REPRODUCTIVE SYSTEM, HUMAN

organ system by which humans reproduce. Humans belong to that group of mammals characterized by the bearing of live offspring that have attained considerable development within the uterus, or womb. Provided all organs are present, normally constructed, and functioning properly, the essential features of human reproduction are (1) liberation of an egg from the ovary at the right time in the reproductive cycle; (2) internal fertilization by spermatozoa (sperm, or male sex cells) of the ovum in the uterine tube; (3) transport of the fertilized ovum along the uterine tube to the uterus; (4) implantation of the blastocyst, the early embryo that develops from the fertilized ovum, in the wall of the uterus; (5) formation of a placenta and maintenance of the intra-uterine existence of the unborn child; (6) birth of the child and expulsion of the placenta; and (7) suckling and care of the child, with an eventual return of the maternal organs virtually to their original state. For this biological process to be carried out, certain organs and structures are required in both male and female bodies. The source of the ova, the female germ cells, is the female gonad or ovary; that of spermatozoa is the testis. In human beings, the two ovaries are situated in the pelvic cavity, and the two testes are enveloped in a sac of skin, the scrotum, lying below and outside the abdomen. Besides producing the germ cells, or gametes, the ovaries and testes are the source of hormones that cause full development of secondary sexual characteristics and also the proper functioning of the genital (reproductive) tracts. These tracts comprise the uterine tube, the uterus, vagina, and associated structures in females, and, in males, the penis, the sperm channelsepididymis, ductus deferens, and ejaculatory ductsand other related structures and glands. The function of the uterine tube is to convey an ovum, which is fertilized in the tube, to the uterus, where gestation (development before birth) takes place. The function of the male ducts is to convey spermatozoa from the testis, to store them, and, when ejaculation occurs, to eject them with secretions from the male glands through the penis. At copulation the erect penis is inserted into the vagina and spermatozoa contained in the seminal fluid are ejaculated into the female genital tract. Spermatozoa then pass from the vagina through the uterus to the uterine tube to fertilize the ovum in the outer part of the tube. Human females exhibit a periodicity in the activity of their ovaries and uterus, which starts at puberty and ends at the menopause. The periodicity is manifested by menstruation at intervals of about 28 days; important changes occur in the ovaries and uterus during each reproductive or menstrual cycle. Periodicity is suppressed during pregnancy and lactation. The sex of a human child is determined at the time of fertilization of the ovum by the spermatozoon. The differences between a man and a woman are genetically determined by the chromosomes that each possesses in the nuclei of the cells. This stage in the development of the individual is detailed in the article human embryology: Early stages. Once the genetic sex has been determined there normally follows a succession of changes that will result, finally, in the development of an adult male or female. There is, however, no external indication of the sex of a human embryo during the first eight weeks of its life within the uterus. This is a neutral or indifferent stage during which the sex of an embryo can be ascertained only by examination of the chromosomes in its cells. The next phase, one of differentiation, begins first in gonads that are to become testes, and a week or so later in those destined to be ovaries. Embryos of the two sexes are initially alike in possessing similar duct systems linking the undifferentiated gonads with the exterior and in having similar external genitalia, represented by three simple protuberances. The embryos each have four ducts, the subsequent fate of which is of great significance in the eventual anatomical differences between men and women. Two ducts closely related to the developing urinary system are called mesonephric, or wolffian, ducts. In males, each mesonephric duct becomes differentiated into four related structures: a duct of the epididymis, a ductus deferens, an ejaculatory duct, and a seminal vesicle (see below). In females, the mesonephric ducts are largely suppressed. The other two ducts, called the paramesonephric or mllerian ducts, persist, in females, to develop into the uterine tubes, the uterus, and part of the vagina; in males they are largely suppressed. Differentiation also occurs in the primitive external genitalia, which in males become the penis and scrotum, and in females the clitoris and labia. At birth the organs appropriate to each sex have developed and are in their adult positions but are not functioning. Various abnormalities can occur during development of sex organs in human embryos, leading to hermaphroditism, pseudohermaphroditism, and other chromosomally induced conditions. During childhood until puberty there is steady growth in all reproductive organs and a gradual development of activity. Puberty marks the onset of increased activity in the sex glands and the steady development of secondary sexual characteristics. In males at puberty the testes enlarge and become active, the external genitalia enlarge, and the capacity to ejaculate develops. Marked changes in height and weight occur as hormonal secretion from the testes increases. The larynx, or voice box, enlarges, with resultant deepening of the voice. Certain features in the skeleton, as seen in the pelvic bones and skull, become accentuated. The hair in the armpit and the pubic hair becomes abundant and thicker. A beard, a moustache, and cheek hair develop, as well as hair on the chest, abdomen, and limbs. Hair at the temple recedes. Skin glands become more active, especially apocrine glands (a type of sweat gland that is found in the armpit and groin and around the anus). These secondary sex characteristics do not develop in individuals castrated before puberty, but the administration of androgens (male sex hormones) to such persons and to males having poorly developed testes can correct, in large measure, some of the poorly developed secondary characteristics. Large amounts of androgen, however, by preventing production of the hormone gonadotrophin by the pituitary, suppress testicular activity, thus depressing formation and release of sperm. Some derivatives of the male sex hormone testosterone can promote general bodily development. In females at puberty, the external genitalia enlarge and the uterus commences its periodic activity with overt menstruation. The mammary glands develop, and there is a deposition of body fat in accordance with the usual contours of the mature female. Growth of axillary (armpit) and pubic hair is more abundant, and the hair becomes thicker. In a female receiving androgens, the typical male secondary sex characteristics may develop, menstruation may be suppressed, and the mammary glands may atrophy. Additional reading The characteristics of human reproduction and reproductive organs are reviewed in Robert H. Williams (ed.), Textbook of Endocrinology, 6th ed. (1981), covering in detail the endocrine aspects of the male and female reproductive systems; Richard E. Jones, Human Reproductive Biology (1991); Ralph M. Wynn and William P. Jollie (eds.), Biology of the Uterus, 2nd ed. (1989), a useful summary of modern research on many aspects of uterine anatomy and physiology; Ethel Sloane, Biology of Women, 3rd ed. (1993); and Claude Gompel and Steven G. Silverberg, Pathology in Gynecology and Obstetrics, 4th ed. (1994), which contains a chapter on each organ of the female genital tract and one on the breast. Reproductive technologies are explained in Lori B. Andrews, New Conceptions (1984); Arthur L. Wisot and David R. Meldrum, New Options for Fertility (1990); and Sherman J. Silber, How to Get Pregnant with the New Technology (1991). Sir Richard J. Harrison The Editors of the Encyclopdia Britannica