Reproductive cycle and development

MDCAT Biology · Biology · Reproduction. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All MDCAT Biology topics

The idea

The menstrual cycle lasts about 28 days and is run by four hormones. FSH from the pituitary makes a follicle grow. The follicle secretes oestrogen, which repairs and thickens the endometrium. A surge of LH at about day 14 causes ovulation. The empty follicle becomes the corpus luteum and secretes progesterone, which keeps the lining ready.

If there is no fertilisation, the corpus luteum breaks down, progesterone falls and the lining is shed.

Fertilisation happens in the Fallopian tube. The zygote divides by cleavage and implants in the uterus as a blastocyst. The placenta then exchanges materials between mother and foetus.

Rules to remember

Order of events: menstruation (days 1 to 5) → follicle growth and oestrogen → LH surge and ovulation (about day 14) → corpus luteum and progesterone.
Oestrogen and progesterone inhibit FSH and LH (negative feedback); this is how the contraceptive pill prevents ovulation.
hCG from the embryo keeps the corpus luteum alive in early pregnancy; later the placenta makes the progesterone.
Cleavage is mitosis with little growth: zygote → morula → blastocyst, which implants.
Germ layers: ectoderm → nervous system and epidermis; mesoderm → muscle, skeleton, blood; endoderm → lining of gut and lungs.
Placenta: maternal and foetal blood do not mix. The umbilical cord has two arteries (deoxygenated blood to the placenta) and one vein.
STDs: gonorrhoea (Neisseria gonorrhoeae), syphilis (Treponema pallidum), AIDS (HIV), genital herpes (herpes simplex virus).

Common mistake

LH, not FSH, triggers ovulation, and menstruation is caused by the fall in progesterone, not by a rise in any hormone.

Worked example

The time from ovulation to the next menstruation is about 14 days in most women. A woman has a regular 32-day cycle. On about which day does she ovulate?

The corpus luteum lasts about 14 days, whatever the length of the cycle.
So ovulation comes about 14 days before the next cycle begins.
Day of ovulation = 32 − 14 = day 18.

Answer: About day 18 of her cycle (not day 14, which is true only for a 28-day cycle).

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Keep going

← Human reproductive systems Skeleton and joints →Take a MDCAT Biology test