Insights into human choroid melanocyte and stromal cells

Abstract

Aims/Purpose: The human choroid is a complex highly vascular pigmented tissue between Bruch's membrane and the sclera. Its dynamic nature is important for eye homeostasis and for in pathology including macular degeneration and myopia. The stroma comprises heterogeneous cell populations including fibroblasts, melanocytes, immune cells, smooth muscle cells and neurons within a loose collagenous extracellular matrix. We used RNA transcriptome and multimarker immunolabelling of human choroidal melanocytes and tissue to further characterize adult choroidal melanocytes.Methods: Human choroidal melanocytes were cultured (n = 6 postmortem donor eyes, <18 h delay, UNSW HREC), and used between passage (P) 2 to 5 for RNA transcriptome and multimarker immunofluorescence labelling and confocal microscopy. Sections and flatmounts from central choroid were also compared. Immunolabelling included melanocyte (Tyrosinase, TRYP1, DCT, MelanA) and stromal cell (S1004A, vimentin, α‐smooth muscle actin, CD45) markers.Results: Cultured melanocytes displayed mostly bipolar morphology with heterogeneous eumelanin density related to donor eye iris colour. Whole transcriptome findings showed enriched melanogenesis genes (high level TYR, PMEL, MITF, TYRP1; MLANA; low level DCT1, MC1R) but not genes reported for RPE (e.g. RPE65), pericytes (e.g. ACTA2) or fibroblasts (e.g. IGF2) confirming melanocyte culture quality. We also found expression for proposed Schwann cell precursors PLP1 and SOX10. Immunolabelling confirmed overall expression of melanogenesis proteins, and subpopulations expressing PLP1 and Sox10, in culture and in choroidal tissue.Conclusions: Melanocytes and stromal fibroblast are among the most numerous and complex heterogeneous populations within the choroid and our study provides further details on characteristics markers and topography. A possible role for Schwann cell precursors in renewing adult melanocyte populations is intriguing and continues to be investigated.