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B. Braunger, M. Zenkel, U. Schlötzer-Schrehardt, F. E. Kruse; Differential Expression of Matrix Glycoproteins by Limbal Fibroblasts Contributes to the Limbal Stem Cell Niche. Invest. Ophthalmol. Vis. Sci. 2007;48(13):445.
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Stem cells are controlled and maintained by various factors creating an unique microenvironment, the stem cell niche. Presumable components of the limbal stem cell niche are specialized matrix and signaling molecules released from fibroblasts in the subepithelial stroma at the limbus. The purpose of this study was to analyze whether limbal fibroblasts differentially express matrix glycoproteins that are known to exert signaling functions in other stem cell compartments.
Stromal fibroblasts were isolated from the central cornea and the limbus of five human donor eyes and directly subjected to RNA isolation or further in vitro expansion in DMEM/F12 containing 15% FCS. mRNA expression of extracellular matrix glycoproteins tenascin-C, SPARC/osteonectin, vitronectin and fibulin-2 as well as the chondroitin sulfate proteoglycan versican was analyzed by quantitative Real-time PCR. Protein expression and localization were examined by immunohistochemistry on limbal/corneal sections of 10 human donor eyes.
Quantitative Real-time PCR showed significantly higher expression levels of vitronectin (10-fold), versican (3-fold) and fibulin-2 (1.5-fold) in cultured limbal fibroblasts compared with cultured corneal fibroblasts, while expression levels of tenascin-C and SPARC did not differ. Fibroblasts isolated directly from the limbal stroma expressed significantly higher levels of tenascin-C (21-fold), versican (9-fold), fibulin-2 (2-fold), and SPARC (2-fold) compared with fibroblasts derived from the central cornea. Vitronectin was only expressed in isolated limbal, but not in corneal fibroblasts. Immunohistochemistry confirmed increased expression of all molecules studied in subepithelial fibroblasts at the limbus compared with the corneal stroma.
Various glycoproteins/proteoglycans appear to be preferentially or selectively secreted by limbal fibroblasts and may contribute to an epithelial-stromal cross-talk in the context of the limbal stem cell niche.
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