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Fertilization in animals can be internal or external Fertilization is the fusion of egg & sperm to form a zygote (1 celled organism) What type of organisms would do each type? Pros and Cons of each type? Human Egg Surrounded by Sperm Fertilization involves mechanisms that prevent polyspermy Cortical granules 11.4 Process of Fertilization Step 1: Capacitation • Wearing down of the membrane surrounding the sperm head. • It becomes fragile as the sperm travel through cervical mucus, uterus, & oviducts in search of • Requires about 6 to 8 hours the egg. 11.4 Process of Fertilization Step 2: Acrosome Reaction • Release of acrosome enzymes • Enzymes digest through surrounding follicle cells and zona pellucida • Success is the result of hundreds of sperm acrosomes rupturing 11.4 Process of Fertilization Step 3: Fusion of sperm head • Sperm membrane fuses with plasma membrane of the egg. Triggers fast block • Opening of Na+ channels causing Na+ to flow into egg depolarizing the membrane. • Prevents other sperm from fusing with membrane. • Occurs 1-3 sec after fusion 11.4 Process of Fertilization Step 4: Cortical reaction • E.R. releases Ca2+ into cytoplasm at site of sperm entry. High Ca2+ concentration triggers cortical granule to fuse with plasma membrane & release contents • Enzymes digest binding proteins so no other sperm can bind. • granule cause the vitelline layer to separate from the plasma membrane & water enters perivitelline space • Vitelline membrane hardens preventing other sperm from entering • Return of normal voltage across egg plasma membrane 11.4 Process of Fertilization Step 4: Cortical reaction 11.4 Process of Fertilization Step 5: Meiosis of egg completed • Ca2+ influx triggers Step 6: Swelling of sperm & egg nucleus • occurs about 20 minutes after entry • Contents merge creating diploid nucleus • Now called zygote • “in vitro” means ‘in glass’ (i.e. a test tube). • Can be used for women whose oviducts are blocked. Photos taken from whereivf-fertilization.com Procedure: 1. Hormone (FSH) is given to increase development of follicles so a larger number can be harvested. 2. Ova are surgically removed from Graffian follicles prior to ovulation 3. Sperm are harvested from the male 4. Fertilization and first few cell divisions occur outside of the body in a petri dish or test tube. 5. After about 5days (when the embryo has reached the blastocyst stage) the embryo is placed into the uterus and allowed to implant. 6. Embryos can also be frozen for later use if the first attempt is unsuccessful. http://www.dreamstime.com/stock-image-vitro-fertilizationimage13398841 www.mountnittany.org Click to see a National Geographic video of IVF click here to go PBS.org to read several articles on IVF Click here for a news interview on how embryos are chosen Read the following articles: • Introduction to test tube babies • Pros and Cons of IVF • In vitro fertilization today • Ethical Questions Who owns the embryo - the ‘donors’ or the laboratory? Is it appropriate to “sell” ova or sperm to other couples for the IVF process? If ‘donor’ ova and/or sperm are used, who are the legal parents? What is to be done with frozen embryos that are no longer needed? From ZYGOTE to BLASTOCYST The Beginning: • Ovum is fertilized in the oviduct. • The resulting cell is called a zygote • 1st cleavage occurs within 36 hours resulting in 2 identical cells. • Zygote continues movement along oviduct and 2nd cleavage occurs with in the next 36 hours resulting in 4 identical cells. Two Cell Stage Eight Cell Stage Morula - Cluster of Cells • Cleavage continues rapidly, resulting in a solid ball of cells. A hollow fluid filled area develops inside this ball of cells creating a blastocyst. • The blastocyst will implant in the endometrium of the uterine lining at approximately day 5 or 8. (it now has between 50 and 100 cells) • This is essential for pregnancy to continue. Human development before implantation. Upon Implantation: Hormone action • Embryo secretes a hormone that sends a signal to mother’s reproductive system. • This hormone is HCG. (Human Chorionic Gonadotropin) • Its function is to maintain the secretion of progesterone and estrogen by the ovary. • HCG can be detected in mother’s urine to determine positive pregnancy test. Effect of high level of Progesterone: Protective mucus plug formed in the cervix Growth of the placenta Enlargement of uterus Ovulation and menstruation stopped Enlargement and sensitivity of breasts.