Stem cells and differentiation

A fertilised egg divides again and again. At first all the new cells are the same, but soon they differentiate: they become specialised for one job, as nerve, muscle or blood cells. A stem cell is a cell that has not yet differentiated. It can divide to make more stem cells and can become other types of cell, which is why stem cells may one day treat many diseases.

Key facts

  • Differentiation is the process by which a cell becomes specialised for a particular job. It gains the sub-cellular structures it needs, such as many mitochondria in a muscle cell.
  • Most animal cells differentiate at an early stage. In mature animals, cell division is mainly for repair and replacement. Many plant cells can differentiate all through the plant's life.
  • Embryonic stem cells, from early human embryos, can become almost any type of human cell. Adult stem cells, such as those in bone marrow, can form only some types, such as blood cells.
  • Meristems at the tips of roots and shoots can make any type of plant cell throughout the plant's life. They are used to clone plants quickly and cheaply: rare species, and crops with useful features such as disease resistance.
  • Stem cells could treat conditions such as diabetes and paralysis. Bone marrow transplants already use adult stem cells to treat leukaemia.
  • In therapeutic cloning, an embryo is made with the same genes as the patient. Stem cells from it are not rejected by the patient's immune system.
  • There are risks and objections: cells grown in a lab could pass on a virus; some people object to destroying embryos, which they believe have a right to life.

Worked examples

Example 1

What makes a stem cell different from a specialised cell?

  1. A specialised cell, such as a red blood cell, is suited to one job and stays that way.
  2. A stem cell has not differentiated yet.
  3. It can divide to make more stem cells, and it can differentiate into specialised cells.

Answer: a stem cell is undifferentiated: it can divide to make more stem cells and can become other types of cell

Example 2

Why can a small cutting from a plant grow into a whole new plant?

  1. Plant cells can differentiate throughout the plant's life.
  2. Meristem cells in the cutting divide and form new roots.
  3. The new plant is a clone: genetically identical to the parent.

Answer: it contains meristem cells that can become any type of plant cell, so it can grow new roots, stems and leaves

Example 3

A patient has diabetes because the cells in their pancreas that make insulin are damaged. Give one benefit and two drawbacks of treating them with embryonic stem cells.

  1. Benefit: embryonic stem cells can become almost any cell, including insulin-making cells.
  2. Ethical drawback: the embryo is destroyed, and some people believe it has a right to life.
  3. Risk: cells grown in the lab could be infected with a virus.
  4. Risk: the patient's immune system could reject the cells, unless they come from therapeutic cloning.

Answer: benefit: the stem cells could become new insulin-making cells; drawbacks: an embryo is destroyed, and the cells could carry a virus or be rejected

Example 4

How does therapeutic cloning stop a patient rejecting stem cells?

  1. The nucleus is taken from one of the patient's body cells.
  2. It is put into an egg cell whose own nucleus has been removed.
  3. The egg divides into an early embryo with the same genes as the patient.
  4. Stem cells from it match the patient, so the immune system does not attack them.

Answer: the embryo is made from the patient's own genes, so the stem cells match the patient