Ch 28 Pregnancy & Development

Ch 28 Pregnancy & Development - CH. 28 : Pr e gna...

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CH. 28: Pregnancy and Human Development I. From Egg to Embryo A. Fertilization 1. Fertilization occurs when a sperm fuses with an egg to form a zygote. 2. Millions of sperm ejaculated into the female reproductive tract are lost due to leakage from the vaginal canal, destruction by the acidic environment of the vagina, inability to pass the cervical mucus, or destruction by defense cells of the uterus. 3. In order to fertilize an egg, sperm must be capacitated , a process involving weakening of the sperm cell membrane in order to allow release of acrosomal hydrolytic enzymes. 4. When sperm cells bind to the zona pellucida surrounding the egg, they undergo an acrosomal reaction , where acrosomal enzymes are released to the oocyte. a. Hundreds of sperm cells must release their acrosomal enzymes before fertilization can occur. b. Once a sperm cell binds to membrane receptors on the oocyte membrane, its nucleus is pulled into the cytoplasm of the oocyte. 5. Blocks to Polyspermy (fertilization by more than one sperm cell): a. The fast block to polyspermy occurs when the membrane of the oocyte depolarizes and prevents similar binding by other sperm cells. b. The slow block to polyspermy results in destruction of sperm receptors, and the formation of a swollen membrane that removes other sperm cells from the surface of the oocyte. 6. Once a sperm enters an oocyte, it loses its tail and midpiece, and migrates to the center of the oocyte while the oocyte completes meiosis II. a. After meiosis II is completed, male and female pronuclei fuse and produce a zygote, which almost immediately enters into mitosis. B. Pre-embryonic Development (day 0-14) 1. Begins with fertilization and continues with the movement of the preembryo to the uterus, where it implants in the uterine wall. 2. Cleavage = The mitotic divisions after fertilization which occur without much growth between divisions, resulting in progressively smaller cells. a. Cleavage forms two identical cells, blastomeres, then four identical cells, etc. This process, completed by 72 hours, produces a solid ball of cells called a morula . b. By the 4 th or 5 th day, the morula has formed a hollow ball of cells called the blastocyst and is about to enter the uterus. 3. Implantation occurs after 6–7 days; the trophoblast adheres to the endometrium, and produces enzymes that irritate the endometrium. a. Uterine capillaries become permeable and leaky, and the trophoblast proliferates, forming the cytotrophoblast and the syncytiotrophoblast . b. Trophoblast cells secrete
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This note was uploaded on 07/23/2009 for the course BIOL 1141 taught by Professor Loripaul during the Summer '09 term at UMSL.

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Ch 28 Pregnancy & Development - CH. 28 : Pr e gna...

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