- Reproductive System
Female reproductive cycles are monthly cycles which occur after puberty to prepare the reproductive system for pregnancy.
The female reproductive cycles involve changes in:
- pituitary gland,
- uterine tubes,
- mammary glands
Hormones involved in female reproductive cycles
Neurosecretory cells in the hypothalamus synthesize Gonadotropin-releasing hormone (GRH) and are carried by the hypophyseal portal system to the anterior lobe of the pituitary gland. This hormone stimulates the release of two hormones:
Follicle-stimulating hormone (FSH) stimulates the development of ovarian follicles and the production of estrogen by the follicular cells.
Luteinizing hormone (LH) serves as the "trigger" for ovulation (release of secondary oocyte) and stimulates the follicular cells and corpus luteum to produce progesterone.
Female reproductive cycles entail includes ovarian cycle. Here the follicle-stimulating hormone and luteinizing hormone produce cyclic changes in the ovaries-the ovarian cycle-development of follicles, ovulation, and corpus luteum formation.
During each cycle, FSH promotes the growth of several primordial follicles into 5 to 12 primary follicles; however, only one primary follicle usually develops into a mature follicle and ruptures through the surface of the ovary, expelling its oocyte.
Development of an ovarian follicle is characterized by:
- Growth and differentiation of primary oocyte
- The proliferation of follicular cells
- Formation of zona pellucida
- Development of the theca follicular
As the primary follicle increases in size, the adjacent connective tissue organizes into a capsule known as theca folliculi. The theca soon differentiates into two layers, an internal vascular and glandular layer, the theca interna, and a capsule-like layer, the theca externa.
Thecal cells are thought to produce an angiogenesis factor that promotes the growth of blood vessels in the theca interna, which provide nutritive support for follicular development.
The follicular cells divide actively, producing a stratified layer around the oocyte.
The ovarian follicle soon becomes oval and the oocyte eccentric in position. Subsequently, fluid-filled spaces appear around the follicular cells, which coalesce to form a single large cavity known as the antrum, which contains follicular fluid. After the antrum forms, the ovarian follicle is called a vesicular or secondary follicle.
The primary oocyte is pushed to one side of the follicle, where it is surrounded by a mound of follicular cells, the cumulus oophorus, that projects into the antrum.
The follicle continues to enlarge until it reaches maturity and produces a swelling on the surface of the ovary. The early development of ovarian follicles is induced by FSH, but the final stages of maturation require LH as well.
Growing follicles produce estrogen, a hormone that regulates the development and function of the reproductive organs.
The vascular theca interna produces:
- follicular fluid.
- Estrogen and
- androgens that are converted by follicular cells into estrogen.
Some estrogen is also produced by widely scattered groups of stromal secretory cells, known collectively as the interstitial gland of the ovary.
Around midcycle, the ovarian follicle, under the influence of FSH and LH, undergoes a sudden growth spurt, producing a cystic swelling or bulge on the surface of the ovary. A small avascular spot, the stigma, soon appears on this swelling.
Before ovulation, the secondary oocyte and some cells of the cumulus oophorus detach from the interior of the distended follicle.
Ovulation is triggered by a surge of LH production and usually follows the LH peak by 12 to 24 hours.
The LH surge, elicited by the high estrogen level in the blood, appears to cause the stigma to balloon out, forming a vesicle. The stigma soon ruptures, expelling the secondary oocyte with the follicular fluid.
Expulsion of the oocyte is the result of intrafollicular pressure and possibly a contraction of smooth muscle in the theca externa owing to stimulation by prostaglandins.
Enzymatic digestion of the follicular wall seems to be one of the principal mechanisms leading to ovulation.
The expelled secondary oocyte is surrounded by the zona pellucida and one or more layers of follicular cells, which are radially arranged as the corona radiata, forming the oocyte-cumulus complex.
The LH surge also seems to induce resumption of the first meiotic division of the primary oocyte. Hence, mature ovarian follicles contain secondary oocytes.
The zona pellucida is composed of three glycoproteins (ZPA, ZPB, ZPC), which usually form a network of filaments with multiple pores.
Binding of the sperm to the zona pellucida (sperm-oocyte interactions) is a complex and critical event during fertilization.
Mittelschmerz and ovulation
A variable amount of abdominal pain, mittelschmerz (German, mittel, mid + schmerz, pain), accompanies ovulation in some women.
In these cases, ovulation results in slight bleeding into the peritoneal cavity, which results in sudden constant pain in the lower abdomen.
Mittelschmerz may be used as a symptom of ovulation, but there are better symptoms, such as a slight drop in basal body temperature.
Other clinical symptoms
- Pains in the abdomen
- Pain in the vagina
- Feeling nauseous and generally unwell
Some women do not ovulate (cessation of ovulation-anovulation) because of an inadequate release of gonadotropins. In some of these women, ovulation can be induced by the administration of gonadotropins or anovulatory agent such as clomiphene citrate.
This drug stimulates the release of pituitary gonadotropins (FSH and LH), resulting in the maturation of several ovarian follicles and multiple ovulations. The incidence of multiple pregnancies increases as much as tenfold when ovulation is induced. Spontaneous abortions occur because there is no chance that more than seven embryos can survive.
Shortly after ovulation, the walls of the ovarian follicle and theca folliculi collapse and are thrown into folds. Under LH influence, they develop into a glandular structure, the corpus luteum, which secretes progesterone and some estrogen, causing the endometrial glands to secrete and prepare the endometrium for implantation of the blastocyst.
If the oocyte is fertilized, the corpus luteum enlarges to form a corpus luteum of pregnancy and increases its hormone production.
Degeneration of the corpus luteum is prevented by human chorionic gonadotropin, a hormone secreted by the syncytiotrophoblast of the blastocyst.
The corpus luteum of pregnancy remains functionally active throughout the first 20 weeks of pregnancy. By this time, the placenta has assumed the production of the estrogen and progesterone that is necessary for the maintenance of pregnancy.
If the oocyte is not fertilized, the corpus luteum involutes and degenerates 10 to 12 days after ovulation. It is then called a corpus luteum of menstruation. The corpus luteum is subsequently transformed into white scar tissue in the ovary, a corpus Albicans. Except during pregnancy, ovarian cycles normally persist throughout the reproductive life of women and terminate at menopause.
The endocrine, somatic (body), and psychological changes occurring at the termination of the reproductive period are called the climacteric.
Another cycle in the female reproductive cycles is the menstrual (endometrial) cycle. This is the time during which the oocyte matures, is ovulated, and enters the uterine tube.
The hormones produced by the ovarian follicles and corpus luteum (estrogen and progesterone) produce cyclic changes in the endometrium.
These monthly changes in the internal layer of the uterus constitute the endometrial cycle, commonly referred to as the menstrual cycle or period because menstruation (flow of blood from the uterus) is an obvious event.
The endometrium is a "mirror" of the ovarian cycle because it responds in