Objective: Similar to other vascular pericyte markers, including smooth muscle (SM) alpha-actin, desmin, and PDGF-beta-receptor, NG2 proteoglycan is not pericyte specific. Therefore, the use of NG2 as a pericyte marker, especially in cell lineage studies, in comparison to other nonspecific pericyte markers requires an understanding of how its expression varies spatially within a microvascular network. The objective of this study was to characterize NG2 expression along vessels within rat microvascular networks and compare this to SM alpha-actin expression.

Methods: Mesenteric tissue, subcutaneous tissue, spinotrapezius muscle, and gracilis muscle were harvested from 250-g, female, Sprague-Dawley rats and stained for NG2 and SM alpha-actin. The distribution of NG2 expression was evaluated in mesenteric networks (n = 28) with complementary observations in subcutaneous tissue and skeletal muscle.

Results: Perivascular cells, including mature smooth muscle cells (SMCs), immature SMCs, and pericytes, expressed NG2. Most importantly, NG2 expression was primarily confined to perivascular cells along arterioles and capillaries, and continuous expression was not observed along venules beyond the immediate postcapillary vessels. The differential expression of NG2 along the arteriolar side of microvascular networks was also observed in rat subcutaneous and skeletal muscle.

Conclusions: The results indicate that NG2 is expressed by all perivascular cells along arterioles, and its absence denotes a venule-specific phenotype. These results identify for the first time a marker that differentiates venous smooth muscle and pericytes from other capillary- and arteriole-associated perivascular cells.