1 mm

Morphogenesis process by which an animal takes shape

determined by genome and differences between embryonic cells

The Acrosomal Reaction

Triggered when sperm reaches egg

releases hydrolytic enzymes that digest material surrounding the egg

Sperm-bindingreceptors

Jelly coat

Acrosome

Actin

Spermhead

Basal body(centriole)

Sperm plasmamembrane

SpermnucleusContact

Acrosomalreaction

Acrosomalprocess

Contact and fusionof sperm and eggmembranes Entry of sperm

nucleus

Cortical reaction

Fertilizationenvelope

Egg plasmamembrane

Vitelline layer

Hydrolytic enzymes

Corticalgranule

Fused plasmamembranes

Perivitellinespace

Cortical granulemembrane

EGG CYTOPLASM

The Cortical Reaction

Initiated by the fusion of egg and sperminduces a rise in Ca2+

stimulates cortical granules to release their contents outside the egg

cause formation of a fertilization envelope block to polyspermy

1 sec beforefertilization

Point ofspermentry

10 sec afterfertilization

Spreading waveof calcium ions

20 sec 30 sec

500 µm

Activation of the Egg

sharp rise in Ca2+ increases the rates of cellular respiration and

protein synthesis by the egg cell = activation of egg cell

Binding of sperm to egg

Acrosomal reaction: plasma membranedepolarization (fast block to polyspermy)

Increased intracellular calcium level

Cortical reaction begins (slow block to polyspermy)

Formation of fertilization envelope complete

Increased intracellular pH

Fusion of egg and sperm nuclei complete

Increased protein synthesis

Onset of DNA synthesis

First cell division

1

Se

co

nd

s

2

3

68

10

4

20

30

501

2

40

34

10

5

20

3040

9060

Min

ute

s

Fertilization in Mammals

the cortical reaction modifies the zona pellucida as a slow block to polyspermy

Folliclecell

Acrosomalvesicle

Egg plasmamembrane

Zonapellucida Sperm

nucleus

Corticalganules

Spermbasalbody

EGG CYTOPLASM

Cleavage

a period of rapid cell division without growth

partitions the cytoplasm of one large cell into many smaller cells blastomeres

Fertilized egg Four-cell stage Morula Blastula

Polarityeggs and zygotes of many animals (except

mammals)Animal poleVegetal pole

Most of yolk

Anterior

Right

Animal pole

Graycrescent

DorsalVentral

Left

Posterior

Body axes Establishing the axes

Futuredorsalside oftadpole

Point ofspermentry

Firstcleavage

Vegetalhemisphere Vegetal pole

Point of sperm entry

Animalhemisphere

Cleavage planes follow a pattern relative to the zygote’s animal and vegetal

poles

Zygote

2-cellstageforming

8-cellstage

4-cellstageforming

Animal pole Blasto-coel

Blastula(crosssection)

Vegetal poleBlastula (at least 128 cells)

0.25 mm

Eight-cell stage (viewedfrom the animal pole)

0.25 mm

Meroblastic cleavageincomplete division of the eggoccurs in species with yolk-rich eggs

Ie. reptiles and birds

Blastocoel

Fertilized egg

BLASTODERM

HypoblastEpiblastYOLK MASS

Cutaway view ofthe blastoderm

Blastoderm

Four-cell stage

Zygote

Disk ofcytoplasm

Holoblastic cleavagecomplete division of the eggoccurs in species whose eggs have little or

moderate amounts of yolk Ie. sea urchins and frogs

Gastrulationrearranges the cells of a blastula into a three-

layered embryoa gastrula

has a primitive gut

The three layers produced by gastrulation are called embryonic germ layersThe ectoderm forms the outer layer The endoderm lines the digestive tractThe mesoderm partly fills the space

between the endoderm and ectoderm

Video: Sea Urchin Embryonic Development

Animalpole

Blastopore

Filopodiapullingarchenterontip

Archenteron

Mesenchymecells

Blastocoel

Future ectoderm

Vegetalpole

Key

Future mesoderm

Future endoderm

Vegetalplate

Blastocoel

Mesenchymecells

Archenteron

Blastocoel

Mesenchume(mesodermforms futureskeleton)

50 µm

Mouth

Ectoderm

Blastopore

Digestive tube (endoderm)

Anus (from blastopore)

Future ectoderm

Key

Future mesoderm

Future endoderm

Archenteron

Blastocoelremnant

Ectoderm

MesodermEndoderm

Yolk plugYolk plugGastrula

Blastocoelshrinking

Blastocoel

Dorsal tip of blastopore

CROSS SECTION

Animal pole

Dorsal lipof blastopore

Vegetal pole Blastula

SURFACE VIEW

Organogenesisregions of the germ layers develop into rudimentary organs

the notochord forms from mesodermthe neural plate forms from ectoderm

curves inward, forming the neural tube

Video: Frog Embryo Development

Neural folds

Neuralplate

LM1 mm

Neuralfold

Notochord

Archenteron

Neural plate formation

Endoderm

Mesoderm

Ectoderm

Neuralfold

Neural plate

Neural tube

Formation of the neural tube

Neural crest

Outer layerof ectoderm

Neural crest

SomitesblocksFormed from mesoderm lateral to the notochord

CoelomLateral to the somitesmesoderm splits to form the coelom

1 mm

Notochord

Archenteron(digestive cavity)

Neural tube

Neural crest

Eye Somites Tail bud

SEM

Coelom Somite

Somites

Notochord

ArchenteronEndoderm

Mesoderm

Ectoderm

Neural tube

Eye

Coelom

Somite

Somites

Neural tube

Lateral fold

Yolk stalkYOLK

Form extraembryonicmembranes

Yolk sac

Early organogenesis

Forebrain

Heart

Bloodvessels

Late organogenesis

Adaptations of Amniotesdevelop in a fluid-filled sac in a shell or the

uterusBirdsReptilesmammals

Germ layers give rise to membranes surrounding embryos

Embryo

Amnioticcavitywithamnioticfluid

AllantoisAmnion

Albumen

Yolk(nutrients)

Yolk sacChorion

Shell

Mammalian Development

eggs of placental mammalsSmallstore few nutrientsExhibit holoblastic cleavageShow no obvious polarity

Gastrulation and organogenesis similar to birds and reptiles

Early cleavage is relatively slow

Cleavage forms the blastocyst formstrophoblast, the outer epithelium of the blastocyst,

initiates implantation in the uterus, and the blastocyst forms a flat disk of cells

As implantation is completed, gastrulation begins

The extraembryonic membranes begin to form

By the end of gastrulation, the embryonic germ layers have formed

Blastocystreaches uterus.

Endometrium(uterine lining)

Maternalbloodvessel

Blastocystimplants.

Inner cell mass

Trophoblast

Blastocoel

Hypoblast

Trophoblast

Epiblast

Expandingregion oftrophoblast

Hypoblast

Chorion (fromtrophoblast

Epiblast

Amnioticcavity

Amnion

Yolk sac (fromhypoblast)

Extraembryonic mesoderm cells(from epiblast)

Extraembryonicmembranes startto form andgastrulationbegins.

Amnion

Chorion

Endoderm

Mesoderm

Ectoderm

Yolk sac

Extraembryonicmesoderm

Gastrulation has produced a three-layered embryo with fourextraembryonic membranes.

Allantois

Expandingregion oftrophoblast

Fate Maps

general territorial diagrams of embryonic development

Fate map of a frog embryo

Epidermis Centralnervoussystem

Blastula

Epidermis

Neural tube stage(transverse section)

Endoderm

Mesoderm

Notochord

DevelopmentDifferentiationSignal molecules

Influence gene

Anterior

Organizer regions

Limb bud

PosteriorZPA

AER

50 µm

Apicalectodermalridge