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Lecture

Hormonal control of reproduction in women

16 Pages
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Department
Physiology
Course Code
PSL201Y1
Professor
Christopher Perumalla

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Description
← Hormonal control of Reproduction in Women ← ← In the years before puberty, the female reproductive organs are immature and incapable of generating ova. At the beginning of puberty, the secretion of GnRH rises dramatically. The increase in GnRH triggers a similar rise in gonadotropin levels. This in turn stimulates the onset of gametogenesis as well as the productions of estrogen and progesterone at the ovaries. In this way, hormone production has driven the chromosomal events that make reproduction possible. ← The female is not only responsible for the production and transport of the ova but also for everything else that enables a new human being to be born. Similarily to the male, in females, gametogenesis and hormone production are performed by the gonads, or the ovaries. ← ← Gametogenesis ← ← Gametogenesis in the female is called oogenesis. After puberty, once every month, the ovaries deliver an early ovum the the uterine or fallopian tube. The end of the tube near the ovaries are trumpet shaped and has featherly projections called fimbria. It is the fimbria that act to catch the early stage ova that exit the ovary once a month on their trip to the uterus. The uterus has muscular walls for contractions which help push the fetus out of the cervix during birth. The other function of the ovaries is hormone production. As in the male, specialized cells of the female gonads secrete various hormones in response to the anterior pituitary hormones, the gonadotropins. After puberty, the uterus and ovaries got hrough cyclic changes in activity every month in conjunction with varying levels of the gonadotropins and ovarian hormones. ← ← Causes of infertility ← ← In females, the most common causes of infertility include those due to mechanical issues such as scar tissue that causes blockage of the uterine tube. Blockage of the uterine tubes usually occurs as a result of a previous infection or endometriosis. ← A common area of injury is the distal end of the tube or the delicate fibria. Injury can be severe enough to result in complete closure of the tube. The uterine tubes normally produce a watery secretion which can build up behind these blockages. When one or both uterine tubes are blocked, the spermatozoa count meet the ovulated ovum and therefore fertilization cannot occur. Endometriosis is another common cause for infertility. It can also be associated with obstruction of the uterine tubes. It refers to a topic or misplaced uterine tissue. ← Another case of fertility is due to problems with ovulation. This may be complete ovarian failure due to problems with levels of hormones or receptor. It can also be a problem of timing or detection of ovulation. ← ← Ovaries and reproductive tract ← ← The inner layer of the uterus in called endometrium and the muscular layer is the myometrium. The endometrial layer is highly granular and goes through substancial morphological changes in response to varying levels of pituitary and ovarian hormones throughout the mestrual cycle. The fimbriae encircle the ovary on either side of the uterus. Each ovary is composed of dense connective tissue but embedded in it are numerous spherical structures called follicles. Each follicle contains one developing ovocyte, an early stage ovum. ← The system exhibits these general characteristics that do not apply to men: ← 1. Displays yclic changes in activity ← 2. Displays restricted periods of fertility ← 3. Displays limited gamete production ← ← ← ← ← Oogenesis ← ← Meiosis occurs on oogenesis. The first step preceeding meiosis is the differenciation of primitive female germ cells to cells called oogonia. The early oogonia are diploid cells that undergo mitosis repeatedly until at 5 months gestation. The structures that will become ovaries house approx 6 million oogonia. After the oogonia have undergone mitosis, at the end of gestation, they differenciate into primary ovocytes. They stay in a period of suspended development called meiotic arrest until just prior to ovulation at puberty. At puberty, one primary ovocyte per month completes the first cell division to yield a secondary ovocyte. The polar body produced will degnerate. Oogensis from this point on occurs in the follicle. If the second ovocyte is fertilized, the second cell division occurs. This second meiotic division yields an ovum and a second polar body which degenerates. The fertilized ovum contains 23 chromosomes. Upon fertilization, the 23 chromosomes in the spermatozoa combine with the 23 chromosomes of the ovum to form a zygote which possesses 46 chromosomes. ← ← Differences between oogenesis and spermatogenesis ← ← The first is the fact that there is a limited window of fertility and that oogenesis displays asymmetric cell division. The third difference is gamete supply. The number of gametes decreases rapidly from birth to puberty. Not every ovocyte reaches maturation to be ovulated. By menopause, only a few thousand ovocytes remain. Therefore, the number of gametes is finite, the ovocytes are not continuously regenerated after puberty. Menopause occurs when there are no more fertile ovocytes remaining. ← ← Menstrual cycle ← ← The menstrual cycle is characterized by cyclic changes in ovarian structure and function which is referred to as the ovarian cycle. It is also characterized by cyclic changes in uterine structure and function which is referred to as the uterine cycle. The uterine cycle is equally characterized as changes in the secretion of ovarian, hypothalamic and pituitary hormones. Each of the phases in the cycles represents changes in the production of hormones that are intimately connected to simultenous changes in the morphology of the ovaries in the uterus. Both ovarian and uterine cycles are 28 days long and occur at the same time. ← ← Ovarian Cycle ← ← The ovarian cycle has two phases: follicular phase and luteal phase. ← • Follicular phase - The follicular phase describes the development of the follicle. This follicular development has 4 stages. At the beginning of the follicular phase in stage 1, about 10 to 25 follicles are recruited to develop under the influence of rising plasma levels of FSH. At this stage, the follicles are simply comprised of a single layer of epithelial cells called granulosis cells which surround a developing ovocyte. Together with the FSH, granulosis cells support the process of oogenesis. As these follicles continue to develop, the granulosis cells proliferate into multiple layers. The outer layer eventually differenciates into theca cells. Theca cells secrete androgens which are converted into estrogen by granulosis cells. After about a week, one of these follicles is selected to develop to full maturity while the remaining follicles undergo degeneration. The dominant follicle is the follicle capable of secreting the most estrogen. Therefore, the developing follicle is the source of increasing amounts of estrogen and will continue to increase in size as it differenciates into the Graagian follicle. Finally, ovulation is triggered in stage 5 at mid cycle by a huge surge in lutenizing hormone secreted by the anterior pituitary. • Luteal phase - After ovulation, the ruptured follicle is then transformed into the corpus luteum. The corpus luteum will mature, actively secreting progesterone and estrogen for several days. If fertilization does not occur, the corpus luteum will degenerate, known as corpus albicans. Through this degenerative process, the secretion of progesterone and estrogen quickly tapers off to virtually nothing. If fertilization occurs, the corpus luteum persists until the end of the first trimester of pregnancy, actively secreting large amounts of progesterone and estrogen throughout this period. ← Uterine cycle The uterine cycle has three phases: the menstrual phase, the proliferative phase and the secretory phase. These are the three levels of change in the development of the endometrial tissue and its blood supply. These changes are brought about by variations of hormone levels produced by the developing follicle o
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