A comparative study of the corneal endothelium in vertebrates
Shaun P Collin* MSc (Melb) PhD (Qld)
H Barry Collin† AM PhD (Melb) DSc (NSW) FRCPath (Lond)
* Department of Zoology, The University of Western Australia
† Department of Optometry and Visual Sciences, The University of Melbourne
Introduction In vertebrates, a corneal endothelium is essential for the maintenance of corneal transparency in a variety of environments, including aerial, terrestrial and aquatic. Knowledge of the surface structure of the corneal endothelium may assist our understanding of this unique tissue and its evolutionary development. Except for humans and some mammals, there have been few studies of other vertebrates, particularly the unique Australian species.
Methods The field emission scanning electron microscope was used to study the corneal endothelium in representatives of four vertebrate classes: Teleostei (five species), Reptilia (two species), Aves (four species) and Mammalia (three species), including Marsupialia (two species). Endothelial cell densities were calculated from micrographs using computer-based image analysis.
Results The cell densities varied considerably from 1,900 ± 197 cells per mm2 for the bream to 11,734 ± 1,687 cells per mm2 for the emu. Most of the corneal endothelia were similar to those reported for mammals. However, in some species such as the koala, the pattern was irregular. Some endothelial cells in birds possessed cilia.
Conclusions The shape of the corneal endothelial cells of vertebrates is typically a mixture of hexagonal and pentagonal cells, in which the cell borders are irregular and interdigitating. An exception is the koala, in which the cells were markedly irregular. Many of the cells have surface microvilli but only in the birds are cilia found in the centre of many endothelial cells. In spite of the range of corneal environments, there are no systematic differences in the cell densities of the various classes and species.