Ultrastructural studies of cornea and sclera

Fullwood, Nigel James (1992). Ultrastructural studies of cornea and sclera. PhD thesis The Open University.


The work of Goodfellow et al., (J. Mol. BioI. 119: 231. 1918) and Sayers et al., (J. Mol. BioI. 160: 593. 1982), was repeated using fresh, equilibrated cornea, confirming their finding that the plot of interfibrillar spacing against liydration is a linear relationship. Intermolecular spacing in cornea, sclera and rat-tail tendon was shown to increase rapidly over the hydration range H=O to H≈1, then tan off (physiological hydration is at H=3.2). Intermolecular and interfibrillar spacings rise in unison over the hydration range H=O to H≈1, after which virtually all of the water goes between the fibrils.

It was found that freezing corneas to 40oC and then thawing had no effect on normally hydrated or dehydrated corneas, but caused damage to swollen corneas. Freezing to -180°C and thawing resulted in permanent damage to normally hydrated corneas .

The changes in intermolecular spacing, D-period spacing, fibril diameter and fibril packing as a result of processing for TEM and SEM have been described. Low temperature embedding in Lowicryl K4M resin produced least disruption in the cornea.

The fibril diameter in cornea at normal hydration was established to be 37.4nm±1.4. X-ray data modelling indicates that fibril packing in the cornea is liquid-like, as proposed by Worthington and Inoyue (Int. J. Macromol. 7: 2. 1985).

It was shown that the water in the corneal stroma is evenly distributed around the fibrils over the hydration range H=O to H=4. Above H=4 more water is entering the stroma than can be "accounted for if its distribution is uniform.

The transmittance of light through the bovine cornea (excluding interference effects), was calculated as T≈12%. Thus, most of the transparency of the cornea must be due to constructive interference as first proposed by Maurice (J. Physiol. 136: 263. 1951).

Examination of keratoconus corneas by the the SEM and TEM and showed abnormal epithelial cells, disrupted lamellae, and abnormally arranged proteoglycans. Analysis of X-ray data proved that stromal thinning in keratoconus was not due to closer fibril packing. Also that keratoconus corneas had increased intermolecular spacing, and an abnormal arrangement of proteoglycans along the fibrils.

No differences were found between the corneal stroma of normal and myopic chicks. Myopic chick sclera was shown to have more aggrecan proteoglycan which was associated with larger empty spaces, than normal sclera. The fibril attachment sites of the proteoglycans was the same in normal and myopic sclera.

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