Classification of brain pericytes based on cell structure, location, and α-smooth muscle actin content

Smooth muscle cells and pericytes, together called mural cells, coordinate numerous vascular functions. Canonically, smooth muscle cells are ring-shaped and cover arterioles with circumferential processes, whereas pericytes extend thin processes that run longitudinally along capillaries. Nearly a century ago, Zimmerman described mural cells with mixed features of smooth muscle cells and pericytes, which he termed “transitional pericytes”. Recent studies suggest that transitional pericytes are critically positioned to regulate cerebral blood flow, but there remains considerable confusion on how to identify and categorize them. Here, we use metrics of cell morphology, vascular territory, and α-smooth muscle actin expression to test the hypothesis that transitional pericytes can be distinguished from canonical smooth muscle cells and pericytes. We first imaged fluorescently-labeled mural cells in large volumes of opticallycleared mouse cortex to elucidate the location of potential transitional pericytes. Subsequent investigation of isolated mural cells revealed that one type of transitional pericyte, which we called “ensheathing pericytes”, could be reliably distinguished. Ensheathing pericytes possessed protruding ovoid cell bodies and had elongated processes that encircled the vessel lumen. They were rich in α-smooth muscle actin and occupied proximal branches of penetrating arteriole offshoots. We provide guidelines to identify ensheathing pericytes and other mural cell types.