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

Wearing down of the membrane surrounding the sperm head. It becomes fragile as the sperm travel through cervical mucus, uterus, & oviducts in search of the egg. Step 1: Capacitation 11.4 Process of Fertilization Requires about 6 to 8 hours

Release of acrosome enzymes Enzymes digest through surrounding follicle cells and zona pellucida Step 2: Acrosome Reaction 11.4 Process of Fertilization Success is the result of hundreds of sperm acrosomes rupturing

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. Step 3: Fusion of sperm head 11.4 Process of Fertilization Prevents other sperm from fusing with membrane. Occurs 1-3 sec after fusion

E.R. releases Ca 2+ into cytoplasm at site of sperm entry. High Ca 2+ 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 Step 4: Cortical reaction 11.4 Process of Fertilization

Step 4: Cortical reaction 11.4 Process of Fertilization

Ca 2+ influx triggers Step 5: Meiosis of egg completed 11.4 Process of Fertilization 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. image

Click here for a news interview on how embryos are chosen click here to go PBS.org to read several articles on IVF Read the following articles: Introduction to test tube babies Pros and Cons of IVF In vitro fertilization today Ethical Questions Click to see a National Geographic video of IVF

Who owns the embryo - the ‘donors’ or the laboratory? What is to be done with frozen embryos that are no longer needed? 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?

From ZYGOTE to BLASTOCYST The Beginning: Ovum is fertilized in the oviduct. The resulting cell is called a zygote

1 st cleavage occurs within 36 hours resulting in 2 identical cells. Zygote continues movement along oviduct and 2 nd 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.

 Protective mucus plug formed in the cervix  Growth of the placenta  Enlargement of uterus  Ovulation and menstruation stopped  Enlargement and sensitivity of breasts. Effect of high level of Progesterone: