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Gary Hin-Fai Yam, Xinyi Seah, Emily Benson, Gary S L Peh, Jodhbir S Mehta; Phenotypic variation of transition zone in human posterior limbus. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4153. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
At the corneal periphery, the corneal endothelium (CE) and Descemet’s membrane (DM) terminate at the Schwalbe’s line, which, under higher magnification, is a thin strip of transition zone (Tz) or smooth zone. The outer Tz border is demarcated by the anterior trabecular meshwork (TM) beam inserts and bridges. Here, we presented an ultrastructural characterization with expression analyses on human Tz.
Research grade cadaveric corneal tissues were obtained. Tz cell distribution was revealed by Alizarin red staining and cell density by quantifying DAPI nuclei (n=4 corneas). Scanning electron micrographs from 20 corneas were obtained for 360° Tz width measurement using method validated by Bland-Altman plots. Serial block face-scanning electron microscopy (SBF-SEM) on 2 limbal tissues was performed for 3D reconstruction of Tz region using Imaris9.2.1 The expression of stem cell and CE differentiation markers were detected by immunofluorescence, immunogold SEM and RNA analysis.
The superficial Tz cells facing to the anterior chamber had irregular shapes, and were loosely arranged with lowered cell density, when compared to the adjacent peripheral endothelium (PE). The Tz width between TM insert and DM border was highly variable in terms of circumferential profile with an average width of 176±40μm (n=15 fully measurable corneas). Among equal-sized quadrants, the narrowest Tz was found in the nasal quadrant (n=5 orientation-marked corneas). In 3D-reconstructed images, the terminal stretch of DM was inserted beneath Tz basement membrane. Confocal immunofluorescence showed that stem cell/progenitor marker expression was localized in the inner Tz (adjacent to PE) with Lgr5 positive cells streaming into PE in the form of narrow cell rows. This region-specific expression was confirmed by immunogold SEM and RNA analysis.
This corroborative study characterizing human Tz showed that Tz was phenotypically different to the adjacent PE. Our results highlight that the inner Tz may represent a niche of CE progenitors. Future work on Tz cell isolation and differentiation may lead to more understanding of the existence and properties of CE progenitors and their roles in CE homeostasis.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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