The menstrual cycle, fertilisation and early development

1Four hormones, one monthly rhythm

Each month from puberty to the menopause, a woman's body prepares for a possible pregnancy: an oocyte matures and is released, and the lining of the uterus thickens to receive an embryo. If no embryo arrives, the lining is shed and the cycle begins again. In Senior 1 you met the menstrual cycle and menstrual hygiene. At A-level we explain how four hormones, two from the pituitary and two from the ovary, control each other by negative and positive feedback, and then follow what happens when an oocyte is fertilised.

2Hormonal control of the menstrual cycle

1. Days 1 to 5. Progesterone and oestrogen are low, so the endometrium is shed: menstruation. Low ovarian hormones remove their inhibition of the pituitary, so FSH rises.
2. Days 5 to 13. FSH makes a follicle grow; the follicle cells secrete rising amounts of oestrogen. Oestrogen makes the endometrium repair and thicken. At low and moderate levels, oestrogen inhibits FSH secretion (negative feedback), so usually only one follicle matures.
3. About day 13. When oestrogen reaches a high level for a day or two, its effect switches to positive feedback: the pituitary releases a surge of LH (and some FSH).
4. About day 14. The LH surge causes ovulation: the mature follicle bursts and releases the secondary oocyte.
5. Days 15 to 24. LH turns the empty follicle into the corpus luteum, which secretes progesterone (and oestrogen). Progesterone keeps the endometrium thick, glandular and full of blood vessels, ready for implantation, and inhibits FSH and LH, so no new follicle develops.
6. Days 25 to 28, if there is no pregnancy. With FSH and LH low, the corpus luteum breaks down. Progesterone and oestrogen fall, the endometrium is shed, and the cycle restarts.

3Copulation and fertilisation

During sexual intercourse (copulation), the penis becomes erect as its spongy tissue fills with blood, and is placed in the vagina. At ejaculation, waves of muscular contraction move sperm from the epididymis through the sperm ducts and urethra, mixing them with the secretions of the glands, and deposit semen at the top of the vagina, near the cervix. Sperm swim through the cervical mucus, through the uterus and into the oviducts. On the way, chemical changes to their membranes, called capacitation, make them able to fertilise an oocyte. Sperm can survive in the female tract for a few days; the oocyte survives for only about a day after ovulation.

1. Acrosome reaction: when a sperm reaches the oocyte, its acrosome releases enzymes that digest a path through the follicle cells and the zona pellucida.
2. Fusion: the membranes of the sperm and the oocyte fuse, and the sperm nucleus enters the oocyte.
3. Cortical reaction: cortical granules release their contents just inside the oocyte membrane, which makes the zona pellucida harden, so no other sperm can enter.
4. Meiosis II is completed, producing the ovum and the second polar body.
5. The haploid nuclei of sperm and ovum fuse, forming a diploid zygote (2n = 46). The sex of the child is set now: an X-carrying sperm gives a girl (XX), a Y-carrying sperm a boy (XY).

4Early development and implantation

As the zygote moves down the oviduct, it divides by mitosis (cleavage) without growing, so the cells get smaller. By about day 5 it has become a blastocyst, which reaches the uterus. Its outer layer, the trophoblast, attaches to the thick endometrium and digests its way in: implantation. The trophoblast then secretes the hormone human chorionic gonadotrophin (hCG). hCG keeps the corpus luteum alive, so progesterone and oestrogen stay high, the endometrium is not shed, and menstruation stops. Pregnancy tests detect hCG in the urine.