Fertilization is the union of the sperm cell nucleus with the egg cell nucleus to produce a single cell, the zygote (Applegate, 2000, 420). During ovulation, a secondary oocyte, surrounded by a zona pellucida and a corona radiata is released from the surface of ovary and enters the uterine tube. As it travels through the female reproductive tract, the sperm undergoes capacitation, which weakens the membrane around the acrosome, allowing the release of enzymes. Thousands of sperm acrosomes release enzymes when they reach the secondary oocyte, and it breaks down the corona radiata and the zona pellucida of the secondary oocyte so that a tiny hole is made in the cell membrane and a sperm can enter the oocyte, after which, changes occur in the cell membrane so no other sperm can enter.
The entry of the sperm stimulates the second meiotic division to resume in the oocyte, creating a polar body which is pushed to the side, and a large ovum with all the cytoplasm and 23 single-stranded chromosomes (Applegate, 2000, 421). The sperm then moves to the center of the egg, their nuclear membranes disintegrate, and the chromosomes combine, completing the fertilization process. The zygote now has a full complement of chromosomes, and is the first cell of the future fetus. The effects of fertilization are the creation of a cell with a new combination of chromosomes, half of which came from the sperm, and half of which came from the egg. This will result in a unique individual if the fertilized zygote implants in the uterus and is allowed to develop to term.
12.2 Once the secondary oocyte has been fertilized, and a zygote has been formed, the cell will undergo a number of mitotic cell divisions which will produce an increasing number of cells (Applegate, 2000, 422). These cell divisions are known as cleavage divisions. The first such cleavage division will be completed about 36 hours after fertilization and result in two cells known a...