Ultrastructural features of tree shrew cornea‐collagen fibrils and proteoglycans

Abstract

Abstract Purpose The unique organisation of collagen fibrils and proteoglycans maintain the transparency of normal cornea. We report here ultrastructural features of collagen fibrils and proteoglycans of tree shrew cornea. Methods Tree shrew corneas of 5, 6 and 10 week old animals were fixed in 2.5% glutaraldehyde containing cuprolinic blue in sodium acetate buffer. The tissue was processed for electron microscopy. Soft imaging programme ‘analySIS LS Professional’ was used to analyse collagen fibril diameter and proteoglycans area. Results Tree shrew corneas consist of 5 layers, the epithelium, Bowman’s layer, stroma, Descemet’s membrane. The epithelium was composed of squamous cells, wing cells and basal cells. The Bowman’s layer was 5.5µm thick and very similar to normal human Bowman’s layer. The stroma was 258 µm thick and consisted of collagen fibril lamellae. Lamellae interlaced each other in the anterior stroma, but ran parallel to each other in the middle and posterior stroma. Longitudinally running collagen fibrils showed banding and were decorated with proteoglycan filaments of area size 390 nm2. The collagen fibril diameter was 39 nm. Very small electron dense particles were observed within the collagen fibrils. Descemet’s membrane has only banded region. Endothelial cells contained all the normal cell organelles. Conclusion The structure of the tree shrew cornea is very similar to normal human cornea. The collagen fibril of the former uniquely showed the presence of electron dense particles within collagen fibrils. These particles could be proteoglycans stained with cuprolinic blue. Similarities of tree shrew cornea with human cornea suggest that it can be a good model to study human corneal diseases.