Ovulation & Uterine Preparation
Ovarian Cycle = Process of follicular growth and selection for ovulation.
Key events prior to ovulation
1.
Primordial follicle
Comprises a single oocyte encircled by flat granulosa cells; the ovary comprises hundreds of these units at any given time. Some of these primordial follicles will enter meiosis and become primary follicles.
2. Primary follicle
Primary oocyte, zona pellucida, and single layer of cuboidal granulosa cells.
3. Secondary follicle
Several layers of granulosa cells surround the primary oocyte in a secondary follicle; thecal cells begin to differentiate and lie at the periphery of the follicle.
4. Follicle-stimulating hormone (FSH)
Released by the anterior pituitary gland; facilitates transition of secondary follicles to the next stage
5. Early tertiary stage (aka, antral stage)
Antrum forms surrounding large primary oocyte. As the follicle grows, it produces increasing quantities of estrogen.
6. Estrogen
Above a given threshold, estrogen triggers the release of luteinizing hormone (LH), from the
anterior pituitary gland.
7. LH "surge"
Induces morphological and physiological changes in preparation for ovulation, including completion of meiosis I and entry into meiosis II.
8. Late tertiary follicle
Secondary oocyte and its polar body are the products of meiosis I.
Granulosa cells form the cumulus oophorus; this "cloud" of cells surrounding the oocyte travels with the oocyte after ovulation.
The corona radiata comprises the granulosa cells immediately surrounding the zona pellucida.
Two layers of thecal cells: the internal layer, which has endocrine functions, and, the external layer, which forms the follicular capsule.
9. Ovulation
The oocyte complex, which comprises the oocyte and cumulus cells, leaves the ovary and enters the
uterine tube, where fertilization may occur.
The follicular cells that remain in the ovary transform to become the corpus luteum ("yellow body").
The corpus luteum is a temporary endocrine gland: it produces progesterone, which acts on the endometrial lining to prepare it for implantation in the case of fertilization.
Uterine Cycle
The uterine cycle = Process of
uterine preparation for implantation in the event of fertilization.
The cycle is marked by two key events:
- Menstruation, during which the top layer of the endometrium is shed.
- Ovulation, which is when the oocyte complex exits the ovary.
1. Begins with onset of menstruation.
2. As FSH induces growth of the ovarian follicles, they produce increasing quantities of estrogen.
3. In turn, estrogen promotes the growth, aka, proliferation, of the endometrium, specifically, of the stratum functionalis (functional layer).
Thus, this phase of high estrogen concentrations and endometrial growth following menstruation is the proliferative phase (notice that it correlates with the follicular phase of the ovarian cycle).
4. Increasing levels of estrogen trigger the LH surge that induces ovulation.
5. After ovulation, the corpus luteum secretes progesterone:
Thus, the secretory phase of the uterine cycle begins (which correlates with the luteal phase of the ovarian cycle).
In this phase, progesterone promotes the differentiation and decidualization of the stratum functionalis so that it becomes rich in glands and vasculature.
6. At this point, we must consider the fate of the oocyte:
If fertilized, it will become a zygote, then an embryo.
If not fertilized, the oocyte will degenerate within ~24 hours of ovulation.
The "fertile window" refers to the days of the menstrual cycle when intercourse is most likely to lead to conception.
Research indicates that the window comprises the 5 days before and the day of ovulation; however, the timing of ovulation is highly unpredictable, as it varies greatly even in women with regular menstrual cycles.
Thus, family planning methods that attempt to predict the fertile window have high failure rates (~24%, according to the CDC*).
7. Regardless of its fertilization status, the oocyte (or embryo) reaches the uterine cavity in 3-4 days.
8. Again, its fate determines the events of the uterine cycle:
If an embryo successfully implants in the endometrial lining, the corpus luteum persists to produce progesterone, which is necessary for endometrial maintenance (until the
placenta can take over this role).
If
implantation does not occur, the corpus luteum regresses; it becomes the corpus albicans ("white body") and progesterone levels fall sharply.
9. In the absence of progesterone, the stratum functionalis breaks down and is shed during menstruation, which marks the beginning of the next menstrual cycle.
Additional Resources:
https://www.cdc.gov/reproductivehealth/contraception/unintendedpregnancy/pdf/contraceptive_methods_508.pdf
https://embryology.med.unsw.edu.au/embryology/index.php/Menstrual_Cycle#cite_note-PMID18440526-1
