Embryological Development Development is fundamentally similar in all eukaryotes Development is fundamentally similar in all eukaryotes There are 3 aspects to embryological development There are 3 aspects to embryological development Cell division Cell division Cell differentiation Cell differentiation Morphogenesis(body shape) Morphogenesis(body shape)

Sea Urchin Mammal Jelly coat Structure of egg Layer of follicle cells Vitelline layer Receptors for sperm Zona pellucida 1.Acrosomal reaction Hydrolytic enzymes in acrosome break down jelly coat & zona pellucida Acrosomal process 2. Sperm contacts egg plasma membrane Proteins on sperm membrane 3. Fast block to polyspermy- electrical reaction prevents multiple sperm entry Modified vitelline layer 4. Cortical reaction-increase in Ca causes formation of fertilization membrane Modified zona pellucida Only nucleus enters 5. Entry of sperm Whole sperm enters 6. Activation of egg-intracellular Ca increases causing increased protein synthesis, pH, cellular respiration Immediate within 20 min.- 1 st cell division in 90 min. 7. Fusion of egg & sperm nucleus Occurs during 1 st division-36 hrs.

Cleavage Zygote rapidly divides-up to 128 cell stage Zygote rapidly divides-up to 128 cell stage These smaller & smaller cells are called blazoners These smaller & smaller cells are called blazoners Each will have different cytoplasm elements Each will have different cytoplasm elements Polarity(except mammals) Polarity(except mammals) Animal pole(rapid) Animal pole(rapid) Vegetal pole(greater cytoplasm/nutrients impedes division Vegetal pole(greater cytoplasm/nutrients impedes division Morula cells Morula cells Develops a fluid filled cavity Develops a fluid filled cavity Blastula-128 cells Blastula-128 cells Internal cavity-blastocoels Internal cavity-blastocoels

Differences in the cleavage process Meroblastic vs holoblastic Meroblastic vs holoblastic Holo-little yolk, even division Holo-little yolk, even division Mero-large yolk, division confined to tiny area called blastodisc Mero-large yolk, division confined to tiny area called blastodisc Radial vs spiral Radial vs spiral Radial in deuterostomes(echinoderms, chordates) Radial in deuterostomes(echinoderms, chordates) Spiral in protostomes(mollusks, arthropods, annelids) Spiral in protostomes(mollusks, arthropods, annelids)

Gastrulation Rearrangement of blastula cells Rearrangement of blastula cells Changes in cell motility, shape, adhesion Changes in cell motility, shape, adhesion Induction- chemical/physical communication Induction- chemical/physical communication Blastopore-cells outside invaginate to form gastrocoel/archenteron Blastopore-cells outside invaginate to form gastrocoel/archenteron Becomes anus(deuterostomes) or mouth(protostomes) Becomes anus(deuterostomes) or mouth(protostomes) 3 germ layers develop(p945) 3 germ layers develop(p945) Later, coelom(body cavity) forms Later, coelom(body cavity) forms

Neurulation Head & nervous system Head & nervous system Neuroectoderm folds into neural tube(CNS) Neuroectoderm folds into neural tube(CNS) Notochord forms(vertebrates) Notochord forms(vertebrates)

Organogenesis Cell division, cell movement, cell differentiation continues Cell division, cell movement, cell differentiation continues Order and time frame varies with organism Order and time frame varies with organism

Amniotic egg reptiles and birds Chorion-gas exchange Chorion-gas exchange Amnion-fluid- filled,protects & cushions Amnion-fluid- filled,protects & cushions Allantois-nitrogenous waste disposal, gas exchange Allantois-nitrogenous waste disposal, gas exchange Yolk sac-digests yolk, blood vessels carry nutrients to embryo Yolk sac-digests yolk, blood vessels carry nutrients to embryo

Morphogenesis-shaping of organs and body plan Caused by 1. Changes in cell shape thru rearrangement of cytoskeleton 2. Migration of cells- ECM may guide cells, convergence is important 3. Cell adhesion- glycoproteins on cell surfaces

Morphogenesis seems to proceed in an orderly fashion: First, body axes are determined First, body axes are determined Right/left, anterior/posterior, dorsal/ventral Right/left, anterior/posterior, dorsal/ventral Second, body segments are determined Second, body segments are determined Head/thorax/abdomen Head/thorax/abdomen Third, tissues and organs are placed Third, tissues and organs are placed

Gene groups important in morphogenesis & development are Segment genes-lose totipotency early on Segment genes-lose totipotency early on Homeotic genes(homeobox, hox)-master control genes which code for transcription factors Homeotic genes(homeobox, hox)-master control genes which code for transcription factors Have been evolutionarily conserved Have been evolutionarily conserved

Loss of totipotency may be influenced by: 1. Cytoplasmic determinants unevenly distributed in early embryo Cells may have different mRNA, protein, etc 2. Induction May be chemical or physical in nature ECM critical-a cell’s fate can be changed by moving it 3. Organizers-sets of cells which influence differentiation Grey crescent of amphibians Dorsal lip of blastopore

Cytoplasmic determinants & induction

organizers