Embryo-AD[1]

Embryo-AD[1] - Human Development Pre-embryonic period...

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Human Development Pre-embryonic period (first 2 weeks) -- includes cleavage, implantation and gastrulation. Embryonic period (3rd - 8th week) -- includes induction. Fetal period (3rd to 9th month) -- includes growth and development.
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Fertilization Only 100 sperm reach ovum. Enzymes from sperm’s acrosome digest their way through the zona pellucida. Change in zona pellucida prevents entry of more than one sperm.
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Male and female nuclei fuse, resulting cell is called a zygote. Sperm penetration initiates “cleavage” of zygote to form a hollow blastula. Blastula implants in uterine lining.
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Three developmental processes Cleavage Gastrulation Induction
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Cleavage Zygote divides repeatedly to produce a blastula, a hollow ball of cells. Increase in cell number, but no increase in overall size. Pattern depends on amount of yolk in ovum; inert yolk divides more slowly.
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Patterns of cleavage Even in sea urchin. Uneven in frog. Disc only in chicken. “Disc”-like in humans, too
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Gastrulation Blastula becomes a 3-layered gastrula. Three primary germ layers: -- ectoderm (outer) -- endoderm (inner) -- mesoderm (middle)
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Induction The 3 primary germ layers interact to form organs and organ systems: Ectoderm = > skin and nervous system. Mesoderm => muscles, skeleton, kidneys, circulatory system. Endoderm = > digestive system, lungs.
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Preembryonic period
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Begins 1.5 days after fertilization. Completed in 5-6 days, before implantation.
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This document was uploaded on 10/31/2011.

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Embryo-AD[1] - Human Development Pre-embryonic period...

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