Human Reproduction — General Introduction

Human reproduction is sexual, involving the fusion of a male gamete (sperm) and a female gamete (ovum) to form a diploid zygote that develops into a new individual over an internal gestation period of about nine months, after which the fully formed baby is delivered — humans are viviparous. Reproduction is not possible until the reproductive organs themselves become functional, which happens at puberty, generally between about 11 and 13 years of age, when a surge of activity along the hypothalamus-pituitary-gonad axis causes the testes in the male and the ovaries in the female to begin producing mature gametes and secreting the sex hormones (chiefly testosterone in the male; oestrogen and progesterone in the female) responsible for the appearance of secondary sexual characters — such as the growth of facial and body hair and deepening of the voice in boys, and the development of the breasts and broadening of the hips in girls — that mark the transition from a sexually immature child to a sexually mature adult capable of reproduction.

The organ systems that make reproduction possible fall broadly into two related groups on each side. The male reproductive system centres on a pair of testes that continuously manufacture sperm from puberty onward, together with a network of ducts (epididymis, vas deferens, ejaculatory duct, urethra) that store, mature and transport sperm, and a set of accessory glands (seminal vesicles, prostate, bulbourethral glands) that add secretions forming semen. The female reproductive system centres on a pair of ovaries that release one mature egg roughly once every 28 days from puberty until menopause, together with the fallopian tubes that carry the egg toward the uterus and are the normal site of fertilisation, the uterus itself, which is where a fertilised egg implants and the foetus develops for the rest of pregnancy, and the cervix and vagina, which together form the lower birth canal.

This chapter works through this machinery in the following order: the detailed anatomy of the male reproductive system and the microscopic structure of the testis; the anatomy of the female reproductive system and the microscopic structure of the ovary; the cellular processes of spermatogenesis and oogenesis by which the gametes themselves are formed; the hormonally driven menstrual cycle that governs female fertility on a monthly basis; and finally the chain of developmental events — fertilisation, cleavage, blastocyst formation, implantation, pregnancy and placenta formation, parturition (childbirth) and lactation — that carries a fertilised egg through to a breastfeeding newborn. Throughout, particular attention is paid to the hormones involved at each stage, since almost every step of human reproduction, from puberty itself to the onset of labour, is under close hormonal (and, for parturition, neuroendocrine) control.