Cell death, differentiation and migration

How cells die. Apoptosis is programmed, controlled cell death. It shapes the body (the cells between a baby's fingers die in the embryo, separating them) and protects against virus infection and cancer. The cell's DNA breaks into pieces, its membrane bulges out in blebs, and macrophages tidy up the remains, with no inflammation. Necrosis is unplanned death from injury or poisons: the cell bursts and its contents spill out, causing inflammation.

How cells specialise. All body cells carry the same DNA, yet a neuron and a red blood cell are very different, because different genes are switched on in each. This is differentiation, guided by signalling molecules called growth factors.

Stem cells stay unspecialised. When one divides, one daughter stays a stem cell and the other differentiates. Embryonic stem cells come from the inner cell mass of a week-old embryo (blastocyst). Adult stem cells wait in the bone marrow, brain, muscle and liver until an injury activates them.

Stem cells differ in how many cell types they can become (their potency):

• Totipotent: the zygote, which can form every cell, including those outside the embryo (like the placenta).
• Pluripotent: the inner cell mass, which can form cells of all three germ layers.
• Multipotent: for example haematopoietic stem cells, which form blood cells only.

In 2006, Shinya Yamanaka turned ordinary adult cells back into stem cells using a few transcription factors: induced pluripotent stem cells (iPSCs). They can give cells that match a patient exactly.

How cells move. A moving cell polarises (gets a front and back), pushes out a leading edge by building actin filaments, grips the surface in front, pulls its body forward and lets go at the rear. Cell movement shapes the embryo, heals wounds and drives immune responses, but it also lets cancer cells spread around the body (metastasis).