Structure of corneal layers, collagen fibrils, and proteoglycans of tree shrew cornea

Purpose The stroma is the major part of the cornea, in which collagen fibrils and proteoglycans are distributed uniformly. We describe the ultrastructure of corneal layers, collagen fibrils (CF), and proteoglycans (PGs) in the tree shrew cornea. Methods Tree shrew corneas (5, 6, and 10 week old animals) and normal human corneas (24, 25, and 54 years old) were fixed in 2.5% glutaraldehyde containing cuprolinic blue in a sodium acetate buffer. The tissue was processed for electron microscopy. The ‘iTEM Olympus Soft Imaging Solutions GmbH’ program was used to measure the corneal layers, collagen fibril diameters and proteoglycan areas. Results The tree shrew cornea consists of 5 layers: the epithelium, Bowman’s layer, stroma, Descemet’s membrane, and endothelium. The epithelium was composed of squamous cells, wing cells and basal cells. The Bowman’s layer was 5.5±1.0 µm thick and very similar to a normal human Bowman’s layer. The stroma was 258±7.00 µm thick and consisted of collagen fibril lamellae. The lamellae were interlaced with one another in the anterior stroma, but ran parallel to one another in the middle and posterior stroma. Collagen fibrils were decorated with proteoglycan filaments with an area size of 390 ±438 nm2. The collagen fibril had a minimum diameter of 39±4.25 nm. The interfibrillar spacing was 52.91±6.07 nm. Within the collagen fibrils, very small electron-dense particles were present. Conclusions The structure of the tree shrew cornea is very similar to that of the normal human cornea. As is the case with the human cornea, the tree shrew cornea had a Bowman's layer, lamellar interlacing in the anterior stroma and electron-dense particles within the collagen fibrils. The similarities of the tree shrew cornea with the human cornea suggest that it could be a good structural model to use when studying changes in collagen fibrils and proteoglycans in non-genetic corneal diseases, such as ectasia caused after LASIK (laser-assisted in situ keratomileusis).

[1]  R. Mayne,et al.  Extracellular matrices in the developing avian eye: type V collagen in corneal and noncorneal tissues. , 1984, Investigative ophthalmology & visual science.

[2]  M. D. Merindano,et al.  Morphological and morphometric aspects of primate cornea: a comparative study with human cornea. , 1997, European journal of morphology.

[3]  N. Mcbrien,et al.  Prevention of collagen crosslinking increases form-deprivation myopia in tree shrew. , 1994, Experimental eye research.

[4]  S. Akhtar,et al.  Role of epidermal growth factor receptor (EGFR) in corneal remodelling in diabetes , 2009, Acta ophthalmologica.

[5]  Koujiro Tohyama,et al.  Comparative observations on corneas, with special reference to bowman's layer and descemet's membrane in mammals and amphibians , 2002, Journal of morphology.

[6]  J. Scott,et al.  Identification of specific binding sites for keratan sulphate proteoglycans and chondroitin-dermatan sulphate proteoglycans on collagen fibrils in cornea by the use of cupromeronic blue in 'critical-electrolyte-concentration' techniques. , 1988, The Biochemical journal.

[7]  Chia-Yang Liu,et al.  Knockdown of Zebrafish Lumican Gene (zlum) Causes Scleral Thinning and Increased Size of Scleral Coats* , 2010, The Journal of Biological Chemistry.

[8]  Saeed Akhtar,et al.  Ultrastructural analysis of collagen fibrils and proteoglycans in keratoconus , 2008, Acta ophthalmologica.

[9]  M. Boulton,et al.  Connective tissue structure of the tree shrew optic nerve and associated ageing changes. , 2007, Investigative ophthalmology & visual science.

[10]  N. Mcbrien,et al.  Isoform-specific Changes in Scleral Transforming Growth Factor-β Expression and the Regulation of Collagen Synthesis during Myopia Progression* , 2004, Journal of Biological Chemistry.

[11]  J. Merayo-Lloves,et al.  Wound healing following refractive surgery in hens. , 2006, Experimental eye research.

[12]  P. Chow,et al.  The tree shrews: adjuncts and alternatives to primates as models for biomedical research , 2003, Journal of medical primatology.

[13]  D. Birk,et al.  Collagen type I and type V are present in the same fibril in the avian corneal stroma , 1988, The Journal of cell biology.

[14]  T. T. Norton,et al.  The susceptible period for deprivation-induced myopia in tree shrew , 1998, Vision Research.

[15]  R. Dahm,et al.  Formation of stromal collagen fibrils and proteoglycans in the developing zebrafish cornea , 2008, Acta ophthalmologica.

[16]  D. Azar,et al.  Confocal and electron microscopic studies of laser subepithelial keratomileusis (LASEK) in the white leghorn chick eye. , 2002, Archives of ophthalmology.

[17]  N. Mcbrien,et al.  Structural and ultrastructural changes to the sclera in a mammalian model of high myopia. , 2001, Investigative ophthalmology & visual science.

[18]  T. T. Norton,et al.  Darkness causes myopia in visually experienced tree shrews. , 2006, Investigative ophthalmology & visual science.

[19]  D. Maurice.  The structure and transparency of the cornea , 1957, The Journal of physiology.

[20]  W M Petroll,et al.  Corneal opacity in lumican-null mice: defects in collagen fibril structure and packing in the posterior stroma. , 2000, Investigative ophthalmology & visual science.