Abstract
The fine structure of 5 naturally occurring and 4 experimentally produced cases of bovine cutaneous fibropapillomatosis was compared to that of normal bovine skin with both light and electron microscopy.
Both degenerative and proliferative changes were found in the affected epithelial cells and fibroblasts. Virus-like particles (approximately 40 mμ) were first seen in the nuclei of degenerating fibroblasts and later in the epithelium. The number of virus-like particles was always small and they were associated with aggregates of chromatin. They were most numerous in the stratum granulosum. The degenerative changes were aggregation and margination of chromatin in the nucleus. In addition, epithelial cells had cytoplasmic vacuolization, loss of tonofibrils, focal necrobiosis, increased electron opaque globules and dilatation of intercellular spaces with changes of desmosomes. When these changes were marked the cells could be recognized as “pale cells” by light microscopy and usually occurred in groups. Viral multiplication appeared confined to these degenerating cells and no viral particles could be found in the proliferating cells. The proliferating epithelial cells were increased in size.
These changes were progressive in the developing fibropapilloma and could be categorized into stages of fibroplasia, immature fibropapilloma, mature fibropapilloma and regression.