Reproductive System

I

Introduction

Reproductive System, term applied to the group of organs necessary for or accessory to the reproductive processes. 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 produced by such division, the primordial germ cells, 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 derived from primitive kidneys located in the anterior, lateral part of the embryo; 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 (gametes), the germ cells undergo a special type of cell division, known as meiosis, which halves the number of chromosomes they carry (see gene). At the time of sexual maturity, the somatic cells in the gonads of higher animals begin to secrete hormones that control the appearance of the various secondary sexual characteristics (see Sex).

III

Gonads

The male gonads—the testes—are organs that contain germ cells which later develop into male gametes (spermatozoa). Ovaries are the organs that contain germ cells which later develop into female gametes—eggs, or ova. 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 gonad size. During the breeding season the ovaries of fish increase in size until they constitute about one-quarter to one-third of the fish's total body weight.

The testes and ovaries of mature animals differ greatly in structure. In the delicate convoluted tubules of the testes—the seminiferous tubules—the primitive germ cells mature into spermatozoa. Mammal testes are generally oval bodies, enclosed by a capsule of tough connective tissue. Projections from this tough capsule into the testis divide it into several compartments, each of which is filled with hundreds of seminiferous tubules. Mature spermatozoa are discharged through a number of (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 for life. 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, both testes are always enclosed in an external scrotum. During foetal life, the testes move through the muscles composing the posterior, ventral portion of the trunk, carrying with them peritoneum and skin surrounding these muscles. The channel in the muscles through which the testis moves is known as the inguinal canal. This usually closes after birth, but sometimes remains open, often becoming the site of herniation. The portion of the peritoneum that the testis carries with it forms a double wall of membrane, the tunica vaginalis, between the scrotum and testis. Occasionally, the testes in the human male do not descend into the scrotal sac. This condition, 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 them 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, which is located in the body cavity, attached to the peritoneum that lines this cavity. 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 nutrients for the ovum contained within it. As the ovum is prepared for release during the breeding season, the tissue surrounding it 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 follicles at various stages of maturity. The number of Graafian follicles present varies between animals; in humans there is usually only one Graafian follicle in one ovary per menstrual cycle. In multiparous animals that give birth to more than one offspring, there may be larger numbers of Graafian follicles in the ovaries.

When the Graafian follicle is completely mature it bursts through the surface of the ovary, releasing the ovum—the process known as ovulation. The ovum is now ready for fertilization. The space formerly occupied by the Graafian follicle is filled by a blood clot known as the corpus haemorrhagicum, which in four or five days is replaced by a mass of yellow cells known as the corpus luteum. This secretes hormones which prepare the uterus for the reception of a fertilized ovum. If the ovum is not fertilized, the corpus luteum is eventually replaced by scar tissue known as the corpus albicans.

The functioning of both male and female gonads is under the hormonal influence of the pituitary gland.

IV

Transport 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 and include the epididymis, which lies attached to the testis and which transports sperm to the vas deferens. This carries the spermatozoa to the ejaculatory duct, which contracts to discharge 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 towards 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, into which the mature egg is drawn 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. 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 open to the exterior.