While endothelial cells are known as the constituent cell class of blood vessels, fibroblast-like cells are a recently-described population with unknown function that inhabit the perivascular space in the brain (Vanlandewijck et al., 2018). We found 7 subclusters each of endothelial (n=16,248) and fibroblast-like (n=1,587) cells (Figure 2D,E). Among the fibroblast-like cells, two subclusters (4 and 5) selectively expressed many genes encoding membrane transporters (e.g., Slc38a2, Slc4a10, Slc26a2, and Slc47a1) and pumps (e.g., Fxyd5 and Atp1b1) (Figure 2D). To varying extents, subclusters 1, 2, and 3 expressed genes involved with extracellular matrix (ECM) secretion, angiogenesis, and contraction (Data S4F), such as the basement membrane collagen genes (Col4a1, Col4a2, and Col15a1) (Figure 2D). Interestingly, cluster 3 expressed higher levels of different collagen genes (Col1a1 and Col3a1) (Data S4F). These examples suggest that fibroblast-like cells are transcriptionally specialized to support membrane transport and ECM production; ECM secretion may involve different sets of co-regulated collagen genes.
