Fertilization & Development of the Early Embryo:

Copulation - Biological term for sexual intercourse.

Fertilization - Fusion (joining) of egg with sperm. During copulation, semen is ejaculated into the vagina. Sperm swims throught eh cervix into the uterus and into the oviducts. If an egg is in the oviduct, a sperm may fuse with the egg to form a zygote (fertilized egg).

Gamete - Sex cells. Male sex cell = sperm. Female sex cell = egg.

Zygote - Fertilized egg. It is a new human.

The zygote divides by mitosis to form a 2 cell embryo. The 2 cells divide by mitosis to create a 4 cell embryo, and so on. The embryo moves down the oviduct as it divides. Cell division within the embryo continues until a hollow ball of cells is formed, creating a blastocyst. About 7 days after fertilization, the embryo implants itself in to the uterine wall and continues to grow and develop.

Implantation: Joining of embryo to the uterine lining.

Human Chorionic Gonadotropin (HCG): A hormone secreted by the placenta by the second week after fertilization which stimulates the corpus luetum to continue producing estrogen and progesterone. Eventually, the placenta takes over the production of these latter two hormones.

Upon implantation in the uterine wall, the embryo begins to form a placenta. The placenta is a disc-shaped structure with many blood vessels in it. It consists of many finger-like extensions which attach to the uterus to increase the surface area for exchange of gases, nutrients and wastes. Note that the blood of the embryo/fetus and mother do not come into direct contact. Exchange of gases and nutrients is largely performed by diffusion and osmosis.

In addition to formation of the placenta, an amniotic sac forms around the embryo/fetus. The amniotic sac contains amniotic fluid which surrounds the embryo/fetus to act as a shock absorber.

When the embryo is about 8 weeks old, it will have begun to form bone tissue and is now known from this point on as a fetus.

Amniocentesis - Process whereby a sample of the amniotic fluid is removed from around the fetus by inserting a hypodermic needle through the abdomen wall. The amniotic fluid contains fetal cells which can be cultured (grown) to make them divide. The chromosomes of some of these fetal cells can then be examined via microscope to test for chromosomal abnormalities such as Down's syndrome.

Progesterone - Hormone which develops and thickens the uterus so it can sustain the developing fetus throughout the nine months of pregnancy.

Progesterone levels within the mother remain high throughout the nine months of pregnancy. At the end of pregnancy, the level of progesterone falls dramatically. This allows the mother's body to release another hormone, oxytocin, which causes the muscle in the uterine walls to contract. The uterine muscle contractions become stronger and stronger as a result of positive feedback.

As contractions strengthen, the cervix relaxes and thus becomes wider. Soon the amniotic sac bursts and the amniotic fluid is released. After many stronger contractions, the baby is pushed out through the cervix and vagina. The umbilical cord is cut and the baby becomes independent of the mother (no longer receives oxygen and nutrients from the mom). Uterine contractions continue for a while in the mom, eventually pushing out the placenta as what we call the afterbirth.