Sex Hormones

Male Sex Hormones

The male sex hormone is called Testosterone and this hormone is required for spermatogenesis. A relatively high concentration of testosterone is maintained within the testicular tissue and testosterone is circulated around the body by diffusion of the hormone from the spermatic cord into the testicular veins and arteries.

Hormone Production

Male sex hormones are regulated by negative feedback systems that operate at various levels within the male sex hormone system. The starting point for the production of testosterone (and therefore the production of spermatozoa)is the hypothalamus. The hypothalamus contains neuroendocrine cells that are capable of secreting a substance called Gonadotropin-releasing hormone or GnRH. GnRH stimulates cells in the anterior pituitary, via the "portal system" to secrete two intermediate hormones within the male sex hormone cycle; Luteinizing hormone (LH) and Follicle-Stimulating Hormone (FSH).

The secretion of GnRH is pulsatile and can vary greatly throughout the day and/or year, and therefore the secretion of LH and FSH are also pulsatile (although the plasma concentration of FSH does not fluctuate as much as LH due to the effect of Inhibin, see below). The activity of GnRH neuroendocrine cells is determined by spontaneous rhythms and by sensory impulses. Cycles such as seasonal sexual activity are controlled by this pulsatile system. In male animals there are generally 4 to 12 GnRH pulses per day.

FSH causes Sertoli cells to produce estradiol and the peptide hormone Inhibin. LH acts on Leydig cells causing them to produce testosterone. Testosterone and estradiol form part of the negative feedback loop on GnRH secretion by the hypothalamus. Inhibin is part of a further negative feedback system, acting on the anterior pituitary to inhibit the secretion of FSH.

Leydig cells produce testosterone using cholesterol as a base molecule. Leydig cells contain various enzymes such as 18-hydroxylase that facilitate the production of steroid hormones. In some species such as pigs, the testes also produce Androstenone which acts as a pheromone and is secreted via the saliva, often producing a copulatory position from sows in oestrous. Androstenone is also responsible for bore taint.

Regulation of Male Sex Hormones

When LH binds to the Leydig cells, it stimulates the cellular messenger cAMP to activate protein kinase A. Protein kinase A undergoes a series of phosphorylations that in turn activate a series of enzymes that synthesis testosterone from the cholesterol base molecule. A portion of the testosterone produced in the Leydig cells diffuses into the Sertoli cells that are positioned adjacent to the Leydig cells in the testes but seperated by a basal lamina. This secreted testosterone is converted to to the female sex hormone estradiol in the Sertoli cell and as with the testosterone, a proportion diffuses into the blood, becoming part of the negative feedback system for LH.

Testosterone inhibits the secretion of GnRH from the hypothalamus and therefore secretion of LH from the pituitary gland. if the testes are removed via castration, blood concentrations of LH and FSH will increase as there is only limited negative feedback.

Effects of Male Sex Hormones

Testosterone plays a crucial role in the development of male sex organs during fetal growth where increased production of testosterone causes penis growth and development of accessory sex glands during puberty. Testosterone also affects a number of other characteristics of the male, often called the "secondary sex characteristics". Testosterone is able to bind to receptors in the cytosol of cells in the same manner as other steroid hormones and these hormone-receptor complexes are then able to bind to DNA in the nucleus resulting in alterations in the level of transcription of specific genes.

Testosterone has a number of anabolic effects stimulating the development and growth of the skeleton and skeletal muscles. Muscle masses show a general increase and in certain body regions such as the neck of stallions or bulls there is obvious hypertrophy. Testosterone also alters behaviour in terms of increasing the degree of sex drive and as a result of the action in several areas of the brain, behaviour can become more aggressive. The larynx of males also enlarges during puberty and the vocal cords lengthen resulting in a deeper and stronger voice.

Testosterone also causes an increase in the level of pheromones to be secreted by glands in the skin which attract and evoke sexual behaviour in females. Glands use in scent marking and territorial marking are also activated by testosterone. In certain species, tusks, antlers and horns are also stimulated to develop.

Female Sex Hormones

Oestrogens are the hormones responsible for inducing oestrous in females. Estradiol is the principle oestrogen in the female.Estrone and Estriol are chemically similar to estradiol but are found in lower concentrations and have a lower estrogenic activity.

Hormone Production

Production of oestrogens occurs in the ovary, placenta, adrenal cortex and in males in Sertoli cells found in the testes (see above). Estradiol is synthesised from cholestrol.

Progesterone is produced in the corpus luteum, also using cholesterol as a base molecule.