Reproductive System.

Reproductive System. I INTRODUCTION Reproductive System, term applied to the group of plant or animal organs that are necessary for or that are accessory to the reproductive processes (see Reproduction). The basic units of sexual reproduction are the male and female germ cells; this article deals with the organs within which the germ cells of animals mature and are stored, the organs through which they are transported in the process of producing a new individual, and accessory glandular organs. For the reproductive organs of plants, see Plant Propagation. II ORIGIN OF THE REPRODUCTIVE CELLS When the embryo of any sexually reproducing animal is undergoing cell division, certain cells, known as primordial germ cells, which are produced by such division, remain in an undifferentiated state. Cells other than primordial germ cells are known as vegetative cells or somatic cells; these cells become differentiated into tissues and organs. In invertebrates, the primordial germ cells congregate in the body cavity or in a section of the circulatory system; in vertebrates, these cells are located in organs that adjoin the excretory system. The tissues in which the germ cells lodge become reproductive organs known as gonads. These organs are embryologically derived from primitive kidneys located in the anterior, lateral part of the body; in most mammals, they shift before birth to the posterior, ventral region of the body. The primordial germ cells remain inactive in the gonads until the animal reaches sexual maturity, when the undifferentiated cells undergo a great number of normal cell divisions or mitoses. In the process of developing into mature reproductive cells or gametes, the germ cells undergo a special type of cell division, known as meiosis, which halves the number of chromosomes they carry. At the time of sexual maturity, the somatic cells composing the gonads of higher animals begin to secrete hormones that control the appearance of the various secondary sexual characteristics (see Sex). III GONADS Organs that contain germ cells which later develop into male gametes or spermatozoa are known as testes or male gonads. Organs that contain germ cells which later develop into female gametes, eggs, or ova are known as ovaries. In many invertebrate species individual animals bear both testes and ovaries (see Hermaphroditism). In some invertebrates, and in most vertebrates, individuals bear either testes or ovaries, but not both sets of organs. In invertebrates, a single animal may have as many as 26 pairs of gonads; in vertebrates, the usual number is 2. Cyclostomes and most birds are unusual among vertebrates in possessing only a single gonad; owls, pigeons, hawks, and parrots are unusual among birds in having two gonads. The size of gonads increases at sexual maturity because of the great number of germ cells produced at that time; many germ cells are also produced during breeding seasons so that many animals have a seasonal increase in size of the gonads. During the breeding season of fish, the ovaries increase in size until they constitute about one-quarter to one-third of the total body weight. The testes and ovaries of mature animals differ greatly in structure. The testes are composed of delicate convoluted tubules, known as seminiferous tubules, in which the primitive germ cells mature into spermatozoa. The testes of mammals are generally oval bodies, enclosed by a capsule of tough connective tissue. Projections from this tough capsule into the testis divide the testis into several compartments, each of which is filled with hundreds of seminiferous tubules. The mature spermatozoa are discharged through a number of ducts, called the efferent ducts, which communicate with the epididymis, a thick-walled, coiled duct in which the sperm are stored. In all vertebrates below marsupials on the zoological scale, and in elephants, sea cows, and whales, the testis remains within the body cavity during the lifetime of the animals. In many mammals, such as rodents, bats, and members of the camel family, the testis remains within the body cavity during periods of quiescence, but moves into an external pocket of skin and muscle, known as the scrotum, during the breeding season. In marsupials, and in most higher mammals, including the human male, the testes are always enclosed in an external scrotum. During fetal life, the testes move through the muscles composing the posterior, ventral portion of the trunk and carry with them the portion of the peritoneum and skin surrounding these muscles. The channel in the muscles through which the testis moves is known as the inguinal canal; it usually closes after birth, but sometimes remains open and is then often the site of herniation (see Hernia). The portion of the peritoneum that the testis carries with it forms a double wall of membrane between the scrotum and testis and is known as the tunica vaginalis. Occasionally, the testes in the human male do not descend into the scrotal sac; this condition of nondescent, which is known as cryptorchidism, may result in sterility if not corrected by surgery or the administration of hormones. Retention of the testes within the body cavity subjects the germ cells to temperatures that are too high for their normal development; the descent of the testes into the scrotum in higher animals keeps the testes at optimum temperatures. Unlike germ cells in the testis, female germ cells originate as single cells in the embryonic tissue that later develops into an ovary. At maturity, after the production of ova from the female germ cells, groups of ovary cells surrounding each ovum develop into "follicle cells" that secrete nutriment for the contained egg. As the ovum is prepared for release during the breeding season, the tissue surrounding the ovum hollows out and becomes filled with fluid and at the same time moves to the surface of the ovary; this mass of tissue, fluid, and ovum is known as a Graafian follicle. The ovary of the adult is merely a mass of glandular and connective tissue containing numerous Graafian follicles at various stages of maturity. When the Graafian follicle is completely mature, it bursts through the surface of the ovary, releasing the ovum, which is then ready for fertilization; the release of the ovum from the ovary is known as ovulation. The space formerly occupied by the Graafian follicle is filled by a blood clot known as the corpus hemorrhagicum; in four or five days this clot is replaced by a mass of yellow cells known as the corpus luteum, which secretes hormones playing an important part in preparation of the uterus for the reception of a fertilized ovum. If the ovum goes unfertilized, the corpus luteum is eventually replaced by scar tissue known as the corpus albicans. The ovary is located in the body cavity, attached to the peritoneum that lines this cavity. The functioning of both male and female gonads is under the hormonal influence of the pituitary gland. IV TRANSPORTATION OF THE REPRODUCTIVE CELLS Before being discharged from the body, the reproductive cells travel from the gonads to an external body opening. In many invertebrates, and in a few aquatic vertebrates, the reproductive cells are discharged into water directly from the gonads through pores in the body wall. In higher animals ducts carry the reproductive cells into the urinary or cloacal excretory systems, or into independent reproductive passages. In male vertebrates, the ducts are directly connected to the testes. The male ducts include the epididymis, which lies attached to the testis, and which transports the male gametes to the vas deferens. The vas deferens carries the spermatozoa to the ejaculatory duct, the contractions of which cause the discharge of sperm into the posterior urethra. In most fishes, the ovary has a hollow expansion through which the ova pass into the cloaca. In most other vertebrates, however, no direct connection exists between the ovary and the oviducts that carry the ova into the cloaca or into the independent external opening. In mammals, when the Graafian follicle bursts, the egg falls toward the interior of the abdominal cavity. The oviduct (known in higher mammals as the fallopian tube) has an open, funnel-shaped end located near the ovary, and the mature egg is drawn into the funnel by ciliary action. Occasionally, the egg misses the open end of the oviduct and falls into the abdominal cavity; such eggs are capable of being fertilized, resulting in ectopic pregnancies (see Pregnancy and Childbirth). In animals lower than marsupials the oviducts open directly into the cloaca; in marsupials and placental mammals, the oviducts, two of which are normally present, fuse at their cloacal ends to form a thick, muscular organ, the uterus or womb, in which the young develop, and a thinner channel, the vagina, which opens exteriorly. V GENITALS In animals that lay their eggs and discharge their sperm into water, the spermatozoa reach the eggs by chemical attraction; the eggs of individuals of a species attract only the sperm of members of the same species. When eggs and sperm are deposited at great distances from each other, the number of eggs fertilized is small. Many amphibians and aquatic vertebrates solve this problem by attaching themselves to their mates by means of holdfast mechanisms, such as claspers; when the female of these animals deposits the eggs, the male can immediately deposit sperm in the same location. In terrestrial animals, various adaptations have been developed whereby internal fertilization of the eggs may be produced. The male snake, which discharges the spermatozoa through a cloaca, has, for example, anal hooks that are inserted into the cloaca of the female during the breeding season. These hooks bind the male and the female together while the spermatozoa are being discharged. External reproductive organs used by animals to facilitate internal fertilization are known as genitals or genitalia. The male genital of all mammals above the monotremes is the penis, a protrusible, erectile organ that directs the spermatozoa into the female cloaca or vagina. In turtles and crocodiles, which are the most primitive animals possessing the organ, the penis is located on the ventral wall of the cloaca and is grooved along its upper side. The spermatozoa travel along the groove into the female cloaca. In marsupials and placental mammals, the penis is a closed tube, composed of three bundles of tissue, bound together by connective tissue and covered with loose skin. Two large bundles of tissue, known as the corpora cavernosa, constitute the upper portion of the penis; these bundles contain numerous compartments that are filled with blood during sexual excitement, causing the penis to become stiff and erect. The flow of blood into the corpora cavernosa is controlled by sacral spinal nerves. Below and between the corpora cavernosa is the third bundle of tissue, which is known as the corpus spongiosum; this bundle is perforated by the urethra and, in several lower mammals, also contains a bone that serves to further stiffen the male genital. At the terminal end of the penis is a sensitive cap known as the glans; in marsupials the glans is forked. In many mammals, the male genital, when not erect, is withdrawn into a sheath in the body. In primates, including the human male, the penis is pendant, and the glans is covered with a layer of retractile skin, known as the prepuce or foreskin, which corresponds to the sheath of lower animals. The chief female genital is known as the vagina. This organ is present in all marsupials and placental mammals. In marsupials, two vaginas and two uteri are present; in primates, including the human female, only one vagina is present; and in mammals intermediate in development between the marsupials and primates, various degrees of partially united, double vaginas are present. The external end of the vagina is covered in virgin primates by a membrane, known as the hymen. Anterior to the hymen is the external opening of the urethra. The urethra and the opening of the vagina are contained in an indented space which is known as the vestibule. In primates, two membranous folds, known as the labia minora, are found on either side of the vestibule. In primates, including the human female, two additional folds, the labia majora, enclose the labia minora. The clitoris, at the front of the labia, is homologous with the penis although greatly reduced in size (see Homology below). VI ACCESSORY GLANDS Among the glands accessory to the reproductive process are those that provide a fluid medium in which the spermatozoa may live, those that produce mucus which reduces friction during copulation, those that emit alluring odors to members of the opposite sex, and those that secrete nourishment for the ova, the embryos, and the newborn young. The seminal vesicles of the male have already been mentioned as organs that secrete mucus. The most important male accessory gland is the prostate gland, a compound gland about the size of a chestnut located at the base of the urethra where the urethra leaves the bladder and enters the penis. The prostate secretes a thin milky fluid with a characteristic odor; this fluid constitutes the greater part of the semen that is deposited in the female vagina and that contains the spermatozoa. The prostate gland is present only in placental mammals, and among these mammals is absent in edentates, martens, otters, and badgers. Cowper's glands are two small glands, about the size of a pea, located on each side of the base of the penis. Their secretion is thick and clear and is believed to protect the spermatozoa against excess vaginal acidity. Cowper's glands are absent in bears, dogs, and aquatic mammals. The primary lubricating glands of the female are the cervical glands, located in the uterus where the uterus communicates with the vagina, and Bartholin's glands, located in the vestibule between the hymen and the labia minora. Both sets of glands secrete mucus. Female placental mammals also have uterine glands that prepare the uterus for the reception of the fertilized egg. The anal glands of many mammals secrete special substances called pheromones, which signal reproductive readiness by scent to members of the opposite sex. Pheromones may occur in other glandular secretions as well. Among the different structures serving to nourish the young, the placenta of placental mammals is unique (see Fetus). The mammary glands of mammals are also included among accessory reproductive glands (see Breast). Female egg-laying animals have albumen glands, which coat the zygote with nutrient albumen before the egg is laid, and nidamental glands, which surround the zygote and albumen with a leathery or calcareous shell. VII HOMOLOGY The sex of a young embryo is indistinguishable, males and females passing through similar embryonic stages. Embryo males and females develop almost a duplicate set of reproductive organs, one set becoming vestigal shortly before birth, and the other set becoming prominent. Most cases of so-called mammalian hermaphroditism are actually cases of abnormal development in which external genitalia similar to those of both sexes have been developed. For example, mammalian females have a small, erectile organ, consisting of two corpora cavernosa, located in the upper portion of the vestibule. This organ, called the clitoris, is homologous with the male penis; except in lemurs and a few rodents it does not contain the urethra, which is usually located beneath the clitoris. In species in which the male possesses a penis bone, the clitoris of the female contains a small bone. Contributed By: Jean Roche Newton E. Morton Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.