Question #064d0
2 Answers
Any terminally differentiated cell type
Explanation:
There are terminally differentiated cells (which do not divide) in every tissue in the body.
The cells that actually divide are transit amplifying cells, precursors/progenitors, and stem cells.
These cells differentiate into their eventual phenotype based on requirement and signaling cues, and they won't divide again (not naturally at least).
They can however be reprogrammed by genetic modification into a more 'stem-like' fate to make them proliferate again.
In higher animals, some cell types are so far 'gone' that you cannot turn back the clock on them. For example, RBCs lose their nucleus and neurons undergo extreme changes to their morphology (elongated axons, dendrites).
Barring cells such as these, upregulating 'stemness' genes such as oct4, sox2, nanog, etc can make a cell regain its proliferating capability. This idea forms the basis of induced pluripotent stem cells.
More advanced concepts (optional reading):
There are exceptions to this rule. Cells that have been thought to be terminally differentiated sometimes naturally 'transdifferentiate' into another cell type, or go into a more primitive mitotic state to compensate for tissue loss. Examples of this are limb regeneration in salamanders, and liver regeneration in mammals. More primitive organisms like the Hydra can regenerate their entire body even if there is only a small chunk left.
Look up these terms: epimorphosis, morphollaxis, transdifferentiation, compensatory regeneration.
germ cells
