Sexual Reproduction in Man
In human beings, reproduction is a very complicated process involving two sexes i.e. Male and Female. In the male, the male gametes or spermatozoa are produced in the testes through the process of spermatogenesis but in the female, the female gametes or ova are produced in the ovaries through the process of oogenesis. So the testes and ovaries constitute primary reproductive or sex organs. But there are some organs which do not produce gametes or secrete sex hormones but perform important functions, are called secondary sex organs. For example, prostate gland, seminal vesicle, vas deferens, penis etc. found in the male and in the female fallopian tube, uterus, vagina, mammary glands are secondary sex organs. The secondary sex organs help in sexual act and in the union of sex cells or gametes. The primary sex organs control the growth and function of secondary sex organs. But the growth, maintenance and function of gonads are controlled by gonadotropin hormones of anterior pituitary.
The human being is a placental mammal and viviparous. The viviparity is a unique feature in all higher animals. The mother usually retains the development foetus inside the uterus. The foetus or embryo gets its nutrition from the body of mother by means of placenta. After gestation period the full grown baby is delivered outside. Even after birth female continues to nourish its young. So mammary glands play an important role in permitting the female to nourish the young with the milk that she produces.
Male Reproductive System
The reproductive role of the male is to produce sperms and deliver them into the vagina of the female. This function requires the following structures.
1. Two testes produce sperm and the male sex hormone testosterone.
2. Accessory glands and tubes furnish a fluid for carrying the sperms into the penis. This fluid together with the sperms is called semen.
3. Accessory ducts store and carry secretions from the testes and accessory glands to the penis.
4. The penis deposits semen into the vagina during sexual intercourse.
The male reproductive system consists of:
1- a pair of testes
2- vasa efferentia
3- the epididymis
4- the vas deferens
6- a copulatory organ called penis
7- Accessory glands
Testes- The human testes are paired oval glandular organs which lie inside the sac like structure called the scrotum. They are held in position by the spermatic cords. Each testis measures 4 cm in length, 3 cm in thickness and 2.5 cm in width. It is covered by a thick connective tissue sheath. The testes in the early foetal life are contained inside the abdominal cavity. Before birth they descend down into the scrotum through a narrow passage called the inguinal canal. The question arises, why testes descend down into the scrotum? This is due to the fact that for the development and maturation of sperms, lower temperature is needed which is available in the scrotum. The temperature of the abdominal cavity is higher than the scrotum. The scrotum is covered externally by hairy skin and is situated at the lateral side of the penis.
The testis is covered by a layer of tough coat of connective tissue called tunica albuginea. The connective tissues are extended in the form of septa or partitions into the substances of the testis; thus dividing into small compartments or lobules. These compartments are occupied by the long, coiled and narrow tubules, known as seminiferous tubules.
The seminiferous tubules are lined with germinal epithelial cells which produce spermatozoa through spermatogenesis. There are large sized cells present in between the germinal epithelial cells, known as sertoli cells which provide nourishment to the spermatozoa. In mature testes, the spermatogenic cells are male germ cells in various stages of development. The spermatogonia lie at the periphery and spermatozoa are found in the lumen of the seminiferous tubules. The spermatogonia are only sex cells present until the age of puberty. In between the seminiferous tubules, there are groups of interstitial cells present in the connective tissue stroma. The interstitial cells secrete male hormone, called testosterone which is responsible for the production and maintenance of secondary male sex characters and also for the normal development and integrity of accessory male reproductive organs.
Vasa efferentia - All seminiferous tubules of the testis open into a system of network of branching tubules called rete testis, from which arise 12 to 20 ducts, called vasa efferentia. These ducts perforate the tunica albuginea of the testis and unite to form a long coiled tube called epididymis.
Epididymis – This structure is a compact body which covers the upper pole of the testis. It consists of a greatly coiled tube called the canal of the epididymis which measures 6 to 7 meters in length. The head of the epididymis is connected with vasa efferentia of the testes and the tail is continues with the vas deferens.
Vas deferens – It is about 20 cm in length and is continuous with the duct of epididymis. It is with the spermatic cord and enters the abdominal cavity through the inguinal canal. At the posterior surface of the urinary bladder, it is joined by the duct of seminal vesicle to form the ejaculatory duct. The ejaculatory duct passes through the prostrate gland and empties into the prostatic urethra.
Urethra- It is divisible into three parts according to the location namely prostatic urethra, membranous urethra and penile urethra. The urethra receives the ducts of prostate and Cowper’s glands, passes through the penis and opens to outside by urethral meatus.
Penis - It is male copulatory organ. It is a highly muscular, cylindrical erectile organ with spongy vascular wall which hangs in front of the scrotum. It is covered by a loose sheath of skin and the skin hang over the tip of the penis called prepuce. The tip of penis covered by the prepuce is called glans penis which is highly sensitive. At the tip of penis, urethra opens by external urethral meatus or orifice. Normally it remains small and flaccid and helps to expel out the urine from the bladder. But on sexual excitement and during the act of intercourse, it gets erected and becomes large and heard. Erect human penis, on an average, is about 15 cm long. Because during intercourse it has to be inserted into the vagina of the female in order to facilitate for the fertilization by ejaculating the semen.
The penis becomes large and hard due to presence of three longitudinal tracts of cylindrical masses of erectile tissues joined together by fibrous tissue and covered by thin skin. The corpora cavernosa are a pair of hard ligamentous bodies but corpus spongiosum is made up of spongy tissue containing large number of sinuses. The corpus spongiosum extends beyond corpora cavernosa and forms a bulging conical part called glans penis. During sexual excitation, there is an increased blood flow into the corpus cavernosa and spongiosum, so that blood fills in the sinuses with great pressure. Thus the penis becomes stiff and enlarged.
Accessory Glands -
1- Seminal vesicles: These are two lobulated membranous sacs located between posterior surface of the urinary bladder and the rectum. They are not reservoirs for spermatozoa but produce a secretion which contributes to the volume of the semen. The secretion comprises mucous, some vitamins, fructose and other nutrients and a large amount of prostglandins. It provides nutrition and protection to sperm in the semen.
2- Prostate gland: This gland surrounds the male urethra as it leaves the urinary bladder. It empties by 15 to 30 ducts into the prostatic urethra. It secrets a thin milky white alkaline substance to the seminal fluid which neutralizes any acidity in the urethra due to residual urine.
3- Cowper’s gland: These are small paired bodies which lie on either side of the membranous urethra at its termination. They are compound tubulo-alveolar glands whose ducts empty into the proximal portion of floor of the cavernous urethra. They secrete a clear viscid fluid which is lubricating in function, i.e. lubricates the vagina of copulating woman to facilitate intercourse.
Semen - It is a mixture of sperms and secretions of vas deferens, seminal vesicles, prostate glands and bulbo urethral glands. A copulating man discharges about 2 to 5 ml of semen into the vagina of women.