The morphological appearances and some tinctorial properties of the cells of the infectious (viral) papilloma of human skin are described. Pathological alterations of the nuclei of affected cells occur in a definite sequence of stages. In the earliest, and acidophilic Feulgen-negative intranuclear inclusion body is recognizable. Subsequently the inclusion body and nucleus enlarge, there is progressive disorganization of nuclear structure, and the inclusion body then becomes basophilic and stains with the Feulgen reaction. The nucleus finally disappears, leaving the inclusion body in the cell remnant.

The relative amounts of Feulgen-colored DNA per cell at each stage in the evolution of the lesion in the infected cells, as well as in normal appearing and hypertrophic cells of the papilloma, were measured microphotometrically in Feulgen preparations. Determinations were made using the "plug" and "two wave length" methods. These are compared with measurements of DNA in cells of the basal and spinous layers of normal human skin.

The frequency distribution curves of relative amount of DNA in cells of normal skin, and normal-appearing cells in hyperplastic epithelium, show the bimodal diploid and tetraploid peaks characteristic of growing tissues.

Infection of the epidermal cell entails prompt synthesis of DNA in the nucleus. Increased amounts of DNA (tetraploid to 16 ploid levels) are found in the earliest recognizable cytopathological stages of infection and do not increase appreciably during the subsequent evolution of the cellular lesion. At a relatively late stage, all the cellular DNA is relocated ("transferred" or "reassembled") in the inclusion body, and is not further significantly increased in amount. Active formation of DNA in affected cells appears, from these measurements, to occur only in relatively intact nuclei.

The inclusion bodies of infected cells are found to contain a relatively basic protein which stains with the alkaline-fast green method for histone.

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