The ovary is a complex, dual-purpose organ that serves as the primary reproductive gland in females, responsible for both producing gametes and synthesizing hormones. Its functions are central to the female reproductive system, governing the menstrual cycle, supporting pregnancy, and influencing many other systemic biological processes. The ovary’s role is not static; it changes dynamically throughout the human lifespan, from fetal development through to the postmenopausal years.
Physical Structure and Location
The human ovaries are paired organs situated within the pelvic cavity, one on each side of the uterus. They are typically solid, ovoid structures, measuring about 3.5 centimeters in length and 2 centimeters in width. They are held in position by several peritoneal ligaments, including the ovarian ligament which connects them directly to the uterus.
Each ovary is encased in a dense connective tissue capsule called the tunica albuginea, covered by a layer of surface epithelium. Internally, the ovary is organized into a central medulla and an outer cortex. The inner medulla serves as the vascular core, containing abundant blood vessels, lymphatic vessels, and nerve fibers.
The outer cortex is the functional area, containing numerous ovarian follicles. These follicles house the female germ cells, or oocytes, and are found in various stages of development. The cortex is the site of hormone production and egg maturation and release.
Primary Function: Hormone Production
The ovary functions as an endocrine gland, producing steroid hormones that regulate reproductive processes. The two major classes of hormones synthesized are estrogens and progestins, with estradiol being the most potent estrogen and progesterone the main progestin. These hormones are secreted by specialized cells within the follicles.
Estrogen is responsible for the development of secondary sex characteristics, such as breast development and the distribution of body fat. Estrogen also plays a role in maintaining bone density by preventing tissue breakdown. It supports the health of the skin and mucous membranes, and has protective effects on the cardiovascular system.
Progesterone’s primary function is to prepare the lining of the uterus, the endometrium, for potential implantation. It causes the uterine lining to mature, increasing its blood supply and nutrient secretions. If fertilization does not occur, the drop in progesterone levels signals the start of menstruation.
The Reproductive Process
Gamete production, known as oogenesis, begins before birth. The cortex contains ovarian follicles, each consisting of an immature egg (oocyte) surrounded by support cells. These follicles exist in a dormant state as primordial follicles until they are periodically activated to begin maturation.
Folliculogenesis is the process where a primordial follicle develops into a mature, pre-ovulatory follicle. This involves the follicle growing, the oocyte completing its first meiotic division, and surrounding cells developing a fluid-filled cavity called the antrum. Maturation from primordial to pre-ovulatory stage can take several months.
During each menstrual cycle, a cohort of follicles develops, but typically only one dominant follicle reaches full maturity. Around the middle of the cycle, a surge in luteinizing hormone triggers ovulation, causing the mature follicle to rupture and release the oocyte into the fallopian tube.
The ruptured follicle transforms into the corpus luteum, which secretes progesterone and some estrogen to sustain the uterine lining in anticipation of pregnancy. If fertilization and implantation do not occur, the corpus luteum degenerates into scar tissue known as the corpus albicans. This degeneration causes a sharp drop in hormone levels, triggering menstruation.
Ovarian Changes Across the Lifespan
The ovary’s reproductive capacity is finite, determined by the ovarian reserve, which is the total number of follicles present. The peak number of oocytes is reached during fetal development, after which the number begins to decline through a process of cell death called atresia. At birth, the ovaries contain about one to two million oocytes, which is the lifetime supply.
By the time a female reaches puberty, the reserve has decreased significantly, with approximately 300,000 to 500,000 follicles remaining. Puberty marks the reactivation of the hypothalamic-pituitary-gonadal axis, leading to the regular cyclical maturation of follicles and the onset of the menstrual cycle. Only about 500 oocytes will ever be released through ovulation during the reproductive years.
The decline in the follicle pool accelerates after the age of 35, reflecting a decrease in both the quantity and quality of the remaining oocytes. This gradual exhaustion of the ovarian reserve eventually leads to menopause, defined as the cessation of menstrual periods for twelve consecutive months.
Menopause typically occurs around the age of 51. At this time, the ovaries contain a minimal number of follicles, approximately 1,000 or fewer. The postmenopausal ovary ceases its primary functions of gamete production and steroid hormone secretion.