Corneal thickness and transparency were comparable to that of normal corneas. Stroma: Cells and Matrix The physical strength and optical properties of the cornea derive mainly from your stroma, a tough connective tissue composed of a combination of specialized extracellular matrix (ECM) components organized with an elegant ultrastructure that provides both strength and transparency to this unique tissue. The stroma is usually populated and managed by keratocytes, neural crest-derived mesenchymal cells, occupying about 3% of the stromal volume. After birth, the number of dividing keratocytes decreases, and in adult mammals, keratocytes have withdrawn from your cell cycle and become quiescent.1C5 Thus, unlike the self-renewing corneal epithelium, homeostasis of the stroma does Rabbit Polyclonal to GPR174 not rely on the presence of an active population of stem cells. B. Corneal Scarring Scarring of the corneal stroma can occur in response to surgery, trauma, or contamination. Corneal scars are long-lasting and disrupt vision for millions of people worldwide.6 Currently, surgical replacement of the stroma is the primary approach to restoration of vision in scarred corneas. The cells responsible for scar deposition are fibroblastic cells derived from stromal keratocytes.7 Upon wounding, keratocytes proximal to the site undergo apoptosis, and keratocytes distal to the wound become motile, mitotically active fibroblasts.2 IDO-IN-3 Expression of -easy muscle actin has become a marker for cells involved in fibrotic ECM deposition.7,8 Secretion of fibrotic components is stable for months after healing in rabbit cornea.9 In humans, corneal scars can remain for decades.10 Damage to the corneal epithelium that does not involve the corneal stroma and retains some of the limbal stem cells can heal without scarring.3 Such epithelial wounds cause keratocyte apoptosis in IDO-IN-3 the anterior stroma, and keratocytes peripheral to the injury migrate into the region and replicate. After epithelial debridement, mouse corneal stromal cells regain expression of stromal matrix components within 12 weeks after wounding.11 Keratocytes derived from the recipient have been identified in human donor keratoplasty tissue, indicating a potential for human keratocytes to repopulate and maintain stromal tissue.12 Such repopulation, however, is slow, sometimes requiring decades. It is obvious from these studies that keratocytes do not conform to the classic definition of terminal differentiation, and at least some cells in the stroma maintain the capability of replication, migration, and regeneration of transparent stromal tissue. II. Stem Cells in the Stroma A. Progenitor Potential of Stromal Cells In vitro growth of adult keratocytes typically prospects to transformation IDO-IN-3 to cells with a fibroblastic morphology, which produce a scar-like ECM rather than the specialized ECM required for corneal transparency.13 This fibroblastic transformation was considered irreversible, but recently it has become apparent that early-passage stromal cells maintain some potential to re-express differentiated keratocyte characteristics.14 However, the ability to differentiate to keratocytes after mitotic expansion is not equally distributed in the stromal cell populace. About 3% of freshly isolated adult bovine stromal cells were found to grow clonally.15 These cells did not show keratocyte morphology or gene expression, rather expressed a number of genes typical of mesenchymal stem cells. When these cloned cells were shifted to a reducedmitogen culture medium, the clonal cells developed dendritic morphology and upregulated expression of keratan sulfate, keratocan, and ALDH3A1, all products highly expressed by differentiated keratocytes. The potential for keratocyte differentiation was managed through greater IDO-IN-3 than 50 populace doublings, indicating that a progenitor phenotype was a stable property of these cells. These stromal progenitor cells exhibited normal karyotype and reached replicative senescence after 70C80 populace doublings, demonstrating that they represent a populace of non-transformed, adult diploid cells. As these cells differentiate to keratocytes, mRNA for several gene products present.