Human Reproduction

Humans reproduce sexually: a male gamete (sperm) and a female gamete (egg/ovum) fuse to form a new individual. The body has specialised reproductive systems for making gametes and supporting the developing baby.

The male reproductive system consists of:

• A pair of testes — held in a sac (the scrotum) outside the body, where it is slightly cooler; the testes make sperm and the male hormone testosterone.
• Ducts — the epididymis, vas deferens and urethra — that store and carry the sperm.
• Glands (such as the prostate and seminal vesicles) that add fluid to make semen.
• The penis, which delivers the sperm.

The female reproductive system consists of:

• A pair of ovaries — which make the eggs (ova) and the female hormones oestrogen and progesterone.
• A pair of oviducts (fallopian tubes) — which carry the egg from the ovary toward the uterus; fertilisation usually happens here.
• The uterus (womb) — a muscular organ where the baby develops.
• The vagina — which receives the sperm and is the birth canal.

So the male system is built to produce and deliver sperm, while the female system produces eggs and nurtures the growing baby.

The formation of gametes is called gametogenesis. The making of sperm is spermatogenesis and the making of eggs is oogenesis; both involve meiosis, so the gametes are haploid (half the chromosome number).

• Spermatogenesis happens in the testes and produces large numbers of tiny, motile sperm continuously from puberty onward. A sperm has a head (with the nucleus), a middle piece (with mitochondria for energy) and a tail (for swimming).
• Oogenesis happens in the ovaries and produces eggs. Usually only one egg is released about once a month; the release of the egg from the ovary is called ovulation.

In females, the reproductive events repeat in a roughly 28-day cycle called the menstrual cycle, controlled by hormones. Its main phases are:

• Menstrual phase (about days 1–5) — if no fertilisation has happened, the thickened lining of the uterus breaks down and is shed as the menstrual flow.
• Follicular/proliferative phase — an egg matures in the ovary and the uterus lining thickens again.
• Ovulation (around day 14) — the mature egg is released.
• Luteal/secretory phase — the uterus is prepared to receive a fertilised egg; if none arrives, the cycle repeats.

The menstrual cycle begins at puberty (menarche) and stops around middle age (menopause).

When sperm are deposited in the female tract, they swim toward the egg in the oviduct. Fertilisation is the fusion of a sperm with the egg to form a single cell, the zygote, which is diploid and carries genes from both parents. Only one sperm normally enters the egg.

The zygote begins to divide by mitosis as it travels down the oviduct, becoming a ball of cells called the blastocyst (embryo). About a week after fertilisation, this embryo embeds itself in the thick, prepared lining of the uterus — a step called implantation. This marks the start of pregnancy (gestation).

After implantation, a special structure called the placenta develops, connecting the embryo to the wall of the uterus through the umbilical cord. The placenta is the lifeline of the baby: it allows oxygen and nutrients to pass from the mother's blood to the baby, and carbon dioxide and wastes to pass back to be removed by the mother. It also makes hormones that maintain the pregnancy.

Over about nine months, the embryo (later called a foetus) grows and all its organs form and mature. The major organs begin to form in the first three months. Finally, the muscular uterus contracts strongly in the process of childbirth (parturition), and the baby is born. After birth the mother's mammary glands produce milk (lactation) to feed the baby; the first milk (colostrum) is rich in antibodies that protect the newborn.

Right after fertilisation, the zygote begins repeated mitotic divisions called cleavage as it moves down the oviduct. These divisions first form a solid ball of cells called the morula, which then develops a cavity to become the hollow blastocyst — the stage that implants in the uterine wall.