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Early Development of the Zygote

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McMaster University
Laura Parker

Early Development of the Zygote to a Blastocyst (1st Week)  Ovulation releases ovum/secondary oocyte  Fertilization in the oviduct → produces zygote  Zygote undergoes cleavage as it moves along oviduct → produces morula o As zygote divides, cells become smaller \ Morula stays same size o Movement by cilia and peristalsis present in oviduct walls  Morula develops into a blastocyst o Trophoblast (outer layer of blastocyst) → nourishes future embryo o Inner cell mass → will become fetus o Fluid filled cavity → for protection (absorbs shocks, resists compression, ...)  Blastocyst (≈100cells) implants itself in uterus lining o Nourished by secretion from uterus o Microvilli provide large surface area (→gas + nutrients exchange) Implantation Of The Blastocyst Into The Uterine Lining (2nd Week)  Trophoblast secretes enzymes → digest tissues and blood vessel of endometrium  Embryo uses released nutrients/products from digestion  Blastocyst becomes buried within endometrium  Microvilli are replaced by placenta  Trophoblast secretes human chorionic gonadotrophin (hCG) hormone The Developing Fetus  During gestation (→length of pregnancy) growth rate is in excess  Placenta is the first organ to develop when blastocyst embeds itself in uterine lining o Growth faster than embryo in early pregnancy  Development increases in complexity o Differentiating of inner cell mass of blastocyst o First month → beginning of a gut, developed kidney, brain, beating heart o Se
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