Chapter 3--Changes By Weeks Of Gestational Age

Infants, Children, and Adolescents (6th Edition)

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Changes By Weeks Of Gestational Age Week 2 (day 1-7 of fertilization) Fertilization of the ovum to form a zygote. (day 1) The zygote undergoes mitotic cellular divisions, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage. (day 1.5-3) The blastocyst contains only a thin rim of trophoblast cells and a clump of cells at one end known as the "embryonic pole" which include embryonic stem cells. The blastocyst hatches from its protein shell (zona pellucida) and performs implantation onto the endometrial lining of the mother's uterus. (day 5-6) If separation into identical twins occurs, 1/3 of the time it will happen before day 5. Week 3 (1 week from fertilization) Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes. The blastocyst is fully implanted day 7-12. Formation of the yolk sac. The embryonic cells flatten into a disk, two-cells thick. If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined. Primitive streak develops. (day 13) Primary stem villi appear. (day 13) Week 4 (2 weeks from fertilization - first missed menstrual period) A notochord forms in the center of the embryonic disk. (day 16) Gastrulation commences. (day 16) A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear. (day 18) Somites, the divisions of the future vertebra, form. (day 20) Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20) Week 5 (3 weeks from fertilization)
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The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape. The heart bulges, further develops, and begins to beat in a regular rhythm. Septum primum appear.
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Chapter 3--Changes By Weeks Of Gestational Age - Changes By...

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