Lecture_3_posted - Early Development in Amphibians Early...

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Early Development in Amphibians
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Early Amphibian Development The good .... The Bad ....
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Early Amphibian Development Site of sperm entry marks the future ventral side of the embryo. Sperm entry triggers the arrangement of microtubules separating the cortical cytoplasm from the internal cytoplasm. These microtubules allow the cortical cytoplasm to rotate with respect to the internal cytoplasm.
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Cleavage Cleavage in most frog and salamander embryos is radially symmetrical and holoblastic. First division begins in animal pole and proceeds toward yolky vegetal region. Vegetal region slows the cleavage down such that the second division begins before the first is complete. Both the first and the second divisions are meridional and are 90 to each other.
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Cleavage The third division is equatorial but displaced towards the animal pole. As division continues, this asymmetric holoblastic cleavage results in two regions. A rapidly dividing region of micromeres near the animal pole and a macromere region at the vegetal pole.
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Cleavage Gastrulation in amphibians has been extensively studied. Vast diversity in how amphibian species accomplish this stage in development. We will focus our discussion on Xenopus. Amphibian embryo with 16-64 cells is called a morula. At 128 cells the blastocoel becomes obvious and is thereafter called a blastula. Blastocoel serves two purposes: 1) Permits cell migration during gastrulation 2) Prevents early cell-cell interactions
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Xenopus Blastula Fate Map
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Stages of Gastrulation Early : The process starts on the future dosal side of the embryo just below the equator in the region of the grey crescent. At this point cells invaginate to form a slit like blastopore. This movement results in the dorsal blastopore lip. Also characteristic of this cell migration is the shape of the cells at the lip. As the main body of the cell is displaced inward it maintains contact with the surface. This results in a thin neck being formed and a bottle shaped appearance. These bottle cells will line the archenteron as it forms.
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Experimental Evidence
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Stages of Gastrulation Mid : The marginal zone cells involute at the dorsal lip and travel along the inner surface of what is an empty sphere. Concurrently this embryo is undergoing epiboly. As involution continues we see the continued expansion of the blastopore lip. The dorsal lip soon expands to form a crescent encompassing the dorsal and lateral blastopore lips.
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Stages of Gastrulation Late : Here we see the formation of the ventral lip and a complete ring around the remaining endoderm cells that remain on the surface. These are called the yolk plug and are eventually internalized so that all the endodermal precursors are internal. At this point the ectoderm completely encircles the embryo with the mesoderm lying between the ecto- and endoderms.
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Epiboly of the Ectoderm
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Stages of Gastrulation
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Early Movements of Xenopus Gastrulation
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Early Movements of Xenopus Gastrulation
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Early Movements of Xenopus Gastrulation
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Axis Formation in Amphibians: The Spemann Organizer
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