Anatomy and Physiology



Hormonal Control of the Menstrual CycleFor a girl, entering puberty not only signifies that she is a young woman, it also signals a dramatic biochemical change in her body heralded by the commencement of the menstrual cycle. In this tough topics section we will discuss the changes that occur and the hormonal regulation that will control a woman's reproductive cycles from the onset of menstruation until she enters menopause about 40 years later. This section will begin with an overview of the reproductive organs affected by or affecting the menstrual cycle. We'll then discuss the female reproductive cycles: the ovarian cycle and the uterine or menstrual cycle. We'll talk about how hormones regulate these cycles. Finally, we will end this topic discussing the chronological order of these events as they occur over the course of a month.The female reproductive system consists of several organs, including a pair of almond-sized ovaries, the uterine tubes, the uterus, and the cervix.The ovaries give rise to the oocytes, cells from which eggs or ova are derived. The ova have the potential to become embryos. They also produce the hormones that determine the secondary sex characteristics, maintain the reproductive system, and determine whether a woman is in the ovarian phase or the menstrual phase of her cycle.The ovaries are surrounded externally by germinal epithelial cells, which are a continuation of the pelvic wall. The ovary has two tissue compartments, the outer cortex, which houses the follicles, and the inner medullary region, which contains large blood vessels and nerves. Let's focus this discussion of the ovaries on the outer cortex and its contents.The cortex holds developing gametes within the tiny saclike structures called ovarian follicles. The follicles contain oocytes, surrounded by one or two layers of cells. The cells immediately around the oocytes are called follicle cells. If a second layer is present on top of the follicle cells, these cells are called granulosa cells.It is changes in the granulosa cells and surrounding spaces that determine the maturation stage of the ovum. A layer of thecal cells forms a connective tissue that surrounds the follicle. The thecal cells are important because they work with the granulosa cells to produce estrogens.When the cells surrounding the ovum have reached the mature Graafian follicle stage, the ovum bursts out of the follicle into the duct system of the female reproductive tract. This event is called ovulation. The glandular structure left behind is the remains of the follicle and is now called the corpus luteum. If no fertilization and implantation occurs, the corpus luteum eventually degenerates and the whole cycle begins again.At ovulation, the ovum is ejected into the uterine tubes, also called the fallopian tubes. These tubes contain smooth muscle cells and ciliated mucosa cells arranged into pleats. As the ovum travels down the uterine tubes, cilia and peristaltic movements of the tube walls propel it forward. From here the ovum travels down to the uterus.The uterus is a hollow organ with thick walls. In premenopausal women who have never been pregnant, the uterus is about the size of a pear. The functions of the uterus include receiving, protecting, and nourishing the fertilized ovum.The ovarian cycle is the monthly series of events that culminates in the maturation of the oocyte into the ovum. The ovarian cycle can be divided into two phases, the follicular phase and the luteal phase. Let's begin our tour of the ovarian cycle at Day 1 of the follicular phase. During the follicular phase, the follicle is growing and maturing so that it can eject an egg. This is marked by six major stages.First, the primordial follicle, which will give rise to the egg, is activated. The primordial follicle is an oocyte surrounded by a single layer of squamous follicular cells. When the follicular cells become rounded or cuboidal and begin to undergo mitosis, the primordial follicle becomes a primary follicle. The primary follicle is the follicle with the egg. This transition is stage number two. The third major stage is the proliferation of follicular cells around the ovum.As these follicular cells continue to multiply, they begin to pile up in multiple layers. It is after this stratification that they are then called granulosa cells. In the fourth stage, thecal cells begin to grow around the follicle. Stage five involves the accumulation of clear fluid between the granulosa cells that forms the antrum. In stage six, the corona radiata, composed of the innermost ring of granulosa cells, forms around the ovum as it prepares for ovulation. This phase lasts about 14 days. Ovulation marks the end of the follicular phase. The signal that initiates the start of the luteal phase is a spike in LH levels.The luteal phase is marked by activity of the corpus luteum. This phase always lasts 14 days. During the luteal phase, the ruptured portion of the follicle fills with blood and is eventually reabsorbed. The cells that made up the follicle increase in size and form an endocrine gland: the corpus luteum. The corpus luteum secretes progesterone and some estrogen. If fertilization were to occur, the corpus luteum would maintain the pregnancy until the placenta has developed and can assume its role as the hormone regulator, towards the end of the third month of pregnancy. However, if, after about 10 days pregnancy has not occurred, the corpus luteum degenerates and its hormone output ceases.In order to fully understand the ovarian cycle, we must understand the hormones involved in female reproduction. Let's discuss them now.The ovarian cycle is controlled by the presence and interactions of several hormones: Gonadotropin-releasing hormone, or GNRH; Follicle stimulating hormone or FSH, Luteinizing hormone or LH; estrogen; and progesterone. GNRH is secreted by the hypothalamus. It stimulates the release of FSH and LH from the pituitary gland. FSH and LH stimulate the ovaries to produce estrogen and progesterone on a cyclical basis.Now let's look specifically at how hormones interact during the ovarian cycle.At the start of the ovarian cycle, which is the follicular phase on Day 1, GNRH stimulates increased production of FSH and LH from the anterior pituitary. The increased levels of circulating FSH and LH stimulate many follicles to begin the oocyte maturation process. However, only one will become the dominant follicle, and develop fully and ovulate. FSH targets the follicle cells, while LH initially targets the thecal cells. During development of the follicle, LH stimulates the thecal cells to produce androgens, which are precursor steroid hormones. The granulosa cells in the follicle convert these precursor hormones to estrogen. The increase in estrogen concentration acts in a negative feedback loop to turn off FSH and LH.Before ovulation on day fourteen, these rising estrogen and progesterone levels cause a negative feedback loop suppressing the release of LH and FSH from the anterior pituitary.Around day fourteen, estrogen concentrations in the blood increase until a critical concentration initiates a positive feedback loop. This positive feedback loop causes a huge pulse of LH. This surge of LH causes the primary oocyte to undergo meiosis and ovulate within five minutes of being released, transitioning from the follicular phase to the luteal phase of the ovarian cycle. The luteal phase will continue from day fourteen to day twenty-eight. LH also transforms the corpus luteum into an endocrine gland, which secretes both progesterone and estrogen. The corpus luteum releases another hormone called inhibin, which functions to help inhibit FSH and LH secretion. Inhibition of FSH and LH are defense mechanisms against development of other follicles and ovulation of multiple ova while the fate of the released ovum has yet to be determined. Very shortly after ovulation, the estrogen levels drop dramatically, most likely because the estrogen-producing follicle has burst to release the ovum.With the declining levels of LH in the blood, luteal activity stops and the corpus luteum shrinks around day twenty-eight. This causes a decline in the levels of ovarian hormones as well as blood levels of estrogen and progesterone. These declining levels cause the inhibition of LH and FSH to be lifted. With the new surge of LH and FSH, the cycle begins again with the maturation of a new batch of follicles.Before we go onto the menstrual cycle, let's summarize what we have discussed so far. The ovaries are both producers of and direct recipients of the effects of hormones. The uterus, too, is directly affected by estrogen and progesterone. The ovaries are sensitive to LH and FSH secreted by the anterior pituitary. Within the ovary, FSH and estrogen are involved in maturing the developing follicle. During this time the amount of estrogen accumulates to such a high concentration in the blood that it initiates a positive feedback loop that climaxes in a huge surge of LH release from the anterior pituitary, initiating ovulation. After ovulation, the surge in LH transforms the ruptured follicle into the corpus luteum, signaling the onset of the fourteen day luteal phase. Once formed, the corpus luteum produces progesterone and estrogen. Also after ovulation, estrogen levels decline rapidly, reducing the blood concentration to a point at which a negative feedback loop can re-establish itself and turn off further LH and FSH production. If no pregnancy occurs, the corpus luteum degenerates and estrogen and progesterone production decline. This causes a new ovarian cycle to begin with the release of FSH and LH from the anterior pituitary. Negative feedback on the hypothalamus also ends, and GnRH signals release of a new round of FSH and LH as well.Now that we have discussed the ovaries and the events surrounding egg production and release, let's turn our attention to the changes happening concurrently during the menstrual cycle, also called the uterine cycle because these changes are associated with the uterus.Implantation and eventual development of the embryo occurs in the endometrium, the lining of the uterus. However, the uterus is only receptive to implantation for a short time each month. When it is receptive is dependent on the uterine cycle, a series of changes that result from fluctuating concentrations of estrogen and progesterone. These changes correspond to development of the ovum in the ovarian follicle.The uterine cycle consists of three phases: the menstrual phase, the proliferative phase, and the secretory phase. Let's start with the first phase: the menstrual phase, which corresponds to Day one of the ovarian phase. It is during this phase that menstruation occurs. Menstruation is the sloughing off of the superficial portion of the endometrium. As this thick layer sloughs off, it passes through the vagina and produces the characteristic vaginal bleeding of menstruation. The menstrual phase corresponds to the lowest point in the concentrations of estrogen and progesterone. At this time, FSH starts to rise, initiating the production of estrogen by the developing follicle.The phase that follows menstruation is the proliferation phase, also called the preovulatory phase. During this time, estrogen levels increase, signaling the basal layer of the endometrium to rebuild itself by generating a new functional layer. In addition, the estrogen stimulates progesterone receptor production so that the endometrium will be sensitive to progesterone later on. As the endometrium increases in size, the arteries increase in number and its glands enlarge as well. Simultaneously, in preparation for sperm entry and potential fertilization, during the proliferation phase the mucous layer of the cervix becomes thin and forms channels to facilitate sperm entry into the uterus. Ovulation occurs at the end of the proliferation stage.The secretory, or postovulatory, phase follows the proliferation phase. During the secretory phase, the endometrium prepares for implantation of an embryo. Progesterone levels rise. The corpus luteum primarily secretes progesterone, which causes the arteries to get bigger and branch out while also changing the functional endometrium to become thicker in order to provide a substantial, nutrient-rich bed for the potential embryo. Increasing progesterone levels also cause the cervical mucus to become viscous and form a cervical plug, which blocks sperm entry and seals the uterus from the vagina. At the same time, rising progesterone and estrogen levels inhibit LH release by the anterior pituitary.If fertilization does not occur, the corpus luteum degenerates towards the end of the secretory phase as LH levels fall. This in turn causes a decrease in progesterone levels. Because of the lack of hormonal support to the endometrium, the arteries spasm and disintegrate. Lack of nutrients causes the endometrial cells to die, initiating menstruation.Okay, let's summarize our tough topic discussion by comparing the concurrent events that occur during the ovarian phase and the menstrual phase over the course of the month due to hormonal regulation.What is happening in the first two weeks of the month? In the ovarian cycle, this fourteen day period corresponds to the follicular phase, the time of the developing ovum. During the same fourteen days, the uterine cycle undergoes two phases: the menstrual phase, which lasts about five days, and the proliferative phase, which lasts nine to ten days. At the end of this 14 day period, ovulation occurs.Hormonally, during the first 5 to 7 days of the cycle, estrogen and progesterone levels are at their lowest and LH and FSH levels begin to rise. By day 14 at ovulation, estrogen is at its highest concentration – eliciting a positive feedback loop that causes a surge in LH concentrations. FSH levels are also at their highest, but still much lower compared to LH, and progesterone remains at basal levels.What changes occur during the last two weeks of the month? The ovarian cycle is in the luteal phase. During this time the corpus luteum is a functional endocrine gland secreting high concentrations of progesterone and sustaining moderately high concentrations of estrogen. The hormones secreted by the corpus luteum cause the secretory phase to occur in the uterine cycle. At this time the endometrial wall becomes thicker and highly vascularized in preparation for implantation of an embryo. The increased levels of estrogen and progesterone also inhibit LH and FSH, which fall to their lowest levels in preparation for the beginning of a new cycle.That's the end of this section. ................
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