BIOL 303 - Feb 19 - Amphibians development very available...

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Amphibians development : very available, large eggs/large embryo - easy for manipulation. Animal and vegetal hemispheres: vegetal (yellowish), animal (dark pigmented). The vegetal cells become endoderm, and animal cells become ectoderm - the divisions are present in the egg before fertilization. Sperm entry ~30º away from the animal pole (preferential zone of entry). Results in cortical rotation, as the membrane rotates towards the site of sperm entry. As cortical rotation occurs, some pigment is left behind and forms the “grey crescent” - future dorsal mesoderm region. 1st cleavage starts at the animal pole, proceeds towards the vegetal, forming blastomeres (sections). Vegetal pole is yolky and viscous, cell division is harder and slows down as it progresses towards the vegetal pole, so the 2nd cleavage starts before the 1st is completed. 3rd division goes around latitudinally, closer to the animal pole - results in smaller blastomeres at the animal side. Eventually forms the morula: larger cells towards the vegetal pole, and the blastocoel forms at the animal side - no zygotic transcription occurs until the blastula stage. Blastocoel function: permits cell migration during gastrulation, and prevents cells closest to the animal pole (animal cap) from interacting with the cells below. Fate mapping:
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