Reproductive System and Organs (Male)
Human reproduction depends on the integrated action of hormones, the nervous system, and the reproductive system. Gametes are the cells involved in sexual reproduction produced by the gonads, and contain only one copy of each chromosome. A failure in the production of healtly sperm in the semen is termed male infertility.
Male gonads are the testes, which produce both sperm (the male mature gamete is spermatozoa) and male sex hormones. The testes are about 1.9 in. (5 cm) in length and 1.2 in. (3 cm) in diameter, and are enclosed in a tough, white fibrous capsule. The testes are complex organs containing a variety of different cell types that are structurally and functionally compartmentalized, thereby properly allowing the production of spermatozoa (spermatogenesis). The testes are microscopically organized into tubules named seminiferous tubules. About 273 yd. (250 m) of tubules are packed into each testis.
Spermatogenesis (sperm production) is controlled by a series of genes located mainly on the sex chromosomes and is performed by the germ cells. Spermatogenesis occurs in the seminiferous tubules. At the outer edge of the tubules are the spermatogonia, the germ cells of the testis. They continue to divide throughout life and reproduce mitotically to maintain their numbers. Some spermatogonia begin meiosis after puberty to produce sperm for the whole reproductive life. A 2n spermatogonial cell replicates its DNA and becomes a 4n primary spermatocyte. Division (first meiotic division) of the primary spermatocyte produces two 2n secondary spermatocytes, and division (second meiotic division) of the secondary spermatocytes produces four 1n spermatids. Each spermatid becomes a sperm cell, with a residual body (excess cytoplasm) left behind. In human beings, spermatogenesis takes 74 days from spermatogonium to released sperm cells. Sperm capacitation (changes that render the sperm able to penetrate the egg) that occurs in the female tract (seven hours in human beings) before a sperm can fertilize an egg, is needed for the acrosome reaction (the release of egg-penetrating enzymes by sperm) to occur at fertilization.
Sertoli cells and Leydig cells are the most peculiar cells of the testicles. Sertoli cells are essential for spermatogenesis, assisting the germs cells in regulating hormonal conditions for their differentiation. Interstitial (Leydig) cells are located in stroma between seminiferous tubules, and represent the endocrine portion (primarily testosterone) of testis.
Male fertility depends on the proper balance of hormones and organs. The first step in fertility takes place in the hypothalamus pituitary axis, by means of the release of gonadotropin-releasing hormone (GnRH) in the hypothalamus that stimulates the pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FHS maintains spermatogenesis and LH stimulates the production of testosterone.
The other reproductive organs include the rete testis, a network of ducts leading to efferent ductules that transport spermatozoa to epididymis. Epididymis is single tube about 6.5 yd. (6 m) long (head, body, tail) that connects the ducts within the testes and is site of sperm maturation. Ductus deferens transport sperm from epididymis to ejaculatory ducts. They are mainly composed by smooth muscle and organized in a tube-shape about 18 in. (45 cm) long that passes upward along the medial side of the testis.
Ejaculatory Ducts are formed by the fusion of the vas deferens and the duct of the seminal vesicle. The ejaculatory duct ejects sperm into urethra. The accessory sex glands are the prostate that is located inferiorly to the urinary bladder and surrounds prostatic urethra seminal vesicles, and the bulbourethral gland (Cowper's) The bulbourethral gland (Cowper's gland) are two small structures about the size of peas, which are located below the prostate gland and enclosed by muscle fibers of the external urethral sphincter.
The prostate, located in the male pelvis, measuring 1.2-1.6 in. (3-4 cm) in length and 1.2-1.9 in. (3-5 cm) in width. The gland, surrounded by the pelvic muscles, is located behind the pubic bone, in front of the rectum, below and at the base of the bladder and surrounds the urethra. On average, the gland weighs about 0.71 oz. (20 g). There are 15-30 excretory ducts from the prostate entering the urethra as it passes through the prostate. Each of these excretory ducts receives prostatic secretions from 4-6 prostatic lobules that contain prostatic acini surrounded by tall columnar epithelium. The prostate has an essential function in human reproduction. Androgens control the growth of the prostate and formation of the prostatic secretions. The testosterone enters the prostatic cells in order to be then metabolized to a more potent metabolite, called dihydrotestosterone (DHT), which then binds to the androgen receptor within the cell. This binding participates in androgen-induced expression of genes such as prostatic specific antigen (PSA).
The seminal vesicles attach to the prostate and produce secrete that mixes with to form semen. They are a convoluted, saclike structure about 1.9 in. (5 cm) long that is attached to the vas deferens near the base of the urinary bladder. The growth of these glands and their secretory activity require androgen production from the testes as well as a functioning androgen receptor within the cells of the sex accessory tissues. They are involved in maintaining the viability and motility of the sperm.
A human male's ejaculate volume is about 3 ml of semen and ranges from 2-6 ml. The sperm constitutes less than 1% of the volume, with the seminal vesicles and prostate producing about 95% of the total volume. The largest portion of the semen, approximately 65%, is secreted from the seminal vesicles and appears in the latter portion of the ejaculate volume. 15-20% comes from the prostate gland and a very small proportion, originates from the bulbourethral gland (Cowper's gland). In the normal male, the ejaculate is rich in proteins and enzymes, as well as prostaglandins, citric acid, spermine and fructose. The significance of many components of prostate fluid is unknown, but is probably involved with reproduction. Thus, the prostate is essentially an organ for human reproduction. The proteins from the seminal vesicle cause the ejaculate to clot and form a coagulum after ejaculation. Subsequently, the PSA, a serine protease secreted from the prostate, lyses (breaks) the clot. Other proteins from the sex accessory tissues coat the sperm and are believed to protect sperm from environmental damage and agglutination, and to mask sperm antigens from the female's immune system. Other proteolytic enzymes in the secretions allow sperm traverse cervical mucus, while the prostaglandins stimulate the transport of the sperm toward the ovum.
The male external reproductive organs include the scrotum and the penis. The scrotum is a pouch of skin and subcutaneous tissue that hangs from the lower abdominal region behind the penis. Scrotum contains and suspends the testes outside the abdominal cavity and far from the body at an optimal temperature for sperm development. Rete testis and epididymis are also contained in the scrotum.
The penis is an erectile organ comprised primarily of two cylinders of sponge-like vascular tissue that fills with blood to create an erection. A series of valves keep the blood in the penis to maintain the erection. A third cylinder is the urethra, the tube that carries the urine and the ejaculate. The devopment of the genitial system includes an undifferentiated and a differentiated stage. This impicates a control of the the differentation toward the male system. Genetic sex is determined by the X and Y sex chromosomes at the time of fertilization, but phenotypic sex is determined by a number of factors as well as masculinizing hormones. Lack of masculinizing hormones results in a female phenotype. Testosterone and dihydrotestosterone promote masculinization of the mesonephric duct and external genitalia, respectively.
Primordial germ cells arise from the wall of the secondary yolk sac and migrate (via peritoneum) to the genital ridge by about six weeks of gestation. Genital ridges are a mesodermic proliferation of the ventromedial surface of the urogenital ridge. Germ cells must enter the genital ridge or the gonads will not develop. They proliferate yielding the medullary cords. Also epithelial cells contribute to form such cords. The primitive cords further develop to become testis cords and they generate a network of thin tubules, the rete testis, which become separated from the surface epithelium by a thickened layer connective tissue called the tunica albuginea. The cords also contain Sertoli cells derived from the surface epithelium of the testis. Leydig cells differentiate from the genital ridge mesenchyme between the testis cords.
Male duct system arises from the mesonephric (Wolffian) ducts, a proliferation from the primitive kidney that serves as excretory duct, and is under hormonal control. Testis determining factor (TDS) triggers the production of Mullerian Inhibiting Substance (MIS) and testosterone. MIS induces the degeneration of the paramesonephric (Muller) ducts in male. The androgens result in the development of the efferent ductules, the epididymis, and the ductus deferens. The seminal vesicles develop as outgrowths from the mesonephric duct. The remaining mesonephric duct becomes the ejaculatory duct. The prostate derives from endoderm proliferation from the developing urethra.
This is the complete article, containing 1,472 words
(approx. 5 pages at 300 words per page).
