Embryological Development


Development is
fundamentally similar in
all eukaryotes
There are 3 aspects to
embryological
development



Cell division
Cell differentiation
Morphogenesis(body
shape)
•
Sea Urchin
Jelly coat
Vitelline layer
Mammal
Structure of egg
Layer of follicle cells
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 Proteins on sperm
membrane
membrane
3. Fast block to polyspermyelectrical reaction prevents multiple sperm entry
Modified vitelline layer
4. Cortical reaction-increase in
Ca causes formation of
fertilization membrane
Only nucleus enters
5. Entry of sperm
Modified zona
pellucida
Whole sperm enters
6. Activation of egg-intracellular Ca increases causing increased protein synthesis,
pH, cellular respiration
Immediate within 20 min.1st cell division in 90 min.
7. Fusion of egg & sperm
nucleus
Occurs during 1st
division-36 hrs.
Cleavage





Zygote rapidly divides-up to 128 cell stage
These smaller & smaller cells are called blazoners
 Each will have different cytoplasm elements
 Polarity(except mammals)
 Animal pole(rapid)
 Vegetal pole(greater cytoplasm/nutrients impedes division
Morula-16-64 cells
 Develops a fluid filled cavity
Blastula-128 cells
 Internal cavity-blastocoels
Differences in the cleavage process

Meroblastic vs holoblastic



Holo-little yolk, even division
Mero-large yolk, division
confined to tiny area called
blastodisc
Radial vs spiral


Radial in
deuterostomes(echinoderms,
chordates)
Spiral in protostomes(mollusks,
arthropods, annelids)
Gastrulation






Rearrangement of blastula
cells
Changes in cell motility,
shape, adhesion
Inductionchemical/physical
communication
Blastopore-cells outside
invaginate to form
gastrocoel/archenteron
 Becomes
anus(deuterostomes) or
mouth(protostomes)
3 germ layers
develop(p945)
Later, coelom(body cavity)
forms
Neurulation



Head & nervous
system
Neuroectoderm folds
into neural tube(CNS)
Notochord
forms(vertebrates)
Organogenesis


Cell division, cell movement, cell differentiation
continues
Order and time frame varies with organism
Amniotic egg
reptiles and birds




Chorion-gas exchange
Amnion-fluidfilled,protects &
cushions
Allantois-nitrogenous
waste disposal, gas
exchange
Yolk sac-digests yolk,
blood vessels carry
nutrients to embryo
Morphogenesis-shaping of organs and body plan
Caused by
1.
2.
3.
Changes in cell shape
thru rearrangement of
cytoskeleton
Migration of cellsECM may guide cells,
convergence is
important
Cell adhesionglycoproteins on cell
surfaces
Morphogenesis seems to proceed in an orderly
fashion:
 First, body axes are determined


Second, body segments are determined


Right/left, anterior/posterior, dorsal/ventral
Head/thorax/abdomen
Third, tissues and organs are placed
Gene groups important in morphogenesis &
development are
 Segment genes-lose totipotency early on
 Homeotic genes(homeobox, hox)-master
control genes which code for transcription
factors

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