HOME
Front page
Credits
Links
MyBodyIndex:
Human Body Guide
Nutrient Guide
Disorder Guide
Remedies & Therapies
The Human Body:
|
Female Reproductive System
The female reproductive system is slightly different from that of the male. The male produces delivers the gametes while the female produces and releases within herself , so able to nourish herself without the aid of any accessory glands and has to undergo the phase of pregnancy if the male gametes had successfully reached the female. The female reproductive system thus, has the gonads(ovaries), the ducts(oviduct, vagina and uterus) and the external organ(vulva and mammary glands).
The gonads are the paired ovaries whose functions include the production and discharge of ova as well as the secretion of female sex hormones. The oviducts are for the transport of ova from ovaries to uterus and are the normal sites of fertilization. The uterus whose functions include menstruation, implantation of fertilized ovum, development of fetus during pregnancy and labour. The vagina serves as a passageway for menstrual flow, while it’s lower canal is the birth canal and is the receptacle for penis during coitus.
The uterus walls consists of 3 layers, the perimetrium/serosa which is the outermost layer, the myometrium, which is the middle layer and forms the bulk of the uterine wall and had 3 layers of smooth musceles, of which during childbirth is sensitive to oxytocin, and the last innermost layer, the endometrium which is soft, smooth and composed of epithelial cells, tubular glads and spiral arterioles, it has 2 layers the stratujm functionalis(next to the uterine cavity, shed during menstruation) and the stratum basalis(permanent, produces new stratum functionalis after menstruation).
The clitoris contains erectile tissue and nerves and is homologous to that of the penis. Tactile stimulation causes enlargement and in plays an essential role in the sexual excitement of the female. The hymen is a thin fold of the membrane which forms a border around the vaginal orifice, partially closing it, so as to allow menstruation. It is what defines virginity. The labia minora and the labia majora form the major part of the vulva. They produce a lubricant mucus during intercourse and protects the clitoris from abrasion. During pregnancy, it will turn bluish due to venous congestion.
The female sex hormones are also under control via the hypothalamus. The hypothalamus when stimulated releases GnRH which trigger the anterior pituitary gland to release the FSH for the maturation of the follicle in the ovary and LH to stimulate cells to secret estrogen precursor molecule. Granulosa cells of the follicle pridcue and secrete estrogen. Estrogen is responsible for the development and maintenance of most of the female secondary sexual characterisitics. Progesterone, another sex hormone, secreted by the ovaries, affects the cyclical changes in the uterus and mammary glands.
Oogenesis is the formation and development of the haploid ova within the ovary. It starts during fetal development with the primodial germ cell that is diploid and under mitosis, divides to form the oogonia, which grows to form the primary occyte. At birth, prior to the ovulation, the primary oocyte undergoes meiosis 1 and forms 1 secondary oocyte and the first polar body. The secondary oocyte goes onto meiosis II and forms an ootid and the second polar body. The ootid goes on and develops to the haploid ovum, while the polar body disintegrates.
At birth, the female baby has a total of 2-4 million follicles in both ovaries. By the age of 20-25, majority would have disintegrated, leaving 70,000 follicles in the ovaries. The primordial follicle are the main ones present before birth and consists of primary oocyte enclosed by follicular cells(membrane ganulosa) whose functions are to provide nourishment to the developing ovum and secrete occtye maturation-inhibiting factor to keep occyte in primordial state. It develops into the primary follicle after birth before the onset of menstruation and the primary oocyte increases 2-3 fold in diameter and has a acellular layer(zona pullucida) around it, internal to the membrane ganulosa. About 20 primary follicles begin development into secondary follicles during a menstrual cycle. The Tertiary follicle has a second mass of cells(theca) arising outside the granulose. The theca is divided into 2 layers, theca interna, capable of secreting steroid hormones and is richly vascularized and the theca externa which becomes the capsule of the follicle. The granulose cells secrete follicular fluid containing oestrogen now. The follicular fluid accumulates withint he follicular antrum. Finally, the Graadian follicle is the mature tertiary follicle which is ready for ovulation. The innermost layer of granulose cells becomes elongated to form the corona radiate. It is the cumulus oophorus(secondary oocyte+ zona pellucida+ corona radiate+membranana granulose). During maturation, the follicle increases the oestrogen production. The follicle then moves slowly to the surface of the ovary. Normally only 1 follicle fully matures each month and the rest degenerates into cyst-like structures. Ovulation(release of mature ovum) is then brought about by the rapid rise in LH and in the presence of FSH. The ovulated secondary oocyte is then swept into the oviduct by the flow of liquid from the abdominopelvic cavity created by the beating of the cilia aatched to the fimbriae surrounding the opening of the oviduct.
After the expulsion of the secondary oocyte, granulose cells and the cells of the theca interna undergo lutenisation under the influence of LH and prolactin to form the corpus luteum located in the cotical region of the ovary. It acts as a temporary endocrine gland that secretes oestrogens, progesterone and relaxin. In the absence of fertilization, the corpus luteum lasts for only 10-14 days before it involutes. This is due to LH secretion being inhibited by progesterone(together with oesterogen) produced by the corpus luteum. It then degenerates into corpus albicans and is gradually resorbed by the stroma.
But In the event of fertilization, the corpus luteum is maintained throughout pregnancy and is mained by the human chrionic gonadotrophin(HCG) produce by the developing placenta. For about 8-10 weeks, the corpus luteum continues to secrete oestrogen and progesterone to prevent the development of new ovarian follicles and ovulation, prevent breakdown of uterine lining and permit continued attachment of fetus to uterus. Once the placenta starts to secrete oestrogens and progesterone, the role of the corpus luteum becomes minor but it still continues to carry out the permission to continue attachment of fetus to uterus.
|