Morphogenesis
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Vertebrate eggs
Early chordate development
Fate of the germ layers
Extraembryonic membranes
Vertebrate Eggs
Size and clevage pattern depends on
amount of yolk present
Amount of yolk
• Microlecithal - “little yolk”
• Mesolecithal - “moderate” yolk
• Macrolecithal - “large” yolk
Distribution of yolk:
• Isolecithal - even distribution
• Telolecithal - yolk is more concentrated in
one hemisphere
Disposition of eggs:
• Oviparity- eggs are shed into the
environment
• Viviparity- eggs are retained internally and
living young delivered
- Ovoviviparous: young nourished by yolk
- Euviviparous: young nourished by
maternal tissues (histotrophic or placental)
Early Embryo Development
Clevage
• Refers to division of cells
• Pattern determined by amount and
distribution of yolk
(1) Holoblastic: entire egg cleaves (equal or
unequal)
(2) Meroblastic: only a small disc of
cytoplasm cleaves
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Blastula
Hollow ball of cells (holoblastic) or a “cap”
of blastoderm (meroblastic)
Each cell is a blastomere
Cavity is the blastocoele
Massive amounts of yolk result in formation
of the blastoderm
Placental mammals produce a blastocyst
consisting of trophoblast and inner cell
mass
Gastrulation
The most important event in your life
• Dynamic process of cellular movements
which form the three primordial germ layers
and the coelom
• Morphogenetic movements depend on the
amount and distribution of yolk.
• Involves some type of involution.
• Regulated by homeobox genes and their
proteins - morphogens
Primary Germ Layers
Formed during gastrulation
• Ectoderm: outerlayers of integument and
nervous system (neural crest and
neurectoderm)
• Mesoderm: muscle, bone and connective
tissue. Dorsal (somite) mesoderm,
intermediate and lateral
• Endoderm: lining of the gut (yolk sac
endoderm - primordial germ cells
Extraembryonic Membranes
• Vitelline: surrounds yolk sac; highly
vascular and confluent with embryonic
circulatory system
• Amnion: encloses embryo to protect against
dessication (water loss); terrestrial
vertebrates
• Chorion: surrounds amnion, contacts
maternal tissue or egg shell
• Allantois: evagination from cloaca; forms
part of mammalian bladder.