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Lecture_2_posted - Early Development in Selected...

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Early Development in Selected Invertebrates
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Early Development in Selected Invertebrates Two major developmental processes occur in succession; cleavage and gastrulation Determine cell fate and major axis
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Cleavage Cleavage represents a period of rapid cell division Division is not accompanied by cell growth Increase in cell number results in a decrease in cell volume Resulting cells called Blastomeres Under control of protein and RNA stored in the egg Division is extremely rapid Frog, 37,000 cells in 43 hours Drosophila, 50,000 cells in 12 hours
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Cleavage Cleavage is driven by mitosis-promoting factor (MPF) Blastomeres have cell cycles with two steps, M and S
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Cleavage MPF has two subunits, CyclinB and cdc2 CyclinB up-regulates cdc2 Induced cdc2 phosphorylates several factors inducing mitosis CyclinB regulated by factors in the cytoplasm When the factors are used up, the embryo has reached the mid-blastula transition (MBT)
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Cleavage Karyokinesis , the mitotic division of the nucleus Cytokinesis , physical division of the cell
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Cleavage The cytoskeleton
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Patterns of Cleavage The pattern of embryonic cleavage is determined by two major factors; 1) The amount and distribution of yolk in the cytoplasm 2) Factors in the egg cytoplasm that influence the angle of the mitotic spindle. Amount and distribution of the yolk effects where cleavage can occur and the resulting size of the blastomeres Yolk free pole is the animal pole , yolk rich pole is the vegetal pole
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Main Patterns of Cleavage Isolecithal ; sparse equal yolk results in holoblastic or complete cleavage Insects, fish, reptiles, birds have large internal yolk. This results in meroblastic or incomplete cleavage Insects have yolk in center, divisions occurring around this central yolk is superficial cleavage
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Specification of Cell Fates Cell fate is determined by cell-cell interactions or asymmetric distribution of patterning molecules Asymmetric distribution by one of three mechanisms: 1) Molecules in cytoplasm are passively acquired during division 2) Molecules are actively transported into individual cells 3) Molecules are associated with centrosomes and follow it into a cell
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Specification of Cell Fates
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Gastrulation Highly coordinated cell and tissue movements where the blastula is dramatically remodeled The three germ layers are established during gastrulation Invagination , in-folding region of cells Involution , inward movement of expanding outer layer over internal side of remaining external cells Ingression , migration of individual cells from surface into interior of embryo
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Gastrulation Highly coordinated cell and tissue movements where the blastula is dramatically remodeled Multilayer body plan established in gastrulation Delamination , splitting of one cellular sheet into two Epiboly , movement of epithelial sheets that spread to enclose deeper layers of the embryo, by division, change in shape, or by several layers intercalating.
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Early Development in Sea Urchin Urchins exhibit radial holoblastic cleavage Divisions one and two are meridional and
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