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Morphogenesis

Morphogenesis. 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.

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Morphogenesis

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  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.

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