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Human Development Fertilization Must occur within 24hrs postovulation Requires capacitated sperm (6-8hrs) Secondary oocyte completes Meiosis II once sperm enters oocyte Zygote formed with fusion of male and female pronuclei Cleavage Rapid mitotic cell division of zygote 2-cell stage; 36 hours 4-cell stage; 48 hours 8-cell stage; 72 hours Morula - solid ball of cells (blastomeres) at 96 hours Takes 4-5 days to travel from uterine tubes to uterine cavity Implantation Morula develops into bastocyst, a hollow ball of cells differentiated into inner cell mass and trophoblast Trophoblast embeds into endometrium – secretes enzymes, HCG – chorionic villi – attachment occurs 7-8 days after fertilization Embryonic Development Extends from the 2-8th week after conception Inner cell mass differentiates into: Ectoderm - epidermis of skin, entire nervous system Endoderm - functional lining of the digestive and respiratory system Mesoderm - forms muscle, bone, blood vessels, and kidneys Fetal Membranes Chorion - fetal part of the placenta, outermost membrane Amnion - fluid filled sac covering embryo and later fetus Yolk sac - source of nourishment for embryo; early site of blood formation Allantois - developing umbilical cord and developing bladder of fetus Placenta Formation Functions to exchange waste products and blood gases Chorionic villi penetrate into the decidua basalis of the endometrium Chorion - fetal portion Decidua basalis - maternal portion Placenta previa Hormones of Pregnancy HCG secreted by the chorion maintains corpus luteum FSH repressed, cycle stops Progesterone maintains endometrial lining Placenta secretes estrogen and progesterone Corpus luteum degenerates Late pregnancy: relaxin secreted