The menstrual cycle The function of the menstrual cycle is to provide a favourable environment for a fetus to grow. If fertilization does not occur, the uterus lining is shed in menstruation. The menstrual cycle is controlled by the following hormones: • progesterone • oestrogen • follicle stimulating hormone (FSH) • luteinizing hormone (LH) The secretion of these hormones is controlled by interacting positive and negative feedback loops.
Hormones • Four hormones involved in controlling menstruation
The Cycle • During the first week of the cycle the pituitary gland stimulated, and releases follicle stimulating hormone (FSH)
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
False colour SEM of human uterus wall, approximately 13 days into cycle. Green cells are secretory, and orange cells are ciliated
Initial concentrations of oestrogen are low. • The low concentration has a negative feedback effect on the secretion of FSH.
As follicle grows it 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)
Positive feedback from increasing oestrogen concentration causes increase in FSH and LH from pituitary gland. Follicle gets bigger – releases more oestrogen
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.
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
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.
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.
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
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
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
TASKS • Answer the questions on the sheet. • Answer the exam questions. • Complete the Homeostasis Glossary.