Fertilization, Implantation, and Development (Begin on Page 463)

Fertilization In human mating, hundreds of millions of sperm are ejaculated into the vagina. The sperm then pass through the cervix, up through the uterus, and into the oviducts. If an egg is passing down one of the oviducts at this time, fertilization may take place. (The oviducts in this diagram are labeled the Uterine Tubes)

Fertilization The egg secretes a chemical that attracts the sperm. One of the sperm breaks through the membranes surrounding the egg, and the sperm cell nucleus enters the cytoplasm of the egg. When this happens, the membranes around the egg change, stopping other sperm from entering the egg. The resulting diploid cell is called a zygote.

In Vitro Fertilization (IVF) The egg of the mother is fertilized in a petri dish outside of the female’s body. This process is used to give infertile women a child of their own. An anonymous surrogate then carries the fertilized zygote in her uterus until birth.

Implantation and Development  What happens to the zygote after fertilization? Cleavage (mitosis) occurs and splits the zygote into a many celled embryo.  When does the embryo enter the uterus? 5-10 days after fertilization.

Implantation and Development Within the uterus, the outer layer of cells of the embryo secretes enzymes that digest part of the thick lining of the uterus. This allows the embryo to attach itself at this spot. (An embryo attaching itself to the uterine wall)

Implantation and Development  What marks the beginning of pregnancy? Implantation of an embryo into the uterine wall.  What is an ectopic pregnancy? When the embryo attaches to a place other than the uterus.  How does an embryo differ from a fetus? An embryo is the early development; not fully formed A Fetus is more developed; looks like a “human”; recognized as its species

Extra-embryonic Membranes Originating Tissue and Function  Chorion: Outer layer of the blastula; Completely surrounds the embryo  Placenta: Chorionic villi and uterine wall; Allows the exchange of O 2, nutrients, and waste  Umbilical Cord: Yolk sac and allantois; Connects the fetus to the placenta

Obtaining Nourishment  Fertilization until Implantation- Food comes from the egg.  After Implantation- Food comes from mother’s body by diffusing through the placenta.

Movement of Materials Through the Umbilical Cord Nutrients and Oxygen  Diffuse from mother’s blood  To placenta  To umbilical cord  To baby’s blood Wastes from baby  Go to umbilical cord  Diffuse through placenta  Go into mother’s blood stream

Substances that can Harm the Fetus  Drugs- Nicotine; Heroin; Narcotics  Alcohol- All kinds of alcohol  Viruses- HIV; German measles  NOTE: These substances are more likely to harm an Embryo or fetus than a fully grown individual because An embryo is still developing; it small and not fully formed.