Dermatosparaxis in a Himalayan cat: II. Ultrastructural studies of dermal collagen.

Dermatosparaxis is a connective tissue disease, primarily of sheep and cattle, that results from deficient activity of the NH2-terminal procollagen peptidase. It is characterized by fragile, loose skin that is easily torn with minor trauma. We have identified a cat twith a defect in this procollagen peptidase which affects only a small proportion of the collagen molecules; the majority of the collagen is processed normally. Nonetheless, as seen by transmission and scanning electron microscopy, this population of aberrant collagen molecules significantly alters the structure of individual collagen fibrils, the assembly of fibrils into fiber bundles and the integration of fiber bundles into a normal, woven network in the reticular dermis of skin. Although the clinical findings are less severe than those in sheep and cattle where the enzymatic defect is more complete, the ultrastructural abnormalities are marked and demonstrate that a minority of abnormal collagen molecules cn have a major effect on the structure and function of connective tissues.