Menstrual cycle

Negative feedback

A primary mechanism of homeostasis, whereby a change in a physiological variable that is being monitored triggers a response that counteracts the initial fluctuation. An example is inhibition of FSH by high levels of oestrogen.

Progesterone

A steroid hormone produced by the corpus luteum in the ovary during the second half of the menstrual cycle. Progesterone maintains and enhances the uterine lining for the possible implantation of a fertilized ovum. It is the primary hormone secreted during pregnancy.

Oestrogen

Female sex hormone produced by follice which stimulates the lining of the womb to build up in preparation for a pregnancy.

FSH

Follicle stimulating hormone - A tropic hormone produced by the anterior pituitary gland that targets the gonads. In females, FSH stimulates the ovaries to develop follicles (oogenesis) and secrete estrogen; in males, FSH stimulates spermatogenesis.

LH

Luteinising hormone - produced by anterior pituitary; has the effect of stimulating ovulation in ovaries, production of testosterone in testes

Follicular phase

The first phase of the ovarian cycle, during which a follicle (an oocyte and its surrounding cells) enlarges and matures. This phase is under the control of FSH from the anterior pituitary, and typically lasts from day 1 to day 14 of the menstrual cycle. The follicle secretes oestrogen during this time period.

Menstruation

The first phase of the uterine (endometrial) cycle, during which the endometrium from the previous cycle is shed off. Oestrogen and progesterone levels are low during this time period. Typically lasts from day 1 to day 5 of the cycle.

Proliferative phase

The second phase of the uterine (endometrial) cycle, during which the endometrium thickens. This phase of the cycle is under the control of oestrogen, secreted from the follicle developing in the ovary during this time period. Typically lasts from day 6 to day 14 of the menstrual cycle.

Luteal phase

The third phase of the ovarian cycle, during which a corpus luteum is formed from the remnants of the follicle that has ovulated its oocyte. The corpus luteum secretes progestrone and estrogen during this time period, which typically lasts from day 15 to day 28 of the menstrual cycle. Formation of the corpul luteum is triggered by the same LH surge that triggers ovulation, however in the absence of LH (levels quickly decline after the surge) the corpus luteum begins to degenerate.

Ovulation

Triggered by a surge in LH and FSH - occurs during the menstrual cycle where an egg is released from an ovarian follicle. Usually occurs around day 14 of the menstrual cycle.

Endometrium

Inner lining of the uterus

Secretory phase

The third phase of the uterine (endometrial) cycle, during which the rebuilt endometrium is enhanced with glycogen and lipid stores. It is primarily under the control of progesterone and oestrogen (secreted from the corpus luteum during this time period), and typically lasts from day 15 to day 28 of the menstrual cycle.