Confident identification of pericytes (PCs) remains an obstacle in the field, as a single molecular marker for these unique perivascular cells remains elusive. Adding to this challenge is the recent appreciation that PC populations may be heterogeneous, displaying a range of morphologies within capillary networks. We found additional support on the ultrastructural level for the classification of these PC subtypes—“thin-strand” (TSP), mesh (MP), and ensheathing (EP)—based on distinct morphological characteristics. Interestingly, we also found several examples of another cell type, likely a vascular smooth muscle cell, in a medial layer between endothelial cells (ECs) and pericytes (PCs) harboring characteristics of the ensheathing type. A conserved feature across the different PC subtypes was the presence of extracellular matrix (ECM) surrounding the vascular unit and distributed in between neighboring cells. The thickness of this vascular basement membrane was remarkably consistent depending on its location, but never strayed beyond a range of 150–300 nm unless thinned to facilitate closer proximity of neighboring cells (suggesting direct contact). The density of PC-EC contact points (“peg-and-socket” structures) was another distinguishing feature across the different PC subtypes, as were the apparent contact locations between vascular cells and brain parenchymal cells. In addition to this thinning, the extracellular matrix (ECM) surrounding EPs displayed another unique configuration in the form of extensions that emitted out radially into the surrounding parenchyma. Knowledge of the origin and function of these structures is still emerging, but their appearance suggests the potential for being mechanical elements and/or perhaps signaling nodes via embedded molecular cues. Overall, this unique ultrastructural perspective provides new insights into PC heterogeneity and the presence of medial cells within the microvessel wall, the consideration of extracellular matrix (ECM) coverage as another PC identification criteria, and unique extracellular matrix (ECM) configurations (i.e., radial extensions) that may reveal additional aspects of PC heterogeneity.
Unequivocal pericyte identification remains a limitation in the field of vascular biology given the lack of unique molecular marker. Compounding this challenge are the recently described heterogeneities in pericyte morphology across microvascular networks. Here, we found further support on the ultrastructural level for classifying pericytes into sub-types - "thin-strand" (TSPs), mesh (MP), and ensheathing (EP) pericytes - based on their architecture in the mouse brain microcirculation. We also observed several instances of an additional cell type in the medial layer between endothelial cells and pericytes, specifically associated with EPs. A conserved characteristic across PC subtypes was extracellular matrix (ECM) encompassing the vascular unit and dispersed among neighboring cells. ECM thicknesses fell within a specific range depending on vessel location, and only thinned where cells were in closer proximity. Pericytes and endothelial cells formed "peg-and-socket" structures at these locations, providing another distinguishing feature across PC subtypes. Unique contact locations seemed to be present between medial and endothelial cells, as well as between vascular cells and the brain parenchyma. The ECM surrounding EPs exhibited another notable configuration in that thin extensions radiated out from the vessel wall into the surrounding parenchyma, suggesting mechanical and/or biochemical roles. Considering these data together, ultrastructural observations may provide an orthogonal perspective on pericyte heterogeneity and the presence of medial cells in cerebrovascular walls as well as assessing ECM coverage as a criterion for PC identification and exploring PC-associated ECM extensions that may have unique relevance in health and disease.
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