Pericytes in Notch3 knockout and diabetic mice form engorged connections with vascular endothelial cells.

Purpose: Pericytes, cells crucially important to maintain a healthy microvasculature, make direct connections with vascular endothelial cells, yet the functional significance of these contacts remains largely unexplored. This study aims to investigate the ultrastructural morphological changes that occur in the interactions between pericytes and endothelial cells in mice lacking the Notch3 receptor and in diabetic retinopathy. Methods: Serial section transmission electron microscopy (ssTEM) was used to image mouse retinal ganglion cell layer capillaries in wild type (WT; 19 vessels), Notch3 knockout (KO; 16 vessels), conditional Notch3 KO (23 vessels), and diabetic mice (18 vessels). Over 2,000 images were manually segmented to trace the boundaries of the basement membrane, endothelial cells, mural cells, and peg-and-socket connections. Automated image analysis was used to measure contact lengths between pericytes and endothelial cells and peg-and-socket features. Results: While the vessels analyzed in each group were of similar diameter and pericyte coverage, Notch3 KO vessels had deeper pegs and increased connectivity between pericytes and endothelial cells. In both Notch3 KO and diabetic conditions, there was also an increase in the size of pericyte pegs. Conclusions: As the Notch3 receptor plays an important role in cell signaling between pericytes and endothelial cells, and diabetes is also known to disrupt Notch3 signaling, our hypothesis for the enlarged peg phenotype is that the pericytes and endothelial cells actively increase their contact surface to compensate for loss of Notch3 signaling.