reproductive system

The bodily system of gonads, associated ducts, and external genitals concerned with sexual reproduction.

The structures concerned with the production of sex cells (gametes) and perpetuation of the species. The reproductive function constitutes the only vertebrate physiological function that necessitates the existence of two morphologically different kinds of individuals in eac animal species, the males and the females (sexual dimorphism). The purpose of the reproductive function is fertilization, that is, the fusion of a male and a female sex cell produced by twodistinct individuals.

Anatomy

Egg cells, or ova, and sperm cells, or spermatozoa, are formed in the primary reproductive organs, which are collectively known as gonads. Those of the male are called testes; those of the female are ovaries. The gonads are paired structures, although in some forms what appears to be an unpaired gonad is the result either of fusion of paired structures or of unilateral degeneration.

The reproductive elements formed in the gonads must be transported to the outside of the body. In most vertebrates, ducts are utilized for this purpose. These ducts, together with the structures that serve to bring the gametes of both sexes together, are known as accessory sex organs. The structures used to transport the reproductive cells in the male are known as deferent ducts and those of the female as oviducts. In a few forms no ducts are present in either sex, and eggs and sperm escape from the body cavity through genital or abdominal pores.

Oviducts, except in teleosts and a few other fishes, are modifications of Möllerian ducts formed during early embryonic development. In all mammals, each differentiates into an anterior, nondistensible Fallopian tube and a posterior, expanded uterus. In all mammals except monotremes the uterus leads to a terminal vagina which serves for the reception of the penis of the male during copulation. The lower part, or neck, of the uterus is usually telescoped into the vagina to a slight degree. This portion is referred to as the cervix.

In most vertebrates the reproductive ducts in both sexes open posteriorly into the cloaca. In some, modifications of the cloacal region occur and the ducts open separately to the outside or, in the male, join the excretory ducts to emerge by a common orifice. See also Copulatory organ; Ovary; Penis; Reproduction (animal).

Physiology

The physiological process by which a living being gives rise to another of its kind is considered one of the outstanding characteristics of plants and animals. It is one of the two great drives of all animals; self-preservation and racial perpetuation.

Estrous and menstrual cycles

The cyclic changes of reproductive activities in mammalian females are known as estrous or menstrual cycles.

Most mammalian females accept males only at estrus (heat). Estrus in mammals can occur several times in one breeding season; the mare, ewe, and rat come to estrus every 21, 16, and 5 days respectively if breeding does not take place. This condition is called poly estrus. The bitch is monestrous; she has only one heat, or estrus, to the breeding season and if not served then, she does not come into heat again for a prolonged interval, 4–6 months according to different breeds. In monestrous and seasonally polyestrous species the period of sexual quiescence between seasons is called anestrus. See also Estrus.

The reproductive cycle of the female in the primate and human is well marked by menstruation, the period of vaginal blood flow. Menstruation does not correspond to estrus but occurs between the periods of ovulation at the time the corpus luteum declines precipitously. See also Menstruation.

Mating

Mating, also called copulation or coitus, is the synchronized bodily activity of the two sexes which enables them to deposit their gametes in close contact. It is essential for successful fertilization because sperm and ovum have a very limited life span.

The logistics of sperm transport to the site of fertilization in the oviduct present many interesting features in mammals, but it is important to distinguish between passive transport of sperm cells in the female genital tract, and sperm migration, which clearly attributes significance to the intrinsic motility of the cell. Viable spermatozoa are actively motile, and although myometrial contractions play a major role in sperm transport through the uterus, progressive motility does contribute to migration into and within the oviducts. Even though a specific attractant substance for spermatozoa has not yet been demonstrated to be released from mammalian eggs or their investments, some form of chemotaxis may contribute to the final phase of sperm transport and orientation toward the egg surface.

Although in most mammalian species the oocyte is shed from the Graafian follicle in a condition suitable for fertilization, ejaculated spermatozoa must undergo some form of physiological change in the female reproductive tract before they can penetrate the egg membranes. The interval required for this change varies according to species, and the process is referred to as capacitation. The precise changes that constitute capacitation remain unknown, although there is strong evidence that they are—at least in part—membrane-associated phenomena, particularly in the region of the sperm head, that permit release of the lytic acrosomal enzymes with which the spermatozoon gains access to the vitelline surface of the egg.

Fertilization takes place in the oviducts of mammals and the fertilized eggs or embryos do not descend to the uterus for some 3 to 4 days in most species. During this interval, the embryo undergoes a series of mitotic divisions until it comprises a sphere of 8 or 16 cells and is termed a morula. Formation of a blastocyst occurs when the cells of the morula rearrange themselves around a central, fluid-filled cavity, the blastocoele. As the blactocyst develops within the uterine environment, it sheds its protective coat and undergoes further differentiation before developing an intimate association with the endometrium, which represents the commencement of implantation or nidation.

Association of the embryo with the uterine epithelium, either by superficial attachment or specific embedding in or beneath the endometrium, leads in due course to the formation of a placenta and complete dependence of the differentiating embryo upon metabolic support from the mother. Implantation and placentation exhibit a variety of forms, but in all instances the hormonal status of the mother is of great importance in determining whether or not implantation can proceed. See also Pregnancy.

Endocrine function

The endocrine glands secrete certain substances (hormones) which are necessary for growth, metabolism, reproduction, response to stress, and various other physiological processes. The endocrine glands most concerned with the process of reproduction are the pituitary and the gonads.

The posterior lobe of the pituitary gland secretes two neuro-humoral agents, vasopressin and oxytocin. These are involved in reproduction only indirectly, through their effect on uterine contractility in labor and on the release of milk from the mammary gland when a suckling stimulus is applied. The anterior lobe secretes a variety of trophic hormones, including two gonadotrophic hormones, the follicle-stimulating hormone (FSH) and the luteinizing or interstitial-cell stimulating hormone (LH or ICSH). These hormones act directly on both ovaries and testes. See also Pituitary gland.

The gonadal (steroidal) hormones control the secretion of gonadotrophins by acting on the hypothalamus. It has been suggested that steroids act by means of a “negative feedback”; that is, high levels of circulating gonadal hormones stop further release of gonadotropins. However, although this is true for experiments involving pharmacological doses of such hormones, it may not be the case with endogenous physiological levels. It is certainly true that less steroid is required to inhibit pituitary function in the female than in the male. Under certain circumstances small doses of gonadal hormones can stimulate release of gonadotropic hormones from the pituitary. Estrogen can simulate the release of LH; hence the occurrence of ovulation in rats, rabbits, sheep, and women. Progesterone can also facilitate ovulation in persistently estrous rats, in chickens, and in estrous sheep and monkeys. See also Estrogen; Progesterone.

The formation of gametes (spermatogenesis and oogenesis) is controlled by anterior pituitary hormones. The differentiation of male and female reproductive tracts is influenced, and mating behavior and estrous cycles are controlled, by male or female hormones. The occurrence of the breeding season is mainly dependent upon the activity of the anterior lobe of the pituitary, which is influenced through the nervous system by external factors, such as light and temperature. The transportation of ova from the ovary to the Fallopian tube and their subsequent transportation, development, and implantation in the uterus are controlled by a balanced ratio between estrogen and progesterone. Furthermore, it is known that estrogens, androgens, and progesterone can all have the effect of inhibiting the production or the secretion, or both, of gonadotrophic hormones, permitting the cyclic changes of reproductive activity among different animals.

Mammary glands are essential for the nursing of young. Their growth, differentiation, and secretion of milk, and in fact the whole process of lactation, are controlled by pituitary hormones as well as by estrogen and progesterone. Other glands and physiological activities also influence lactation, although this is largely via the trophic support of other pituitary hormones.

The gonads, which produce the GERM CELLS, and all associated organs and tissues in either sex that provide the mechanisms for fertilization and development of an embryo: in the male, the testes, vasa deferentia, prostate, seminal vesicles, and penis; in the female, the ovaries, Fallopian tubes, uterus, vagina, and mammary glands (breasts). Also, in both sexes, the relevant sex hormones and the tissues that produce them.

— Stuart Judge

See urogenital system.

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Organs of the human reproductive system. In a male, the scrotum, a pouch of skin, is divided into … (credit: © Merriam-Webster Inc.)

Organ system by which humans reproduce. In females, the ovaries sit near the openings of the fallopian tubes, which carry eggs from the ovaries to the uterus. The cervix extends from the lower end of the uterus into the vagina, whose opening, as well as that of the urethra (see urinary system), is covered by four folds of skin (the labia); the clitoris, a small erectile organ, is located where the labia join in front. The activity of the ovaries and uterus goes through a monthly cycle of changes (see menstruation) throughout the reproductive years except during pregnancy and nursing. In males, the testes lie in a sac of skin (the scrotum). A long duct (the vas deferens) leads from each testis and carries sperm to the ejaculatory ducts in the prostate gland; these join the urethra, which continues through the penis. In the urethra, sperm mixes with secretions from the seminal vesicles, prostate gland, and Cowper gland to form semen. In early embryos, the reproductive systems are undetermined. By birth the organs appropriate to each sex have typically developed but are not functioning. They continue to grow, and at puberty their activity increases and maturation occurs, enabling sexual reproduction.

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The organs and glands in the body that aid in the production of new individuals (reproduction). (See illustrations below and next page.)

In the male, sperm are produced in the testes and conveyed to the female in a fluid called semen, which passes out of the body through the penis. Other parts of the male reproductive system include the prostate gland, the scrotum, and the urethra.

In the female, the eggs, or ova are produced in the ovaries and released during ovulation into the fallopian tubes about halfway through the menstrual cycle. If fertilization occurs, the resulting zygote travels down the fallopian tube to the uterus, where it implants and continues development. If the ovum is not fertilized, it continues its journey toward the uterus, where it degenerates and is released in the menstrual flow through the vagina during menstruation.

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