Morphological classification of RALDH2-positive cells in the human choroid

Abstract

Purpose : A key signal molecule in postnatal eye growth is all-trans-retinoic acid and it has been speculated that all-trans-retinoic acid is involved in the regulation of scleral matrix regulation. Apparently choroidal RALDH2 represents the key-player in ocular growth inhibition, also in postnatal eyes. While RALDH2-positive cells have been recently detected within the choroid, it is not clear up to now if these RALDH2-expressing cells are an independent cell population or a sub-type of cells in the already known canon of choroidal cells. Therefore, aim of this study is to morphologically identify and classify this cell-population with immunohistochemical methods in the human choroid.

Methods : Human eyes were obtained from the Salzburg Eye Bank, in full accordance with the Declaration of Helsinki. Choroids were dissected free and prepared for double-immunohistochemistry of RALDH2 and CD31 (vascular endothelium), CD146 and NG2 (pericytes), smooth-muscle actin (ASMA), CD68 and LYVE1 (macrophages), IBA1 (microglia), PGP9.5 (pan-neuronal) and vimentin. Confocal laser-scanning microscopy in single optical section mode was used for documentation.

Results : Within the choroidal stroma, RALDH2+ cells displayed short and more circumferential arranged processes. Others were located adjacent to choroidal blood vessels, these displayed scare cytoplasm and long processes with polar arrangement. Vessel-associated cells were negative for CD146 and NG2, and were further not co-localized with CD31 or ASMA. RALDH2+ cells were negative for LYVE1 and CD68, were negative for CD146 and NG2, and were lacking IBA1 and PGP9.5. Some, but not all RALDH2+ cells were co-localized with vimentin. Intrinsic choroidal neurons were lacking RALDH2, as were melanocytes when identified in epi-detection mode.

Conclusions : With the markers used here, RALDH2+ cells seem to represent an independent cell-population within the human choroid. While some RALDH2+ cells were positive for the intermediate filament vimentin, this could indicate a mesenchymal origin. Additional markers are necessary to further classify this new cell-population and the possible contribution in ocular pathologies

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.