The normal reproductive system

Alcohol Health & Research World,  Spring, 1991  by John Doria

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To understand how alcohol affects the reproductive system, it is important to understand how the reproductive system functions normally. The primary reproductive organs are the gonads: the testes in the male and the ovaries in the female. In both sexes, the gonads serve dual functions: production of the reproductive cells, or gametes, and secretion of the sex hormones. Additional hormones produced at distant locations in the body exert various levels of control over these processes; these hormones travel through the bloodstream to reach their target organs. The interactions of the various organs, structures, and hormones involved in reproduction provide several potential sites of action for substances, such as alcohol, that alter reproductive function.

Male Reproductive Physiology

Functions of the Testes

The essential male reproductive functions are the production of male gametes, or sperm (spermatogenesis), and the deposition of sperm in the female for the purpose of fertilization. Additional functions include the production of male sex hormones and the effects of those hormones on reproduction and other bodily functions. Sperm production and hormone production are carried out by two different types of cells in the testes.

Spermatogenesis.

Sperm cells are produced within tiny, convoluted structures known as seminiferous tubules. These tubules have a combined length of 250 meters and occupy 95 percent of the volume of the testes. They produce hundreds of millions of sperm cells daily until late adult life, or even until death.

Hormone Production.

The most important male sex hormone is testosterone, a steroid that can be synthesized in the body from cholesterol. Testosterone is produced by Leydig cells. These cells are found in the small spaces surrounding the seminiferous tubules, where they occupy 2 percent to 3 percent of the testicular volume. The Leydig cells produce about 7 milligrams of testosterone daily.

Erection and Ejaculation

Erection of the penis and ejaculation of the sperm result from reflex mechanisms involving the lower third (sacral and lumbar portions) of the spinal cord. These reflex mechanisms can be initiated by direct genital stimulation or by mental arousal. The degree of erection is proportional to the degree of stimulation or arousal.

Hormonal Control of Male Reproductive Functions

Male reproductive function is controlled by testosterone in concert with hormones known as gonadotropins. Gonadotropins are produced by the anterior (front) portion of the pituitary gland, which is located at the base of the brain. The gonadotropins affect only the testes; testosterone acts not only on the testes but also on various organ systems throughout the body.

Effects of Testosterone.

Testosterone is necessary for spermatogenesis; a deficiency will cause sterility. Testosterone also affects the development of male secondary sexual characteristics, such as muscular development, bone growth, basal metabolic rate, and the distribution of body hair. Male sex hormones are also found in women, where they help stimulate general body growth and maintain sexual drive.

Effects of Gonadotropins.

The pituitary gonadotropins include follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Unlike testosterone, these hormones are polypeptides, molecules composed of chains of amino acids. They were named for their effects in the female, although their respective molecular structures are the same in both sexes. In the male, FSH and LH are essential for both testosterone secretion and spermatogenesis. The Leydig cells produce testosterone only when stimulated by LH; the quantity of testosterone secreted is roughly proportional to the amount of LH available. The function of FSH is to stimulate spermatogenesis within the seminiferous tubules. However, sperm cells will not achieve final maturity without testosterone. Therefore, both FSH and LH must be secreted by the anterior pituitary for spermatogenesis to occur.

The Hypothalamic-Pituitary-Gonadal Axis.

The pituitary is often referred to as the "master gland," because its hormones control the activities of other glands. However, the pituitary in turn is regulated by a "higher" control center, a region of the brain known as the hypothalamus. Located directly above the pituitary, the hypothalamus produces substances that travel via a special set of blood vessels to the pituitary to control the secretion of pituitary hormones. The hypothalamic substance that controls FSH and LH secretion is known as gonadotropin-releasing hormone; like the gonadotropins, this hormone is a polypeptide. Gonadotropin-releasing hormone is emitted in regular bursts, resulting in fairly stable blood levels of both LH and FSH. It is important to bear in mind that the hypothalamus does not affect the testes directly, but rather influences them indirectly by its control of the pituitary.

The hypothalamus is influenced by concentrations of steroid sex hormones and gonadotropins circulating in the bloodstream. Other substances, including chemical messengers from other parts of the brain, also modulate secretion of releasing hormones, thereby frequently altering the degree of fertility. The term "hypothalamic-pituitary-gonadal (HPG) axis" refers to the integrated functioning of these three levels of hormonal control.