The follicular phase of the menstrual cycle

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, estrogen, begins from the very first day of menstruation and lasts until the ovaries develop one or several dominant follicles. The end of this phase is ovulation.

The duration of it can be completely different. Most often, the delay in menstruation occurs precisely because of the slow maturation of the follicle. The main factor that affects the duration of this process is the time required for the body to reach the limit level of estrogens.

According to functional and morphological changes in the ovary, the monthly cycle can be divided into such 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 only takes the first half of the cycle, but it can sometimes last longer. The average cycle time is 28 days. Any deviation from this number is 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 increases not only in the systemic circulation, but also in the follicular fluid. Somewhere 2 days before the onset of menstruation begins the development of the follicle from its resting or primordial shape.

Just at this time in the ovary, the yellow body dies , which formed in the previous cycle. The result of this process is a sharp decrease in the concentration of estradiol, progesterone and inhibin (the last two to a lesser extent).

During 4-5 days of this phase, the primordial ovarian follicles continue to leave and remain in rest. The follicle-stimulating hormone (FSH) begins to induce the proliferation of cells, whose activity also increases. The result is an increase in the formation of androgens from estrogens, which are synthesized by interstitial cells.

Forming follicles by this time usually have not 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 receptors for LH and its own receptors. And this increases their sensitivity to the luteinizing hormone.

The follicular phase is also characterized by the fact that on the 6th-7th day of the cycle one of the emerging follicles begins to dominate. It just will be further developed and after ovulating approximately to the 13-15 day 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 suggests that aromatase activity 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 ceases. The increase in the content of estradiol also leads to an increase in the secretion of luteinizing hormone and an increase in the ovary sensitivity to gonadotropins. In addition, a high estrogen content causes the growth of the endometrium. Such changes are called the "proliferation phase".