Male Reproductive System PDF

Reproductive Physiology Course Prof. Dr. Muhammad-Baqir M-R. Fakhrildin 1. Introduction: Normally, the presence of a Y-chromosome leads to the formation of a pair of fetal testes from undifferentiated ridges. The secretions of these testes are crucial to the establishment o...

Reproductive Physiology Course Prof. Dr. Muhammad-Baqir M-R. Fakhrildin 1. Introduction: Normally, the presence of a Y-chromosome leads to the formation of a pair of fetal testes from undifferentiated ridges. The secretions of these testes are crucial to the establishment of the male phenotype. The testis first develops from the genital ridge in the upper lumbar region of the fetus. During embryogenesis, and after migration of the germ cells to the gonadal ridge, the total number of germ cells is approximately 3X10 6 per gonad. This number increases to about six millions spermatogonia per testis by the time of puberty. Actually, differentiation of the primordial germ cells in the male at puberty, it begins at an average age of 13 years as the result of stimulation by adenhypophysial gonadotropic hormones and continuing throughout the remainder of life, with declining in old age. It is estimated that there are 2000 genes that regulate spermatogenesis, most of these being present on the autosomes, but with approximately 30 genes on the Y-chromosome. Male reproductive organs form spermatozoa, suspend them in secretions produced by accessory glands, and conduct them via seminal pathways to the female reproductive tract by mating behavior. These activities are influenced by hormones, including ones formed by the testes. 2. Classification of male reproductive organs The sex organs in the male can be divided as: 2.1. Primary sex organ: In male, a pair of testes producing spermatozoa (male gametes). Also, testes secrete male sex hormones as well. In the testis, sperm originate from spermatogonia that divide through successive stages to form primary spermatocytes, then secondary spermatocytes, and finally spermatids, which mature to form spermatozoa (the sperm). Between the primary and secondary spermatocyte stages, the 23 pairs of chromosomes of the spermatogonia divide to form 23 unpaired chromosomes in the sperm. 2.1.1. Testes 1  Reproductive Physiology Course Prof. Dr. Muhammad-Baqir M-R. Fakhrildin The testes are paired oval glands measuring about 5 cm in length and 2.5 cm in diameter. They weight between 15-25 gm. The testes develop high on the embryo’s posterior abdominal wall, and usually enter the scrotum by 32 weeks. Full descent is not complete until just prior to birth. The testes are covered by a dense layer of white fibrous tissue called tunica albuginea, that extends inward and divides each testis into a series of internal compartments called lobules. Each of the 200-300 lobules contains one to three tightly coiled tubules, the convoluted seminiferous tubules, that produce sperm by a process called spermatogenesis. A cross section through a seminiferous tubule reveals that is packed with sperm cells in various stages of development. The most immature cells, the spermatogonia, are located against the basement membrane. These cells collectively constitute the male germinal epithelium cells. Toward the lumen of the tube, one can see layers of progressively more mature cells. In order of advancing maturity, these are primary spermatocytes, secondary spermatocytes and spermatids. By the time, spermatozoon has reached full majority, it is in the lumen of the tubule and begins to be moved through a series of ducts. Embedded between the developing sperm cells in the tubules are called Sertoli (or sustentacular or nurse) cells. These cells produce secretions that supply nutrients to the spermatozoa, and secrete the hormone inhibin. These cells may be play a major role in the development of the spermatozoa as: a-) To provide a special local environment for division and metabolism of the cells. b-) Probably to provide special nutrients and perhaps local hormones that are needed for sperm development. c-) To remove most of the cytoplasm from the spermatids to cause compaction of the head of the sperm and formation of its tail. Between the seminiferous tubules are clusters of Leydig interstitial cells. These cells secrete the male hormone testosterone, considered the most important androgen. 2.2. Secondary (or male accessory) sex organs: These are epididymis, vas deferens, seminal vesicles, prostate, Cowper’s (bulbo- urethral) glands, urethra and penis. The bulk of semen is made up by seminal vesicles secretion, which forms 80-90 % of semen, which is the last to be ejaculated and serves to wash the sperm of the ejaculatory duct and urethra. The rest of 10-12 % is formed by spermatozoa, and 2  Reproductive Physiology Course Prof. Dr. Muhammad-Baqir M-R. Fakhrildin secretions of prostate gland, epididymis and Cowper’s glands. The secretions from various glands mixed in semen help the spermatozoa in providing nourishment, chemical protection and support their swimming motions. This information, combined with clinical information on the patient’s status and with basic semen measurements, can often elucidate male accessory glands dysfunction. There are various biochemical markers of accessory gland function, e.g., fructose and prostaglandins for the seminal vesicles; citric acid, zinc, -glutamyl transpeptidase and acid phosphatase for the prostate gland; free L-carnitine, glycerophosphocholine and neutral -glucosidase for epididymal secretion. 2.2.1. Epididymis The epididymis, which is a system of coiled tubes (4-6 meters long) through which the sperm slowly pass while they mature. It consists of head, body and tail. It stores the spermatozoa until they are ejaculated. The spermatozoa remain metabolically inactive in epididymis and are capable of surviving for months. The spermatozoa mature within epididymis, which also increases their motility and power of fertility. The epididymis secretes glycerylphosphorylcholine (GPC) and phosphorylcholine. The GPC may be improve the fertilizing ability of spermatozoa. 2.2.2. Vas deferens The vas deferens is a fibroelastic duct 30-40 cm in length. The vas deferens, which stores sperm and also conducts the sperm from the epididymis to the urethra. The upper end of the vas is tortuous and enlarged and is called the ampulla of the vas deferens. Additional sperm are stored here prior to ejaculation. 2.2.3. Seminal vesicles The seminal vesicle, which is a coiled tubular gland that secretes a viscoid yellowish colored fluid so-called seminal fluid into the upper end of the vas deferens at the time of sexual intercourse (coitus). The secretion of seminal vesicles is yellowish in appearance due to flavin substance. The seminal vesicles provide fructose, seminal clot-forming proteins, basic secretions and prostaglandins to the seminal plasma. Although the seminal plasma contains variety of sugars such as glucose, ribose and others, but fructose is the principal sugar providing a readily available exogenous energy source for the spermatozoa. The secretion of seminal vesicles has very high creatine phosphokinase activity. 2.2.4. Prostate gland The prostate gland, which is a large gland that surrounds the urethra where it leaves the neck of the urinary bladder; it empties still another type of fluid into the urethra during sexual intercourse, a thin milky, alkaline fluid so-called prostatic fluid. 3  Reproductive Physiology Course Prof. Dr. Muhammad-Baqir M-R. Fakhrildin It is reported that the ejaculatory process, initiated by contractions of the epididymes, and is followed immediately by contractions of the prostate gland so that the first split of ejaculation is composed of both epididymal and prostatic secretion. It is concluded that the prostatic secretion is slightly acidic (pH: 6.5) and particularly rich in enzymes including: acid phosphatase, -glucoronidase, lysozyme, -amylase, -glutamyl transferase and seminine. Other typical substances are derived from prostatic secretion such as: citric acid, calcium, zinc and spermine; the later being responsible for the typical odor of seminal fluid. 2.2.5. Bulbo-urethral (Cowper’s) glands The glands form a mucoid secretion for lubrication, and a substance that neutralizes urine acidity, which is discharged into the anterior (penile) urethra. These glands add an alkaline secretion to the semen. Sperm survive and remain fertile longer in an alkaline fluid. 2.2.6. Penis The penis, which is the external male sexual organ. This erectile tissue organ consists of two corpora cavernosa and one corpus spongiosum, each of which is composed of literally of thousands of cavernous spaces that contain blood. It is pressure of excess blood filling these spaces that swells the penis and results in erection. At the tip of the penis is the glans penis, which is composed mainly of an enlarged end of the corpus spongiosum. Therefore, it shares in the erectile process. This portion of the penis is highly sensitive and therefore is the source of most sexual sensations during sexual intercourse. 3. Spermatogenesis and spermiogenesis The seminiferous tubules contains a large number of small to medium-sized germinal epithelium cells called spermatogonia, which are located in two to three layers along the outer border of the tubular epithelium. These continually proliferate to replenish themselves, and a portion of them differentiate through definite stages of development to form spermatozoa. The first stage in spermatogenesis is growth of some spermatogonia to form considerably enlarged cells called spermatocytes. After the chromosomes of these cells reduplicate themselves to form chromatids (making a total of 92 chromatids in the cell), the spermatocyte then undergoes two rapid meiotic divisions without formation of any new chromatids. The net result of these two divisions is formation of four cells called spermatids, each containing only 23 unpaired chromosomes. The spermatids do not 4  Reproductive Physiology Course Prof. Dr. Muhammad-Baqir M-R. Fakhrildin divide again but instead mature in an average of 74 days to become spermatozoa by process of spermiogenesis. Spermiogenesis whereby the spermatid as a typical cell becomes a spermatozoon through achievement many steps involving: a-) Construction of the acrosome by the Golgi complex. b-) The nucleus, thus polarized at one end, condenses and elongates. c-) At the other end, one of the centrioles initiates formation of the flagellar tail. d-) Mitochonderia migrate to form a sheath in the tail. e-) Excess cytoplasm is shed as a residual body. f-) The head of the spermatid throughout spermiogenesis stays held in a recess in a Sertoli cells. 4. Structure and characteristics of the spermatozoa Spermatozoa are produced or matured at the rate of about 300 million per day, and one ejaculated have a life expectancy of about 48 hours within the female reproductive tract. A spermatozoon is highly adapted for reaching and penetrating a female ovum. It is measured 50-53 in length and composed of a head, a midpiece and a tail. Within the head are nuclear material and the acrosome contains number of enzymes that effect penetration of the spermatozoon into the ovum. Numerous mitochondria in the midpiece carry on the metabolism that provides energy for movement. The tail as a typical flagellum propels the sperm along its way. 5  Reproductive Physiology Course Prof. Dr. Muhammad-Baqir M-R. Fakhrildin After formation in the seminiferous tubules, the spermatozoa begin a long passage through 6 meters long coiled epididymis, usually requiring several days for this passage. While the sperm are still in the seminiferous tubules or in the first part of the epididymis, they are unable to fertilize an ovum, but after they have passed through most of the epididymis, they then become capable of fertilization; this is called sperm maturation. 5. Semen (seminal fluid) Semen is a mixture of spermatozoa and the secretions of the seminal vesicles, the prostate gland and the bulbo-urethral glands. The average volume of semen for each ejaculation is (2.5-6) mL, and the average range of spermatozoa ejaculated is (50- 100) million/ml. When the number of spermatozoa falls below 20 million/ml, the male is likely to be sterile. The very large number is required because only a small percentage eventually reach the ovum. And, although only a single spermatozoon fertilizes an ovum, fertilization seems to require the combined action at the ovum of a larger number of them. The intercellular material of the cells covering the ovum presents a barrier to the sperm. This barrier is digested by the hyaluronidase and other proteinases secreted by the acrosome of the spermatozoon. However, it appears that a single sperm does not produce enough of these enzymes to dissolve the barrier. A passageway through which one may enter can be created only by the action of many. Semen has a slightly alkaline pH (7.3-7.5). The prostatic secretion gives semen a milky appearance, and fluids from the seminal vesicles and bulbo-urethral glands give it a mucoid consistency. Semen provides spermatozoa with a transportation medium and nutrients. It neutralizes the acid environment of the male urethra and the female vagina. It also contains enzymes which activate sperm after ejaculation, and antibiotic called seminal plasmin, which has the ability to destroy a number of bacteria. Once semen ejaculated into the vagina, liquid semen coagulates rapidly because of a clotting enzyme produced by the prostate that acts on a substance produced by the seminal vesicles. This clot liquefies by the prostate glands. 6