Zoology Notes: 016 Chapter 12

Chapter 12. The Reproductive System Reproduction is the process by which human beings, plants, and animals create more of their own kind. There are two main kinds of reproduction: sexual and asexual. Generally, in sexual reproduction, a new organism is formed from the union of two sex cells that come from two different parents. Most of the higher forms of animals, including man, reproduce sexually. In asexual reproduction, a new organism develops from parts of, or parts produced by one parent. Types of Asexual Reproduction Binary Fission. This involves the splitting of one-celled organisms (ex. Amoeba) into two cells. The new cells grow to the size of the parent cell and each splits into new organisms. Fragmentation. This is the process wherein the animal (ex. Planaria) breaks into two or more parts and each develop into a complete new adult. Budding. This occurs in sponges. The new organism develops as an outgrowth of the parent organism. Sporulation. This occurs when an organism produces and releases one-celled structures called spores. Each spore develops into a complete new organism. Male Reproductive System Internal Organs The male reproductive organs can be divided into three groups. First are the gonads or the testes. The second group consists of a series of ducts including the epididymis, ductus deferens, and the urethra. The third group consists of the accessory glands: seminal vesicles, prostrate and the bulbourethral (cowper’s) gland. The spermatic cord serves as the internal supporting structure. Testes. The testes correspond to the ovaries in the female. Each is an oval organ lying within the abdominal cavity in early life. About two months prior to birth, the testes descend into the scrotum. Occasionally, the testes do not descend, remaining in the abdomen, giving rise to the condition known as cryptorchism. The scrotum maintains a temperature compatible with the viability of spermatozoa therefore a cryptorchid person is sterile. The testes perform two Fig. 12.1. The male reproductive system. functions which to a large extent are complementary: 1. 2.

production of spermatozoa secretion of testosterone (male sex hormone)

Epdidymis. The epididymis is the first part of the duct system of the testis. It is a coiled tube lying on the posterior aspect of the testis. The epididymis is the site of maturation of the sperm cells. Ductus Deferens (Vas Deferens). This is a muscular tube which, at the time of ejaculation, propels the spermatozoa from the epididymis to the ejaculatory duct in the prostatic urethra. 53

Seminal Vesicles. There are two seminal vesicles each consisting of a single tube coiled upon itself. The seminal vesicles are membranous pouches lying posterior to the bladder near its base. They secrete a fluid giving motility to the spermatozoa. The tube of each seminal vesicle ends in a straight, narrow duct joining the ductus deferens to form the ejaculatory duct. The ejaculatory duct actually ejects the spermatozoa and seminal fluid into the urethra. Prostate Gland. This is a conical body about the size of a chestnut lying inferior to the bladder. It surrounds the first inch of urethra and secretes a thin, milky, alkaline fluid which aids in maintaining the viability of sperm cells. Bulbourethral Gland (Cowper’s Gland). The bulbourethral glands are paired pea-sized glands, located inferior to the prostate gland on either side of the urethra. They discharge a lubricating mucous secretion prior to ejaculation which also becomes a part of the semen. External Organs The scrotum and the penis are the external male organs of reproduction. The scrotum is a pouch which hangs behind the penis and is suspended from the pubis. It is a continuation of the abdominal wall and is divided by a septum into two sacs each containing and supporting a testis with its epididymis. The penis is the male organ of copulation. It is a flaccid structure when not stimulated. The phenomenon of erection occurs with sexual stimulation. The arteries supplying the penis dilate and a large quantity of blood under pressure enters the cavernous spaces of the erectile tissue. As these spaces expand, they compress the veins thus retaining all the entering blood allowing the penis to become firm facilitating entry into the vagina during sexual intercourse. When the arteries constrict, blood leaves the penis and the organ returns to its flaccid state. The skin of the penis is thinner and more highly pigmented than the skin of the remainder of the body. Terminally, the skin folds inward and backward upon itself, overhanging the glans penis as the prepuce or foreskin. In current practice, this foreskin may be removed in the newborn by a surgical procedure known as circumcision. Female Reproductive System

Internal Organs

54

The internal organs of reproduction in the female include the ovaries, uterine tubes, uterus, and the vagina. Ovaries. The female gonads or ovaries are two oval-shaped structures located in the upper part of the pelvic cavity, one on each side of the uterus. The ovaries are suspended from the broad ligaments of the uterus by the mesovarium, a fold of peritoneum, and are anchored to the uterus by the ovarian ligament. A thin layer of columnar cells, the germinal epithelium, covers each ovary. The inner structure of Fig. 11.2. The female reproductive system.

stroma of the ovary consists of a meshwork of spindle-shaped cells, connective tissue, and blood vessels. Minute vesicular follicles (Graafian follicles) at various stages of development are present in each ovary. The ova develop within these follicles. The two major functions of ovaries are: 1. 2.

development and expulsion of the female ova elaboration of female sex hormones

Uterine Tubes (Fallopian Tubes). The uterine or Fallopian tubes are two flexible, trumpetshaped, muscular tubes whose function is to convey the ovum to the uterus by muscular contraction and ciliary action. It is also the site of fertilization. Uterus. The uterus is a pear-shaped, thick-walled muscular organ suspended in the anterior part of the pelvic cavity above the bladder and in front of the rectum. The uterine tubes enter into the upper ends (horns of the uterus) and the lower end (cervix) projects into the vagina corresponding to the stem end of an inverted pear. The uterus is the site of fetal implantation and development. Vagina. The vagina is a tubular canal 4 to 6 inches in length, directed upward and backward extending from the vestibule to the uterus. It is situated between the bladder and the rectum. The vagina serves as part of the birth canal and represents the female organ of copulation. External Organs Vulva. The external female reproductive organs are collectively called the vulva which includes the mons pubis, labia majora, labia minora, clitoris, vestibular glands, and hymen. The mons pubis is a firm, cushion-like elevation of adipose tissue over the symphysis pubis, covered by pubic hair. The labia majora are homologous to the scrotum in the male. The labia minora are folds of skin lying medial to the labia majora enclosing the vestibule. The clitoris is a pea-shaped projection of erectile tissue, nerves, and blood vessels occupying the apex of the vestibule anterior to the vagina. The clitoris is important in the sexual excitation of the female and represents the homologue of the penis in the male although it is not traversed by Fig. 11.3. The external organ of the female reproductive system. the urethra. The vestibule is the cleft between the labia minora. Situated within the vestibule are the hymen, the vaginal orifice, the urethral orifice,

55

and the opening of the vestibular glands. The hymen is a thin fold of vascularized mucous membrane separating the vagina from the vestibule. Accessory Reproductive Organs The two mammary glands, or breasts, are accessory reproductive organs responsible for secreting milk for nourishment of the newborn child. Physiologic Processes Related to Reproduction Gametogenesis. Gametogenesis is the maturation of sex cells. It can be considered the first phase of reproduction (Spermatogenesis for spermatozoa; Oogenesis for egg cells) Menstruation. Menstruation is a recurrent discharge of blood, mucus, and epithelial cells from the lining of the uterus. Fertilization. Fertilization is the union of sperm cells with egg cells. Pregnancy. Pregnancy begins when a fertilized egg attaches itself to the lining of the uterus. The fertilized egg begins dividing until the end of the second month of its development. Up to this point it is called an embryo. There are three structures formed during the embryonic stage: the placenta, the umbilical cord, and the amniotic sac. After five weeks, the embryo resembles a human and is thus called a fetus. By the end of two months, all the internal organs have formed. Growth and development continue on until birth at nine months. Birth. Birth begins when the muscles of the uterus become active causing the mother to have labor pains. Muscles force the baby out of the uterus through the vagina and out of the mother’s body. After the baby has been born, the umbilical cord is cut leaving a permanent scar (navel). Muscle action continues until the placenta (now called afterbirth) is discharged through the vagina. Reproductive Hormones Estrogen and Progesterone. Estrogen and progesterone are two essential ovarian hormones. Estrogen is secreted by maturing graafian follicles, progesterone by the corpus luteum, a yellow mass formed from follicles that have matured and have discharged their ova. Testosterone. Testosterone is the male reproductive hormone secreted by interstitial cells of Leydig.

56