I. Embryology
A. Processes of Development
1. Cell division (mitosis)
2. Differentiation
3. Morphogenesis (pattern formation, organogenesis)
Process: fertilization cleavage gastrulation neurulation organogenesis
Stage: Zygote โ 2,4,8-cell,โฆ โ Blastula โ Gastrula โ Neurula โ โ Organism
Patterns of Cleavage and Gastrulation - depends on egg type:
isolecithal - sparse yolk, evenly distributed โ radial, spiral or rotational cleavage
mesolecithal - moderate amount of yolk at one end โ displaced radial cleavage (frogs)
telolecithal - large amount of yolk โ discoidal cleavage (fishes, reptiles, birds)
Gastrulation
- invagination of cell layers from surface to interior
- forms gut cavity (archenteron); blastocoel disappears
- establishes 3 germ layers:
Ectoderm โ epidermis of skin, neural tube โ CNS, neural crest โ PNS + other structures
Mesoderm โ dermis of skin, notochord, connective tissues, bones, muscles, heart, blood,
blood vessels, kidneys, gonads, coelomic membranes
Endoderm โ lining of GI tract, lungs, gills, liver, pancreas, lining of bladder
Frog embryo - gastrulation proceeds from the dorsal lip of the blastopore
Discoidal cleavage - gastrulation occurs along the primitive streak
Neurulation (chordates)
- infolding of dorsal ectoderm to form neural tube
- neural crest cells break off, migrate to form nerves, ganglia, adrenal medulla, melanocytes
Mesoderm Development
notochord - embryonic โbackboneโ, forms early in development
- characteristic structure of chordates
somites - segmented blocks of mesoderm, lateral to notochord
- form vertebrae, most muscles, dermis of skin
intermediate mesoderm - forms kidneys and gonads
lateral plate mesoderm - splits to form coelomic membranes (parietal and visceral layers)
- migrating visceral mesoderm forms heart, blood vessels, muscle of GI tract
mesenchyme - undifferentiated embryonic connective tissue, gives rise to most mature CTs
[Cross-sectional diagram of vertebrate embryo after neurulation]
Amniotic egg - extraembryonic membranes, characteristic of amniotes (reptiles, birds, mammals)
1. amnion - encloses fluid-filled cavity around the embryo, fx. for support & hydration
2. chorion - outermost membrane, fx. for gas exchange (embryonic placenta in mammals)
3. yolk sac - surrounds yolk in birds & reptiles, empty in mammals (gives rise to germ cells)
4. allantois - surrounds cavity off embryonic gut, fx. for waste storage and gas exchange
(chorioallantoic membrane), forms embryonic bladder and umbilical cord in mammals
Note embryological homology of extraembryonic membranes of amniotes
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