EMBRIOLOGIA&

CICLO CELULAR

JUAN CARLOS MUNEVAR

APPLICATIONS OF MITOSIS AND MEIOSIS

A Characteristic of Living things

Growth, Division, Reproduction

New Terms:• Development – progress through lifetime• Growth – increase in size, volume, mass• Morphogenesis – development of shapes• Epigenesis – development from a formless zygote• Differentiation – cells become different from each

other• Determination – fate of cells is set (before

differentiation), usually at gene control level• Induction – One tissue or substance causes

determination, then differentiation, of other cells

APOPTOSIS

EVOLTUION OF EMBRYOS

•Requires multicellularity

•Only becomes embryology when cells are differentiated.

•Follows a common pattern in the Animal Kingdom

Stages in Embryology

Animal Examples

The Egg

• Large, sessile gamete

• Yolk – the phospholipid lecithin

• Haploid nucleus (notice timing for humans and Ascaris)

• Oolemma – two membranes plus jelly layer

• Jelly layer -- Hyaluronic Acid and Proteins

• Cortical granules – vesicles in cortex

The Sperm

• Small, motile gamete

• Flagellated in animals and lower plants

• Haploid nucleus

• Acrosome – specialized cytoplasmic vesicle for digesting jelly layer (hyaluronidase and proteases) and for attaching to Oolemma (bindin)

Fertilization

• Sperm activated by egg hormones (gynagamones)

• Formation of Acrosomal Process (microfilaments)

• Release of hyaluronidase and proteases

• Bindin attach to bindin recognition site on oolemma

Egg Activation

• Depolarization of membrane

• Evacuation of cortical granules

• Elevation of vitelline membrane (separation of vitelline membrane from egg cell membrane in oolemma)

• Blocking of all other bindin sites

• Activation of metabolism in cytoplasm

• Completion of meiosis in some organisms

Early cleavage

• First cleavage – Two-cell stage

• Second cleavage – Four-cell stage

• Additional cleavages in ball shape

• Morula

• Blastula – first morphogenetic movements (cells migrate to form hollow ball)

• Blastocoel (space) and Protoderm (tissue)

Gastrulation• Second Morphogenetic step (first is

formation of the blastula).

• Tissues MOVE relative to each other and relative to their position in the embryo!

• Different cells move different amounts and change shape by different amounts.

• Protein gradients in embryo control this.

• HOX and Homeobox genes in animal kingdom

Morphogenesis

Example of invagination

Cells change shape

Coordinated by position

Role of Cytoskeleton

Early Cleavage Differences

• Protostomes

– Spiral cleavage

– Determinate

– Mosaic pattern

– Blastopore mouth

– Arthropods, mollusks, annelids

• Deuterostomes

– Radial cleavage

– Indeterminate

– Regulated development

– Blastopore anus

– Echinoderms, chordates

Blastula Differences

• Small eggs – symmetrical blastula (Echinoderms and Mammals)

• Large eggs – asymmetrical blastula (Amphibians, Reptiles, Birds)

• Animal Pole – active side ectoderm

• Vegetal Pole – inactive endoderm

Gastrulation in Frog• Morphogenesis: invagination

• Epiboly and Involution for asymmetrical eggs

• Triploblastic – three tissue layers– Ectoderm – outer skin, CNS in vertebrates– Endoderm – lining of the gut– Mesoderm – inner organs and tissues– Coelom – body space in proto- and

Deuterostomes.

Notocord in Chordates

• First tissue to fully differentiate

• Forms in mid-dorsal mesoderm (chordamesoderm)

• Induces formation of neural tube and cranial space in nearby ectoderm (dorsal side)

• Replaced by spinal column (cartilage and bone from mesoderm) in vertebrates

Differentiation of Mesoderm• Mid-dorsal – chordamesoderm• Dorsal – somites – segmented parts of body• Intermediate – thin layer, contributes to kidney

and/or testicular ducts (labeled “nephrotome” in previous slide)

• Lateral Plate – two sheets join at mid-ventral line– Somatic – body side

– Splanchnic – surroundings of gut

– Coelom – body space between two layers of mesoderm

Pattern formation

• Studied first in Drosophila

• HOX and Homeobox genes found throughout animal kingdom

• Frequent repeated inductions followed by determination, then differentiation

• Stimulates morphogenesis, part of development

Chick

• 3-D slide from dorsal side

• Note: – Somites, – Neural tube and cranial space– Notocord

• Compare to Frog slices

CLONING

• Dedifferentiation of carrot cells.

• Demonstration that frog cells are not terminally differentiated.

• More difficult in mammals!!

Summary

• Development is part of a life cycle

• Involves mitosis for nuclear division

• Changes in control of DNA cause differentiation (different proteins made in different concentrations)

• Genes and development steps are regulated by other genes throughout life

LIFE CYCLES

Reproduction

• To “produce again”

• Life cycle includes both diploid and haploid phases

• You, as an individual, are the diploid phase of a life cycle

• Emphasis on phases leads to classification of life cycles

Generalized Life Cycle

Diploid phase

Haploid phase

meiosisFertilization/symgamy

zygote

gametes

Diploid adult??

Haploid adult??

Representative Examples

• Chlamydomonas – haplontic (haploid dominant) life cycle, single celled

• Ulothrix – haplontic life cycle, filamentous (colony?)

• Ulva – diplohaplontic life cycle, multicellular in both haploid and diploid phases, phases equal.

• Fern – diplontic (diploid dominant) life cycle• Animal Kingdom – extreme diplontic life cycle;

learn Echinoderm, Ascaris, human

Haploid Phase

Diploid Phase

HAPLONTIC STYLE OF LIFE CYCLE

Chlamydomonas

• Protist

• Chloroplast

• Haplontic life cycle (Haploid dominant)

• No mitosis in diploid phase of cycle

• Represents very early Eukaryotic life cycles

Chlamydomonas

ULOTHRIX

• Haplontic life cycle

• “Multicellular” in haploid phase (colonial?)

• Both growth and asexual reproduction by mitosis in haploid phase.

• Diploid phase (resting spore) can only perform meiosis (no mitosis in diploid phase)

New Terms

• Gametophyte – the gamete-producing plant. This describes a multicellular HAPLOID phase in plant life cycles. This plant produces gametes by MITOSIS (chromosome number stays the same!).

• Spore – a haploid cell that will divide by mitosis (usually to produce a gametophyte).

More New Terms

• Gametangium – in Ulothrix and many similar algae, it is a specialized cell that produces gametes by mitosis.

• Sporangium, or zoosporangium – a specialized cell that produces spores (zoospores) by mitosis.

• Zoospore – a spore that is flagellated.

Diploid phase

Resting spore only

Haploid phase

Includes spores, a multicellular filament (gametophyte), and gametes

Ulothrix Haplontic style life cycle

Alternation of generations

• Ulva is our example

• Introduction of mitosis in the diploid phase!

• Eons of evolutionary time to develop• Loss of asexual reproduction in the haploid

phase for some species.• Mitosis still in haploid phase for growth and

production of gametes.

New Terms

• Sporophyte – the spore producing plant. This is the multicellular DIPLOID plant. It produces spores by MEIOSIS (chromosome number reduced from diploid to haploid).

• Sporangium – specialized cells or ORGAN (multicellular structure) that produces spores by meiosis. Located on the sporophyte.

Emphasis of Diploid Phase

• Ferns, Conifers, Flowering Plants

• Gametophyte becomes less important in the life cycle (smaller than sporophyte and with a shorter lifetime).

• Sporophyte develops complex tissues and organs.

New Terms for Fern• Thalus – name for the gametophyte (n).

• Archegonium – gametophyte (haploid) organ that produces eggs by mitosis.

• Antheridium – gametophyte (haploid) organ that produces sperm by mitosis.

• Sorus – structure on underside of sporophyte leaf that contains many sporangia (diploid) that produce spores by meiosis.

Evolutionary Trends

• Shift in emphasis from haploid dominant to diploid dominant life cycles.

• Shift from haploid to diploid organisms.• Single to multicellular; increased tissue

differentiation; longer lives• Emphasis on mitosis for reproduction in haploid

phase (asexual) to use of it for growth, then growth in diploid phase

• Meiosis/fertilization ONCE per life cycle (sexual reproduction)

SUMMARY

• Switch in emphasis from dominant haploid to dominant diploid phases.

• Development of multicellularity, then of tissue differentiation.

• Role of MITOSIS undergoes significant change.

• Roles of meiosis and fertilization do NOT change.