Once fertilization has occurred, the secondary oocyte completes meiotic division. It then undergoes mitosis, first forming a ball of cells and then an early embryonic structure called a blastocyst. Cells of the blastocyst secrete human chorionic gonadotropin, a hormone that maintains the mother's corpus luteum and its production of estradiol and progesterone.This prevents menstruation so that the embryo can implant into the endometrium,develop,and form a placenta. Birth is dependent upon strong contractions of the uterus, which are stimulated by oxytocin from the posterior pituitary.
■ Figure 20.39 The process of fertilization. As the head of the sperm cell encounters the gelatinous corona radiata of the secondary oocyte, the acrosomal vesicle ruptures and the sperm cell digests a path for itself by the action of the enzymes released from the acrosome. When the plasma membrane of the sperm cell contacts the plasma membrane of the ovum, they become continuous, and the nucleus of the sperm cell moves into the cytoplasm of the ovum.
During the act of sexual intercourse, the male ejaculates an average of 300 million sperm into the vagina of the female. This tremendous number is needed because of the high sperm fatality— only about 100 survive to enter each fallopian tube. During their passage through the female reproductive tract, the sperm gain the ability to fertilize an ovum. This process is called capacitation. Although the changes that occur in capacitation are incompletely understood, experiments have shown that freshly ejaculated sperm are infertile; they must be present in the female tract for at least 7 hours before they can fertilize an ovum.
A woman usually ovulates only one ovum a month, for a total of less than 450 ova during her reproductive years. Each ovulation releases a secondary oocyte arrested at metaphase of the second meiotic division. The secondary oocyte, as previously described, enters the uterine tube surrounded by its zona pellucida (a thin transparent layer of protein and polysaccha-rides) and corona radiata of granulosa cells (fig. 20.39).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.