Characterization of heterogeneous glial cell populations involved in dehydration-induced proliferation in the adult rat neurohypophysis.

The adult neurohypophysis (NH) is a well-established site of CNS plasticity: its glial cells, the pituicytes, reorganize their structure and undergo increased proliferation in response to stimulations such as dehydration. However, it remains to be clarified whether the newly-formed cells derive from pituicytes re-entering the cell cycle or from glial precursors or stem cells. Here, we first analyze the expression of several glial markers in the adult rat NH and demonstrate that the pituicytes constitute a heterogeneous population. In particular, we identify a distinct subtype of glial cells expressing the oligodendrocyte precursor marker platelet-derived growth factor receptor alpha (pdgfralpha). In addition, adult NH explants can give rise to migratory precursors able to differentiate into mature oligodendrocytes, unlike NH cells in vivo. This led us to hypothesize that the adult NH could contain immature cells, therefore we used a neurosphere-forming assay to test for the presence of stem or progenitor cells. Adult NH cells can generate bipotent primary neurospheres but not secondary ones, suggesting that the structure contains glial progenitors but probably not stem cells. Finally, when the NH is stimulated by dehydration, we observe an increase in cell proliferation associated with an increase in cell death. By identifying the cells incorporating bromodeoxyuridine (BrdU) or positive for Ki67, we demonstrate that this increased proliferation concerns all glial cell types in the adult NH, including the pdgfralpha+ cells. Our study shows that the NH is a complex structure composed of multiple glial subtypes, which all participate in the physiological response to dehydration.