What is the follicular phase of the menstrual cycle?

Updated: Jan 08, 2019
  • Author: Kristi A Tough DeSapri, MD; Chief Editor: Richard Scott Lucidi, MD, FACOG  more...
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Answer

In physiologic terms, the first day of menses is considered the first day of the menstrual cycle. The following 13 days of the cycle are designated the follicular phase. As levels of progesterone, estradiol, and inhibin decline 2-3 days before menses, the pituitary begins to release higher levels of follicle-stimulating hormone (FSH), which recruits oocytes for the next menstrual cycle. The hypothalamus is the initiator of the follicular phase via gonadotropin-releasing hormone (GnRH).

The GnRH pump in the hypothalamus releases GnRH in a pulsatile fashion into the portal vessel system surrounding the anterior pituitary gland. GnRH interacts with the anterior pituitary gland to stimulate release of FSH in the follicular phase. FSH is secreted into the circulation and communicates with the granulosa cells surrounding the developing oocytes.

As FSH increases during the early portion of the follicular phase, it meshes with granulosa cells to stimulate the aromatization of androgens into estradiol. The increase in estradiol and FSH leads to an increase in FSH-receptor content in the many developing follicles.

Over the next several days, the steady increase of estradiol (E2) levels exerts a progressively greater suppressive influence on pituitary FSH release. Only one selected lead follicle, with the largest reservoir of estrogen, can withstand the declining FSH environment. The remaining oocytes that were initially recruited with the lead follicle undergo atresia.

Immediately prior to ovulation, the combination of E2 and FSH leads to the production of luteinizing-hormone (LH) receptors on the granulosa cells surrounding the lead follicle.

During the late follicular phase, estrogen has a positive influence on LH secretion, instead of suppressing pituitary LH secretion as it does early in the follicular phase. To have this positive effect, the E2 level must achieve a sustained elevation for several days. The LH surge promotes maturation of the dominant oocyte, the release of the oocyte and then the luteinization of the granulosa cells and the surrounding theca cells of the dominant follicle resulting in progesterone production.

The appropriate level of progesterone arising from the maturing dominant follicle contributes to the precise timing of the mid cycle surge of LH. E2 promotes uterine endometrial gland growth, which allows for future implantation.


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