Reproduction and embryology

The reproductive system enables humans to produce offspring, and embryology follows the development of a new individual from a single fertilised cell. Sexual reproduction combines genetic material from two parents, so the offspring is genetically distinct from either. The sex cells, or gametes — egg and sperm — are produced by a special form of cell division called meiosis, which halves the chromosome number so that fertilisation restores the normal amount.

Fertilisation occurs when a sperm cell joins with an egg, forming a single cell that carries a full set of chromosomes from both parents. This cell then divides repeatedly, first into a small cluster of cells and then into an organised structure in which cells begin to specialise. Over the following weeks the developing embryo forms the foundations of the body's tissues and organs, before later being referred to as a fetus as development continues.

Understanding fertilisation and early development underpins assisted reproductive technologies. In in vitro fertilisation (IVF), eggs and sperm are brought together in a laboratory and a resulting early embryo is transferred to the womb, which can help people who have difficulty conceiving naturally. The early embryo is also the source of embryonic stem cells, unspecialised cells able to develop into many different cell types; their ability to specialise makes them a major focus of medical research, alongside the ethical questions their use raises.

Frequently asked questions

What happens at fertilisation?

A sperm joins with an egg to form a single cell containing chromosomes from both parents, which then begins to divide and develop into an embryo.

What is in vitro fertilisation?

IVF is a technique in which eggs and sperm are combined in a laboratory and an early embryo is then placed in the womb to establish a pregnancy.

What makes stem cells special?

Stem cells are unspecialised cells that can develop into many different cell types, which is why they are studied for repairing or replacing damaged tissue.