Characterization of smooth muscle cell and pericyte differentiation in the rat retina in vivo

Purpose. To identify and apply a range of suitable mural cell markers and undertake an in vivo characterization of pericyte and smooth muscle cell (SMC) differentiation in the developing rat retina. Methods. Pericyte and SMC differentiation was characterized by immunohistochemistry with antibodies to NG2, desmin, alpha-smooth muscle actin (SMA), calponin, and caldesmon. Results. Immature mural precursor cells (MPCs) were scattered throughout the primitive capillary plexus in the rat retina at embryonic day (E)20. The postnatal differentiation of pericytes and arteriolar and venous SMCs followed with distinct intermediate phenotypes. SMC differentiation coincided with selection of major vessels from the primordial capillary bed. Maturation of radial arteriolar SMCs was indicated by the expression of calponin and caldesmon, proteins that play a role in the regulation of SMC contraction. The mere presence of immature mural cells did not confer vessel stability; rather vessel stability in the developing rat retina coincided with caldesmon and calponin expression in arteriolar SMCs. Conclusions. This normative data and the identification of suitable in vivo markers of pericytes and SMCs will allow meaningful interpretation of the changes in these cell types. When examining the role of mural cells in developmental and pathologic vascularization, the results show that there is a need to use multiple-marker immunohistochemistry because of significant mural cell heterogeneity. The observation that the expression of caldesmon and calponin in arteriolar SMCs coincides with resistance to hyperoxia in the developing rat retina, lead us to suggest that maturation of SMCs and their consequent ability to regulate blood flow may play a key role in vessel stabilization.