Morphogenesis

1
Morphogenesis

• Vertebrate eggs
• Early chordate development
• Fate of the germ layers
• Extraembryonic membranes

2
Vertebrate EggsSize and clevage pattern depends
on amount of yolk present

• Amount of yolk
• Microlecithal - little yolk
• Mesolecithal - moderate yolk
• Macrolecithal - large yolk

3

• Distribution of yolk
• Isolecithal - even distribution
• Telolecithal - yolk is more concentrated in one
  hemisphere

4

• 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)

5
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

6

• 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

7
GastrulationThe 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

8
Primary Germ LayersFormed 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

9
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.