Stem cells are unspecialized cells that can divide repeatedly and differentiate into specialized cell types. They are special because they combine long-term self-renewal with the potential to produce cells with different structures and functions.
This is required for both SL and HL in IB Biology subtopic B2.3, Cell specialization.
Why Stem Cells Are Special
Stem cells have two defining properties:
- Self-renewal: they can undergo mitosis repeatedly while retaining their unspecialized state. This maintains a supply of stem cells.
- Potency: they can differentiate along one or more developmental pathways. During differentiation, changes in gene expression cause cells to produce different proteins, giving them specialized structures and functions.
In early animal development, the fertilized egg divides to produce unspecialized cells. These initially have very broad developmental potential, but their potency becomes more restricted as development proceeds.
| Stem cell type | Developmental potential | Example |
|---|---|---|
| Totipotent | Can form every cell type, including tissues needed to support embryonic development | Cells of a very early embryo |
| Pluripotent | Can form many body cell types, but not an entire organism | Embryonic stem cells at a later stage |
| Multipotent | Can form a limited range of related cell types | Adult stem cells in bone marrow |
In adults, stem cells occur in stem cell niches, such as bone marrow and hair follicles. A niche provides conditions that maintain stem cells or stimulate their proliferation and differentiation. Bone marrow stem cells, for example, replace blood cells that are continually lost.
A common misconception is that all stem cells can become any cell type. Only totipotent cells have the widest potential; adult stem cells are generally multipotent and therefore more restricted.
