Lecture_4 - Early Development in Mammals Relevant Readings:...

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Early Development in Mammals Relevant Readings: 348-364
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Early Development in Mammals Studying development in mammals is signifcantly more diFfcult then in other lower organisms. This is in part due to the size oF the human zygote which contains 1/1000th oF the volume oF an Xenopus egg. Number oF eggs produced is several orders oF magnitude lower than amphibians. ±ewer than 10 eggs are ovulated in any given instance. Another diFFerence is the location oF development.
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Fertilization to Implantation The mammalian oocyte is released from the ovary and swept by the Fmbriae into the oviduct. ±ertilization occurs in a region close to the ovary in the ampulla of the oviduct. Meiosis is completed at roughly the same time. Cleavage begins about 24 hours later with subsequent divisions occurring 12-24 hours apart. All the while the cilia in the oviduct push the embryo towards the uterus. At the proper time the blastocyst will shed the zona pellucida and adhere to the uterine wall.
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Early Cleavage in Mammals Early cleavage in mammals differentiates itself from our other examples in Fve primary ways; 1) Division is very slow. 2) The orientation of blastomere cleavage varies relative to each other. The second cleavage undergoes rotational cleavage. 3) Asynchrony of divisions. 4) Unlike most embryos we have discussed, the mammalian genome is activated early in cleavage.
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Compaction Finally, and potentially most important, is the phenomenon of compaction . In mouse, cells are loosely packed until about the eight cell stage. Following the third cleavage the blastomeres undergo a radical change. Blastomeres huddle together, forming a compact ball. Dependent upon adhesion molecules such as E-Cadherin . Tight junctions form between the cells and seal off the inside of the sphere. Internally the cells form gap junctions to allow molecules and ions to pass.
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Continued Division The compacted 8 cell embryo divides to form the 16 cell morula . The morula consists of a large group of surface cells and a small
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This note was uploaded on 07/15/2010 for the course BIOL 301 taught by Professor Pickard during the Fall '09 term at Waterloo.

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Lecture_4 - Early Development in Mammals Relevant Readings:...

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