Stem cells by source

Stem cells are grouped by where they come from: embryonic, fetal and adult.

Embryonic stem cells (ESCs) come from the inner cell mass of the blastocyst, about five days after fertilisation. They are:

• pluripotent: they can form cells of all three germ layers, and so almost any cell of the body;
• possibly immortal: they can self-renew for a very long time while keeping a stable, normal (diploid) set of chromosomes;
• flexible: depending on the conditions, they either keep renewing or start specialising.

They hold great promise for medicine, but their use is limited by safety concerns and by ethical questions, because obtaining them destroys an early embryo.

Fetal stem cells come from the fetus (an embryo is called a fetus after the eighth week of development): from fetal blood, bone marrow, liver and kidney. Fetal blood is rich in blood-forming stem cells, which multiply faster than those from cord blood or adult bone marrow.

Adult (somatic) stem cells are found after embryonic development is complete, even in children and in cord blood, and most of all in bone marrow. Their normal job is to maintain and repair their own tissue. They were once thought to make only the cells of their own tissue, but some show plasticity (see the next lesson). Two important kinds:

• Haematopoietic stem cells (HSCs), in the bone marrow, make all the blood cells.
• Mesenchymal stem cells (MSCs), from bone marrow, fat and the umbilical cord, multiply well, are genetically stable, tend not to be rejected, and can repair cartilage, bone and fat.