menstrual cycle

Question 1

Describe the menstrual cycle with reference to the alternation of menstruation and ovulation, the natural variation in its length, and the fertile and infertile phases of the cycle with references to the effects of progesterone, oestrogen, luteinising hormone and follicle-stimulating hormone.

Answer 1

Onset of puberty is triggered by a hormone secreted by the hypothalamus in the brain whose the target organ is the anterior pituitary gland in the brain, which then releases two hormones: Follicle-stimulating hormone and Luteinising hormone
First effect of FSH and LH: enhanced secretion of sex hormones, oestrogen and testosterone by the ovaries and testes
In the presence of FSH and LH and the respective sex hormone(s), there follows sexual development of the body and the preparation of the body for its role in sexual reproduction
The continually changing concentration of all four hormones bring about a repeating cycle of changes → Menstrual Cycle

Question 2

Menstrual cycle: ovulation + menstruation

Answer 2

The reproductive cycle repeats every 28 days on average
Two cycles coordinated by hormonal messages:
Ovarian cycle: monthly preparation and shedding of an egg cell from an ovary
Uterine cycle: buildup of the lining of the uterus (endometrium)

Question 3

Ovulation (def.)

Answer 3

release of a matured egg from an ovary, into the fallopian tube

Question 4

Menstruation (def.)

Answer 4

uterine bleeding caused by the breakdown of the endometrium (blood-rich inner lining of uterus)

Question 5

Menstruation (features)

Answer 5

Usually lasts 3-5 days
Menstrual discharge, which leaves the body through the vagina, consists of blood, clusters of cells and mucus
After menstruation, the endometrium regrows, reaching its maximum thickness in 20-25 days
If the embryo implants in the endometrium, it will obtain nutrients from the endometrium, and the thickened lining will not be discharged

Question 6

Negative and positive feedback of LH, FSH, oestrogen and progesterone

Answer 6

1. FSH stimulates growth of follicles in ovary
2. Developing follicles secrete oestrogen, which
   - Stimulate buildup of endometrium
   - Prevent further secretion of FSH
   Exerts a negative feedback on the anterior pituitary gland
3. Peak levels of oestrogen occurs about midpoint of the cycle. High and sudden peak stimulates secretion of LH
   Exerts a positive feedback on the anterior pituitary gland
4. Surge in LH stimulates ovulation
5. LH stimulates conversion of follicle tissue to corpus luteum
6. Corpus luteum secretes progesterone and some oestrogen. This
   - Continues buildup of endometrium, allowing it to become thickened
   - Inhibits further secretion of LH and FSH
   Exerts a negative feedback on the anterior pituitary gland
7. Falling levels of LH and FSH causes corpus luteum to degenerate. As a result,
   - Progesterone and oestrogen decreases
   - Endometrium breaks down
   - Menstrual cycle starts from beginning

Question 7

No implantation (no fertilisation) (implications)

Answer 7

Degeneration of the corpus luteum causes the levels of oestrogen and progesterone to fall below a critical level
The endometrium begins to shed, starting menstruation on day 1 of the next cycle

Question 8

Implantation (with fertilisation) (implications)

Answer 8

Fertilised egg/embryo secretes a hormone called human chorionic gonadotropin (HCG)
HCG maintains the corpus luteum, which continues to secrete oestrogen and progesterone, keeping the endometrium intact
Levels of FSH and LH would remain low because the formation of the placenta/maintenance of corpus luteum would produce progesterone to inhibit the secretion of FSH and LH for the maintenance of the lining to sustain pregnancy
Most home pregnancy tests work by detecting HCG in urine
In males, HCG boosts testosterone production, so its use is banned by many sports organisations