Characterizing pericytes and the vascular transition zones of the human macula in postmortem eyes

To quantify the pericyte density in the macular capillaries, define the distribution of the transition zones, and characterize inter-pericyte tunneling nanotubules (IP-TNTs) in postmortem donor eyes. Six healthy postmortem donor retinas were immunostained with NG2 (neural/glial antigen 2; pericytes), αSMA (alpha-smooth muscle actin; vascular smooth muscle cells), and CD31 (cluster of differentiation 31, endothelial cells), along with DAPI nuclear staining. Pericytes were identified by their NG2 staining and their density quantified in the superficial, middle, and deep capillary plexuses (SCP, MCP, DCP). The transition zone was identified by pericytes double-labeled with αSMA/NG2 along downstream branches of the major arterioles. IP-TNTs were identified as tubular structures, marked by NG2 staining and devoid of CD31 staining, that connect pericytes on separate capillary systems. Pericyte density as well as the ratio of pericytes per junction were significantly higher in the transition zones than within the capillary plexuses (51.6 ± 14.7 versus 22.7 ± 5.9 pericytes per 1000 μm, P < 0.001; 1.22 ± 0.35 vs. 0.73 ± 0.3, P < 0.001, respectively). A lower density of pericytes was observed in the DCP (20.1, 17–23) compared to the SCP (22.9, 18–29) and MCP (23.6, 19–27, P = 0.002). While they were more abundant in the SCP, transition zone vessels were identified in all the capillary plexuses, and IP-TNTs were more abundant within the MCP. Our results suggest that the macular capillary layers have unique structural attributes, with transition zones likely contributing to independent functional regulation of each layer. The significantly higher pericyte density and junctional coverage in the transition zones and their extension into the different plexuses underscores their important regulatory role in the human macula. The lower pericyte density in the DCP, and prevalence of IP-TNTs in the MCP suggest these layers have unique functional adaptations.