Novel aspects of the ultrastructural organization of human corneal keratocytes

Abstract

Purpose: Proper functioning of the endothelium and proper structural organization of the keratocytes and collagen bundles are of ultimate importance for transparency of the cornea. The role of the endothelium has been investigated extensively, whereas the role of the keratocytes is still unclear. Detailed knowledge on the ultrastructural organization of keratocytes and the relationship between keratocytes and collagen bundles is as essential for understanding corneal transparency as is knowledge of endothelial functioning.

Methods: Thirty-five corneas (30 postmortem donor corneas and 5 fresh corneas from the operating theater; age range, 28 to 90 years) were used for light microscopy, transmission electron microscopy, and scanning electron microscopy. Serial frontal sections of the central stroma reaching from epithelium to endothelium and cross-sections were studied. At three levels, reconstructions of the mutual arrangement of keratocytes were made using semithin sections.

Results: Keratocytes have the appearance of highly active cells with an abundancy of organelles. Between the dendritic ramifications of these cells, large amounts of amorphous material is observed. One of the most remarkable observations is the presence of an extensive network of fenestrations along the surface of the keratocytes. Another important observation is the circular arrangement of keratocytes gradually turning clockwise like a corkscrew from epithelium to endothelium.

Conclusions: From the current study, the following conclusions can be drawn: Keratocytes are not quiescent but are highly active cells probably involved in turnover of the extracellular matrix; fenestrations may be of functional relevance with respect to facilitation of diffusion and mechanical attachment of the collagen fibers to the keratocytes; the corkscrew organization of keratocytes suggests that they form completely closed sheets of communicating cells throughout the depth of the cornea, creating equal chances for all light rays to pass one or more keratocytes and thus minimizing variation in light scattering over the entire cornea.