(a) Give an account of hormonal regulation of menstruation in human females. (20 marks) (b) Explain the source(s) and functions…

(a) Hormonal regulation of menstruation in human females

The menstrual cycle of about 28 days is controlled by the hypothalamus, anterior pituitary and ovary. Menstrual phase (days 1 to 5): the fall of oestrogen and progesterone at the end of the previous cycle causes the functional layer of the endometrium to break down and be shed, with bleeding. Follicular (proliferative) phase (days 6 to 13): the hypothalamus releases GnRH, which makes the pituitary secrete FSH and LH. FSH stimulates growth of several follicles, and the growing follicle secretes oestrogen. Oestrogen rebuilds the endometrium and at first inhibits FSH by negative feedback. Ovulation (about day 14): a high level of oestrogen, maintained for about two days, gives positive feedback and causes the surge of LH, with a smaller FSH surge. The LH surge ruptures the mature Graafian follicle and releases the secondary oocyte. Luteal (secretory) phase (days 15 to 28): LH changes the ruptured follicle into the corpus luteum, which secretes progesterone and some oestrogen. Progesterone makes the endometrium glandular and ready for implantation, and inhibits FSH and LH. If there is no fertilisation, the corpus luteum degenerates after about 10 days, the hormone levels fall, and the endometrium is shed so that the next cycle begins. If fertilisation occurs, hCG from the embryo maintains the corpus luteum, and menstruation does not occur.

(b)(i) Vitamin B₁ (thiamine)

Sources: whole grains and unpolished rice, pulses, nuts, yeast, pork and eggs. Functions: it is changed to thiamine pyrophosphate, a coenzyme for pyruvate dehydrogenase and α-ketoglutarate dehydrogenase in carbohydrate oxidation, and for transketolase in the pentose phosphate pathway. It is needed for energy release and nerve function. Deficiency: beriberi, with weakness and nerve inflammation, and Wernicke-Korsakoff syndrome in alcoholics.

(b)(ii) Vitamin B₂ (riboflavin)

Sources: milk, eggs, liver, green leafy vegetables and yeast. Functions: it forms the coenzymes FMN and FAD, which carry hydrogen in the electron transport chain and in the oxidation of fatty acids and in the Krebs cycle (for example succinate dehydrogenase). FAD is also needed for glutathione reductase, which protects cells from oxidative damage. Deficiency: cracks at the corners of the mouth (angular stomatitis), inflamed tongue and skin and eye disorders.

(b)(iii) Vitamin B₁₂ (cobalamin)

Sources: only foods of animal origin such as liver, meat, fish, eggs and milk, and gut bacteria make some. Vegetarians may be short of it. Functions: it is a coenzyme for methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA, and for methionine synthase, which uses a methyl group from methyltetrahydrofolate to make methionine from homocysteine. It is needed for DNA synthesis, normal red blood cell formation and the myelin sheath of nerves. Absorption needs the intrinsic factor of the stomach. Deficiency: megaloblastic (pernicious) anaemia and nerve damage.

(c) Three phases of oogenesis in human females

Oogenesis is the formation of ova in the ovary. It has three phases. Multiplication phase: in the foetal ovary, primordial germ cells divide mitotically to give millions of oogonia. About the fifth month, they begin meiosis I and become primary oocytes, which stop in prophase I (diplotene stage) and are surrounded by follicle cells as primordial follicles. No new oocytes are formed after birth. Growth phase: after puberty, under FSH, a few primordial follicles grow each cycle. The primary oocyte enlarges, stores nutrients and gets a zona pellucida, and the follicle becomes primary, secondary and then the tertiary or Graafian follicle with an antrum. Usually one becomes dominant. Maturation phase: the LH surge makes the primary oocyte complete meiosis I just before ovulation. It divides unequally into a large secondary oocyte and a small first polar body. The secondary oocyte starts meiosis II and stops at metaphase II, and is ovulated. Meiosis II is finished only if a sperm enters, giving the ovum and the second polar body. Thus each oogonium gives one ovum and three polar bodies.