Reproductive and Hormonal Functions of the Male:
The reproductive functions of the male can be divided into three major subdivisions: (1) spermatogenesis, which means simply the formation of sperm; (2) performance of the male sexual act; and (3) regulation of male reproductive functions by the various hormones.Physiologic Anatomy of the Male Sexual Organs:
Spermatogenesis
During formation of the embryo the testes contain immature germ cells called spermatogonia which lie in two or three layers of the inner surfaces of the seminiferous tubules. The spermatogonia begin to undergo mitotic division, beginning at puberty, and continually proliferate and differentiate through definite stages of development to form spermSteps of Spermatogenesis
Spermatogenesis occurs in the seminiferous tubules during active sexual life as the result of stimulation by anterior pituitary gonadotropic hormones, beginning at an average age of 13 years and continuing throughout most of the remainder of life but decreasing markedly in old age. In the first stage of spermatogenesis, the spermatogonia migrate among Sertoli cells toward the central lumen of the seminiferous tubule.
Meiosis: Spermatogonia that become progressively modified and enlarged to form large primary spermatocytes. Each of these, in turn, undergoes meiotic division to form two secondary spermatocytes. After another few days, these too divide to form spermatids that are eventually modified to become spermatozoa (sperm).
During the change from the spermatocyte stage to the spermatid stage, the 46 chromosomes (23 pairs of chromosomes) of the spermatocyte are divided, so that 23 chromosomes go to one spermatid and the other 23 to the second spermatid. This also divides the chromosomal genes so that only one half of the genetic characteristics of the eventual fetus are provided by the father, while the other half are derived from the oocyte provided by the mother. The entire period of spermatogenesis, from spermatogonia to spermatozoa, takes about 74 days.
In each spermatogonium, one of the 23 pairs of chromosomes carries the genetic information that determines the sex of each eventual offspring. This pair is composed of one X chromosome, which is called the female chromosome, and one Y chromosome, the male chromosome. During meiotic division, the male Y chromosome goes to one spermatid that then becomes a male sperm, and the female X chromosome goes to another spermatid that becomes a female sperm.
Formation of Sperm: When the spermatids are first formed, they still have the usual characteristics of epithelioid cells, but soon they begin to differentiate and elongate into spermatozoa. The head comprises the condensed nucleus of the cell with only a thin cytoplasmic and cell membrane layer around its surface. On the outside of the anterior two thirds of the head is a thick cap called the acrosome that is formed mainly from the Golgi apparatus. This contains a number of enzymes that play important roles in allowing the sperm to enter the ovum and fertilize it. The tail of the sperm, called the flagellum, which is used for sperm movement (flagellar movement). The energy for this process is supplied in the form of ATP that is synthesized by the mitochondria in the body of the tail. Normal sperm move in a fluid medium at a velocity of 1 to 4 mm/min. This allows them to move through the female genital tract in quest of the ovum.
Hormonal Factors That Stimulate Spermatogenesis
Several hormones play essential roles in spermatogenesis. Some of these are as follows:
1. Testosterone, secreted by the Leydig cells located in the testis, is essential for growth and division of the testicular germinal cells, which is the first stage in forming sperm.
2. Luteinizing hormone, secreted by the anteriorpituitary gland, stimulates the Leydig cells to secrete testosterone.
3. Follicle-stimulating hormone FSH, also secreted by the anterior pituitary gland, stimulates the Sertoli cells; without this stimulation, the conversion of the spermatids to sperm (the process of spermiogenesis) will not occur.
4. Estrogens, formed from testosterone by the Sertoli cells when they are stimulated by FSH hormone, are probably also essential for spermiogenesis.
5. Growth hormone (as well as most of the other body hormones) is necessary for controlling background metabolic functions of the testes. Growth hormone specifically promotes early division of the spermtogonia themselves; in its absence, as in pituitary dwarfs, spermatogenesis is severely deficient or absent, thus causing infertility.
Maturation of Sperm in the Epididymis
After formation in the seminiferous tubules, the sperm require several days to pass through the 6-meter-long tubule of the epididymis. Sperm removed from the seminiferous tubules and from the early portions of the epididymis are non-motile, and they cannot fertilize an ovum. However, after the sperm have been in the epididymis for some 18 to 24 hours, they develop the capability of motility. Even though several inhibitory proteins in the epididymal fluid still prevent final motility until after ejaculation.
Storage of Sperm: The two testes of the human adult form up to 120 million sperm each day. A small quantity of these can be stored in the epididymis, but most are stored in the vas deferens. They can remain stored, maintaining their fertility, for at least a month. During this time, they are kept in a deeply suppressed inactive state by multiple inhibitory substances in the secretions of the ducts. Conversely, with a high level of sexual activity and ejaculations, storage may be no longer than a few days.
After ejaculation, the sperm become motile, and they also become capable of fertilizing the ovum, a process called maturation.
Physiology of the Mature Sperm: The normal motile, fertile sperm are capable of flagellated movement though the fluid medium at velocities of 1 to 4 mm/min. The activity of sperm is greatly enhanced in a neutral and slightly alkaline medium, as exists in the ejaculated semen, but it is greatly depressed in a mildly acidic medium. A strong acidic medium can cause rapid death of sperm. The activity of sperm increases markedly with increasing temperature, but so does the rate of metabolism, causing the life of the sperm to be considerably shortened. Although sperm can live for many weeks in the suppressed state in the genital ducts of the testes, life expectancy of ejaculated sperm in the female genital tract is only 1 to 2 days.
Function of the Seminal Vesicles: Each seminal vesicle is a tortuous tube lined with a secretory epithelium that secretes a mucous material containing an abundance of fructose, citric acid, and other nutrient substances, as well as large quantities of prostaglandins and fibrinogen. During the process of emission and ejaculation, each seminal vesicle empties its contents into the ejaculatory duct. This adds greatly to the bulk of the ejaculated semen, and the fructose and other substances in the seminal fluid are of considerable nutrient value for the ejaculated sperm until one of the sperm fertilizes the ovum.
Function of the Prostate Gland: The prostate gland secretes a thin, milky fluid that contains calcium, citrate ion, phosphate ion, a clotting enzyme, and a profibrinolysin. During emission, the capsule of the prostate gland contracts simultaneously with the contractions of the vas deferens so that the thin, milky fluid of the prostate gland adds further to the bulk of the semen. A slightly alkaline characteristic of the prostatic fluid may be quite important for successful fertilization of the ovum, because the fluid of the vas deferens is relatively acidic owing to the presence of citric acid and metabolic end products of the sperm and, consequently, helps to inhibit sperm fertility. Also, the vaginal secretions of the female are acidic (pH of 3.5 to 4.0). Sperm do not become optimally motile until the pH of the surrounding fluids rises to about 6.0 to 6.5. Consequently, it is probable that the slightly alkaline prostatic fluid helps to neutralize the acidity of the other seminal fluids during ejaculation, and thus enhances the motility and fertility of the sperm.
Semen
Semen, which is ejaculated during the male sexual act which is about 2 to 7ml, is composed of the fluid and sperm from the vas deferens (about 10 per cent of the total), fluid from the seminal vesicles (almost 60 per cent), fluid from the prostate gland (about 30 per cent), and small amounts from the mucous glands, especially the bulbourethral glands. Thus, the bulk of the semen is seminal vesicle fluid, which is the last to be ejaculated and serves to wash the sperm through the ejaculatory duct and urethra.
The average pH of the combined semen is about 7.5, the alkaline prostatic fluid having more than neutralized the mild acidity of the other portions of the semen. The prostatic fluid gives the semen a milky appearance. Also, a clotting enzyme from the prostatic fluid causes the fibrinogen of the seminal vesicle fluid to form a weak fibrin coagulum that holds the semen in the deeper regions of the vagina where the uterine cervix lies. The coagulum then dissolves during the next 15 to 30 minutes because of lysis by fibrinolysin formed from the prostatic profibrinolysin. In the early minutes after ejaculation, the sperm remain relatively immobile, possibly because of the viscosity of the coagulum. As the coagulum dissolves, the sperm simultaneously become highly motile.
Although sperm can live for many weeks in the male genital ducts, once they are ejaculated in the semen, their maximal life span is only 24 to 48 hours at body temperature. At lowered temperatures, however, semen can be stored for several weeks, and when frozen at temperatures below -100C, sperm have been preserved for years.
Erection, Emission, and Ejaculation: The Male Sexual Act:
Erection, accompanied by increases in the length and width of the penis, is achieved as a result of blood flow into the erectile tissues of the penis. These erectile tissues include two paired structuresthe corpora cavernosalocated on the dorsal side of the penis, and one unpaired corpus spongiosum on the ventralside. The urethra runs through the center of the corpus spongiosum. The erectile tissue forms columns that extend the length of the penis.Erection is achieved by parasympathetic nerve-induced vasodilation of arterioles that allows blood to flow into the corpora cavernosa of the penis. The neurotransmitter that mediates this increased blood flow is now believed to be nitric oxide. As the erectile tissues become engorged with blood and the penis becomes turgid, venous outflow of blood is partially occluded, thus aiding erection.
The term emission refers to the movement of semen into the urethra, and ejaculation refers to the forcible expulsion of semen from the urethra out of the penis. Emission and ejaculation are stimulated by sympathetic nerves, which cause peristaltic contractions of the tubular system, contractions of the seminal vesicles and prostate, and contractions of muscles at the base of the penis. Sexual function in the male thus requires the synergistic action (rather than antagonistic action) of the parasympathetic and sympathetic systems.
Erection is controlled by two portions of the central nervous systemthe hypothalamus in the brain and the sacral portion of the spinal cord. Conscious sexual thoughts originating in the cerebral cortex act via the hypothalamus to control the sacral region, which in turn increases parasympathetic nerve activity to promote vasodilatation and erection of the penis.
Testosterone:
The testes secrete several male sex hormones, which are collectively called androgens. Testosterone is eventually converted into the more active hormone dihydrotestosterone in the target tissues.Testosterone is formed by the interstitial cells of Leydig, which lie in the interstices between the seminiferous tubules and constitute about 20 per cent of the mass of the adult testes. Leydig cells are almost nonexistent in the testes during childhood when the testes secrete almost no testosterone, but they are numerous in the newborn male infant for the first few months of life and in the adult male any time after puberty; at both these times the testes secrete large quantities of testosterone.
Functions of Testosterone:
Functions of Testosterone during Fetal Development:
Testosterone secreted first by the genital ridges and later by the fetal testes is responsible for the development of the male body characteristics, including the formation of a penis and a scrotum rather than formation of a clitoris and a vagina. Also, it causes formation of the prostate gland, seminal vesicles, and male genital ducts, while at the same time suppressing the formation of female genital organs.
Testosterone begins to be elaborated by the male fetal testes at about the seventh week of embryonic life.
Also Testosterone causes descent of the testes. The testes usually descend into the scrotum during the last 2 to 3 months of gestation when the testes begin secreting reasonable quantities of testosterone. The process of tsetse decent needs normal amount of testosterone.
Effect of Testosterone on Development of Adult Primary and Secondary Sexual Characteristics
After puberty, the increasing amounts of testosterone secretion cause the penis, scrotum, and testes to enlarge about eightfold before the age of 20 years. In addition, testosterone causes the secondary sexual characteristics of the male to develop, beginning at puberty and ending at maturity. These secondary sexual characteristics, in addition to the sexual organs themselves, distinguish the male from the female.
*Effect on the Distribution of Body Hair: Testosterone decreases the growth of hair on the top of the head; a man who does not have functional testes does not become bald. However, many virile men never become bald because baldness is a result of two factors: first, a genetic background for the development of baldness and, second, superimposed on this genetic background, large quantities of androgenic hormones.
*Effect on the Voice: Testosterone causes hypertrophy of the laryngeal mucosa and enlargement of the larynx. *Testosterone Increases Thickness of the Skin and Can Contribute to Development of Acne: Testosterone increases the thickness of the skin over the entire body and increases the ruggedness of the subcutaneous tissues. Testosterone also increases the rate of secretion of all the bodys sebaceous glands. Especially important is excessive secretion by the sebaceous glands of the face, because this can result in acne.
*Testosterone Increases Protein Formation and Muscle Development: One of the most important male characteristics is development of increasing musculature after puberty, averaging about a 50 per cent increase in muscle mass over that in the female. This is associated with increased protein in the non-muscle parts of the body as well. Many of the changes in the skin are due to deposition of proteins in the skin, and the changes in the voice also result partly from this protein anabolic function of testosterone.
*Testosterone Increases Bone Matrix and Causes Calcium Retention: After the great increase in circulating testosterone that occurs at puberty the bones grow considerably thicker and deposit considerable additional calcium salts. Thus, testosterone increases the total quantity of bone matrix and causes calcium retention. The increase in bone matrix is believed to result from the general protein anabolic function of testosterone plus deposition of calcium salts in response to the increased protein.
Testosterone has a specific effect on the pelvis to narrow its outlet and lengthen it. In the absence of testosterone, the male pelvis develops into a pelvis that is similar to that of the female.
The testosterone also causes the epiphyses of the long bones to unite with the shafts of the bones at an early age. Therefore, despite the rapidity of growth, this early uniting of the epiphyses prevents the person from growing tall.
*Testosterone Increases Basal Metabolism: the usual quantity of testosterone secreted by the testes during adolescence and early adult life increases the rate of metabolism some 5 to 10 per cent above the value that it would be were the testes not active.
*Effect on Red Blood Cells: the average man has about 700,000 more red blood cells per cubic millimeter than the average woman so the level of Hb is higher. This difference may be due partly to the increased metabolic rate by testosterone and may be from the direct effect of testosterone on red blood cell production.
Control of Male Sexual Functions by Hormones from the Hypothalamus and Anterior Pituitary Gland: A major share of the control of sexual functions in both the male and the female begins with secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus. This hormone in turn stimulates the anterior pituitary gland to secrete two other hormones called gonadotropic hormones: (1) luteinizing hormone (LH) and (2) follicle-stimulating hormone (FSH). In turn, LH is the primary stimulus for the secretion of testosterone by the testes, and FSH mainly stimulates spermatogenesis.
Gonadotropic Hormones: LH and FSH
Both of the gonadotropic hormones, LH and FSH, are secreted by the same cells, called gonadotropes, in the anterior pituitary gland. In the absence of GnRH secretion from the hypothalamus, the gonadotropes in the pituitary gland secrete almost no LH or FSH. LH and FSH are glycoproteins. They exert their effects on their target tissues in the testes mainly by activating the cyclic adenosine monophosphate second messenger system, which in turn activates specific enzyme systems in the respective target cells.