Phagocytosis of O4+ axonal fragments in vitro by p75− neonatal rat olfactory ensheathing cells

Olfactory ensheathing cells (OECs) have gained wide interest because of their unique regeneration‐promoting capacity. However, despite their frequent use in regeneration studies, the characterization of the cells has remained fragmentary. In the present study, we analyzed freshly dissociated neonatal rat OECs at the light and electron microscopic level and studied their fate in vitro using a novel two‐step labeling protocol based on antibody internalization. We report the identification and characterization of two distinct OEC populations in situ and in primary cell suspensions that differed in number, p75 NGF receptor expression, and O4 immunoreactivity. The major OEC population in primary cells suspensions did not express p75 but stained positive for the glycolipid O4 (p75−/O4+). During culturing, these cells upregulated p75 expression and lost O4 immunoreactivity. Conversely, the minor OEC population consisted of p75+/O4− OECs that maintained p75 expression in vitro. Interestingly, ultrastructural analysis revealed not only that O4 immunoreactivity of p75− OECs was, in fact, due to O4+ axonal fragments adhering to the cell surface but also that p75− OECs rapidly phagocytosed these fragments in vitro. Taken together, the identification of two distinct OEC populations in the neonatal olfactory bulb that converge into single p75+ phenotype in vitro is reported. The observation that upregulation of p75 receptor expression in vitro was only apparent in those OECs closely associated with O4+ axonal processes may suggest that axonal signalling in vivo negatively regulates p75 receptor expression. The strong phagocytic activity of OECs in vitro may reflect one important aspect of their physiological function. © 2004 Wiley‐Liss, Inc.