Discuss the role of hormones in menstrual Cycle?
In this article we will discuss about the Menstrual Cycle: Ovarian Cycle & Uterine Cycle.
2.History of Uterus
In human males, the formation and release of gametes is a continuous process, beginning at puberty and lasting throughout life. In contrast, human females show cyclic changes in ovulation and reproductive activities. The monthly changes in the secretion of female hormones and corresponding changes in the whole reproductive system occurs in a cyclic manner.. These rhythmical changes in the female sexual organ is called menstrual cycle. But it would be more appropriate to call it female sexual cycle. The menstrual cycle is of 28 days in duration but it may be as short as 20 days or as long as 45 days. This cycle includes an ovarian cycle and an uterine cycle which involves changes in ovary and uterus respectively. The gonadotrophin hormones such as the follicle stimulating hormone (FSH) and the luteinizing hormone (LH) secreted from the anterior lobe of pituitary regulate all the events in the menstrual cycle. The FSH and the LH are secreted at the age of 9-10 years which initiate monthly cycle at the age of 11-16 years.
The unset of sexual cycle is called puberty or menarch. The fertile life extends upto 45-50 years and then becomes irregular and finally stops. This is called menopause.
It consists of three phages: (i) follicular phage, (ii) ovulatory phase and (iii) luteal phase.
Follicular phase :
The follicular phase in characterized by the following events.
(i) Two to three fold enlargement of the ovum.
(ii) Growth of additional layers of granulosa cells and the development of primary follicle.
(iii) Growth of 6 to 12 primary follicles per month under the influence of FSH and LH.
(iv) Formation of theca externa and theca interna around the granulosa layer.
(v) Secretion of a fluid by the granulosa cells and formation of a fluid-filled cavity called antrum.
(vi) Further growth of the follicle and enlargement of the ovum to form cumulus oophorus.
(vii) Progressive growth of a single follicular cell faster than others and secretion of estrogen by the follicular cells.
(viii) The growth of the enlarged follicle and its maturation.
(i) Ovulatory phase begins after the formation of a matured follicle called Graafian follicle.
(ii) Swelling of the outer wall of the matured follicle under the influence of LH to form a protruding nipple-like body called stigma.
(iii) Loss of folliculer fluid through the stigma.
(iv) Rupture of stigma and release of the ovum in a mass of fluid.
(v) In women with a normal 28 days of female sexual cycle, ovulation occurs on the 14th day after the beginning of menstruation.
Luteal phase (progestational phase):
- The luteal phase begins immediately after the expulsion of the ovum.
- The granulosa and the theca interna cells change into specialized luteal cells.
- Next, the luteal cells become enlarged and filled with lipid material. This is called luteinization and the mass of developed cells together form corpus luteum.
- The corpus luteum becomes fully developed under the influence of LH i.e., luteinizing hormone or yellowing hormone.
- The corpus luteum becomes a secretory (endocrine) organ and starts secreting a large amount of progesterone and lesser amount of estrogen.
- In addition, it secretes another hormone called inhibin which inhibits further secretion of FSH.
- The progesterone and estrogen promote the development of uterine endometrium for implantation of embryo. Progesterone inhibits further development of ovarian follicles and in association with prolactin prepares the mammary glands for secretion of milk.
- In the event of fertilization, the fertilized embryo is implanted in the uterine bed of endometrium and development of uterine endometrium for implantation of embryo. Progesterone inhibits further development of ovarian follicles and in association with prolactin prepares the mammary glands for secretion of milk.
- In the event of fertilization, the fertilized embryo is implanted in the uterine bed of endometrium and the corpus luteum remains functional throughout pregnancy.
- In the absence of fertilization, the corpus luteum degenerates into a white fibrous structure called corpus albicans.
Histology of uterus:
The uterus is thick-walled and composed of three layers. The antermost layer is called serous coat, the middle layer is the myometrium which forms the bulk of the uterine wall. The smooth muscle fibres present in this layer contract under the influence of oxytocin during birth. The innermost layer is called the endometrium. It is soft and smooth; made up of simple tubular glands, epithelial cells and spiral arterioles. The uterine cavity has great power of extension, extending 500 times during pregnancy.
Uterine cycle consists of three phases which involves structural and physiological changes in the endometrium. The three phases are: (i) proliferative phase, (ii) secretory phase, and (iii) menstrual phase.
(i) This phase coincides with the follicular phase of ovarian cycle .
(ii) The stromal and epithelial cells proliferate under the influence of estrogen.
(iii) The endometrium thickens within 4-7 days.
(iv) The endometrium becomes glandular and richly vascular.
(i) The secretory phase begins after the discharge of ovum from the matured follicle.
(ii) Increased secretion of progesterone from the corpus luteum prepares the endometrium to receive the fertilized ovum.
(iii) The estrogen stimulates additional cellular proliferation.
(i) Twelve days after ovulation and in the absence of fertilization, the corpus luteum begins to lose its secretory function and starts degenerating.
(ii) It develops into corpus albicans.
(iii) There is a sharp decrease in the secretion of progesterone and estrogen, rapid regression and necrosis of endometrium, contraction of blood capillaries, separation of necrotic tissue, and shedding of the epithelial lining along with blood and mucous which leads to menstrual flow. A mass of desquamated epithelial and uterine tissues are flushed out of the uterus by the action of prostaglandin. Menstruation continues for a period of 4-7 days. The menstrual fluid is non-clotting due to the presence of fibrinolysin. A large number of leucocytes are released in the menstrual fluid.
The next cycle begins after 28 days. Under the influence of FSH and LH, new primary follicles start developing. Uterine endometrium is repaired and gradually thickens. The follicles secrete progesteronc and estrogen which stops the flow of menstrual fluid.