Phases of the Menstrual Cycle - A Medical Perspective Overview

The menstrual cycle can be divided into the stages of follicular phase (14 days during which proliferative phase occurs), ovulation, luteal phase (14 days during with secretory phase occurs), and menstruation.

Follicular phase and ovulation::

Follicular Phase:

During the follicular phase, release of FSH (follicle stimulating hormone) from the pituitary results in the development of a primary ovarian follicle. This primary ovarian follicle produces estrogen which causes the uterine lining to proliferate (proliferative phase).

Ovulation:

During the middle of the menstrual cycle (14 days), an increase in LH (LH spike) results because of the estrogen produced during the proliferative phase. This LH spike causes ovulation (relase of ovum from the follicle) to occur.

Luteal Phase:

Ovulation is followed by the Luteal phase, during with the remnants (theca interna and granulosa cells) of the follicle (left behind after ovulation of the ovum) develop into the corpus luteum. Corpus luteum produces progesterone, which then maintains the lining of the endometrium (that had proliferated in response to the estrogen in the proliferative phase). This is basically getting ready for the the ovum to implant.

Menstruation:

If the ovum does not implant, the corpus luteum degnerates and the progesterone levels decrease resulting in the sloughing off of the endometrium lining (Menstruation). If the ovum does implant (after fertilization occurs), then the developing trophoblast will maintain the corpus luteum by producing hCG (human chorionic gonadotropin) until about 8 weeks when the placenta can maintain production of estrogen and progesterone.

Restarting the Cycle:

A subsequent decrease in estrogen and progesterone (after degeneration of the corpus luteum and menstruation) results in an increase in FSH which then stimulates development of more primary ovarian follicles to start the menstrual cycle all over again. Nearly 5-15 primordial follicles are formed of which only one (dominant) will mature into ovulation. It will stimulate itself with estrogen (autocrine stimulation by the combined work of theca interna cells (produce andrestenedione in response to LH stimulation) and granulosa cells (convert the androstenedione into estradiol in response to FSH stimulation) of the follicle.