 | This articleneeds morereliable medical references forverification or relies too heavily onprimary sources. Please review the contents of the article andadd the appropriate references if you can. Unsourced or poorly sourced material may be challenged andremoved.Find sources: "Follicular phase" – news ·newspapers ·books ·scholar ·JSTOR(December 2017) |
| Follicular phase | |
|---|---|
 Figure 1. Menstrual cycle illustrating hormone levels, follicle development and uterine cycle | |
| Biological system | Urogenital system(reproductive) |
Thefollicular phase, also known as thepreovulatory phase orproliferative phase,[1] is the phase of theestrous cycle (or, in primates[2] for example, themenstrual cycle) during whichfollicles in theovary mature from primary follicle to a fully mature Graafian follicle. It ends withovulation. The main hormones controlling this stage are secretion ofgonadotropin-releasing hormones, which arefollicle-stimulating hormones andluteinising hormones. They are released bypulsatile secretion.[1] The duration of the follicular phase can differ depending on the length of the menstrual cycle, while theluteal phase is usually stable, does not really change and lasts 14 days.
Due to the increase of FSH, the proteininhibin B will be secreted by the granulosa cells. Inhibin B will eventually blunt the secretion of FSH toward the end of the follicular phase. Inhibin B levels will be highest during the LH surge before ovulation and will quickly decrease after.[1]
Follicle-stimulating hormone (FSH) is secreted by theanterior pituitarygland (Figure 2). FSH secretion begins to rise in the last few days of the previous menstrual cycle,[3] and is the highest and most important during the first week of the follicular phase[4] (Figure 1). The rise in FSH levels recruits five to seven tertiary-stageovarian follicles (this stage follicle is also known as aGraafian follicle orantral follicle) for entry into the menstrual cycle. These follicles, that have been growing for the better part of a year in a process known asfolliculogenesis, compete with each other for dominance.[5]
FSH induces the proliferation ofgranulosa cells in the developing follicles, and the expression ofluteinizing hormone (LH) receptors on these granulosa cells (Figure 1). Under the influence of FSH,aromatase andp450 enzymes are activated, causing the granulosa cells to begin to secreteestrogen. This increased level of estrogen stimulates production ofgonadotropin-releasing hormone (GnRH), which increases production of LH.[4] LH inducesandrogen synthesis bythecal cells, stimulates proliferation, differentiation, and secretion of follicular thecal cells and increases LH receptor expression on granulosa cells.[4]
Throughout the entire follicular phase, rising estrogen levels in the blood stimulates growth of theendometrium andmyometrium of theuterus.[6] It also causes endometrial cells to produce receptors forprogesterone,[6] which helps prime the endometrium to respond to rising levels of progesterone during the late proliferative phase and throughout the luteal phase.
Two or three days before LH levels begin to increase,[7] usually by day seven of the cycle,[8] one (or occasionally two) of the recruited follicles has emerged as dominant. Many endocrinologists believe that the estrogen secretion of the dominant follicle has increased to a level that GnRH production is suppressed, which lowers the levels of LH and FSH. This slowdown in LH and FSH production leads to theatresia (death) of most of the recruited follicles, though the dominant follicle continues to mature. Estrogen levels will continue to increase for several days (on average, six days, but this varies widely).[7]
These high estrogen levels initiate the formation of a new layer ofendometrium in the uterus, histologically identified as the proliferative endometrium. Crypts in thecervix are also stimulated to produce fertilecervical mucus.[7] This mucus reduces the acidity of thevagina, creating a more hospitable environment forsperm.[9] It also has a characteristic texture that helps guide sperm through the cervix[10] and to thefallopian tubes, where they wait for ovulation.[medical citation needed] In addition,basal body temperature may lower slightly under the influence of high estrogen levels.[11]
Estrogen levels are highest right before theLH surge begins (Figure 1). The short-term drop in steroid hormones between the beginning of the LH surge and the event of ovulation may cause mid-cycle spotting or bleeding.[12] Under the influence of the preovulatory LH surge, the firstmeiotic division of theoocytes is completed. The surge also initiates luteinization of thecal and granulosa cells.[4] Ovulation normally occurs 30 (± 2) hours after the beginning of the LH surge (when LH is first detectable in urine).[13]
Follicular waves are best described as the phase when follicles have matured sufficiently and rupture, leading to ovulation. Recent findings into the menstrual cycle in mammals has discovered that 2 or more follicles can develop but only one of the follicles fully matures to release the egg.[14][15] This follicular wave involves multiple surges in the levels of FSH to initiate follicular development. A study has found that 68% of women tended to display two follicular wave developments before ovulation while the remaining had three waves.[16]
This article incorporates text available under theCC BY 4.0 license.{{cite book}}: CS1 maint: multiple names: authors list (link)