Male Reproductive System PDF

Physiology Reproductive System Chapter 23 | Male Reproductive System Reproductive System | Male Reproductive System Contents : Introduction 3 Blood-Testis Barrier 13 Spermatogenesis 22 Development of Spermatozoa 34 Effect of Temperature 41 Semen 44 Erection 51 Ejaculation 54 Reproductive System | Ma...

Physiology Reproductive System Chapter 23 | Male Reproductive System Reproductive System | Male Reproductive System Contents : Introduction 3 Blood-Testis Barrier 13 Spermatogenesis 22 Development of Spermatozoa 34 Effect of Temperature 41 Semen 44 Erection 51 Ejaculation 54 Reproductive System | Male Reproductive System Contents : Endocrine Function Of The Testes 59 Secondary Sex Characteristics 67 Control Of Testicular Function 79 Inhibins 81 Steroid Feedback 86 Chapter Summary 91 Reproductive System | Male Reproductive System Introduction : The physiology of the mature male reproductive system is related to the gonads. The male gonads have a dual function: the production of germ cells (gametogenesis) and the secretion of sex hormones. The androgens are the steroid sex hormones that are masculinizing in their action. Reproductive System | Male Reproductive System The testes secrete large amounts of androgens, principally testosterone, but they also secrete small amounts of estrogens. Unlike that observed in females, male gonadotropin secretion is noncyclical, and once mature, male gonadal function slowly declines with advancing age, but the ability to produce viable gametes persists. Reproductive System | Male Reproductive System The Male Reproductive System : The testes are the male source of DNA which is packed in the head of the sperm and it is the main source of male hormones. There are two testes each is ovoid and encloses by a capsule of connective tissue & smooth muscle called tonica albuginea. Thin partitions of connective T. extend from tonica Albuginea across the testes dividing them into partitions called lobules. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System The lobules are the center of testicular activity because the lobules in the two testes contain about 500m of convoluted seminiferous tubules. The testes are made up of loops of convoluted seminiferous tubules, in the walls of which the spermatozoa are formed from the primitive germ cells (spermatogenesis). Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System Both ends of each loop drain into a network of ducts in the head of the epididymis. From there, spermatozoa pass through the tail of the epididymis into the vas deferens. They enter through the ejaculatory ducts into the urethra in the body of the prostate at the time of ejaculation. Reproductive System | Male Reproductive System Between the tubules in the testes are nests of cells containing lipid granules, the interstitial cells of Leydig (Leydig cells) which secrete testosterone into the bloodstream. The spermatic arteries to the testes are tortuous, and blood in them runs parallel but in the opposite direction to blood in the pampiniform plexus of spermatic veins. This anatomic arrangement may permit countercurrent exchange of heat and testosterone. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System Blood-Testis Barrier Reproductive System | Male Reproductive System Gametogenesis & Ejaculation : Blood-Testis Barrier The walls of the seminiferous tubules are lined by primitive germ cells and Sertoli cells, large, complex glycogen-containing cells that stretch from the basal lamina of the tubule to the lumen. Germ cells must stay in contact with Sertoli cells to survive; this contact is maintained by cytoplasmic bridges. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System Tight junctions between adjacent Sertoli cells near the basal lamina form a blood-testis barrier that prevents many large molecules from passing from the interstitial tissue and the part of the tubule near the basal lamina (basal compartment) to the region near the tubular lumen (adluminal compartment) and the lumen. However, steroids penetrate this barrier with ease, and evidence suggests that some proteins also pass from the Sertoli cells to the Leydig cells, and vice versa, to function in a paracrine manner. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System In addition, maturing germ cells must pass through the barrier as they move to the lumen. This appears to occur without disruption of the barrier by coordinated breakdown of the tight junctions above the germ cells and formation of new tight junctions below them. Reproductive System | Male Reproductive System The fluid in the lumen of the seminiferous tubules is quite different from plasma; it contains very little protein and glucose but is rich in androgens, estrogens, K+, inositol, and glutamic and aspartic acids. Maintenance of its composition depends on the blood-testis barrier. Reproductive System | Male Reproductive System The barrier also protects the germ cells from bloodborne noxious agents, prevents antigenic products of germ cell division and maturation from entering the circulation and generating an autoimmune response, and may help establish an osmotic gradient that facilitates movement of fluid into the tubular lumen. Spermatogenesis Reproductive System | Male Reproductive System Spermatogenesis : Spermatogonia, the primitive germ cells next to the basal lamina of the seminiferous tubules, mature into primary spermatocytes. This process begins during adolescence. The primary spermatocytes undergo meiotic division, reducing the number of chromosomes Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System In this two-stage process, they divide into secondary spermatocytes and then into spermatids, which contain the haploid number of 23 chromosomes. The spermatids mature into spermatozoa (sperm). The formation of a mature sperm from a primitive germ cell by spermatogenesis in humans spans approximately 74 days. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System Each sperm is an intricate motile cell, rich in DNA, with a head that is made up mostly of chromosomal material. Covering the head like a cap is the acrosome, a lysosome-like organelle rich in enzymes involved in sperm penetration of the ovum and other events associated with fertilization. Reproductive System | Male Reproductive System The motile tail of the sperm is wrapped in its proximal portion by a sheath holding numerous mitochondria. Spermatids mature into spermatozoa in deep folds of the cytoplasm of the Sertoli cells. Mature spermatozoa are released from the Sertoli cells and become free in the lumens of the tubules. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System The Sertoli cells secrete :Androgen-Binding Protein (ABP) , Inhibin and MIS. They do not synthesize androgens, but they contain aromatase (CYP19), the enzyme responsible for conversion of androgens to estrogens, and they can produce estrogens. Reproductive System | Male Reproductive System ABP probably functions to maintain a high, stable supply of androgen in the tubular fluid. Inhibin inhibits follicle-stimulating hormone (FSH) secretion. FSH and androgens maintain the gametogenic function of the testis. The stages from spermatogonia to spermatids appear to be androgen-independent. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System However, the maturation from spermatids to spermatozoa depends on androgen acting on the Sertoli cells in which the developing spermatozoa are embedded. FSH acts on the Sertoli cells to facilitate the last stages of spermatid maturation. In addition, it promotes the production of ABP. Development of Spermatozoa Reproductive System | Male Reproductive System Further Development of Spermatozoa : Spermatozoa leaving the testes are not fully mobile. They continue their maturation and acquire motility during their passage through the epididymis. Motility is obviously important in vivo, but fertilization occurs in vitro if an immotile spermatozoon from the head of the epididymis is microinjected directly into an ovum. Reproductive System | Male Reproductive System The ability to move forward (progressive motility), which is acquired in the epididymis, involves activation of a unique set of proteins from the CatSper family, which are localized to the principal piece of the sperm tail. CatSpers form an alkaline-sensitive Ca2+ channel that becomes more active as the sperm go from the acidic vagina (pH ~5) to the cervical mucus (pH ~8). Reproductive System | Male Reproductive System Sperm from knockout mice that do not express CatSper1-4 have altered motility and are infertile, emphasizing the important role of these proteins. In addition, spermatozoa express olfactory receptors, and ovaries produce odorant-like molecules. Recent evidence indicates that these molecules and their receptors interact, fostering movement of the spermatozoa toward the ovary (chemotaxis). Reproductive System | Male Reproductive System Ejaculation of the spermatozoon involves contractions of the vas deferens mediated in part by P2X receptors, ligand gated cation channels that respond to ATP and fertility is reduced in mice in which these receptors are knocked out. Once ejaculated into the female, the spermatozoa move up the uterus to the isthmus of the uterine tubes, where they slow down and undergo capacitation. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System This further maturation process involves two components: increasing the motility of the spermatozoa and facilitating their preparation for the acrosome reaction. Effect of Temperature Reproductive System | Male Reproductive System Effect of Temperature : Spermatogenesis requires a temperature considerably lower than that of the interior of the body. The testes are normally maintained at a temperature of about 32°C. They are kept cool by air circulating around the scrotum and probably by heat exchange in a countercurrent manner between the spermatic arteries and veins. Reproductive System | Male Reproductive System Situations that increase heat around the testes in humans (eg, hot baths [43–45°C for 30 min/d] and insulated athletic supporters) can reduce sperm counts In addition, evidence suggests a seasonal effect in men, with sperm counts being greater in the winter regardless of the temperature to which the scrotum is exposed. Semen Reproductive System | Male Reproductive System Semen : The fluid that is ejaculated at the time of orgasm, the semen, contains sperm and the secretions of the seminal vesicles, prostate, Cowper glands, and, probably, the urethral glands (Table). An average volume per ejaculate is 2.5–3.5 mL after several days of abstinence from sexual activity. Reproductive System | Male Reproductive System Table: Composition of human semen. Composition of human semen Color: White, opalescent Specific gravity: 1.028 pH: 7.35–7.50 Sperm count: Average about 100 million/mL, with fewer than 20% abnormal forms Reproductive System | Male Reproductive System Table: Composition of human semen. Composition of human semen Other components: Fructose (1.5–6.5 mg/mL), Phosphorylcholine, Ergothioneinem Ascorbic acid, Flavins, Prostaglandins From seminal vesicles (contributes 60% of total volume) Spermine, Citric acid, Cholesterol, phospholipids, Fibrinolysin, fibrinogenase Zinc, Acid phosphatase From prostate (contributes 20% of total volume) Phosphate, Bicarbonate, Hyaluronidase Buffers Reproductive System | Male Reproductive System The volume of semen and the sperm count decrease rapidly with repeated ejaculation. Even though it takes only one sperm to fertilize the ovum, each milliliter of semen normally contains about 100 million sperm. Reduction in sperm production is associated with infertility: 50% of men with counts of 20–40 million/mL and essentially all of those with counts under 20 million/mL are sterile. Reproductive System | Male Reproductive System The presence of many morphologically abnormal or immotile spermatozoa also correlates with infertility. The prostaglandins in semen, which come from the seminal vesicles, are at high concentrations, but their function in semen is unknown. Human sperm move at a speed of about 3 mm/min through the female genital tract. Reproductive System | Male Reproductive System Sperm reach the uterine tubes 30–60 min after copulation. Contractions of the female organs may facilitate the transport of the sperm to the uterine tubes. Erection Reproductive System | Male Reproductive System Erection : Erection is initiated by dilation of the arterioles of the penis. As the erectile tissue of the penis fills with blood, the veins are compressed, blocking outflow and adding to the turgor of the organ. The integrating centers in the lumbar segments of the spinal cord are activated by impulses in afferents from the genitalia and descending tracts that mediate erection in response to erotic psychological stimuli. Reproductive System | Male Reproductive System The efferent parasympathetic fibers are in the pelvic splanchnic nerves (nervi erigentes). The fibers presumably release acetylcholine and the vasodilator vasoactive intestinal polypeptide (VIP) as cotransmitters. NO as a vasodilator plays a prominent role in the production of erection. Normally, erection is terminated by sympathetic vasoconstrictor impulses to the penile arterioles. Ejaculation Reproductive System | Male Reproductive System Ejaculation : Ejaculation is a two-part spinal reflex that involves emission, the movement of the semen into the urethra; and ejaculation proper, the propulsion of the semen out of the urethra at the time of orgasm. The afferent pathways are mostly fibers from touch receptors in the glans penis that reach the spinal cord through the internal pudendal nerves. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System Emission is a sympathetic response, integrated in the upper lumbar segments of the spinal cord and effected by contraction of the smooth muscle of the vasa deferentia and seminal vesicles in response to stimuli in the hypogastric nerves. The semen is propelled out of the urethra by contraction of the bulbocavernosus muscle, a skeletal muscle. The spinal reflex centers for this part of the reflex are in the upper sacral and lowest lumbar segments of the spinal cord. Reproductive System | Male Reproductive System Endocrine Function Of The Testes Reproductive System | Male Reproductive System Endocrine Function Of The Testes : Testosterone Testosterone, the principal hormone of the testes is a steroid ,It is synthesized from cholesterol in the Leydig cells and is also formed from androstenedione secreted by the adrenal cortex. The secretion of testosterone is under the control of LH, and the mechanism by which LH stimulates Leydig cells involves increased formation of cyclic adenosine monophosphate (cAMP) via the G-protein–coupled LH receptor and Gs. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System Cyclic AMP increases the formation of cholesterol from cholesterol esters and the conversion of cholesterol to pregnenolone via the activation of protein kinase A. Reproductive System | Male Reproductive System Secretion The testosterone secretion rate is 4–9 mg/d (13.9–31.33 μmol/d) in normal adult males. Small amounts of testosterone are also secreted in females, with the major source being the ovary, but possibly from the adrenal as well. Reproductive System | Male Reproductive System Transport & Metabolism Ninety-eight percent of the testosterone in plasma is bound to protein: 65% is bound to a β-globulin called gonadal steroid–binding globulin (GBG) or sex steroid– binding globulin, and 33% to albumin. GBG also binds estradiol. The plasma testosterone level (free and bound) is 300–1000 ng/dL (10.4–34.7 nmol/L) in adult men, compared with 30–70 ng/dL (1.04–2.43 nmol/L) in adult women. Reproductive System | Male Reproductive System It declines somewhat with age in men. A small amount of circulating testosterone is converted to estradiol, but most of the testosterone is converted to 17-ketosteroids, principally androsterone and its isomer etiocholanolone and excreted in the urine. Reproductive System | Male Reproductive System Actions : In addition to their actions during development, testosterone and other androgens exert an inhibitory feedback effect on pituitary LH secretion; develop and maintain the male secondary sex characteristics; exert an important protein-anabolic, growth promoting effect; and, along with FSH, maintain spermatogenesis. Secondary Sex Characteristics: Reproductive System | Male Reproductive System Secondary Sex Characteristics: The widespread changes in hair distribution, body configuration, and genital size that develop in boys at puberty—the male secondary sex characteristics—are summarized in Table. Reproductive System | Male Reproductive System Table: Changes at puberty in boys (male secondary sex characteristics). Male Secondary Sex Characteristics External genitalia: Internal genitalia: Voice: Sperm count: Penis increases in length and width. Scrotum becomes pigmented and rugose. Seminal vesicles enlarge and secrete and begin to form fructose. Prostate and bulbourethral glands enlarge and secrete Larynx enlarges, vocal cords increase in length and thickness, and voice becomes deeper. Beard appears. Hairline on scalp recedes anterolaterally. Pubic hair grows with male (triangle with apex up) pattern. Hair appears in axillas, on chest, and around anus; general body hair increases. Reproductive System | Male Reproductive System Table: Changes at puberty in boys (male secondary sex characteristics). Male Secondary Sex Characteristics Mental: More aggressive, active attitude. Interest in opposite sex develops. Body conformation: Shoulders broaden; muscles enlarge. Skin: Sebaceous gland secretion thickens and increases (predisposing to acne). Reproductive System | Male Reproductive System The prostate and seminal vesicles enlarge, and the seminal vesicles begin to secrete fructose. This sugar appears to function as the main nutritional supply for the spermatozoa. Although body hair is increased by androgens, scalp hair is decreased. Hereditary baldness often fails to develop unless dihydrotestosterone (DHT) is present. Reproductive System | Male Reproductive System Anabolic Effects : Androgens increase the synthesis and decrease the breakdown of protein, leading to an increase in the rate of growth. It used to be argued that they cause the epiphyses to fuse to the long bones, thus eventually stopping growth, but it now appears that epiphysial closure is due to estrogens. Secondary to their anabolic effects, androgens cause moderate Na+, K+, H2O, Ca2+, SO4–, and PO4– retention; and they also increase the size of the kidneys. Reproductive System | Male Reproductive System Doses of exogenous testosterone that exert significant anabolic effects are also masculinizing and increase libido, which limits the usefulness of the hormone as an anabolic agent in patients with wasting diseases. Reproductive System | Male Reproductive System Mechanism of Action : Like other steroids, testosteronebinds to an intracellular receptor, and the receptor/steroid complex then binds to DNA in the nucleus, facilitating transcription of various genes. In addition, testosterone is converted to DHT by 5α-reductase in some target cells, and DHT binds to the same intracellular receptor as testosterone. Reproductive System | Male Reproductive System Reproductive System | Male Reproductive System DHT also circulates, with a plasma level that is about 10% of the testosterone level. Testosterone–receptor complexes are less stable than DHT– receptor complexes in target cells, and they conform less well to the DNA-binding state. Thus, DHT formation is a way of amplifying the action of testosterone in target tissues. Reproductive System | Male Reproductive System Humans have two 5α-reductases that are encoded by different genes. 1. Type 1 5α-reductase is present in skin throughout the body and is the dominant enzyme in the scalp. 2. Type 2 5α-reductase is present in genital skin, the prostate, and other genital tissues. Testosterone–receptor complexes are responsible for the maturation of Wolffian duct structures and consequently for the formation of male internal genitalia during development, but DHT–receptor complexes are needed to form male external genitalia. Reproductive System | Male Reproductive System DHT–receptor complexes are also primarily responsible for enlargement of the prostate and probably of the penis at the time of puberty, as well as for the facial hair, the acne, and the temporal recession of the hairline. On the other hand, the increase in muscle mass and the development of male sex drive and libido depend primarily on testosterone rather than DHT. Control Of Testicular Function Reproductive System | Male Reproductive System Control Of Testicular Function : FSH is tropic for Sertoli cells, and FSH and androgens maintain the gametogenic function of the testes. FSH also stimulates the secretion of ABP and inhibin. Inhibin feeds back to inhibit FSH secretion. LH is tropic for Leydig cells and stimulates the secretion of testosterone, which in turn feeds back to inhibit LH secretion. Hypothalamic lesions in animals and hypothalamic disease in humans lead to atrophy of the testes and loss of their function. Inhibins Reproductive System | Male Reproductive System Inhibins : Testosterone reduces plasmaLH but, except in large doses, it has no effect on plasma FSH. Plasma FSH is elevated in patients who have atrophy of the seminiferous tubules but normal levels of testosterone and LH secretion. These observations led to the search for inhibin, a factor of testicular origin that inhibits FSH secretion. Reproductive System | Male Reproductive System There are two inhibins in extracts of testes in men and in antral fluid from ovarian follicles in women. They are formed from three polypeptide subunits: a glycosylated α subunit with a molecular weight of 18,000; and two nonglycosylated β subunits, βA and βB, each with a molecular weight of 14,000. The subunits are formed from precursor proteins. Reproductive System | Male Reproductive System The α subunit combines with βA to form a heterodimer and with βB to form another heterodimer, with the subunits linked by disulfide bonds. Both αβA (inhibin A) and αβB (inhibin B) inhibit FSH secretion by a direct action on the pituitary, although it appears that it is inhibin B that is the FSH-regulating inhibin in adult men and women. Reproductive System | Male Reproductive System Inhibins are produced by Sertoli cells in males and granulosa cells in females. The heterodimer βAβB and the homodimers βAβA and βBβB are also formed. They stimulate rather than inhibit FSH secretion and consequently are called activins. Inhibins and activins are found not only in the gonads but also in the brain and many other tissues. In the bone marrow, activins are involved in the development of white blood cells. Steroid Feedback Reproductive System | Male Reproductive System Steroid Feedback A current “working hypothesis” of the way the functions of the testes are regulated by steroids is shown in Figure. Castration is followed by a rise in the pituitary content and secretion of FSH and LH, and hypothalamic lesions prevent on the anterior pituitary and by inhibiting the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. Reproductive System | Male Reproductive System Inhibin acts directly on the anterior pituitary to inhibit FSH secretion. In response to LH, some of the testosterone secreted from the Leydig cells bathes the seminiferous epithelium and provides the high local concentration of androgen to the Sertoli cells that is necessary for normal spermatogenesis. Reproductive System | Male Reproductive System Systemically administered testosteronedoes not raise the androgen level in the testes to as great a degree, and it inhibits LH secretion. Consequently, the net effect of systemically administered testosterone is generally a decrease in sperm count. Reproductive System | Male Reproductive System Testosterone therapy has been suggested as a means of male contraception. However, the dose of testosterone needed to suppress spermatogenesis causes sodium and water retention. The possible use of inhibins as male contraceptives is being explored. Chapter Summary Reproductive System | Male Reproductive System Chapter Summary The gonads have a dual function: the production of germ cells (gametogenesis) and the secretion of sex hormones. The testes secrete large amounts of androgens, principally testosterone, but they also secrete small amounts of estrogens. Spermatogonia develop into mature spermatozoa in the seminiferous tubules via a process called spermatogenesis. Reproductive System | Male Reproductive System This is a multistep process that includes maturation of spermatogonia into primary spermatocytes, which undergo meiotic division, resulting in haploid secondary spermatocytes. Several further divisions result in spermatids. Each cell division from a spermatogonium to a spermatid is incomplete, with cells remaining connected via cytoplasmic bridges. Spermatids eventually mature into motile spermatozoa to complete spermatogenesis; this last part of maturation is called spermiogenesis. Reproductive System | Male Reproductive System Testosterone is the principal hormone of the testis. It is synthesized from cholesterol in Leydig cells. The secretion of testosterone from Leydig cells is under control of luteinizing hormone at a rate of 4–9 mg/day in adult males. Most testosterone is bound to albumin or to gonadal steroid– binding globulin in the plasma. Testosterone plays an important role in the development and maintenance of male secondary sex characteristics, as well as other defined functions.