Technische Universität München

The Entrepreneurial University

The endocrine and nervous systems are two major systems involved in the control and regulation of body functions. The endocrine system consists of endocrine glands and endocrine cells in other organs (kidney, placenta, etc.) that secrete biologically active substances called hormones. In contrast to exocrine glands, which have secretory ducts, the endocrine glands are “ductless” and secrete hormones into the blood. Hormones are distant chemical messengers that are transported via the circulating blood to the target cells where they exert their specific effects. Most hormones are derivatives of cholesterol (steroid hormones) or are amino acids (thyroid hormones, adrenaline) and polypeptide derivatives. Depending on the nature and size of the hormone molecule, hormones bind to specific receptors that are either on the surface or in the cytoplasm/the nucleus of target cells, triggering sequences of intracellular changes that produce their physiological effects. The main endocrine glands in the body are the pituitary (hypophysis), thyroid, parathyroid, and adrenal glands, the pancreas, ovaries, and testes. The rate of hormone secretion is often regulated by a homeostatic negative feedback control mechanism. The major hormones and their effects are presented in tables (see picture box).

Note: Some hormones reach the target cells by extracellular fluid and have local, paracrine effects (prostaglandins, endothelins, etc.).  

Male and female reproductive systems
The differences between male and female organisms depend primarily on the sex chromosomes (XY or XX) and a pair of endocrine structures, the testes in the male and the ovaries in the female. The differentiation of gonads in utero is genetically determined, but the formation of male genitalia depends upon the presence of testes, secreting testosterone, and if there is no testicular tissue the development is female. After birth the gonads remain inactive until adolescence when they are activated by gonadotrophins (LH and FSH) from the anterior pituitary. In adolescence, as a result of the action of the gonads-secreted hormones, the secondary sexual characteristics develop and the final maturation of the reproductive system occurs, including mammary development and onset of menstrual cycles in the female.  

In both sexes, the gonads have gametogenic (production of germ cells) and endocrine (secretion of sex hormones) functions. Steroid hormones secreted by the gonads (androgens, principally testosterone, from testes and estrogen and progesterone from ovaries) promote the sex-specific physical characteristics and initiate and maintain reproductive function. Androgens are steroid hormones with masculinizing effects and estrogens are steroid hormones with feminizing effects. Both types of sex hormones are normally secreted in males and females, but there are general differences in hormone concentrations between sexes. There is a slow decline in the functions of male reproductive system with advancing age, but the ability to father children persists. The functions of female reproductive system regress after a number of years and the menstrual cycles cease (menopause).