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Anatomy and Cell Biology
Anatomy and Cell Biology 3319
Peter Merrifield

Anatomy NotesSept 1511 The embryonic period is the first 8 weeks of development The fetal period happens after that and involves mostly growth Week 1from zygote to blastocyst An ovary releases an immature egg called an oocyte Fimbria at the distal end of the fallopian tube uterine tube sweep the egg along Fertilization occurs when sperm meets the egg The fertilized oocyte is now called a zygote Sperm and egg are haploid and they make a diploid zygote The zygote undergoes repeated cell division as it moves along the tube The large number of cells produced provides the building blocks for the embryo After 3 days a morula forms around the proximal end of the tube At 4 days the morula enters the uterus and takes up water and forms a cavity called the blastocyst cavity The morula is a cluster of cells inner and outer cells The morula now becomes the blastocyst A cluster of cells on one side of the blastocyst cavity is called the inner cell mass and the layer of cells surrounding the cavity is called the trophoblast This is the first example of specializationthe inner cell mass will form the embryo and the trophoblast will form the placenta On day 6 the blastocyst begins implanting upon the endometrial lining of the uterus Implantation takes about a week to complete During the first week the embryo is not affected by teratogens Ectopic pregnancy If it is a tubal pregnancy it will not produce a placenta Sometimes to embryo can fall back into the body cavity the mesentery of the large intestine In placenta previa the embryo grows near the cervix and the placenta doesnt have the support of the uterus The baby is still brought to term though by Csection Week 2implantation of the blastocyst and the twolayered embryo At day 7 the trophoblast outer cells sends out projections that invade the uterine wall At day 9 the endometrial surrounds the embryo The inner cell mass has now divided into two sheets of cells the epiblast and the hypoblast The epiblast and hypoblast together make up the bilaminar
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