Each primary oocyte undergoes the first meiotic division while still in the ovarian follicle. This division forms a secondary oocyte and the first polar body, each containing 23 chromosomes. At ovulation, the secondary oocyte and first polar body, still enclosed within a sphere of granulosa cells, are released into a uterine tube. They are slowly moved toward the uterus through peristalsis and the beating cilia of epithelial cells lining the uterine tube.
After semen is deposited in the vagina, sperm begin their long journey into the uterus and on into the uterine tubes. Sperm inherently swim against the slight current of fluid that flows from the uterine tubes through the uterus and into the vagina, which helps to guide sperm towards the uterine tubes. of the millions of sperm in the semen, only a few hundred sperm actually enter the uterine tubes. Prostaglandins in semen stimulate reverse peristalsis of the uterus and uterine tubes that greatly aids the migration of sperm. Sperm reach the superior portions of the uterine tubes within one hour after sexual intercourse. Usually, only one uterine tube contains a secondary oocyte because only one secondary oocyte is usually released at ovulation. Sperm entering an empty uterine tube have no chance for fertilization.
Within the uterine tube, sperm are chemically attracted to the secondary oocyte. Many sperm cluster around the oocyte and attempt to penetrate the granulosa cells. The acrosomes of the sperm release enzymes that dissolve the “glue” holding granulosa cells
together so they can reach the oocyte. It takes many sperm to disperse the granulosa cells, so that one sperm can eventually wriggle between them to contact the oocyte. The acrosome then releases a different enzyme that enables the sperm to penetrate the oocyte membrane and enter the oocyte. Once this happens, changes in the oocyte plasma membrane prevent other sperm from entering.
When a sperm enters the secondary oocyte, it triggers the second meiotic division, which forms the ovum and a second polar body. Then, the sperm nucleus and ovum nucleus unite in fertilization to form a zygote, the first cell of the infant-to-be. The zygote is the first stage of development. The zygote contains 46 chromosomes, 23 from the sperm and 23 from the ovum.
A secondary oocyte remains viable for about 24 hours after ovulation. Most sperm remain viable in the female reproductive tract for about 72 hours, although some may be viable for up to five days. Therefore, fertilization is most likely to occur when sexual intercourse occurs from three days before ovulation to one day after ovulation.
Immediately after fertilization, the zygote begins to divide by mitotic cell division. The early cell divisions are collectively known as cleavage. These divisions occur so rapidly that maximum cellular growth between divisions is not possible, which results in increasingly smaller cells. During this time, the preembryo is carried along the uterine tube by peristalsis and the beating cilia of epithelial cells lining it. By the time the preembryo reaches the uterus, it consists of a solid ball of cells called a morula (mor’-u-lah) that is not much larger than the zygote.
Continued but slightly slower mitotic divisions form a larger hollow ball of cells called a blastocyst (blas ‘-to-sist). Located within the blastocyst is the embryoblast or inner cell mass, a specialized group of cells from which the embryo later develops. The superficial wall of the blastocyst is called the trophoblast, which later will form the embryonic portion of the placenta.
About the seventh day of development, the blastocyst attaches to the endometrium. Digestive enzymes, released from the trophoblast, enable the blastocyst to penetrate into the endometrium, where it is soon covered by the superficial endometrium. This entire process is called implantation and is completed by the fourteenth day.
Identical, or monozygotic, twins develop from a single zygote; this means that the twins possess identical genetic characteristics. The embryoblast of the blastocyst separates completely, usually by the end of the first week of embryonic development, and results in two embryos within separate amnion sacs yet sharing a common chorion and placenta. Fraternal, or dizygotic, twins develop from two zygotes: two different secondary oocytes are fertilized by different sperm. These twins do not possess identical genetic characteristics and develop within separate amnions and chorions. Each embryo develops its own placenta, though the placentas may fuse if they are located near each other within the uterus.