The ovaries are small, oval-shaped glands located in the pelvis, one on each side of the uterus. As a central part of the female reproductive system, they are responsible for producing and releasing eggs, known as oocytes. They also synthesize and secrete hormones that regulate the menstrual cycle and support pregnancy. The ovary undergoes significant structural and functional changes throughout a person’s life, from before birth through menopause.
The Two Primary Types of Ovarian Cells
The ovary is composed of two main categories of cells: germ cells and somatic cells. The germ cell is the oocyte, or egg cell, which can develop into an embryo if fertilized. Each oocyte is housed within a functional unit called a follicle, a cluster of supportive somatic cells.
Surrounding the oocyte are the somatic cells, which perform support functions. The most prominent of these are the granulosa cells and theca cells. Granulosa cells directly encircle the oocyte, providing nourishment and secreting hormones that influence its development. Theca cells form the outer layer of the follicle and work with the granulosa cells to produce hormones. A third type, ovarian surface epithelial cells, forms the outer lining of the ovary.
The Follicle and Ovulation
The development and release of an egg is a process known as folliculogenesis. It begins with a primordial follicle, an immature oocyte surrounded by a single layer of granulosa cells. During the menstrual cycle, hormonal signals prompt a cohort of these follicles to grow. This maturation involves the rapid multiplication of granulosa cells and the formation of a fluid-filled cavity called the antrum.
The follicle that becomes dominant continues to expand, while the others that began developing will degenerate in a process called atresia. The culmination of this process is ovulation, where the mature follicle ruptures and releases the egg from the ovary. Following ovulation, the remaining follicular cells transform into a new structure called the corpus luteum.
Hormone Synthesis and Its Impact
The ovary’s role extends beyond releasing an egg; it is also an endocrine organ. The theca and granulosa cells within the follicle cooperate to synthesize reproductive hormones. Theca cells produce androgens, which are then converted into estrogens by the granulosa cells. Estrogen is responsible for developing female secondary sexual characteristics and prompts the uterine lining to thicken for potential pregnancy.
After ovulation, the corpus luteum, formed from the follicle’s remnants, produces large amounts of progesterone. Progesterone further prepares the uterus for the implantation of a fertilized egg and is necessary for maintaining early pregnancy. This cyclical production of estrogen and progesterone by ovarian cells drives the menstrual cycle, orchestrating the interplay between the ovaries, pituitary gland, and uterus.
Cellular Aging and Menopause
A person is born with a finite number of oocytes, around one to two million, known as the ovarian reserve. New eggs cannot be generated after birth. This reserve continuously declines as follicles are recruited for maturation or undergo atresia each month. By puberty, only about 25% of the original pool of eggs remains.
This gradual depletion of the ovarian reserve is a natural part of aging. As the number of healthy follicles diminishes, the ovary’s ability to produce hormones wanes. Menopause marks the point when the ovarian reserve is exhausted. Consequently, the production of estrogen and progesterone drops significantly, leading to the cessation of the menstrual cycle.
When Ovarian Cells Malfunction
Cellular dysfunction in the ovary can lead to common health issues. For instance, ovarian cysts are frequently related to follicular structures. These fluid-filled sacs can form when a follicle fails to mature or does not rupture to release its egg. While most cysts are harmless and resolve on their own, they result from a disruption in the ovulation cycle.
More serious conditions can also arise. The majority of ovarian cancers originate from the surface epithelial cells that form the outer layer of the organ, not from germ or hormone-producing somatic cells. Alterations in these cells can lead to uncontrolled growth, forming tumors. While the specific triggers for these changes are complex, it shows how malfunctions in one cell population can have significant health consequences.