Identifying Stem Cells

As early as 1961, scientists knew that adult bone mar- row contained  cells that  could make all of the blood cell types. But it wasn’t until 1988 that those stem cells were isolated as pure populations. Why did it take so long? The techniques  for identifying  stem  cells have only recently  been developed.  Partly,  this  is because adult stem cells are, by their very nature, inconspicu-  ous in shape, size, and function. They also tend to hide deep in tissues and are present only in very low num- bers,  making  their  identification  and  isolation   like finding a needle in a haystack.

Fluorescent markers can be used to identify stem cells hidden among ordinary adult cells. Here, human embryonic stem cells are recognized by the marker proteins they express (green). Courtesy of Paul J. Tesar, Laboratory of Molecular Biology, NINDS and the  NIH  Stem Cell  Unit.

How do scientists know when they have found a stem cell? Every cell displays an array of proteins on its surface; different cell types have different proteins. Scientists can use these surface proteins  as “markers” that characterize individual  cell types—a type of “molecular ID.”  For  example,  using  molecules  that recognize and attach  to specific surface proteins  and that can fluoresce under certain wavelengths  of light, scientists  can  visually  tell  the  difference  between  a blood stem cell and a mature white blood cell. Unfortunately, not all stem cells can now be identified in this manner  because scientists have not yet identi- fied markers for all stem cell types. Scientists also iden- tify stem cells by observing their behavior  in the laboratory: stem cells must be able to remain  unspe- cialized and self-renew for long periods of time. Scientists  believe that  there  might  be more  types  of adult  stem cells than  the handful  that  have  already been identified, but finding them is a difficult process.