Review of Medical Physiology - William F. Ganong 2002

Endocrine System, Metabolism, and Reproduction
The Gonads: Development and Function of the Reproductive System
Introduction

Modern genetics and experimental Embryology have revealed that in most mammalian species, the manifold differences between male and male-to-female organisms primarily depend on a single chromosome (the Y chromosome) and a single pair of endocrine structures: the Testes in males and the Ovaries in females. The differentiation of immature Gonads into testes or ovaries is genetically determined in humans during embryonic development. However, The formation of male genitalia relies on the presence of functional testes; in the absence of testicular tissue, female-pattern development occurs. There is evidence that male Sexual Behavior in certain species, as well as their characteristic gonadotropin secretion pattern, is driven by the action of male Hormones on the Brain during early development. After birth, the gonads remain quiescent until Puberty, when they are activated by anterior pituitary gonadotropins. The hormones secreted by the gonads during this period trigger the appearance of adult male or female secondary sex characteristics and the female sexual cycle. In women, ovarian function regresses after many years, and sexual cycles cease (menopause). In men, gonadal function gradually declines with age, but The ability to father children is retained.

In individuals of both sexes, the gonads perform a dual function: they produce Germ Cells (gametogenesis) and facilitate the secretion of Sex Hormones. Androgens are steroid sex hormones that exert masculinizing effects, whereas estrogens are feminizing. Normally, both types of hormones are secreted in both sexes. The testes secrete large amounts of androgens—predominantly testosterone—as well as estrogens. Conversely, the ovaries secrete large amounts of estrogens and minor quantities of androgens. Androgens are also produced in the adrenal cortex and can be converted into estrogens within adipose and other Tissues. The ovaries additionally produce progesterone, a steroid that fulfills specialized Functions in preparing the Uterus for Pregnancy. During pregnancy, the ovaries secrete the polypeptide hormone relaxin, which relaxes the Pubic Symphysis ligaments and softens the cervix, thereby facilitating childbirth. In both sexes, the gonads also secrete other Polypeptides, such as inhibin, which suppresses FSH secretion.

The secretory and gametogenic Functions of the gonads depend on the secretion of the gonadotropins FSH and LH by the anterior Pituitary Gland. Sex hormones and inhibin interact via feedback mechanisms to inhibit gonadotropin secretion. In males, gonadotropin secretion is non-cyclic, whereas in postpubertal females, it is regular and sequential, which is essential for menstruation, pregnancy, and Lactation.