CONTROL OF THE OESTROUS CYCLEDaniel Laverick| Andrew Westgarth |Andrew Watkis

DEFINITION The Oestrus Cycle : The regular

pattern of changes that takes place in the reproductive system of female mammals, which is controlled through the interaction of a number of hormones!

The Menstrual Cycle : In humans and some other primates, the lining of the uterus is shed along with some blood between each cycle.

HORMONAL CONTROL OF THE MENSTRUAL CYCLE

• There are four main hormones that control the human oestrous cycle:

• Follicle-stimulating hormone (FSH): Stimulates the development of follicles. Stimulates production of oestrogen.

• Luteinising hormone (LH): Causes ovulation to occur. Stimulates production of progesterone.

• Oestrogen: Causes rebuilding of uterus lining. Stimulates production of LH.

• Progesterone: Maintains uterus lining. Inhibits production of FSH.

• Oestrogen and progesterone are produced in the ovaries, while FSH and LH are produced in the pituitary gland of the brain.

THE CYCLEThe menstrual cycle

begins when the uterus lining is shed (along with blood)

DAY 1-5

The pituitary gland releases into the blood

which stimulates follicles in the ovary to

grow and mature. Each follicle contains

an egg

DAY 1

The growing follicles secrete small amounts of oestrogen into the blood. This low level of oestrogen causes the

uterus lining to build up, and also inhibits(stops) the release of FSH and LH.

This is Negative Feedback.

As the follicles grow, more oestrogen is produced. The level of

oestrogen increases until, it reaches it’s critical point. This then inhibits the production of more FSH

and stimulate production of LH, which is Positive Feedback

DAY 10

The LH then causes one of the

follicles in the ovary to release its egg. (Ovulation)

DAY 14

THE CYCLE CONTINUEDAfter Ovulation, LH stimulates the

now empty follicle to develop into a structure known as the Corpus

Luteum, which secretes progesterone.

Progesterone maintains the thick lining of the

uterus, and also inhibits release of FSH and LH (Negative Feedback)

If the egg is not fertilised, the corpus luteum

degenerates and so no longer produces progesterone.

With less progesterone, the lining of the uterus is no longer

maintained and so breaks down (menstruation). Less

progesterone also means that FSH release is no longer inhibited.

FSH release therefore resumes and the cycle repeats.

MENSTRUAL PHASE• It lasts for about 3 – 5 days.• If the ovum is not fertilized, corpus luteum

regresses from 24th day of the cycle. • This causes a decrease in estrogen and

progesterone levels. • It results in shrinkage of endometrium,

coiling and spasm of arteries leading to breakage of the endometrium.

• The necrosed endometrium is shed along with blood and other secretions.

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MENSTRUAL CYCLE

DIAGRAM