NATS 1610 Lecture Notes - Inner Cell Mass, Conjoined Twins, Ectopic Pregnancy
Document Summary
Living body: the pre embryonic period: cleavage and implantation. 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. On the first day: fertilization occurred in the upper third of oviduct. Is at the morula stage a solid ball of 12 or more cells: the pre-embryo reaches the uterus, cells are small and tightly bound within the zone pellucida, morula is about the same size as zygote was. About the 5th 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. About the 6th day: now called a blastocyst, 2 parts inner cell mass and trophoblast blastocyst floats freely in uterus until implantation, will free itself from zone pellucida and increase in size.
