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Bi Sci 001 Monday March 29, 1999 Announcements: Review Session tonight Lecture notes: Cleavage Division of the fertilized egg into a ball of undifferentiated cells. Day 1 2 cells Day 4-7 ?? Arrival at uterus. It has now become a hollow ball of cells called a blastocyst. Implantation At about 1 week, the blastocyst burrows into the endometrium (inner layer of the uterus). The process takes about 1 week. Some of the outer layer of the blastocyst forms the chorion which aids in implantation. Gastrulation Migration of cells in the blastocyst (blastula) to form three layers. It occurs during the week 2 of pregnancy. Extremely important step in development. (Some say most important) One third of all embryos either don't "do" gastrulation of don't do it correctly. (This results in the loss of the embryo in most cases) Siamese twins are a result of "double gastrulation". Cells loose their general function at this time and become committed to certain cell types. "Repair" becomes more difficult now. Tissues arising from gastrula Ectoderm - outer layer Nervous system Mesoderm - middle layer Muscle Endoderm - inner layer Intestinal organs Organogenesis Cell layers split into "subpopulations" that are destined to become differnet organ systems. The major organ systems are formed by the 7th week of pregnancy. (Most women don't know they are pregnant yet.) At 3 weeks the embryo is about 1/8 inch long. From 2nd week until the 8th week, the developing baby is called and embryo. This is another critical stage in human development. This is a time when major - many lethal - birth defects occur. Some of these as a result of chemicals or toxins. A teratogen is a chemical capable of causing birth defects.
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