3.10 Menstrual Cycle

3.10 Understand the role of oestrogen and progesterone in the menstrual cycle.

- Oestrogen and Progesterone are both hormones. Hormones are produced in a structure known as an endocrine gland in the ovary. Hormones travel through the blood from this gland to its target tissue. This is where the hormone will have it's effect. Sometimes even multiple effects.

- The Ovary (as shown in diagram) is the endocrine gland for oestrogen.

1) The first effect of Oestrogen takes place in the lining of the uterus. Oestrogen is sent through the blood stream to the endometrium.  The diagram shows the difference in the thickness of the lining of the uterus. In the first half of the cycle the lining becomes increasingly thick.

2) The second effect of Oestrogen is the release of a hormone named LH. This is caused when Oestrogen flows through the blood stream to the brain. The peak of this hormone LH is about day 13 in the cycle which causes the ovary to release an egg. This is when Fertilisation is now possible to occur.

Closer look at what happens in the ovary:

- During the first 13 days of the cycle a structure called a follicle begins to grow larger and larger. The center of this follicle is the egg. It is the cells around the follicle which produce the Oestrogen. Around about day 13 the follicle reaches its greatest size at the same time as LH causes the wall of the ovary to rupture. This releases the egg into the oviducts. This process is called Ovulation.

- Once this process is complete the now empty follicle changes function and begins to turn yellow. This gives us the new name Corpus Luteum. This is known to produce Progesterone, so Corpus Luteum is the endocrine gland for Progesterone. This hormone travels through the blood stream to the lining of the uterus. This is effect 3 which makes sure that the lining of the uterus does not break down. It maintains the lining of the uterus. In this condition it is possible for the fertilised egg to implant into the wall and develop into a pregnancy (between the two lines left and right of number 3 on diagram)

- However if this does not occur the Corpus Lutiem breaks down and Progesterone levels fall. This results in effect number 4 which is the break down of the lining of the uterus. This is known as the menstrual period or bleeding. This marks the end of one menstrual cycle and the beginning of another.