3.5.5 Control of the Oestrous Cycle

1. 3.5.5 Control of the Oestrous Cycle L.O: to state and explain the hormones involved in the oestrous (menstrual) cycle to interpret graphs of hormones

2. The Menstrual Cycle

3. The oestrus cycle • When a female is born pvaries contain all of her eggs cells • Approx 300,000 cells that have potential to mature into ova • Function of cycle = to co-ordinate release of an ovum from an ovary with the preparation of the wall of the uterus for the fertilised egg

6. The Cycle • During the first week of the cycle the pituitary gland stimulated, and releases follicle stimulating hormone (FSH)

8. FSH stimulates a potential egg cell in the ovary • The cells around the ovum also develop. This is called a follicle. • Follicle cells secrete oestrogen. • Oestrogen stimulates rebuilding of uterus wall. An oocyte (pre-ovum) surrounded by follicle cells

11. Initial concentrations of oestrogen are low. • The low concentration has a negative feedback effect on the secretion of FSH.

12. Negative feedback lowers FSH concentration

13. As follicle grows is produces larger concentrations of oestrogen. • At a certain threshold, its effect reverses. • It now has a positive feedback effect on secretion of FSH from pituitary. • Also stimulates pituitary gland to release luteinising hormone (LH)

15. Positive feedback from increasing oestrogen concentration causes increase in FSH and LH from pituitary gland. Follicle gets bigger – releases more oestrogen

16. Peak of LH causes follicle to burst and release ovum • This is ovulation (on day 14 of the cycle) • Follicle reforms to become structure called corpus luteum (‘yellow body’) • LH stimulates corpus luteum to produce progesterone.

17. Peak of LH causes ovulation (day 14) Oestrogen falls because follicle is gone, but corpus luteum still produces some. Ovulation – ovum released from follicle. Follicle become corpus luteum

18. Progesterone completes uterus wall. Increases blood supply and promotes glycogen storage. • Rising concentrations of progesterone and oestrogen have negative feedback effect on FSH and LH. This prevents new follicles forming.

19. Fall in FSH and LH due to negative feedback with oestrogen and progesterone Progesterone produced by corpus luteum. Inhibits FSH and LH Corpus luteum – produces progesterone. Uterus wall fully completed.

20. Fertilisation must occur within 2 days of ovulation. • Embryo takes 3 days to reach the uterus and implant. • If no embryo implants within a week the corpus luteum starts to break down. • Progesterone and oestrogen concentrations fall. Uterus wall begins to break down. • FSH no longer inhibited, so begins to rise. • Cycle begins again

21. FSH and LH totally inhibited Breakdown of corpus luteum causes fall in progesterone and oestrogen Corpus luteum begins to break down if no embryo has implanted a week after ovulation

22. FSH begins to rise since it’s not inhibited by oestrogen and progesterone Low levels of progesterone and oestrogen because there is no follicle or corpus luteum Uterus wall breaks down due to low levels of oestrogen and progesterone New egg cell stimulated by rising FSH levels

25. Hormones • Four hormones involved in controlling menstruation