Three Main Points About The Follicular Phase
The follicular phase is one of the phases of the menstrual cycle.
Based on a 4-phase division of the menstrual cycle, this phase is considered the second after the menstrual phase and leads to ovulation.
There are three key points that underlie the importance of the follicular phase in the menstrual cycle
- It generates the ovum( egg). This is called ovogenesis (ovum- generation).
- It is the main phase that produces estrogen, the major female hormone.
- It is the phase that determines the length of the entire cycle.
This phase is also called the proliferative phase.
The term follicular is based on the changes occuring in the ovaries while the synonym word proliferative (meaning multiplication) is based on the changes occurring in the endometrium of the uterus at the same time.
What Are Follicles?
Follicles are basically the “guardians” and “home” of maturity of the developing ovum or egg. They are a collection of cells called granulosa cells that surround the ovum “helping” it in its maturation process.
The follicles in a woman first appear during embryo development while she was yet in the womb. These develop to a stage of maturity where they are called primordial follicles. Each female baby is born with a fixed number of these follicles in her ovaries. There are normally about 400,000 follicles in this early development in the ovaries of every female child after birth. This number does not increase. No fear though, they are usually enough for all the reproductive years, from menarche to menopause!
At puberty and beyond, development progresses to the fully mature follicle called the Graafian follicle which is quite large and contains some fluid, and is ready to release the egg. As the ovum undergoes changes, the granulosa cells surrounding it also increase in number and morphology.
From birth to puberty, the primordial follicles in the ovaries are inactive, in a state of “sleep”.
There are two basic processes occurring in the follicles at the same moment: ovum production and estrogen secretion.
The follicular phase normally begins as FSH levels begin to rise in blood. This occurs as early as the first day of menses. FSH then recruits about 6 to 12 follicles in the ovaries to start growing. Note that it is not all the primordial follicles that grow at same time under the influence of FSH but only the number recruited. The others remain in the inactive state. Should all the follicles grow at once, then they will get exhausted in one cycle and none left for the next. Result? Premature menopause!
The recruited follicles then progress in development, as the granulosa cells multiply. The ovum within also increases in size and matures. Despite the concurrent growth of the 6 to 12 follicles, only one finally gets mature to form the Graafian follicle while the others shrink.
As the follicles mature, the cells surrounding the ovum are prompted by FSH to begin producing estrogen which acts on the endometrium. It also makes these cells sensitive to Luteinizing hormone which is the ovulation that causes the mature follicle to rupture and release the egg( ovulation).
Length of the Follicular Phase
The time from recruitment of primordial follicles to ovulation is normally about two weeks. In a 28-day cycle, it comprises days 1-14.
The time it takes for follicles to mature varies greatly from one woman to another and is mainly responsible for the variations of cycle lengths amongst women. It may also be affected by stress, exercise or concurrent medical illness.
In contrast to the Luteal phase, the lenght of this phase may vary from cycle to cycle in the same woman.
The follicular phase can thus vary from about 7 days in a 21-day cycle to about 21 days in a 35-day cycle.
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