Menstruation (Period or Menstrual Cycle)

What is menstruation?
- Menstruation calculator
The hypothalamus
- GnRH

The pituitary gland
Ovaries
Endometrium
- Anatomy
- Physiology

What is menstruation?

Menstruation describes the female period. The menstruation cycle involves the monthly release of an egg (ovum) in a process called ovulation, with bleeding due to shedding of the uterine lining at the end of the cycle. Normal menstruation is a highly complex interactions between a number of hormones produced by three organs of the body; the hypothalamus; the pituitary gland and the ovaries. The interactions between these organs are referred to as the hypothalamic-pituitary-ovarian axis (HPO axis). The cycle has an average duration of 28 days, but the normal range is between 21 to 35 days. Menstruation usually starts at an average age of 13 (called menarche) and lasts on average till age 51 (called menopause).

Plasma hormone levels

The inner lining of the uterus (the endometrium) goes through three phases during the menstrual cycle: the proliferative phase, the secretory phase and the menstrual phase. The changes in this lining is needed for provision of an optimal environment for the fertilised egg to grow, which is why is needs to be shed and renewed monthly. The ovarian cycle is divided into two phases: the follicular phase and the luteal phase, during which different levels of hormones are released. Bleeding as a result of menstruation lasts 2 to 7 days on average, from which there is an average loss of 20 to 80mL of blood.

Menstruation Calculator - Calculate when your next period is due.

The hypothalamus

There are five different hormones released from the hypothalamus that have an effect on the menstrual cycle. These hormones include:

Gonadotrophin releasing hormone (GnRH)
Thyrotropin releasing hormone (TRH)
Somatotropin release-inhibitory factor (SRIF) aka Somatostatin
Corticotrophin releasing factor (CRF)
Prolactin release-inhibiting factor (PIF)

Each of these have a different effect on the anterior pituitary gland, stimulating it to release or stop releasing a particular hormone.

Gonadotrophin releasing hormone

Release of this hormone is responsible for the stimulation of specific cells called gonadotrophs in the pituitary gland. This stimulation results in the production of two important hormones called luteinising hormone (LH) and follicular stimulating hormone (FSH) from the pituitary. GnRH is of great importance in the menstrual cycle. One of the most important features of GnRH release is the fact that its release occurs in a pulsatile fashion. At the start of puberty there is a marked increase in the frequency and amplitude of GnRH release.

A part of the brain called the surge centre controls the timing of this increased release of GnRH. The surge centre is present in females very early in life, however it is only as puberty approaches that this centre becomes more responsive to hormonal changes. Throughout the menstrual cycle there is pulsatile release of GnRH. Anything that interferes with the pulse frequency of GnRH can stop the menstrual cycle from occurring. Restoration of this pulsatile GnRH by administering hormones can produce a return to ovulation.

The pituitary gland

Anatomy

The pituitary gland is an outpouching of the base of the brain which lies under the hypothalamus. The close proximity of these two parts of the brain is a reflection of their closely linked function. The pituitary gland is divided into two different parts, each of which have different functions. The anterior pituitary is responsible for housing the gonadotrophs, these are the cells that release hormones important in controlling the menstrual cycle.

Physiology

The anterior pituitary gland is composed of six different cell types and produces six different hormones. The cell type that is of importance in menstruation is the gonadotroph. These cells release follicle stimulating hormone (FSH) and luteinising hormone (LH) and are also responsible for production and storage of these hormones.

FSH

The granulosa in the ovaries are the main target for the action of FSH. In response to FSH stimulation the granulosa cells release oestrogen. The combined effect of oestrogen and FSH is to cause growth and increased oestrogen production.

LH stimulates cells in the ovary, called the theca cells, to produce hormones called androgens which are then transported to the granulosa cells in the ovary for conversion into oestrogens.

Gonadotrophin secretory patterns

The normal ovulatory cycle is divided into two phases called the follicular and luteal phases.

Follicular phase: is initiated from the day bleeding stops and finishes with a mid cycle surge of LH.
Luteal phase: this is initiated with the mid-cycle surge of LH which coincides with ovulation and ends with the first day of onset of the period.

Ovaries

Anatomy

The female ovaries are paired, flat, elliptical structures which measure approximately 5cm in diameter. The ovaries are in the abdomen and are suspended by various ligaments. The ovary itself consists of two parts, the outer cortex and the inner medulla. The cortex is where development of the eggs occurs, and the medulla carries nerves and blood vessels.

Physiology

Females are born with approximately 2 to 4 million primary follicles. These fetal follicles contain a developing egg called a primary oocyte surrounded by a layer of granulosa cells. These primary oocytes are part way through a cell division. This process of division doesn't resume until the time of ovulation. With each ovarian cycle, a handful of ovarian follicles are recruited and usually only one of these ovulates, the remaining unrecruited follicles remain in an inactive state. Development of follicles occurs until menopause.

Hormones in the ovarian cycle

Oestrogen

This is low at the beginning of the menstrual cycle and peaks at the middle and then once again towards the end.

Progesterone

There is little production of this in the first half of menstruation but a significant increase in the second half. The progesterone remains high if pregnancy occurs. Progesterone is responsible for an increased body temperature in pregnancy as well.

Endometrium

Anatomy

The endometrium is the inner layer of the uterus and is attached to the muscle layer of the uterus. It is functionally divided into two distinct zones. The outer part is the part that sheds during the menstrual cycle, and the inner part contains stem cells that helps to regenerate the lost cells.

Physiology

The endometrium goes through three stages during the menstrual cycle:

Menstrual phase

This phase begins with the first day of menstruation. Contraction of the muscle layer occurs expelling the blood and endometrial cells through the vagina. Occurs when oestrogen and progesterone are at their lowest levels.

Proliferative phase

There is oestrogen mediated renewal of the endometrial tissue due to the migration of stem cells from the inner layer. There are new blood vessels and glands that form during this phase.

Secretory phase

Increased secretory activity by the endometrial glands is stimulated by progesterone. The endometrial glands in this phase become more developed. The increased secretory activity in this phase of menstruation creates an ideal environment in the uterus for development of an embryo.

More information

For information on different types of contraception, female anatomy and related health issues, see Contraception.

Reference

Collier J. Longmore M. & Scally P. Oxford Handbook of Clinical Specialties. 6th ed. Oxford. Oxford University Press. 2003
Hacker & Moore. Essentials of Obstetrics & Gynaecology. 3rd ed. Philadelphia. WB Suanders. 1998
Johnson L. R. Essential Medical Physiology. 2nd ed. Philadelphia. Lippincott- Raven. 1998.
McCarthy A. & Hunter B. Master Medicine: Obstetrics & Gynaecology. 2nd ed. Edinburgh. Churchill Livingstone. 2003
Moore KL & Dalley AF. Clinically Oriented Anatomy 4th ed. Philadelphia. Lippincott Williams & Wilkins. 1999