Ovaries

Like the testes, the ovaries are endocrine glands and not only produce eggs for reproduction, but also secrete the sex hormones estrogens and progestins. Both ovaries and testes develop from the mesoderm, from the genital ridge located on the ventral side of the primary kidney. The ovaries are paired bean-shaped structures about 4 cm in length, held on the dorsal side of the abdominal cavity by mesenteries.

The outer layer of each of these glands is formed by the germinal epithelium from which the eggs develop, and the inner part consists of connective tissue and blood vessels. Directly below the germinal epithelium lies a thick layer of spherical cellular formations - follicles, each of which contains an egg. By the time of birth, there are already several hundred thousand follicles formed from the germinal epithelium.

Some of them degenerate, but most remain dormant until puberty, when the process of growth and development begins. Each month, one or more follicles begin to enlarge and become stretched by follicular fluid until they finally protrude above the surface of the ovary and burst, releasing the egg they contain. This process is called ovulation.

The released egg travels through a canal called the oviduct (fallopian tube) into the uterus. If an egg meets a sperm and is fertilized at the top of the oviduct, it will eventually implant into the wall of the uterus and begin to develop into an embryo. If fertilization does not occur, the egg degenerates. Regardless of whether fertilization occurs, the cells of the follicle remaining after its rupture during ovulation quickly multiply and fill the cavity at the site of the former follicle. Since these cells are yellow in color, the resulting formation is called the corpus luteum. Being the size of a pea, it protrudes above the surface of the ovary and is visible to the naked eye.

If fertilization has occurred, the corpus luteum remains for many months; otherwise, it will atrophy and turn into a small white patch of scar tissue after about two weeks. The main female sex hormone, 17β-estradiol, is produced by the cells lining the cavity of each follicle. This substance causes the physical changes that occur in a woman during puberty: expansion of the pelvis, development of the mammary glands, growth of the uterus and vagina, the appearance of pubic hair, changes in the timbre of the voice and the onset of menstruation. The second female sex hormone, progesterone, is produced in the cells of the corpus luteum. It is necessary to complete each menstrual cycle, as it completes the changes that occur in the uterus under the influence of estradiol. In addition, it makes possible the implantation of a fertilized egg into the wall of the uterus and stimulates the development of mammary glands in the last months of pregnancy.

Chemically, progesterone is related to the hormones of the adrenal cortex and is believed to be formed as an intermediate product in their synthesis, as well as in the synthesis of estradiol and testosterone. Both male and female sex hormones are produced in individuals of both sexes; in fact, one of the richest sources of female sex hormone used for practical purposes is the urine of stallions. Recently, female sex hormones have been found in palm oil and in the tissues of the willow Salix discolor. What role they play there is unknown.