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The corneal endothelium is a monolayer of cells in the posterior cornea that is responsible for maintaining a clear cornea. Corneal endothelial cells may be induced to divide, but it has been held that they do not divide in the normal cornea of an adult human. Some studies have suggested that a stem cell population for the corneal endothelium exists. This population could give rise to mature corneal endothelial cells and may reside either in the peripheral corneal endothelium or in the adjacent posterior limbus. This study was initiated to demonstrate the presence of such stem cells in the region of the posterior limbus and to show the response of these cells to corneal wounding.Unwounded and wounded corneas with their attached limbal sections were analyzed by immunofluorescence for the presence of nestin, telomerase, Oct-3/4, Pax-6, Wnt-1, and Sox-2. Alkaline phosphatase activity was observed with an enzyme-based reaction that produced a fluorescent product.In the unwounded cornea, stem cell markers nestin, alkaline phosphatase, and telomerase were found in the trabecular meshwork (TM) and in the transition zone between the TM and the corneal endothelial periphery (including Schwalbe's line). Telomerase was also present in the peripheral corneal endothelium. When wounded corneas and their attached limbii were tested, the same markers were found. However, after wounding, additional stem cell markers, Oct-3/4 (in the TM) and Wnt-1 (in both the TM and the transition zone), appeared. Moreover, the differentiation markers Pax-6 and Sox-2 were seen. Pax-6 and Sox-2 were also manifest in the peripheral endothelium post-wounding.Well documented specific stem cell markers were found in the TM and the transition zone of the human posterior limbus. Wounding of the corneas activated the production of two additional stem cell markers (Oct-3/4, Wnt-1) as well as two differentiation markers (Pax-6, Sox-2), the latter of which also appeared in the corneal endothelial periphery. It is suggested that stem cells reside in the posterior limbus and respond to corneal wounding to initiate an endothelial repair process. The stem cells may also contribute to a normal, slow replacement of corneal endothelial cells.

The well-known reluctance of human corneal endothelial cells (HCECs) to divide has continually intrigued investigators. Related to this, the discovery of an increased endothelial cell population in the periphery of the human cornea has prompted an investigation for evidence of the existence of stem-like cells in the endothelial periphery. Showing that stem cells or transient amplifying cells may exist in the periphery might explain the origin of HCECs and indicate a source for these cells in wound repair. In addition, these cells might be of value in culturing or as a source for the synthesis of artificial corneas.Human corneas with attached scleral rims were obtained from eye banks and were assayed for telomerase activity and BrdU (bromodeoxyridine) incorporation to determine, respectively, the presence of a stem-like cell marker and replicative activity. In the case of telomerase activity, the tissues were divided into central, intermediate and peripheral areas by the use of trephines. BrdU staining (using alkaline phosphatase bound secondary antibody) was performed on whole corneas plus scleral rims exposed to BrdU antibodies on the endothelial side whereas BrdU fluorescence (using fluorescein bound secondary antibody) was obtained from transverse sections of the these tissues by the same procedure. Some corneas were wounded to determine whether the wounded areas stimulated BrdU (by staining or fluorescence) followed by the synthesis of transforming growth factor beta (TGFbeta). The latter was determined by quantitative ELISA. Rabbit corneas were also assayed for BrdU incorporation to compare their evidence of cell division with that of humans.After dividing corneas into central, intermediate, and peripheral sections, the dissected endothelial tissues exhibited positive telomerase activity in the peripheral and intermediate sections. No activity was observed in the central endothelial tissues or the limbus between the trabecular meshwork and Schwalbe's line. BrdU staining with alkaline phosphatase was occasionally observed in the wounded area's human corneal endothelial cells after wounding. When BrdU fluorescence assays were made on corneal transverse sections with fluorescein, fluorescence occurred in an area just at and adjacent to the trabecular meshwork, but was not seen at the corneal endothelium. After wounding, BrdU fluorescence extended into the corneal endothelium. TGF-beta levels were increased in fluids bathing the endothelium following wounding, but the increases lagged behind the wounding event.It is suggested stem-like cells may be sequestered in a niche at the junctional region where the corneal endothelial cells and the trabecular meshwork come together. These putative stem cells may supply new cells for both the corneal endothelium and the trabeculae. Evidence suggests that cells from this area migrate (perhaps as transient amplifying cells) to the endothelial periphery and, perhaps, to wounded areas of the corneal endothelium when needed. The migration may not be constant and may be age dependent.

To determine the protein and mRNA expression of p53, p63, and p73 in central and peripheral human corneal endothelial cells. Since these proteins are known to be involved in the regulation of cell division, this study seeks information about their influence in regulating cell proliferation in the human corneal endothelium.Human donor corneas were separated into central and peripheral sections. The endothelial tissue from these samples was dissected and samples were analyzed for mRNA transcription of p53, transactivating p63 (TAp63), delta N p63 (DeltaNp63), transactivating p73 (TAp73), and delta N p73 (DeltaNp73) via the reverse transcriptase-polymerase chain reaction (RT-PCR). Additional samples were analyzed for p53, p63, and p73 protein expression via SDS-PAGE, western blotting, and immunodetection. Frozen corneal sections were immunostained for p53 and analyzed via fluorescence microscopy.p53 and TAp63 mRNA and protein expression were detected in central and peripheral human corneal endothelium. p53 and TAp63 protein expression were greater in central than in peripheral tissue. DeltaNp63 and all isoforms of p73 were not detected in either central or peripheral corneal endothelium.p53 is expressed in both peripheral and central human corneal endothelium, although it is more highly expressed in the central endothelium. Similarly, TAp63 is more highly expressed in central rather than in peripheral endothelium. This suggests that the peripheral endothelium may have more potential for cell division than the central endothelium. DeltaNp63, a stem cell marker, was not detected in the corneal endothelium. Neither the TAp73 nor the DeltaNp73 isoforms were detected in either central or peripheral human corneal endothelium.