Circulating progenitor cells are a special kind of cell that can travel through the body and differentiate into many types of tissue. There are many chemical factors that can affect the migration and development of circulating progenitor cells. Neural progenitor cells, for example, can develop into neurons (grey matter) or glial cells (white matter) in the presence of certain growth factors found in the brain. Progenitor cells have receptors for these factors that help the cells identify when and where they are most needed.
Neural progenitors are affected by the same molecules that help other tissue types to grow and differentiate. These molecules include growth factors that occur naturally in fetal development. Putting neural progenitor cells near factors like epidermal growth factor and fibroblast growth factor-2 causes them to multiply rapidly.
When the growth factors are removed, the progenitor cells begin to differentiate into both neurons and glial cells. Other growth factors can encourage circulating progenitor cells to become muscle, bone, or other tissue types. This system allows the body to carefully control the number of cells it has available for injury recovery and tissue growth. When new tissue is needed, cells release the appropriate growth factor to attract progenitor cells.
A peptide called substance P is another factor that attracts circulating progenitor cells. Substance P normally causes an increase in neural progenitor cells upon exposure. Research has found that when the brain is injured, cells near the injured area release substance P to attract more progenitor cells.
These progenitor cells developed into glial cells that helped repair damage to the injured area. The glial cells also strengthened connections between neurons, allowing the neurons to continue sending signals. Substance P, therefore, attracts progenitor cells to help prevent injured tissue from dying, and is one method the brain uses to recover from trauma.
Following an injury, the body needs a way to attract progenitor cells to the injury site. Progenitor cells are created in the bone marrow, but only travel through the bloodstream when signaled. This signaling is often performed by chemicals called cytokines, such as stromal-derived factor-1 (SDF-1).
Cells at the injury site release SDF-1, and direct progenitor cells into the bloodstream. The circulating progenitor cells seek out higher concentrations of SDF-1, leading them to the injury site. Once there, other growth factors tell the progenitor cells which types of tissue are needed, and the progenitors differentiate appropriately.