gms | German Medical Science

Recent studies suggest that hematopoietic stem cells can be found in several tissues of mesodermal origin, such as muscle or adipose tissue. We examined the corneal stroma for the presence of cells expressing hematopoietic, endothelial, and stem cell markers.

Corneas from human donors that were excluded from transplantation because of low endothelial cell count were obtained from the local cornea bank. Corneal endothelium and epithelium were removed by collagenase digestion and by mechanical scraping. Collagenase-isolated stromal cells were subjected to two-color flow cytometry and methylcellulose cultures. Expression of CD133, CD34, CD14, CD45, CD105, CXCR4, c-kit, bcrp-1, VE-Cadherin, CD31 and various cytokine receptors was analyzed. Explant cultures were grown from mechanically dissected corneal tissue in medium selective for hematopoetic or endothelial differentiation. The human embryonic keratocyte cell line Ek1Br, adult corneal tissue and dermal fibroblasts were analyzed for the presence of CD133 mRNA.

On average, 5 % of the stromal cells expressed the stem cell marker CD133, and 3 % coexpressed CD34. Further phenotypic studies revealed expression of the leucocyte antigen CD45, the monocytic markers CD14 and CD105 as well as the receptors for the hematopoietic cytokines G-CSF and GM-CSF. Upon transfer to methylcellulose, stromal cells generated CD45+ colony-forming units of the monocytic lineage that could be further differentiated into cells with keratocyte morphology. In suspension culture supplemented with hematopoietic growth factors, increased numbers of CD133+ progenitor cells as well as CD133-CD45+CD14+ monocytic cells were observed. In contrast, stromal cells were not able to differentiate into the endothelial lineage when stimulated with angiogenic growth factors and did not generate any endothelial colonies in methylcellulose. mRNA expression of CD133 was found in human corneal stroma and in EK1Br cells.

Taking into account that CD133+ cells coexpressed hematopoietic markers, and generated monocytic colonies, CD133-expressing stromal cells can be considered as monocytic progenitor cells. Based on the findings that CD133+ cells have the capacity to proliferate in culture, and that CD133-derived colonies could be differentiated into fibroblastic cells, we conclude that CD133+ cells could represent the stem cell of the corneal stroma.