The changes in ultrastructure and transparency in chemically or physically altered rabbit cornea

This thesis describes an x-ray diffraction (XRD) and transmission electron microscopy (TEM) study of rabbit comeal ultrastructure. The healing of partial and full thickness wounds in both old and young subjects, with or without the application of mannose-6-phosphate (M6P) was followed. The implications for light scattering following the refractive surgery photorefractive keratectomy (PRK) or comeaplasty (hyaluronidase treatment) was also studied. XRD was used to accurately measure the spacing and alignment of collagen and its constituent molecules whereas TEM was used to study the degree of stromal disorder and the measurement of either proteoglycans (PGs), or the positions and diameter of collagen fibrils following comeal alteration. Resultant haze was measured by slit- lamp or normal photography. From these measurements, comeal transparency was estimated using a theoretical light scattering model, the 'direct summation of fields' model, (Freund et al., 1986; 1995). Both full and partial thickness wounds (caused by trephine or PRK respectively) showed a similar response to wound healing. In both types of wound fibrillar order moved towards a normal value with time. The mean PG size increased in response to both types of wound and more so in full thickness wounds. Neither type of wound returned to its normal haze value but stromal order did increase, indicating that healing took place. This was to a slower extent in the full thickness wounds probably due to increased wound volume. Full thickness wounds also demonstrated fibrils spreading across the wound from a tightly packed position at the wound edge, and an inability to swell. PRK on different age groups suggested an age dependent response to healing with younger animals displaying less haze, less PG content and having a wider variation in interfibrillar spacings. Tissue in this type of wound was shown to be highly hydrated, this was reduced by M6P with a concurrent decrease in PG content but no reduction in associated haze. Comeaplasty resulted in a compression of the fibril packing leading to a small loss of transparency but deemed unlikely to cause visual impairment. PRK of both old and young corneas also resulted in a small loss of transparency leading to the conclusion that haze following PRK is not necessarily caused by fibril disorder.