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The Living Body 6 - Cleavage and Implantation.docx

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York University
Natural Science
NATS 1610
Barbara Czaban

NATS 1610 WINTER TERM L25 – Cleavage and Implantation After sperm enters the egg, the 2 nuclei move toward each other • Pronucleic fusion − Fertilization is completed − Zygote is formed Cleavage partitions the zygote into a ball of many smaller cells • Each new cell contains − Different regions of the original cytoplasm − Thus having different cytoplasmic determinants − Leading to different developmental fates for different cells st • On 1 day, fertilization occurred in the upper third of oviduct nd • On 2 day single-celled zygote undergoes cleavage (Mitotic divisions) − Cleavage continues rapidly as the pre-embryo moves through the rest of oviduct toward uterus • Sometimes mass of cells splits into 2 − Monozygotic twins – identical twins – from 1 zygote o Genetically identical – same gender − Incomplete splitting results in conjoined twins • Fraternal twins occur when two seconday oocytes are released during ovulation – fertilized by two separate sperm − Dizygotic twins – non-identical twins – from 2 zygotes o Not genetically identical – may be different gender • On 4 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 the zygote was th • On 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 hollow sphere of cells – blastocyst • On 6 day − No called a blastocyst o 2 parts – inner cell mass and trophoblast − Blastocyst floats freely in uterus until implantation − Will free itself from zona pellucida and increase in size Structure of Blastocyst • Outer trophoblast layer − Gives rise to extraembryonic membrane that will be the embryo’s part of placenta • Inner cell mass − Becomes the embryo − & some extraembryonic membranes that will extend from or surround embryo • Blastocoel − Fluid-filled cavity − Diminishes in size as embryo develops Implantation • About the 6 day, blastocyst begins embedding into inner lining of uterus • Trophoblast cells secrete enzymes − Digest away some of the tissue & blood vessels of the endometrium − Blastocyst begins to burrow into maternal tissues • 1/3 to ½ of all zygotes fail to become blastocyst and implant An ectopic pregnancy • Occurs when a blastocyst implants somewhere other than i
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