Stem cell niches are specialized microenvironments within tissues. For IB Biology at both SL and HL, the two key human examples are bone marrow and hair follicles.
A stem cell niche is the immediate environment surrounding a population of stem cells. It includes neighbouring cells, extracellular material and signalling molecules that regulate stem-cell behaviour.
The niche has two main possible effects. It can maintain stem cells in an undifferentiated state, preserving their ability to divide, or it can promote their proliferation and differentiation when replacement cells are required.
| Location of niche | Function |
|---|---|
| Bone marrow | Contains haematopoietic stem cells, which can divide and differentiate to replace blood cells. Signals from the niche regulate whether these cells remain as stem cells or develop into specialized blood-cell types. |
| Hair follicle | Contains stem cells that contribute to repeated hair growth and the replacement of cells associated with the follicle. The niche regulates when these cells remain inactive and when they divide and differentiate. |
These locations illustrate why niches are essential. Stem cells must be protected and maintained so that the supply is not exhausted, but they must also respond when growth or tissue repair requires new specialized cells.
A common misconception is that a stem cell niche is simply a place where stem cells are stored. It is an active regulatory environment: interactions within the niche influence gene expression, division and differentiation.
Exam technique: For subtopic B2.3 Cell specialization, be ready to identify bone marrow and hair follicles as two niche locations and explain that a niche can either maintain stem cells or promote their proliferation and differentiation. If the command term is “explain,” link the niche’s signals to the production of specialized replacement cells rather than merely naming the locations.