Jethalal S. Zaveri
Gamete (Sex cell)
The ability to reproduce is one of the essential characteristics of life. In human beings the process is one of sexual reproduction. That is, it involves two sexes, each contributing a gamete (mature sex cell) for the formation of the new individual. The female produces an egg-cell or ovum which is fertilized by the germ cell (spermatozon) produced by the male. The nucleus of every human cell contains 44 somatic and 2 sex chromosomes. Each gamete undergoes a special type of cell division called meiosis, so that each contains twenty-three chromosomes (22 somatic and one sex chromosome). Thus the zygote resulting from the fusion of an ovum and a sperm results in a cell with forty-six chromosomes.
The reproductive organs of the male and the female differ anatomically and physiologically, each being adapted to the functional activities they are required to perform. They are stimulated by the gonadotroph hormones from the anterior pituitary gland.
The function of the male system is to form and transmit spermatozoa and implant them in the female so that they can meet the ova. The female system is adapted to form ova which, if fertilized by the sperm, remains in the cavity of the uterus where it is nurtured until it is born.
THE MALE REPRODUCTIVE SYSTEM
The Testes and Their Functions
The testes are small ovoid glands suspended in the scrotum. They are the reproductive glands or gonads of the male. Each testis consists of from 200 to 300 lobules composed of tubules. They provide the spermatogenic cells with nutrition.
The function of the testes is controlled by the hypothalamic-pituitary system. The follicle-stimulating hormone (FSH) stimulates growth of seminiferous tubules.
Two of the important function of the testes are:
1. Spermatogenesis is the process by which primitive male gametes (spermatogonia) become mature sperm. At puberty, spermatogonia begin to divide under the influence of FSH, and will continue to do so throughout life with a decrease in the later years.
The gametes undergo mitotic division to form primary spermatocytes, each containing 44 somatic and 2 sex chromosomes. By a meiotic division it produces a pair of secondary spermatocytes, each containing 22 somatic and one sex chromosome. The sex-chromosomes are called X and Y. They can be distinguished from the others, which are somatic (body) chromosomes. An ovum has two X chromosomes (XX), whereas a sperm has X and X chromosome (XY). The fertilized cell will have either an 'X' or a 'Y' chromosome, but not both, from the sperm. The sex of a child clearly depends on whether it inherits an X or a Y chromosome from its father.
2. Secretion of Testosterone. Very little testosterone (the principal male hormone) is secreted in childhood. At puberty, hormone from the anterior pituitary stimulates its production. Its secretion promotes maturation of the reproductive organs and causes the appearance of secondary sexual characteristics (hair over the chest, pubis and on the face. Its secretion is essential for development of the spermatozoa beyond the primary stage.
Functions of the System
The male system is concerned with spermatogenesis and the introduction of spermatozoa into the female system during sexual intercourse. Erection of the male organ, emission of spermatozoa and other constituents of semen into the urethra and ejaculation of the semen out of the urethra are performed by the male system.
THE FEMALE REPRODUCTIVE SYSTEM
The Ovaries and their Functions
The Ovaries are the female gonads or sex glands. They are small ovoid structures, lying one on either side of the uterus. Each ovary is suspended from below by an ovarian ligament.
1. Oogenesis
Oogenesis is the process by which primitive female gametes become mature ova. Very little development takes place between childhood and puberty. During puberty the internal organs reach maturity, become active and menstruation begins. The mature ovary has a cycle of activity which occupies approximately 28 days.
At birth, the ovaries of every girl contain about 2 million primordial follicles, each containing a primary oocyte. During the reproductive years, some 500 of the follicles will mature and expel their ova.
The secretion of follicle stimulating hormone (FSH) by the anterior pituitary gland stimulates primordial follicle, which begins to grow and develop. By the time ovulation occurs, the oocyte undergoes the first phase of meiosis.
2. Ovulation
Luteinizing hormone (LH), also from the anterior pituitary, assists FSH to promote final follicular growth and ovulation. The follicle ruptures expelling the ovum which enters the uterine tube.
If the ovum is fertilised by the male sperm, it implants itself in the uterus. Large amount of oestrogen and progesterone are produced during the early months of pregnancy. If the ovum is not fertilized, production of hormones ceases, menstruation occurs and the next cycle begins.
3. Secretion of Hormones
Ovarian endocrine activity is mainly concerned with the secretion of oestrogen and progesterone. Oestrogen is a collective name for a group of hormones which are of similar structure. It is responsible for the development of secondary sexual characteristics, female reproductive tract and mammary glands.
Progesteron acts in close collaboration with oestrogen and affect tissues which have already been influenced by the latter. Its principal function is to prepare the system for pregnancy.
Uterus and Its Functions
The uterus is a hollow, pear-shaped organ situated in the pelvic cavity. It is held in place by four pairs of supporting ligaments and by the muscles of the pelvic floor. Two uterine tubes open into the cavity.
During the reproductive years each menstrual cycle prepares the uterus to receive the fertilized ovum, and to retain and nourish the developing fetus throughout the duration of pregnancy. At the end of pregnancy the muscular walls of the uterus contract to expel the fetus.
The uterine tubes (Fallopian tubes) are about 10 cm long. They extend from the uterus to curve round the ovaries. These tubes serve as ducts to convey the ovum from the ovary to the uterus. The ovum is prepelied towards the uterus by peristaltic contractions of the smooth muscles.
Fertilization of an ovum by a sperm usually occurs in one of these tubes, and the fertilized ovum normally continues its journey towards uterus where it implants. Occasionally a tubal pregnancy occurs put rarely proceeds beyond six weeks and is aborted.
Mammary Glands (The Breasts)
The breasts are accessory organs of the female system. They begin to enlarge and develop at puberty due to the influence of the ovarian hormones. During pregnancy there is further growth and development and changes occur in preparation for lactation. After childbirth release of prolactin from the posterior pituitary stimulates the process of lactation while oxytocin (from the same gland) causes the expulsion of milk to the ducts. The secretion of oxytocin is stimulated by the infant sucking at the breast. Regular sucking is necessary to maintain lactation.
Menopause
The child-bearing period usually lasts for about 35 years. After that the processes which occured at puberty are reversed. The changes are caused by the change in the concentration of the sex hormones. The ovaries gradually become less responsive to the FSH and LH. Ovulation and the menstrual cycle become irregular and eventually stop altogether. This is called menopause. Several other phenomena may also occur at the same time including episodes of unpredictable behaviour sometimes.
In the male, fertility and sexual ability tend to decline gradually with ageing There is no period comparable to the menopause in the male.
