Reproductive systems

What the exam asks

Expect a graph of hormone concentrations against cycle day, with events to identify, or a described intervention whose effect you predict. The recurring items are: name the hormone at a labelled peak, say what triggers ovulation, explain why a contraceptive containing the cycle's own hormones prevents it, and locate a lesion from a pattern of high and low hormones. The feature that makes this topic worth real study is the sign reversal of oestrogen feedback, because nothing else in the course does it and candidates who have learned 'oestrogen inhibits FSH' as a fact cannot then explain the LH surge at all. Carry in that the same hormone does both, and that the variable is how high and for how long. The other trap is menstruation. Asked what causes it, candidates look for something switching on. Nothing does. The corpus luteum dies on schedule, progesterone withdraws, and the endometrium it was maintaining is no longer maintained. When a graph question asks what caused an event, check whether the answer is a fall rather than a rise.

Three things sit under this topic: how gametes are made, how the menstrual cycle is controlled, and what happens at fertilisation. Only the middle one is examined in depth, and it is examined as a control system rather than as a set of events on a calendar.

Gametogenesis is best carried in as a contrast. Both sexes run meiosis, but they run it differently. Spermatogenesis divides the cytoplasm evenly and yields four sperm from each primary spermatocyte, continuously, from puberty. Oogenesis divides the cytoplasm very unevenly and yields one ovum, because the point is to keep the contents in one cell rather than to share them out. The three discarded sets go into polar bodies. Oogenesis also stops twice: primary oocytes arrest in prophase I before birth and wait years, and the secondary oocyte arrests in metaphase II and is only released from it by fertilisation.

The menstrual cycle is the best feedback system in the syllabus, because it does something no other endocrine axis in the course does: it reverses its own sign. Oestrogen from the growing follicle inhibits FSH and LH while it is low, which is ordinary negative feedback. Once it is high and stays high for about two days, the same hormone acting on the same pituitary switches to positive feedback and drives the LH surge that triggers ovulation. Then the corpus luteum makes progesterone, feedback goes negative again and stays there. Almost every question on this topic is asking you to locate a moment in that sequence.

Fertilisation is examined lightly, and mostly for where it happens and how a second sperm is kept out.

What to hold

• One primary spermatocyte yields four functional sperm; one primary oocyte yields one ovum plus polar bodies, because the cytoplasm is divided unequally to keep the resources in a single cell.
• Primary oocytes arrest in prophase I before birth and stay arrested until the cycle in which they are ovulated, which can be decades later.
• The oocyte released at ovulation is a secondary oocyte arrested in metaphase II, and it completes meiosis II only if a sperm penetrates it.
• In the male, LH acts on Leydig cells to produce testosterone, while FSH acts on Sertoli cells to support developing sperm.
• In the follicular phase, FSH drives follicle growth and the granulosa cells of the follicle secrete oestrogen.
• Oestrogen exerts negative feedback on the pituitary at low concentrations but switches to positive feedback when it is high and sustained, and that switch produces the LH surge.
• The LH surge triggers ovulation, which follows it by roughly a day.
• After ovulation the follicle becomes the corpus luteum, which secretes progesterone; progesterone maintains the secretory endometrium and restores negative feedback so no new follicle matures.
• Menstruation is caused by progesterone and oestrogen falling as the corpus luteum regresses, not by any hormone rising.
• The luteal phase is close to fixed at about 14 days, so variation in cycle length is variation in the follicular phase.
• An implanting embryo secretes hCG, which acts like LH to rescue the corpus luteum, so progesterone stays high and menstruation does not occur.
• Fertilisation normally occurs in the ampulla of the uterine tube, while implantation occurs in the endometrium several days later.
• The cortical reaction hardens the zona pellucida after the first sperm fuses, which is the lasting block to polyspermy.