Brent Cornell

    
The menstrual cycle describes recurring changes that occur within the female reproductive system to make pregnancy possible

• Each menstrual cycle lasts roughly one month (~28 days) and begins at puberty (menarche) before ending with menopause

There are two key groups of hormones which control and coordinate the menstrual cycle:

• Pituitary hormones (FSH and LH) are released from the anterior pituitary gland and act on the ovaries to develop follicles
• Ovarian hormones (estrogen and progesterone) are released from the ovaries and act on the uterus to prepare for pregnancy

Menstrual Hormones

Flowchart of Hormonal Actions During Menstrual Cycle

Key Events in the Menstrual Cycle

There are four key events that comprise a typical menstrual cycle: follicular phase, ovulation, luteal phase and menstruation

• These events are distinguished by changes to hormonal levels, follicular development and the status of the endometrium

1.  Follicular Phase

• Follicle stimulating hormone (FSH) is secreted from the anterior pituitary and stimulates growth of ovarian follicles
• The dominant follicle produces estrogen, which inhibits FSH secretion (negative feedback) to prevent other follicles growing
• Estrogen acts on the uterus to stimulate the thickening of the endometrial layer

2.  Ovulation

• Midway through the cycle (~ day 12), estrogen stimulates the anterior pituitary to secrete hormones (positive feedback)
• This positive feedback results in a large surge of luteinizing hormone (LH) and a lesser surge of FSH
• LH causes the dominant follicle to rupture and release an egg (secondary oocyte) – this is called ovulation

3.  Luteal Phase

• The ruptured follicle develops into a slowly degenerating corpus luteum
• The corpus luteum secretes high levels of progesterone, as well as lower levels of oestrogen
• Estrogen and progesterone act on the uterus to thicken the endometrial lining (in preparation for pregnancy)
• Estrogen and progesterone also inhibit secretion of FSH and LH, preventing any follicles from developing

4.  Menstruation

• If fertilisation occurs, the developing embryo will implant in the endometrium and release hormones to sustain the corpus luteum
• If fertilisation doesn’t occur, the corpus luteum eventually degenerates (forming a corpus albicans after ~ 2 weeks)
• When the corpus luteum degenerates, estrogen and progesteron levels drop and the endometrium can no longer be maintained
• The endometrial layer is sloughed away and eliminated from the body as menstrual blood (i.e. a woman’s period)
• As estrogen and progesterone levels are too now low to inhibit the anterior pituitary, the cycle can now begin again

Stages of the Menstrual Cycle

   Event:            Cycle Overview             Pituitary Hormones             Ovarian Cycle             Ovarian Hormones             Uterine Cycle