The Female Reproductive System
The female gonads are the ovaries, where the female sex cells, or ova, are formed (Fig. 19-8). The remainder of the female reproductive tract consists of an organ (uterus) to hold and nourish a developing infant, various passageways, and the external genital organs.
The ovary is a small, somewhat flattened oval body measuring about 4 cm (1.6 inches) in length, 2 cm (0.8 inch) in width, and 1 cm (0.4 inch) in depth. Like the testes, the ovaries descend, but only as far as the pelvic portion of the abdomen. Here, they are held in place by ligaments, including the broad ligament, the ovarian ligament, and others, that attach them to the uterus and the body wall.
The Ova and Ovulation
The outer layer of the ovary is made of a single layer of epithelium. Beneath this layer, the female gametes, the ova, are produced. The ovaries of a newborn female contain a large number of potential ova. Each month during the reproductive years, several ripen, but usually only one is released.
The complicated process of maturation, or “ripening,” of an ovum takes place in a small fluid-filled cluster of cells called the ovarian follicle or graafian follicle (Fig. 19-9). As the follicle develops, cells in its wall secrete the hormone estrogen, which stimulates growth of the uterine lining. When an ovum has ripened, the ovarian follicle may rupture and discharge the egg cell from the ovary’s surface. The rupture of a follicle allowing the escape of an ovum is called ovulation. Any developing ova that are not released simply degenerate. After it is released, the egg cell makes its way to the nearest oviduct, a tube that arches over the ovary and leads to the uterus (see Fig. 19-8).
Figure 19-8 Female reproductive system. The enlargement (right) shows ovulation.
Figure 19-9 Microscopic view of the ovary. The photomicrograph shows egg cells (ova) developing within ovarian (graafian) follicles.
The Corpus Luteum: After the ovum has been expelled, the remaining follicle is transformed into a solid glandular mass called the corpus luteum. This structure secretes estrogen and also progesterone, another hormone needed in the reproductive cycle. Commonly, the corpus luteum shrinks and is replaced by scar tissue. When a pregnancy occurs, however, this structure remains active. Sometimes, as a result of normal ovulation, the corpus luteum persists and forms a small ovarian cyst (fluid-filled sac). This condition usually resolves without treatment.