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Embryonic Cleavage

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Embryonic Cleavage

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    1. Embryonic Cleavage Rapid division to produce many cells Reduction of nuclear to cytoplasm ratio Distribution of cytoplasmic determinants Initial induction/differentiation Initial axis formation

    2. The Cell Cycle

    9. Cleavage patterns Amount and distribution of yolk Alecithal, isolecithal: Holoblastic Centrolecithal, telolecithal: Meroblastic Orientation of mitotic spindle Categories: Radial, bilateral, rotational, superficial, spiral

    11. Sea Urchin: Radial, Holoblastic

    14. Amphibian: Bilateral (Radial?), Holoblastic

    19. Extreme telolecithal: Fish, reptiles, birds Meroblastic or discoidal cleavage

    23. Mammals Slow cell cycles: 12-24 h Rotational cleavage Early cell division asynchronous Early activation of embryonic genome Compaction and cavitation

    25. Compaction Polarization events in blastomeres Localization of proteins, microvilli, junctions Starts cell differentiation: trophoblast vs. inner cell mass

    30. Conjoined Twins Single placenta, amnion, egg Same sex (usually female) 1/40,000 total births, 1/200,000 live births Genetic/environmental causes Example Categories: Craniophagus Pygophagus Thoracophagus Ischiopagus Parapagus

    32. Spiral Cleavage Annelids, molluscs Mitotic apparatus oblique with respect to A-V axis Spiral stack of cells with largest on bottom No blastocoel: stereoblastula Dextral vs. sinistral Maternal inheritance

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