Stem cells: Stem cells from adult bone marrow can form many types of cells including blood cells.

The idea

Adult stem cells are found in tissues such as bone marrow. They are unspecialised cells that can divide to replace worn-out or damaged cells, but they are more limited than embryonic stem cells: they can form many cell types, including blood cells, rather than most cell types. In bone marrow, stem cells can differentiate into red blood cells, which carry oxygen, and white blood cells, which defend against pathogens. They can also produce platelets, but platelets are cell fragments, not whole cells, so they are not counted as blood cells. This is why bone marrow transplants can restore blood cell production in patients whose marrow is damaged or diseased. Using a patient's own adult stem cells avoids the ethical concerns linked to embryos and reduces the risk of immune rejection, but adult stem cells are harder to grow in large numbers and cannot form as many cell types.

What earns the marks

• Adult stem cells are found in body tissues such as bone marrow and remain unspecialised.
• Their potency is more limited than that of embryonic stem cells, so they cannot form most cell types.
• Bone marrow stem cells are used in transplants to restore blood cell production, and using a patient's own cells avoids embryo-related ethical issues and reduces rejection risk.
• Platelets are cell fragments, not cells, so they should not be listed as an example of a blood cell formed by adult stem cells.

Common mistakes and how to put them right

• Saying adult stem cells can form any cell type.

  Put it right by stating they can form many cell types, including blood cells, but not most cell types.

• Confusing adult stem cells with embryonic stem cells.

  Put it right: Adult stem cells are found in tissues such as bone marrow, whereas embryonic stem cells come from early embryos.

• Thinking adult stem cells only make red blood cells.

  Put it right by including white blood cells as a second blood cell example, and note that platelets are cell fragments rather than cells.

Exam tips

• Name bone marrow as the source and give at least two blood cell examples, such as red blood cells and white blood cells.
• Compare adult and embryonic stem cells directly using the words many and most to show the difference in potency.
• When discussing treatment, link bone marrow stem cells to restoring blood cell production and mention one advantage such as reduced rejection risk.