NATS 1610 Lecture Notes - Inner Cell Mass, Zona Pellucida, Conjoined Twins
5 Feb 2014
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Document Summary
After sperm enters the egg, the 2 nuclei move toward each other: pronucleic fusion. Cleavage partitions the zygote into a ball of many smaller cells: each new cell contains. Leading to different developmental fates for different cells: on 1st day, fertilization occurred in the upper third of oviduct, on 2nd 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: 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: not genetically identical may be different gender, on 4th day. Is at the morula stage a solid ball of 12 or more cells. Cells are small and tightly bound within the zone pellucida.
