The fine structure of equine papillomas and the equine papilloma virus

Combined light and electron microscopy were used to follow the sequence of virus development in equine papillomas. The deepest layer in which virus was observed was the stratum spinosum of the epidermis. In this layer virus was scattered throughout the nuclei and was occasionally found in association with the nucleolus. In the stratum granulosum virus particles were more numerous, often forming isolated nuclear aggregates. Virus inclusions observed in the stratum granulosum by electron microscopy were correlated with nuclear inclusions seen by light microscopy. In the stratum corneum closely packed arrays of virus were embedded in dense keratinous material. Negatively stained viruses from papillomatous extracts exhibited capsomeres apparently arranged in accordance with icosahedral (5:3:2) symmetry. The mean diameter of the intact virion was 57.2 m μ. On the basis of morphology and cellular location, it has been concluded that equine papilloma virus qualifies for membership in the papovavirus group.