The gonads (testes and ovaries) secrete sex steroids. These include male sex hormones, or androgens, and female sex hormones—estrogens and progesterone. The androgens and estrogens are families of hormones. The principal androgen secreted by the testes is testosterone, and the principal estrogen secreted by the ovaries is estradiol-17$. The principal estrogen during pregnancy, however, is a weaker estrogen called estriol, secreted by the placenta. After menopause, the principal estrogen is estrone, produced primarily by fat cells.
The testes consist of two compartments: seminiferous tubules, which produce sperm cells, and interstitial tissue between the convolutions of the tubules. Within the interstitial tissue are Leydig cells, which secrete testosterone. Testosterone is needed for the development and maintenance of the male genitalia (penis and scrotum) and the male accessory sex organs (prostate, seminal vesicles, epididymides, and vas deferens), as well as for the development of male secondary sex characteristics.
During the first half of the menstrual cycle, estradiol-17P is secreted by small structures within the ovary called ovarian follicles. These follicles contain the egg cell, or ovum, and granulosa cells that secrete estrogen. By about midcycle, one of these follicles grows very large and, in the process of ovulation, extrudes its ovum from the ovary. The empty follicle, under the influence of luteinizing hormone (LH) from the anterior pituitary, then becomes a new endocrine structure called a corpus luteum. The corpus luteum secretes progesterone as well as estradiol-17p.
The placenta—the organ responsible for nutrient and waste exchange between the fetus and mother—is also an endocrine gland that secretes large amounts of estrogens and
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