Comparative studies on the corneal structural adaptation of two rodents inhabiting different environments

Abstract

The corneas of Rattus rattus and Meriones shawi are composed of three main layers; an outer epithelium, a middle stroma (basement membrane, total stroma and Descemet’s membrane) and an inner endothelium. The mean thickness of the epithelium, total stroma, Descemet’s membrane, and endothelium was about 52 ± 7.3 μm, 275 ±18.7 μm, 5.5 ± 0.7 μm, and 7.5±0.23μminR.rattus,whereasitwas38 ± 5.8μm,124 ±4.7μm,4 ±0.21μm,and 4.2 ± 0.17 μm in M. shawi. In R. rattus and M. shawi, the outermost cells of the corneal epithelium are mostly polygonal and hexagonal in shape with nearly regular borders and show a dense pattern of microplicae with different scatter electron that exhibits three and two polymorphic appearances, respectively. Type A: numerous light cells with dense microplicae; type B: many dark cells with a moderate density of microplicae, and type C: few dark cells with a less density of microplicae are found in R. rattus; whereas, types A and B are found in M. shawi. In both investigated species, the epithelial cells are characterized by some structural components, such as glycocalyx, fibrous components and tight junction between these cells, to resist the impact of the external stressed factors and to protect the underlying tissue, as well as to maintain an excellent transparency of the cornea. Among these structures, the cytokeratin filaments are the major components of the cytoplasm of the corneal epithelial cells (basal, polygonal, wing and squamous cells). Actin filaments are also found in the corneal epithelial cells, but they are prominent within the apical epithelial cells. In R. rattus, the stroma is formed of an outer lamellar zone and an inner lamellar one; the latter is thicker and characterized by its interfibrillar spaces between the bundles of wavy dissociated collagen fibrils, which are arranged in an orthogonal manner. In M. shawi, however, the stroma is formed of one lamellar zone of flattened bundles of highly wavy and branched collagen fibrils, which are composed of perpendicular fibrillae alternating with longitudinal ones. In R. rattus, the SEM showed that the endothelial cell surfaces are slightly bulging with many blebs, whereas in M. shawi, it showed that the surfaces of the endothelial cells are flattened and nearly smooth. In conclusion, the transparency of the cornea, may be highly attributed to the increase in the thickness of the stroma, the presence of stromal interfibrillar spaces and the case of the stromal swelling. These aforementioned features are found in the corneal stroma of R. rattus, which live in different habitats of varying degrees of density such as water and dry or humid air, whereas these features are lacking in M. shawi, which live only in arid zones.