The Menstrual Cycle Flashcards
What are the two key functions of the ovary?
1) Generation of a fertilizable oocyte with full competence
2) Secretion of steroid hormones required for preparation of the reproductive tract for fertilization and subsequent establishment of pregnancy
Describe what is seen in this ovarian cortex
There are four main types of follicles:
Premordial follicles- consisting of a small primary oocyte with very little cytoplasm surrounded by flat granulosa cells
Primary follicles- which are the next step formed from the premordial follicles (via lack of AMH inhibition from other secondary follicles) and still consist of a single oocyte and a layer of cuboidal granulosa cells, but the cytoplasm is larger
Secondary follicles- more granulosa layers (from 2-100 layers)(the main producer of AMH to feedback inhibit on premordial follicles)
Tertiary follicles- marked by accumulation of fluid as more estrogen is made by the oocyte (begin to produce more estradiol than AMH)
What happens once all premordial follicles are depleted?
Menopause
Describe the development of the dominant follicle during a menstrual cycle
The primary role of the follicle is oocyte support. From birth, the ovaries of the human female contain a number of immature, primordial follicles. These follicles each contain a similarly immature primary oocyte. After puberty and commencing with the first menstruation, a clutch of follicles begins folliculogenesis, entering a growth pattern that will end in death or in ovulation (the process where the oocyte leaves the follicle).
During post-pubescent follicular development, and over the course of roughly a year, primordial follicles that have begun development undergo a series of critical changes in character, both histologically and hormonally. Two-thirds of the way through this process, the follicles have transitioned to tertiary, or antral, follicles. At this stage in development, they become dependent on hormones emanating from the host body, causing a substantial increase in their growth rate.
With a little more than ten days until the end of the period of follicular development, most of the original group of follicles have died (a process known as atresia). The remaining cohort of follicles enter the menstrual cycle, competing with each other until only one follicle is left. This remaining follicle, the late tertiary or pre-ovulatory follicle, ruptures and discharges the oocyte (that has since grown into a secondary oocyte), ending folliculogenesis.
What are the two key functions of the endometrium?
- Cyclic regeneration and secretion of substances that foster fertilization and implanation
- Development of vascular support and immunitary protection for establishment and progression of pregnancy
What causes the endometrium to grow during the follicular phase?
FSH stimulates the follicular growth and more estrogen production which causes endometrium growth until ovulation.
After ovulation, the anterior pituitary begins producing more LH which promotes progesterone production for the luteal phase of the menstrual cycle
What causes the variation in the menstrual cycle in women?
While the luteal phase is typically fixed at a 14 day length, the follicular phase varies from 14-21 days and accounts for the change
What happens to the menstrual cycle around age 40?
There begins to be less and less follicles so estrogen is decreased and the brain senses this and tells the pituitary to produce more FSH causing ovulation to occur more quickly and cycles to be shorter
FSH and LH levels skyrocket in peri/post-menopausal women!
What happens after a few years of shorter cycles?
The ovaries begin to decrease less follicles and responds less and less to FSH leading to longer cycles and eventually the disappearance of periods
Give an overview of the follicular phase of menstruation
At puberty, the follicular phase begins with GnRH being released from the hypothalamus due to release of negative feedback effects of progesterone from the previous luteal phase and causes secretion of FSH that stimulates folliculogenesis and estradiol production, leading to progressive growth of the follicles selected during that cycle.
This increase in estradiol production slowly rises then feeds back negatively on the hypothalamus, resulting in suppression of FSH (and LH). Similarly, tertary follicles secrete Inhibin B which inhibits FSH production. Thus, FSH levels fall as the follicle matures (after about 7-8 days). Before FSH levels drop, a dominant follicle is selected via FSH upregulating LH receptors on its surface and AMH causing agenesis of all others.
Near ovulation, there is an abrupt shift from negative feedback to positive feedback of estrogen (near at least 200pg for 2 days to promote LH surge), causing a surge of LH (and FSH) that stimulates ovulation. The frequency of LH pulses continues to be approx. 1/hr, but the amplitude of the LH pulses increases dramatically
How would the follicular phase be different in older women?
She will have lower production of Inhibin B during the follicular phase due a lower number of follicles, as well as estrogen being produced at a much faster pace
What is Day 1 of the menstrual cycle?
The first day of menses, when the follicular phase begins again and FSH begins to rise (until day 7-8 usually)
What does the surge in LH cause in the oocyte?
The oocyte, which is tetraploid (4n, 96), will be stimulated to finish meiosis I and lose the polar body before arresting again in metaphase of meiosis II before ovulation
What happens in the luteal phase of menstruation?
Progesterone conc. rise in the middle to late luteal phase and LH pulses slow to 1 pulse every 4 hrs and pulses of progesterone occur soon after these. Progesterone works to stabilize the endormetrium in case pregnancy occurs
T or F. Inhibin A levels peak in the mid-luteal phase
T. While Inhibin B secretion is virtually absent during the luteal phase
Serum leptin conc are highest in the ______ phase
luteal
What causes Menses?
The decline in estradiol and progesterone release from the corpus luteum results in the loss of endometrial blood supply, endometrial sloughing, and the onset of menese approx. 14 days after ovulation.
In response to falling corpus luteum steroud production, the HPO axis resets and FSH levels begin to rise again
Late tertiary phase
At this point, the majority of the group of follicles that started growth 360 days ago have already died. This process of follicle death is known as atresia, and it is characterized by radical apoptosis of all constituent cells and the oocyte. Although it is not known what causes atresia, the presence of high concentrations of FSH has been shown to prevent it.
A rise in pituitary FSH caused by the disintegration of the corpus luteum at the conclusion of the previous menstrual cycle (lack of neg feedback) precipitates the selection of five to seven class 5 follicles to participate in the next. The selected follicles, called antral follicles, compete with each other for growth-inducing FSH.
In response to the rise of FSH, the antral follicles begin to secrete estrogen and inhibin, which have a negative feedback effect on FSH. Follicles that have fewer FSH-receptors will not be able to develop further; they will show retardation of their growth rate and become atretic. Eventually, only one follicle will be viable. This remaining follicle, called the dominant follicle, will grow quickly and dramatically—up to 20 mm in diameter—to become the preovulatory follicle.
Note: Many sources misrepresent the pace of follicle growth, some even suggesting that it takes only fourteen days for a primordial follicle to become preovulatory. Actually, the follicular phase of the menstrual cycle means the time between selection of a tertiary follicle and its subsequent growth into a preovulatory follicle.The actual time for development of a follicle is varied among cases.
