One of the fundamental laws of nature is that every species makes a vigorous attempt to reproduce its kind. The higher animals reproduce sexually. This involves the production of potentially differentiated sex cells by the male and female. In animals, this has led to the development of highly specialized body parts for the specific purpose of reproduction.
Male Reproductive System:
|Male reproductive tract of a bull (Courtesy: iaea.org)|
The primary sex organs of the male are the two testes, which are normally located in an external sac of skin called the scrotum. The secondary sex organs are the duct system (the vas deferens), the epididymis, and the penis. The penis is trans-versed by the urethra, which is common passageway for urine and semen. The semen is composed of sexual secretions and spermatozoa. Besides the reproductive tract, there are a few accessory organs like the prostate gland, a pair of seminal vesicles, and two Cowper's glands, or bulbo-urethral glands. The primary, secondary and accessory sex organs are collectively called male reproductive tract.
(a) Scrotum: The scrotum is a two-lobed sac developed from the invagination of inguinal skin to accommodate the testes. The scrotum is pendulous and situated just behind the rear part of rudimentary teats in the inguinal region. Although the scrotum is apparently divided into two almost equal halves by the median vertical band, the left half is slightly longer and more voluminous than the right half.
The main function of the scrotum is to support and protect the testes suspended by the spermatic cord in the scrotal sac. The scrotum functions as a heat-regulating mechanism in the male. It keeps the testicles 4 - 5 degree celsius below normal body temperature. This lowered temperature is essential for spermatogenesis. The large number of sebaceous and sweat glands on the scrotum help in lowering the scrotal temperature. During the hot season, the thermoregulatory action of the tunica dartos muscles causes it to relax, allowing the scrotum to elongate, dropping the testes far from the heat of the body. During the cold season, the scrotal muscle contracts, retracting the scrotum and bringing the testes nearer to the body. This thermoregulatory action, however, does not begin until the animal approaches puberty.
(b) Testes: The testes are the primary sex organs in the male. They are found in pairs suspended in the scrotal sac by the spermatic cord outside the abdominal cavity in the inguinal region. Each testis is an independent unit, separated from the other in the scrotal sac. The testes are firm and compact masses of parenchymatous tissue. In the buffalo bull, the average length, breadth and circumference of the testes without epididymis have been reported to be 7.73, 4.32, and 12.22 cm respectively.
The testes develop within the abdominal region near the kidneys. They commence their descent from the abdomen into the scrotal pouches during fetal development. Migration is normally completed by the time of or soon after birth. Either one or both of the testes may, however, fail to descend into scrotum during maturity. This condition is known as unilateral or bilateral cryptorchidism. Bulls affected by bilateral cryptorchidism are sterile. This condition is thought to be an inherited trait, hence such bulls are not selected for further breeding.
The main functions of the testes are,
(i) Production of viable, potentially fertile spermatozoa,
(ii) Production of androgens or the male hormone, testosteroone.
The seminiferous tubules produce spermatozoa from the germinal epithelial layer by a series of cell division. The seminiferous tubules join to form the rete testes in the mediastinum, and these, in turn, lead in to a dozen efferent ducts, the vasa efferentia, which finally converge at the dorsal part of the mediastinum to form the begining of the epididymis.
The main sex hormone, tesstosterone, is secreted by the Leyding cells or interstitial cells of the testes. The secretion of this hormone is regulated by the luteinizing hormone of the anterior pituitary gland. Testosterone is responsible for the development and maintenance of the functions of the male reproductive tract, secondary sex characteristics and sexual behaviour.
(c) Epididymis: The epididymis emerges from the joining of the vas efferentia at the dorsal part of the testis. It is a very long single duct, highly vonvoluted and appearing as a mass of tubes. It is comprised of three parts: the caput epididymis (head), the corpus epididymis (body) and the cauda epidiymis (tail). The tail of the epididymis opens into the vas deferens.
Throughout most of its length, the epididymal tube is lined with secretory cells. Spermatozoa accumulate in the epididymis and mature during their passage through it. In the epididymis, the spermatozoa mature and become able to move spontaneously and fertilize the ovum (egg) when they come in contact with it.
(d) Vas deferens (ductus deferens): The vas deferens is a tube emerging from the tail end of the epididymis. It starts from the base of the testes, extends upward, and in association with the spermatic cord runs through the inguinal ring, where it separates itself from the arteries, veins, nerves, and other cord tissues. It passes through the abdominal cavity towards the pelvis and finally empties into the urethra. The lumen of the vas deferens is narrow and lined with mucous membrane. The wall is made up of longitudinal and circular layers of involuntary muscles covered by the outer layer of the peritoneum. The muscles of the vas deferens contract involuntarily during ejaculation of semen and help in the expulsion of spermatozoa. In the pelvic region, the vas deferens enlarges to form the ampulla of Henle which is 4 - 7 inches long. The ampulla has numerous glands, and spermatozoa often accumulate heree before ejaculation. The glands of the ampulla secrete fructose and citric acid which provide nutrition for the spermatozoa.
(e) Urethra: The urethra is the common passage for the excretion of urine and semen. It extends through the pelvis and the penis and ends at the tip of the glans penis as the external urethral orifice. In the urethra, spermatozoa mix with the seminal plasma of the accessory sex fluids at the time of ejaculation.
(f) Penis: The penis is essentially composed of erectile tissue. It is divided into three portions: the attached portion is called the root, the main portion is called the body, and the free portion is called the glans penis. The erectile tissue is a sponge-like system of blood vessels which becomes filled with blood under pressure when the bull is sexually stimulated. This helps the penis to enlarge and become rigid, thus enabling it to enter into the vagina of the female.
The penis of the bull contains very little erectile tissue. The length of the penis from the neck of the pelvic urethra to the tip ranges from 65 - 111 cm. The penis is a cylindrical organ with a tapering end; the tapering portion opens at the angular end of the triangular sheath. The sheath of the buffalo bull is a pendulous triangular fold of skin extending backward from the umilicus. When the bull is not sexually excited, the penis is in a "S" - shaped from tknown as a sigmoid flexure. During erection, the longitudinal flexure straightens, thereby increasing the length of the organ. The erector muscles pull the penis against the pelvis and aids in erection by compressing the veins of the penis. The retractor muscles help to return the extended penis to flexed state.
(g) Accessory sex glands: The accessory sex glands of the male reproductive system are a pair of seminal vesicles, the prostate gland, and Cowper's glands or transverse urethral glands. They provide the bulk of seminal plasma.
Seminal Vesicles. Each of the two seminal vesicles are located on either side of the ampulla. The seminal vesicles are lobulated and highly secretory. They open either above or below the opening of the vas deferens. The secretions of the seminal vesicles contains a large amount of fructose and citric acid, which are required for the nourishment of spermatozoa.
Prostate gland. The body of the prostate is situated in front of the vesicula seminalis on the dorsal surface of the pelvic urethra near the neck of the urinary bladder. It secretes a mineral rich fluid.
Cowper's glands or transverse-urethral glands. The Cowper's glands are two in number and lie one on each side of the pelvic urethra, partially buried in the transverse-cavernous muscle. They produce a viscid, mucous like lubricating substance.
Semen. The reproductive sexual fluids containing spermatozoa (product of seminiferous tubules of testes), secoretion of the epididymis, and the secretions of the accessory sex glands mixed in the urethra is known as semin. The semen is creamy-white to yellowish in color, and its consistency varies with the number of spermatozoa in the semen. A buffalo bull produces 1.5 - 6 ml of semen per ejaculation.
The discharge of semen during mating with a female or in an artificial vagina is called ejaculation. Nervous stimulation creates muscular contractions of seminal tract which ejects spermatozoa into the penis through the pelvic urethra. As a result of rhythmic contractions of the urethra, the semen is forced out through the orifice of the penis with a sudden thrust of very short duration.