"While the language may be lovely and the reasoning just, the ideas themselves may prove trivial." -Lu Chi (from Wen Fu)

Writing Prizes:
Winning Essays

Three Generations Prize for Writing in the Sciences

"Hormonal Control of the Menstrual Cycle"
Bronwen E. Bromberger '01

Introduction

S.C. Yen, Professor of Reproductive Endocrinology at the University of California, San Diego, defines the menstrual cycle as "a repetitive expression of the interaction of the hypothalamic -pituitary-ovarian system with associated structural and functional changes in the target tissues ... of the reproductive tract" (1). In this approximately 28-day cycle, which takes place solely in primates, the hypothalamus, pituitary, and ovaries participate in a defined cycle of hormonal release that causes an egg to mature and prepares the lining of the uterus to receive the egg should it be fertilized. With multiple hormones acting over both long and short ranges to cause finely-regulated effects, it is essential that sensitive control mechanisms act to ensure that all of the steps in the cycle occur in concert. Several hypotheses concerning loci of control have been developed over the past twenty years that begin to paint a picture of the menstrual cycle as a process that integrates both neurological and endocrine control, as well as feedback from the body about metabolic considerations such as nutritional state. The menstrual cycle encompasses the complete maturation of a follicle, ovulation, and the maintenance of the uterine lining for a predetermined amount of time before it is shed. Because it lies as an easily-definable point in the cycle, the onset of menstruation is conventionally defined as Day 1. It is at about this time that the follicle that eventually will yield the egg and produce the ovarian hormones for the upcoming cycle enters its final stage of development. The follicles begin their growth from resting cells more than 60 days before the beginning of the cycles in which they are used, meaning that several cycles' worth of eggs are continuously maturing in the ovary at any one time (2). The first phase of the cycle, called the follicular phase, is marked by the maturation of a group of follicles and the selection of one which will eventually provide the egg and in which the majority of the ovaries' hormonal biosynthesis for the cycle will take place. Around Day 14, the mature follicle releases an egg, leaving behind the cells that will become the corpus luteum, which will release a different complement of hormones for the remainder of the cycle. If fertilization and implantation successfully occur, the fertilized egg releases human chorionic gonadotropin, a hormone that maintains production of estrogen and progesterone by the corpus luteum throughout the beginning of pregnancy. Progesterone is responsible for the maintenance of the uterine lining. Without pregnancy, apoptosis of the corpus luteum cells around 14 days after ovulation results in a drop ii circulating level of progesterone and the endometrial lining is shed during menstruation. This paper outlines the signals that coordinate these dramatic events. As stated above, the hormones that control the menstrual cycle are synthesized at three main sites: the hypothalamus and the pituitary in the brain, and the developing follicles and later the corpus luteum of the ovary. Gonadotropin releasing hormone (GnRH) is the product the hypothalamic neurons, while the pituitary responds to GnRH by producing follic-stimulating hormone (FSH) and luteinizing hormone (LH), two protein hormones also known as the gonadotropins. During the follicular phase, the gonadotropins stimulate ovarian follicle maturation and the concomitant release of estrogen. FSH is primarily responsible for the maturation of granulosa cells in the follicle and their development of receptors for LH and human chorionic gonadotropin. LH affects the theca cells, stimulating them to produce androgens, the substrates for estrogen production in the granulosa cells. When these granulosa cells' estrogen synthesis becomes sufficient raise estrogen levels in the bloodstream to a concentration of between 300-500 pg/rf,, and maintain it for approximately 48 hours, the high estrogen level signifies that the follicle is mature. At this point, positive feedback by estrogen causes the levels of If and LH surge to more than four times their previous concentrations. This surge of pituitary hormones initiates ovulation within 35-44 hours (1), leaving behind the theca and granulosa cells of the follicle. These become the corpus luteum, which acts as major source of ovarian hormones for the luteal phase (Days 14 through 28) of the These products of the corpus luteum include progesterone, estrogen, and the glycor hormone inhibin. High progesterone and estrogen levels during the middle of the h phase cause changes in the endometrium that facilitate the implantation of a fertilized egg. As stated above, without implantation, the corpus luteum expires two weeks a ovulation, the levels of the ovarian hormones decrease, and the cycle repeats itself.

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• Lynne Viti and Wini Wood
• Wellesley College Writing Program
• Created by: Jerina Hajno '04 and Nora Jarrah '02
• Date Created: June 29, 2000
• Last Modified: August 4, 2008
• Expires: July 31, 2009

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