The follicular phase is the phase of the estrus cycle, (or in humans and great apes, menstrual cycles) as long as the follicles in the ovaries mature. It ends with ovulation. The main hormone that controls this stage is estradiol. There are several steps in the follicle development process that occur in the follicular phase: Initiation, Recruitment and Progression, Pre-ovulation and Ovulation ripening. These processes occur in different areas of the body and will be described briefly below.
Video Follicular phase
Hormonal events
Recruitment of follicles
Follicle-stimulating hormone (FSH) is secreted by the anterior pituitary gland (Fig. 2). FSH secretion begins to increase in the last few days of the previous menstrual cycle, and is highest and most important during the first week of the follicular phase (Fig. 1). The increase in FSH levels recruits five to seven tertiary-grade ovarian follicles (this follicle stage is also known as the Graafian follicle or antral follicle) to enter the menstrual cycle. These follicles, which have grown for the better part of a year in a process known as folliculogenesis, compete against each other for dominance.
FSH induces the proliferation of granulosa cells in developing follicles, and expression of luteinizing hormone (LH) receptors in these granulosa cells (Fig. 1). Under the influence of FSH, aromatase and p450 enzymes are activated, causing granulosa cells to start secreting estrogen. This increase in estrogen levels stimulates the production of the gonadotropin-releasing hormone (GnRH), which increases the production of LH. LH induces androgen synthesis by thecalcal cells, stimulates the proliferation, differentiation, and secretion of follicular thecal cells and increases the expression of LH receptors on granulosa cells.
Throughout the follicular phase, increased levels of estrogen in the blood stimulate the growth of endometrium and uterine myometrium. It also causes endometrial cells to produce receptors for progesterone, which helps to enlarge the endometrium to respond to an increase in progesterone levels during the final proliferation phase and throughout the luteal phase.
Estrogen spike
Two or three days before the LH level begins to rise, usually on the seventh day of the cycle, one (or sometimes two) recruited follicles have emerged as dominant. Many endocrinologists believe that estrogen secretion in the dominant follicle has increased to a level where GnRH production is suppressed, which lowers LH and FSH levels. This slowing of LH and FSH production leads to atresia (death) of most of the recruited follicles, although the dominant follicle continues to mature. Estrogen levels will continue to rise for several days (on average, six days, but this varies greatly).
This high level of estrogen initiates the formation of a new layer of endometrium in the uterus, histologically identified as a proliferative endometrium. Crypt in the cervix is ​​also stimulated to produce fertile cervical mucus. This mucus reduces vaginal acidity, creating a more friendly environment for sperm. It also has a distinctive texture that helps guide sperm through the cervix and into the fallopian tubes, where they wait for ovulation. In addition, basal body temperature may be slightly lower under the influence of high estrogen levels.
LH surge and ovulation
The highest estrogen level just before the LH spike begins (Figure 1). Decreased short-term steroid hormone between early LH surges and ovulatory events can cause spotting or bleeding in the middle of the cycle. Under the influence of preovulatory LH surges, the first meiotic division of the oocyte is complete. The spike also initiates luteinization of thecal and granulosa cells. Ovulation usually occurs 30 (Â ± 2) hours after the start of LH surges (when LH is first detected in the urine).
Maps Follicular phase
Follicular wave
Previously, it was understood that a single follicle matures until it breaks to release the egg during ovulation. Recent findings into the menstrual cycle in mammals have found that 2 or more follicles may develop but only one of the fully mature follicles to release the egg. This follicular wave involves some spikes in the FSH level to initiate follicular development. A study has found that 68% of women tend to display two follicle waves before ovulation while the remaining woman has three waves.
References
Source of the article : Wikipedia
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