The follicular phase is the first phase of the ovarian cycle, which is characterized by the growth and maturation of the follicle.
The follicular phase, or, as it is also called, the estrogen phase, begins from the very first day of menstruation and lasts until the moment when one or several dominant follicles mature in the ovaries. The end of this phase is ovulation.
Its duration can be completely different. Most often, menstruation delays occur precisely due to the slow maturation of the follicle. The main factor that affects the duration of this process is the time it takes the body to reach the maximum level of estrogen.
According to the functional and morphological changes in the ovary, the monthly cycle can be divided into the following phases:
1) The follicular phase;
2) Ovulatory;
3) Luteal;
4) Menstrual.
The phase of the cycle that leads to ovulation is called follicular. Usually it takes only the first half of the cycle, but can sometimes last longer. The average cycle time is 28 days. Any deviations from this number are caused only by the duration of the follicular phase.
The follicular phase is characterized by a growing increase in synthesis and the release of inhibin and estradiol. Moreover, the concentration of these hormones is growing not only in the systemic circulation, but also in the follicular fluid. Somewhere 2 days before the onset of menstruation, the development of the follicle begins from its resting or primordial form.
Just at this time period, the corpus luteum, which was formed in the previous cycle, dies in the ovary. The result of this process is a sharp decrease in the concentration of estradiol, progesterone and inhibin (the last two to a lesser extent).
Over the course of 4-5 days of this phase, primordial ovarian follicles continue to emerge from the dormant state. Follicle-stimulating hormone (FSH) begins to induce proliferation of cells, the activity of which also increases. The result of this is an increase in the formation of androgens of estrogens, which are synthesized by interstitial cells.
Forming follicles at this point usually have more than one layer of follicular cells surrounding the oocyte. In addition, a small accumulation of follicular fluid appears between these cells. The follicle-stimulating hormone begins to induce the appearance of an even greater number of LH receptors and its own receptors. And this increases their sensitivity to luteinizing hormone.
The follicular phase is also characterized by the fact that on the 6-7th day of the cycle, one of the forming follicles begins to dominate. Just he will receive further development and after ovulate by about 13-15 days of the cycle. The dominant follicle is characterized by the highest mitotic activity of follicular cells, the highest accumulation of follicle-stimulating hormone in the follicular fluid.
Non-dominant follicles have an increased ratio of androgens and estrogens, and this indicates that the activity of aromatase is reduced in follicular cells. Such non-dominant follicles undergo reverse development (atresia). Androgens are a key factor in the induction of atresia.
The late and middle follicular phase of the menstrual cycle is characterized by an even greater increase in the synthesis of inhibin B and estradiol, and this leads to inhibition of the production of follicle-stimulating hormone by the pituitary gland and the growth of new follicles stops. An increase in estradiol also leads to an increase in the secretion of luteinizing hormone and an increase in ovary sensitivity to gonadotropins. In addition, a high estrogen content causes endometrial growth. Such changes are called the "proliferation phase."