Life Sciences: Human Reproduction- The Menstrual Cycle

Hi

WHAT IS IT?

A series of changes in the endometrium & can be divided into 3 phases:

1. Menstrual and development of Graafian follicle (days 1-5)
2. Building phase (days 6-14)
3. Luteal and secretory phase (days 15-28)

PHASE 1

The breakdown & shedding of the endometrium occurs as there is no progesterone to maintain the endometrium. The detached tissue as well as blood pass out of the body. The hormone FSH is secreted from the anterior lobe of the pituitary and this stimulates the growth and development of a primary follicle into a Graafian follicle.

PHASE 2

Oestrogen is secreted from the maturing follicles and this begins to repair the endometrium. High levels of oestrogen also stimulates the secretion of LH from the anterior lobe of the pituitary gland. LH causes the Graafian follicle to rupture and this releases an ovum (ovulation) at day 14.

PHASE 3

Still influenced by LH< the ruptured Graafian follicle then changes into a corpus luteum- this secretes progesterone and a small amount of oestrogen. Under the influence of these 2 hormones, the endometrium thickens in preparation of implantation. The high levels of progesterone and oestrogen will causes a negative feedback and the FSH and LH secretion by the anterior lobe of the pituitary gland will be inhibited. If fertilisation doesn’t occur, the lack of LH will cause the corpus luteum to degenerate and this the amount of progesterone will drop and the endometrium won’t be able to continue to develop. The cycle will then begin again.