Female reproductive organs
The female reproductive organs include a pair of ovaries, a pair of oviducts (fallopian tubes), the uterus and the vagina. The human ovaries are two small-almond-like flattened bodies lying on the sides of vertebral column behind the kidneys in the pelvic cavity.
Each ovary is composed of stroma of fibrous connective tissue and is lined by germinal epithelial cells.
Each ovary is differentiated into an outer cortex and an inner medulla. In mature ovary, the cortex contains follicles and coipora lutea while the medulla contains only the large blood vessels.
During the growth of ovary, the stroma forms a mass of epithelial cells which becomes separated from the main layer. One cell of this mass gives rise to an immature ovum or oocyte while the remaining cells form a layer surrounding the ovum called granulosa. Immature ovum and granulosa constitute the primordial follicle. The stroma of the ovary surrounding the granulosa becomes organised into connective tissue layers, the theca external and thee an internal.
Ovulation occurs when follicle bursts liberating the egg. Normally, only one ovulation occurs during each monthly reproductive cycle of a woman. But sometimes, two or more can occur resulting in the fraternal type of twins. Ovulation occurs about 14 days before the beginning of menstruation.
The egg or ova are usually shed in the body cavity from where they are picked up by the fallopian tubes and laid down into the tube. The fallopian tubes lead to a central pear-shaped uterus which is connected to vagina through cervix. Near the opening of the vagina are found bulbourethral glands which secrete a fluid to lubricate and neutralize any acid which may be present there.
Vagina leads these to the exterior through vaginal orifice which is flanked by an inner pair of moist folds, the labia minora and an outer hair- covered the labia majora. These structures enclose clitoris which is homologous to the penis of male.
Fertilization and Implantation:
During copulation, the continued stimulation of the penis triggers the muscles of the genital organs which results in the discharge of semen from the urethra. When semen is deposited within the vagina, the sperms tend to move in all directions. About three days are required for an ovum to travel down the oviducts to the uterus. Some of the sperms find their way up the uterus where they meet the descending ovum.
As the sperm and ovum meet, fertilization may take place in the oviduct. It is believed that a woman's fertility period lasts only a day or two after ovulation. If the ovum is not fertilized during this time, it passes from the body and if it is fertilized, it becomes implanted in the endometrium.
Human milk to feed the baby is made by the mother's mammary glands from constituents in the maternal blood and its formation is under control of prolactin and oxytocin hormones of the pituitary. The human mammary gland consists of about 15 separate milk producing systems which are arranged radially around the nipple.
The milk is produced in the milk-producing alveoli which are connected to a main milk duct. There are thus 15 openings at the nipple, each leading to its own milk producing alveoli. The main milk duct has a dilatation just below the surface which is known as the lactiferous sinus in which some milk is stored. Baby can take milk from this sinus.
Menstrual cycle is characteristic of primates and it does not occur in other vertebrates. In mammals other than primates, the sexual cycle is known as oestrus cycle.
Menstrual cycle begins with the casting-off of endometrial lining of the uterus and bleeding. Under the influence of follicle stimulating hormone (FSH), an ovum and its follicle mature and the follicle secretes increased amounts of oestrogen which stimulates the regrowth of endometrium. It occurs from 5th to the 14th day of the cycle.
At this time, ovulation occurs and the follicle is transformed into the progesterone-producing corpus luteum. If pregnancy does not occur at this stage, it is followed by menstruation which lasts for about 13 to 14 days. During menstruation, the uterus endometrial lining is sloughed off and bleeding takes place due to rupture of blood vessels.
Menstruation lasts about 4 days and happens only when fertilization is not accompanied. The menstrual cycle remains under the control of hormonal secretion and is the sign of puberty in human females. In human female, the cycle usually lasts about 28 days.