphioxus, An Animal with a Microlecithal Egg• As a result, these presumptive endodermal cells are

termed Presumptive Notochord Cells.• The early forming notochord will temporarily form

the roof of the archenteron.• Presumptive Mesoderm Cells will begin to aggregate

lateral to the newly forming notochord and become undifferentiated mesoderm.

• At this stage the amphioxus embryo is composed of an outer layer of ectoderm and an inner layer of endoderm surrounding the archenteron.

Gastrulation in Amphioxus, An Animal with a Microlecithal Egg

– The roof of the archenteron has the notochord flanked by undifferentiated mesoderm.

– Once the mesoderm has become established, the undifferentiated mesodermal cells will be arranged in bands.

• These mesodermal bands will fold and move upwards due to the driving force of mitosis that is still occurring rapidly.

• The upwardly folding mesoderm will form Mesodermal Pouches(aka; Coelomic Pouches posterior of the animal.– Each mesodermal band will produce its own mesodermal

pouch.– At this point, the amphioxus larva has hatched and is swimming

about in its environment.

Gastrulation in Amphioxus, An Animal with a Microlecithal Egg• Once the mesodermal pouches form, they begin to extend ventrally

between the ectoderm and endoderm.– The right and left mesodermal pouches will eventually extend to and

meet at the ventral aspect of the larva to form the coelom.• The fate of the mesoderm in the mesodermal pouches:

– The outer wall of each pouch lays against the ectoderm and is now termed Somatic Mesoderm.

– Together with the ectoderm somatic mesoderm makes up the body wall or Somatopleure.

– The inner wall lays against the endoderm and is now termed Splanchnic Mesoderm.

– The coelom is the space between the splanchnic and somatic mesoderm.

Gastrulation in Amphioxus, An Animal with a Microlecithal Egg• The development of the mesodermal pouches occurred in a

segmental pattern in amphioxus.– The pouches on the right and left hand sides will fuse

ventrally.– Later the fused pouches will begin to fuse to one another

between adjacent segments starting anteriorly and progressing posteriorly.

• The germ layers are now established and Organogenesis (the formation of organs and organ systems) can occur.

Gastrulation in Frogs, An Animal with a Mesolecithal Egg• In vertebrates other than therian mammals yolk in the

vegetal pole affects Gastrulation.– In frogs a process called Epiboly occurs.

• The small, rapidly dividing cells of the animal pole will grow downwards over the large, slower dividing, cells of the vegetal pole.

• This group of rapidly dividing cells will flow inwards, over the yolk forming a blastopore and an archenteron. This is called Involution.– The yolk cells will make up the floor of the archenteron.– The rest of embryogenesis is similar to that seen in

amphioxus .. .

Notochord and Dorsal Mesoderm Formation in Frogs/MesolecithalEggs

• As involution progresses, a stream of undifferentiated cells will flow over the dorsal lip of the blastopore.

• These cells establish a narrow band on the roof of the archenteron called the Chordomesoderm.– A notochord develops from the midline of the

chordomesodermand will temporarily remain on the roof of the archenteron.

Notochord and Dorsal Mesoderm Formation in Frogs/Mesolecithal Eggs

– The chordomesoderm cells lateral to the developing notochord will give rise to a pair of undifferentiated mesodermal bands (lateral to the notochord).

• These bands of mesoderm form the dorsal mesoderm.

• The dorsal mesoderm will become segmental and form hollow mesodermal somites.– The somites will form the musculature of the body

wall.

Endoderm and Ectoderm Development Formation in Frogs/Mesolecithal Eggs

• While the notochord and dorsal mesoderm are developing, presumptive endodermal cells will migrate into the interior of the embryo, spread out, and provide (with the yolk cells) the lining of the archenteron. These cells will be the endoderm.

• The cells on the surface of the gastrula become the ectoderm.

Intermediate and Lateral Plate Mesoderm Development Formation in Frogs/Mesolecithal Eggs• Mesodermal cells (other than those in the

somites) will migrate into the gastrula through the blastopore.

• These mesoderm cells expand out as a sheet moving cephaladbetween the ectoderm and the endoderm.– This mesoderm is termed Lateral Plate .

Intermediate and Lateral Plate Mesoderm Development Formation in Frogs/Mesolecithal Eggs

– Lateral Plate Mesoderm will divide into two sheets:

• Somatic Mesodermwill be the outer sheet.• Splanchnic Mesoderm will be the inner sheet.• The coelom is the space between the somatic and

splanchnic mesoderm.

Intermediate and Lateral Plate Mesoderm Development Formation in Frogs/Mesolecithal Eggs• Somatic mesoderm and the ectoderm together form

the somatopleure.• Splanchnic mesoderm and the endoderm together

form the splanchnopleure.• A portion o the lateral plate mesoderm does not

follow this pattern.– This portion of the lateral plate mesoderm is located dorsal

to therest of the lateral plate mesoderm and immediately lateral to the somites.

• These cells will give rise to much of the urogenital system.• This mesoderm is called Intermediate Mesoderm or Nephrogenic

Mesoderm.

Gastrulation in Birds, An Animal With a Macrolecithal Egg• Due to the greater yolk content of macrolecithal eggs, epiboly

cannot occur.– Remember cleavage does not occur in the yolk in macrolecithal eggs.

• Only the cells of the animal pole will demonstrate cleavage and give rise to a Blastoderm.

– The blastoderm organizes into an upper sheet of cells (theEpiblast) and a lower sheet of cells (the

– Hypoblast) separated by the blastocoel forming a Bilaminar Embryonic Disc.

• Cells from the periphery of the hypoblast will grow downward overthe yolk sac.• They will become part of the endoderm lining the yolk sac.• The hypoblast cells do not become a portion of the developing embryo.

– Instead the hypoblast cells are limited to the yolk sac and its stalk.– The cells of the epiblast will give rise to all of the structuresof the

developing embryo.

Notochord and Mesoderm Formation in Birds/Macrolecithal Egg• Cells from the epiblast develop into a Primitive Streak.

– The primitive streak is a thin, multilayered structure running the length of the embryo.

• A thickened nodule of closely packed blastoderm cells forms along the primitive streak called Hensen's Node.– Hensen`s node defines the caudal end of the future embryo.

• The primitive streak and Hensen`s node are the functional equivalent of the blastopore in eggs having less yolk.

• Epiblast cells migrate from the primitive streak into the hypoblastwhere they will displace the hypoblast cells to establish the endoderm.– Epiblasts cells will then migrate a second time. These cells will situate

between the epiblast and endoderm to form the mesoderm.– The Remaining portion of the epiblast is now the ectoderm.– The result of these migrations is the formation of a Trilaminar Embryonic Disc.

A Notochord Process develops from the primitive steak and pushes through the epiblast.• Other cells move forward along the newly forming

notochord to become the dorsal mesoderm.– Simultaneously the lateral plate mesoderm streams in between

the ectoderm and endoderm.– The dorsal mesoderm will eventually segment to form somites.

• Simultaneously the lateral plate mesoderm differentiates into somatic and splanchnic mesoderm.

• The splanchnopleure will extend down over the yolk to form a yolk sac.– By the second day of incubation the splanchnopleure produces the

first blood cells and a delicate network of Vitelline Vessels (aka; Omphalomesenteric Vessels) on the Area Opacaof the yolk sac.

• Vitelline vessels carry yolk globules into the embryo.– Carries yolk to the early, “S” shaped heart of the embryo for further distribution.

Gastrulation in Placental Mammals

• Gastrulation begins with the Blastocyte, a hollow ball of cells havinga blastocoel and being divided into a trophoblast and an inner cell mass.– The trophoblast does not contribute to the embryo, It

contributes to the extraembryonic membranes.• It will establish contact with and implant in the uterine wall.

– The cells of the inner cell mass will develop into embryonic structures.

• Using a variety of mechanisms in different placental mammals, the inner cell mass will divide into a Bilaminar Embryonic Disc/Blastodiskhaving an epiblast and primitive endoderm.

Gastrulation in Placental Mammals

• Then epiblast cells will migrate into the space between the epiblast and primitive endoderm to form the mesoderm and a Trilaminar Embryonic Disc.– The epiblast cells at this point will become ectoderm.– Mesoderm cells will develop into dorsal mesoderm,

forming along the developing notochord, and lateral plate mesoderm.

• 1] The lateral plate mesoderm will develop into somatic and splanchnic mesoderm.

Gastrulation in Placental Mammals

• Notochord formation is similar in mammals as it is in macrolecithal eggs which demonstrates our reptilian ancestry.

• In placental mammals the three gem layers begin to form at the same time as the extraembryonic membranes begin to form.

Neuralation

• Neural tissue develops from the ectoderm.– Presumptive neural ectoderm I found in a wide

band on the dorsal aspect of the embryo laying above the notochord.

• As the gastrula elongates, this band thickens forming the Neural Plate/Floor Plate.

• The neural plate develops a pair of folds called Neural Folds on either side of the Neural Groove.

• The formation of the neural folds vs. the neural groove is due to different mitotic rates.

• At this point the embryo is now termed a Neurala.

Neuralation• The neural groove is widest at the anterior extremity of the

gastrula.– This wider region will become the brain.– The remainder of the neural groove will form the spinal cord.

• The neural plate will sink deeper into the neural groove and the two neural folds will grow closer together.

• Eventually the neural folds will meet over the neural tube and fuse together.

• `Now the neural groove is a Neural Tube.• The cavity enclosed by the neural tube is called the

Neurocoel.– The neurocoel will give rise to the ventricle system of thebrain and

the central canal of the spinal cord.

Neuralation

• As a result of the developmental processes involved, the CNS is ectodermal, dorsal, and hollow.– Neuralation will vary a bit between different

vertebrates. The differences are minor overall.• One of the greatest differences is seen in agnathans and

actinopterygians where neural folds do not form.• Instead the neural ectoderm becomes a wedge-shaped

structure called a Neural Keel.– The neural keel will push down into the body above the notochord.– The neural keel will then detach from the surface and cellular

rearrangements will allow for a neurocoelto form. This gives it a structure similar to that in the typical vertebrate CNS.

Mesenchyme

• Mesenchyme is an undifferentiated embryonic tissue.– It is composed of totipotent cells called Mesenchymal

Cells residing in an amorphous extracellular matrix.– Mesenchymal cells can develop into any other cell

type.• They have branching, dendritic processes.• Mesenchyme makes up much of the early embryonic body.• Most mesenchyme is of mesodermal origin.

Mesenchyme

– Some, called Mesectoderm, is of ectodermal origin.

– During organogenesis mesenchyme will become organized into clusters called Blastemas.

• These blastemas will gradually assume the basic outline of future organs and their mesenchymal cells will differentiate to from the appropriate tissues.

– Some Mesenchymal cells are present in the adult.• They will be pleuripotent in nature allowing them to

replacedamaged/lost cells so as to repair tissues.

The Fate of the Ectoderm

• Ectoderm will give rise to:– CNS,PNS, and some of the associated membranes,– sensory epithelium of the special senses and the

retina of the eye,– epidermis, epidermal derivatives, integumentary

glands,– the lining of the stomodeum and the proctodeum

and their derivatives,– numerous other tissues especially those of the head

and pharynx.

Stomodeum and Proctodeum

• The oral cavity begins to form as a midventral invagination of the ectoderm at the anterior end of the foregut on the embryonic head.– This invagination is called the Stomodeum.

• The foregut and stomodeum are initially separated by a thin partition called the Oral Plate.– The oral plate will rupture to allow for communication between

the digestive tract and the external environment.• The stomodeum ectoderm lines the anterior portion of the

oral cavity including at least part of the tongue.– Some of this stomodeal ectoderm will be invaded by endoderm

making it hard to distinguish which cells produced which of the glands located at he junction of ectoderm and endoderm.

Stomodeum and Proctodeum

– In mammals stomodealectoderm gives rise to cells that secrete tooth enamel.

• In other vertebrates neural crest cells secrete enamel.

– An evagination called Rathke`s Pouchforms from the roof of the stomodeum prior to the rupturing of the oral plate.

• Rathke`s pouch will give rise to the adenohypophysis.• The proctodeum is another ectodermal invagination (like

the stomodeum) but it is associated with the hindgut.• The proctodeum and hindgut are separated by a thin

partition called the Cloacal Plate temporarily.

The Neural Crest

• When the neural tube formed some of the neighboring ectodermseparated from the surface ectoderm and the neural tube becoming theNeural Crest.– The neural crest first appears as neuroectodermal

clusters.• One pair ofneural crest will occur in each metamere

and several clusters will occur lateral to the hindbrain and midbrain.

• Neural crest cells proliferate rapidly and give rise to a great number of structures.

The Neural Crest

• When the neural tube formed some of the neighboring ectoderm separated from the surface ectoderm and the neural tube becoming the Neural Crest.– The neural crest first appears as neuroectodermal

clusters.• One pair of neural crest will occur in each metamere

and several clusters will occur lateral to the hindbrain and midbrain.

• Neural crest cells proliferate rapidly and give rise to a great number of structures

The Neural Crest

• Some cells in the head form blastemas for the skeleton of the arches including the jaws.

• Some form portions of the neurocranium and some of the membrane bones that will surround the neurocranium.

• Some are involved in the development of the endocrine glands (The thyroid, parathyroids, and thymus) and glands derived from the pharyngeal/branchial pouches called Ultimobranchial Bodies.

• In agnathans, neural crest cells will give rise to the branchial basket and lingual cartilages.

• Neural crest mesenchyme in the head or the trunk moves ventrally around the notochord to form the autonomic ganglia and adrenal medulla.

The Neural Crest

• Neural crest cells around the neural tube produce the meninges (excepting the dura mater).

• Some neural crest cells will become a variety of pigment cells including those of the skin.

• Some neural crest cells remain close to their point of origin to become the perikarya of sensory neurons whose perikarya are located in PNS ganglia.

• Other neural crest cells will migrates distally along the course of the PNS to give rise to the neurilemma.

Ectodermal Placodes• Ectodermal placodes are paired thickenings of localized portions of

theectoderm that sink beneath the skin and will give rise to neurectoderm and the lens of the eye.– The resulting neuroectoderm will produce neuroblasts and certain

sensory epithelia.• Ectodermal placodes can be grouped a:

– one pair ofOlfactory Placodes/Nasal Placodes• They form above the stomodeum.• They will become part of the lining of the nasal pits (that open into the nostrils).• Some of these cells will develop into the neurosensory cells of theolfactory

epithelium.

– one pair of Otic Placodes in the ectoderm lateral to the hindbrain• The ectoderm of the oticplacodes will become theOtocystsas theysink into the head.

– The otocysts will give rise to the sensory epithelium of the internal ear.

Ectodermal Placodes– one pair of Optic Placodes/Lens Placodes which will develop

into the lens of the eye.– a series of Epibranchial Placodes on the side of the head that

will sink into the head along the midbrain and hindbrain.• These placodes will contribute neuroblasts that become cellbodies in

one pair or more of the ganglia of CN 7, 9, and 10.• Their nerve fibers will innervate the taste buds.

– in fishes and amphibians a series of Linear Series of Placodes will extend the length of he trunk and tail.

• Some of these placodes will migrate into the head.• They will sink into the body to become the Neuromast Organs of the

cephalic and lateral lines.

– Other placodes on the heads of fishes become electroreceptive epithelium.

The Fate of the Endoderm• Endoderm will give rise to the epithelium of the entire digestive tube and

its evaginations.• Endoderm will also give rise to the epithelia of the parathyroids,

ultimobranchial glands, thymus, lining of eustachian tubes, and the lining of the middle ear.– This endoderm derives from the pharyngeal pouches.

• Midventralevaginations of the pharynx such as the thyroid, lungs, and swim bladders, when present, are lined by endoderm.

• Caudal to the pharynx the endoderm will evaginate to give rise to the liver, gall bladder, pancreas, crop sacs, and a variety of gastric and intestinal ceca.

• Since they are derived from the cloaca, mammalian urinary bladders and urogenital sinuses are typically lined by endoderm.

The Fate of the Mesoderm• Dorsal Mesoderm or the Epimere

– Most of the dorsal mesoderm will develop into block - like segments running in pairs lateral to the notochord called Somites.

• They run along the notochord and neural tube throughout the tail, trunk, and, to a lesser degree, the head.

• The somite will differentiate into three regions (running superficial to deep):

– Dermatome – gives rise to the dermis but only of the dorsal body.

» The remainder of the dermis arises from the lateral platemesoderm.

– Myotome – gives rise to skeletal muscles.– Sclerotome – gives rise to the vertebrae, ribs, and portions

of the neurocranium.

The Fate of the Mesoderm

– Some of the dorsal mesoderm located anterior to the somites arearranged into incomplete segments called Somitomeres.• Somitomeres lack the dermatome and

sclerotome components.– So somitomeres give rise to skeletal muscles.

» These muscles are the extrinsic ocular muscles and thebranchniomeric muscles

Lateral Plate Mesoderm or Hypomere

• Lateral plate mesoderm is confined to the trunk and divides into somatic mesoderm and splanchnic mesoderm (which is separated by the coelom).

• Somatic mesoderm will give rise to:– connective tissues of the body wall– blood vessels of the body wall– the skeleton of the body wall, limb girdles, and limbs– dermis of the body wall– and the parietal peritoneum.

Lateral Plate Mesoderm or Hypomere

• Splanchnic mesoderm gives rise to:– smooth muscles and connective tissue system of the

digestive tube and its outpocketings– smooth muscles and connective tissue system of the

heart and visceral blood vessels– The Intermediate Mesoderm or Mesomere

• Intermediate mesoderm is a pair of unsegmented mesodermal ribbons running the length of the trunk between the lateral plate mesoderm and the somites.

• It will give rise to kidney tubules and the longitudinal ducts of the urogenital system.

– For this reason it is also known as “Nephrogenic Mesoderm”.

The Extraembryonic Membranes

• Most vertebrates possess specialized membranes that extend beyond the embryonic body.– They arise early during development and provide

a number of essentialfunctions.– The chief extraembryonic membranes are: yolk

sac, amnion, chorion, and allantois.• The yolk sac is the most primitive and is founding all

vertebrates.• The other three are found only in the amniotes

(reptiles, birds, andmammals).

The Yolk Sac

The yolk sac surrounds the yolk and empties into the midgut.– It is lined by endoderm.– It is highly vascularized.

• The associated blood vessels are termedVitellinearteries, veins.• Vitelline vessels are confluent with blood vessels of the

embryonic body.

– As the embryo grows the yolk sac will diminish.• As it atrophies, the yolk sac will disappear into the ventral body

wall.– The remaining intracoelomic yolk sac will either becomeincorporated

into the wall of the midgut or remain as an outpocketing.

The Yolk Sac

• Embryonic sharks develop a large yolk diverticulum (or outpocketing) within the coelom.– This yolk diverticulum is still present when the shark is born and

will sustain it for several days.• In viviparous species of fish and amphibians the yolk sac,

being highly vascularized and close to internal tissues, will often serve as an oxygen absorbing membrane to receive oxygen from the mother.– In this case it is sometimes referred to as a “Simple Yolk Sac

Placenta”.• Therian mammals lack yolk but still will have a yolk sac.

– It is called Meckelʼs Diverticulum.– It is a sign of our reptilian heritage.

The Amnion and Chorion

• Amniotic embryos develop within two membranous sacs, the amniotic and chorionic sacs.

• Both the amnion and chorion form when upfoldings of the somatopleure meet above the embryo and fuse together.– This will form the amniotic sac that will surround the embryo.– The somatopleure continues to extend and will grow around both

the amniotic sac and the yolk sac to give rise to the chorionic sac.• The evolution of these membranes allowed for the

development of the amniotic egg.– The amniotic sac surrounds the embryo and holds a fluid called

Amniotic Fluid.• Amniotic fluid is mostly metabolic water from embryonic tissues.• It will protect the embryo from dehydration and mechanical trauma.

The Amnion and Chorion

• The chorion will lay in close association with either the eggshell or the maternal body wall.– The chorion will keep the embryo supplied with

oxygen either through the eggshell or from the mother.

– In viviparous species the chorion also supplies the embryo with nourishment from the mother.

The Allantois

• The allantois arises as a midventral evagination of the embryo’s cloaca.– Normally it grows until it comes in contact with the chorion.– This forms the Chorioallantoic Membrane.

• In monotremes and most reptiles the chorioallantoic membrane serves for respiration.

• In eutherian mammals it is in contact with the maternal lining and makes up the embryonic portion of the placenta called the Chorioallantoic Placenta.

– The base of the placenta, that portion closest to the cloaca, becomes the urinary bladder in amniotes.

– In mammals a portion forms a portion of the umbilicus called the Urachus or Middle Umbilical Ligament.

The Placenta

• The placenta, in its broadest sense, is any region in a viviparous animal where maternal and embryonic tissues of any type are in intimate contact and allow for an exchange of materials.– This would include the simple yolk sac placenta of some non -

amnioticspecies.– Most people consider a true placenta to exist only in amniotes.

• A true placenta would consist of:– a highly vascularized region of extraembryonic membrane

suchas yolk sac, chorionic membrane, chorioallantoic membrane, orchorion

– associated highly vascularized maternal lining

The Placenta

• Eutherian mammals have a chorioallantoic membrane.• In most marsupials the yolk sac, not the allantois, lays

against the chorion forming a Choriovitelline Placenta.– The extraembryonic membrane only lays closely against the

endometrium and can simply peel away from the endometrium during parturition.

– This is called a Contact or Nondeciduous Placenta.• Placental mammals have finger-like extensions called

Chorionic Villi that invade into the endometrium.– As a result during parturition a portion of the endometrium

islost.– This is termed a Deciduous Placenta.