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January 21, 2014 cleavage and implantation.docx

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Natural Science
NATS 1610
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Living body: the pre embryonic period: cleavage and implantation January 21, 2014 -cleavage partitions the zygote into a ball of many smaller cells • Each new cell contains o Different regions of the original cytoplasm o Thus having different cytoplasmic determinants o Leading to different developmental fates for different cells -on the first day: fertilization occurred in the upper third of oviduct -on the second day: single celled zygote undergoes cleavage (mitotic divisions) • Cleavage continues rapidly as the pre-embryo moves through rest of oviduct towards uterus • Sometimes mass of cells splits into 2, monozygotic twins, identical twins, 1 zygote (twins, incomplete splitting results in conjoined twins) • Fraternal twins occur when 2 secondary oocytes are released during ovulation – fertilized by 2 separate sperm (dizygotic twins – non identical twins – from 2 zygotes (may be diff gender) -By the fourth day: • The pre-embryo reaches the uterus • Is at the morula stage – a solid ball of 12 or more cells • Cells are small and tightly bound within the zone pellucida • Morula is about the same size as zygote was -about the 5 day: • Morula enters the uterus • Spaces begin to appear between cells at the centre of the ball • Fluid from uterine cavity accumulates between spaces • Creating a hallow sphere of cells – blastocyst th -About the 6 day: • Now called a blastocyst o 2 parts – inner cell mass and trophoblast • blastocyst floats freely in uterus until implantation • Will free itself from zone pellucida and increase in size -outer trophoblast layer: gives rise to extraembryonic membrane that will be the embryos part of placenta -Inner cell mass: becomes the embryo, and some extraembryonic membranes that will extend from or surround embryo -Blastocoel: fluid filled cavity, diminishes in size -1/3 to 1/2 of all zygotes fails to become blastocysts and implant -An ectopic pregnancy occurs when a blastocyst implants somewhere other than in the uterus -tro
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