reproductive system


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Related to reproductive system: female reproductive system, endocrine system, male reproductive system

reproductive system,

in animals, the anatomical organs concerned with production of offspring. In humans and other mammals the female reproductive system produces the female reproductive cells (the eggs, or ova) and contains an organ in which development of the fetus takes place; the male reproductive system produces the male reproductive cells, the sperm, and contains an organ that deposits the sperm within the female.

In the Human Female

In the human female reproductive system, ova are produced in the ovariesovary,
ductless gland of the female in which the ova (female reproductive cells) are produced. In vertebrate animals the ovary also secretes the sex hormones estrogen and progesterone, which control the development of the sexual organs and the secondary sexual characteristics.
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, two small organs set in the pelvic cavity below and to either side of the navel. The ovaries also secrete, in cyclic fashion, the hormones estrogenestrogen
, any one of a group of hormones synthesized by the reproductive organs and adrenal glands in females and, in lesser quantities, in males. The estrogens cause the thickening of the lining of the uterus and vagina in the early phase of the ovulatory, or menstrual, cycle
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 and progesteroneprogesterone
, female sex hormone that induces secretory changes in the lining of the uterus essential for successful implantation of a fertilized egg. A steroid, progesterone is secreted chiefly by the corpus luteum, a group of cells formed in the ovary after the follicle
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 (see menstruationmenstruation,
periodic flow of blood and cells from the lining of the uterus in humans and most other primates, occurring about every 28 days in women. Menstruation commences at puberty (usually between age 10 and 17).
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). After an ovum matures, it passes into the uterine tube, or fallopian tubefallopian tube
, either of a pair of tubes extending from the uterus to the paired ovaries in the human female, also called oviducts, technically known as the uterine tube.
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. If sperm are present as a result of sexual intercourse or artificial inseminationartificial insemination,
technique involving the artificial injection of sperm-containing semen from a male into a female to cause pregnancy. Artificial insemination is often used in animals to multiply the possible offspring of a prized animal and for the breeding of endangered
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, fertilization occurs within the tube. The ovum, either fertilized or unfertilized, then passes down the fallopian tube, aided by cilia in the tube, and into the womb, or uterusuterus,
in most female mammals, hollow muscular organ in which the fetus develops and from which it is delivered at the end of pregnancy. The human uterus is pear-shaped and about 3 in. (7.
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, a pear-shaped organ specialized for development of a fertilized egg.

An inner uterine layer of tissue, the endometrium, undergoes cyclic changes as a result of the changing levels of the hormones secreted by the ovaries. The endometrium is thickest during the part of the menstrual cycle in which a fertilized ovum would be expected to enter the uterus and is thinnest just after menstruation. If no fertilized egg is present toward the end of the cycle, the thickened endometrium degenerates and sloughs off and menstruation occurs; if a fertilized egg is present it becomes embedded in the endometrium about a week after fertilization. The developing embryo produces trophoblastic cells and these, along with cells from the endometrium, form the placenta, the organ in which gas, food, and waste exchange between mother and embryo takes place. The embryo also forms the amniotic sac within which it develops.

The lower end of the uterus is called the cervix. The vagina, a passage connecting the uterus with the external genitals, receives the penis and the sperm ejaculated from it during sexual intercourse. It also serves as an exit passageway for menstrual blood and for the baby during birthbirth
or labor,
delivery of the fetus by the viviparous mammal. Birth is also known as parturition. Human birth normally occurs about 280 days after onset of the last menstrual period before conception.
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. The external genitals, or vulva, include the clitoris, erectile tissue that responds to sexual stimulation, and the labia, which are composed of elongated folds of skin. After birth the infant is fed with milk from the breasts, or mammary glandsmammary gland,
organ of the female mammal that produces and secretes milk for the nourishment of the young. A mammal may have from 1 to 11 pairs of mammary glands, depending on the species. Generally, those mammals that bear larger litters have more glands.
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, which are also sometimes considered part of the reproductive system.

In the Human Male

In the male reproductive system sperm are produced in the seminiferous tubules of the testestestis
or testicle
, one of a pair of glands that produce the male reproductive cells, or sperm. In fetal life the testes develop in the abdomen, then descend into an external sac, the scrotum.
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, two organs contained in the scrotum, an external sac in the groin. The testes also produce the male hormone testosteronetestosterone
, principal androgen, or male sex hormone. One of the group of compounds known as anabolic steroids, testosterone is secreted by the testes (see testis) but is also synthesized in small quantities in the ovaries, cortices of the adrenal glands, and placenta, usually
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 and a portion of the seminal fluid, the liquid in which sperm are carried. The external location of the scrotum ensures the relatively low temperature that is necessary for the normal development of sperm. After formation, the sperm pass from the testes into the tubular epididymis, and from there into another passage, the vas deferens. The seminal vesicle, which produces nutrient seminal fluid, and the prostate glandprostate gland,
gland that is part of the male reproductive system. It is an organ about the size of a chestnut and consists of glandular and muscular tissue. It is situated below the neck of the bladder, encircling the urethra.
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, which produces alkaline prostatic fluid, are both connected to the ejaculatory duct leading into the urethraurethra
, canal in most mammals that carries urine from the bladder to the outside of the body; in the male it also serves as a genital duct. The urethra is about 1 1-2 in. long (3.8 cm) in women, terminating above the vaginal opening. In men the urethra is about 8 in.
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.

The first stage of the male sexual act, erection, results from nerve impulses from the autonomic nervous system that dilate the arteries of the penis, thus allowing arterial blood to flow into erectile tissues of the organ. During intercourse, contractions in the ducts of the testes, epididymis, and ductus deferens cause expulsion of sperm into the urethra and their mixture with the seminal and prostatic fluids. These substances, together with mucus secreted by accessory glands known as Cowper's glands, form the semen, which is discharged from the penile urethra during ejaculation.

Human Reproductive Disorders

Disorders that may affect the proper functioning of the reproductive system include abnormal hormone secretion, sexually transmitted diseasessexually transmitted disease
(STD) or venereal disease,
term for infections acquired mainly through sexual contact. Five diseases were traditionally known as venereal diseases: gonorrhea, syphilis, and the less common granuloma inguinale, lymphogranuloma venereum, and
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, and the presence of cancerous tissue in the region. Such problems frequently affect fertility and may complicate pregnancy.

See infertilityinfertility,
inability to conceive or carry a child to delivery. The term is usually limited to situations where the couple has had intercourse regularly for one year without using birth control.
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. See also fertility drugfertility drug,
any of a variety of substances used to increase the possibility of conception and successful pregnancy. Different methods are used to correct or circumvent the many different functional disorders of both males and females that can interfere with conception and
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; in vitro fertilizationin vitro fertilization
(IVF), technique for conception of a human embryo outside the mother's body. Several ova, or eggs, are removed from the mother's body and placed in special laboratory culture dishes (Petri dishes); sperm from the father are then added, or in many cases a
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.

Reproductive system

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 each 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 two distinct 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 Ovary, Reproduction (animal), Testis

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 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 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.

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 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 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.

reproductive system

[¦rē·prə¦dək·tiv ‚sis·təm]
(anatomy)
The structures concerned with the production of sex cells and perpetuation of the species.
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