Menstrual Cycle - The pain of anticipation
There is apparently no greater joy than the joy of childbirth and women are exclusively privy to this joy. But to experience this once (or more) in a lifetime experience, their bodies prepare themselves for an anticipated pregnancy, every month from menarche to menopause, putting themselves and to an extent, the people around them through a lot! In this article we are going to understand the basics of the female reproductive physiology as part of the gynecology series.
The ovaries, fallopian tubes, uterus, and the vagina form the reproductive apparatus in the female body. The ovaries are responsible for the production of ova and hormones in a cyclical fashion; this hormonal response is controlled by the hypothalamus and the pituitary gland.
The ovaries are not the only organs to respond to these cyclical changes in hormone levels in circulation. The uterus, specifically the endometrium, also responds to these hormones, and so presumably, we have cyclical ovarian changes and cyclical endometrial changes in response to the varying levels of the hormones.
Gonadotropin releasing hormone (GnRH), released from the hypothalamus is responsible for the release of 2 hormones from the pituitary gland; the Follicular Stimulating Hormone (FSH) and the Leutinizing Hormone (LH). These two hormones are the major players in the female reproductive physiology.
The menstrual cycle, is divided into two parts by the process of ovulation:
- The proliferative or follicular phase
- The secretory phase or luteal phase
Though variable, the average duration of a normal menstrual cycle is 28 days; the first day calculated from the first day of bleeding. Day-5 to day-14 is the proliferative phase; the secretory phase is almost always 14 days long, and ovulation occurs on day-14 in a 28 day cycle.
During the proliferative phase, under the influence of the FSH, the follicles in the ovary start to grow and mature; these maturing follicles releases Estrogen (from the granulosa cells), a steroidal hormone that acts on the endometrium and causes it to proliferate. This is necessary, since the endometrial glands would have sloughed off as a consequence of the previous cycle. Among these maturing follicles, one follicle grows bigger than the others; this is the Graffian Follicle, which houses the ovum that eventually gets released.
At the time of ovulation, there is a sudden increase in the LH levels; this is called “the LH surge”; at this point, the follicle ruptures and releases the ovum into the abdominal cavity. This is accompanied by some bleeding which causes peritoneal irritation and hence the mid-cycle pain (called mittelshmerz) and ovum from the abdominal cavity is picked up by the fimbriae of the fallopian tube.
Once the oocyte gets released, the remaining part of the follicle, starts developing a yellow pigment, lutein, under the influence of LH from the pituitary. This corpus luteum, secretes progesterone, another steroidal hormone, which acts on the endometrium and stabilizes it; makes it spongy and richly vascular for the anticipated implantation of the fertilized ovum, the zygote. This part of the cycle is called the secretory phase, since the endometrial glands start their secretory activity. The corpus luteum secretes the progesterone required for maintaining a pregnancy (if in case it occurs), till the placenta takes over sometime around the 9th week of pregnancy.
If the implantation doesn’t happen, the corpus luteum regresses; this causes a withdrawal of all the hormones, leading a loss of support to the endometrium, which gets shed off due to ischemic necrosis, leading to the passing of menses.
During the secretory phase, the endometrial blood supply comes from highly convoluted blood vessels called spiral arteries. When the hormones get withdrawn, these arteries undergo vasospasm which aids in the necrosis and shedding of the endometrium; this vasospasm is mediated by prostaglandins, which is the culprit behind the terrible cramps associated with the process of menstruation.
Author: Anirudh Murali (Facebook)
Sources and citations
Barman, Susan M., et al. “Chapter-22: Reproductive Development & Function of the Female Reproductive System.” Ganong's Review of Medical Physiology, by Kim E. Barrett, 24th ed., McGraw Hill- Lang publications, 2012, pp. 401–404.