Ultrastructural findings of feline eosinophilic dermatoses

Abstract

Despite the increasing evidence that degeneration of collagen is not involved in the histogenesis of the eosinophilic debris observed with light microscopy in feline eosinophilic dermatoses (FED), flame figures and foci of ‘collagen degeneration’ are still reported as degenerated collagen mixed with degranulated eosinophils. The purpose of this study was to investigate ultrastructurally eosinophil degranulation pathways and dermal collagen morphology in FED. Four patients with histopathological diagnosis of FED, three clinically diagnosed with eosinophilic granulomas and one with an eosinophilic plaque, and one healthy control were studied. Areas that contained eosinophils, collagen, flame figures and ‘collagen degeneration’ were ultrasectioned and examined with transmission electron microscope. Eosinophils (48 to 123 per cat) were classified into resting (intact cells), undergoing piecemeal degranulation (PMD) (viable cells with partially empty granules) and cytolytic [disrupted cells with clusters of free eosinphil granules (Cfegs)]. Longitudinal and transverse sections of collagen fibrils were also evaluated. 9.01% of eosinophils examined were resting, whereas 90.98% had released their content, 70.14% by cytolysis and 20.84% by PMD. Collagen fibrils showed no ultrastructural abnormalities and appeared identical to those observed in the control. Flame figures and foci of ‘collagen degeneration’ appeared constituted by collagen fibrils–fibers interspersed with cytolytic eosinophils. Based on these findings we conclude that eosinophils play an active role in the pathogenesis of FED through their granule content release, by cytolysis and PMD. Moreover, based on the normal collagen ultrastructure, the use of the term ‘collagen degeneration’ to describe the eosinophilic debris observed with light microscopy in FED should be definitely avoided. The study was funded by a grant of the European Society of Veterinary Dermatology.