Epithelioid Cell Granulomas Experimentally Induced by Prototheca in the Skin of Mice : A Light Microscopic Study

Prototheca wickerhamii, an achlorophyllous algae, was previously found to induce massive epithelioid cell granulomas in the skin of mice. By means of light microscopy, examination was made of the histological reactions involved in epithelioid cell granulomas induced by intradermal and/or subcutaneous inoculation of Prototheca wickerhamii in BALB/c and ICR mice. Six BALB/c mice showed granuloma nodules while only three of six ICR mice did so. Based on the results of the present and previous studies, BALB/c mice may be considered a strain particularly vulnerable to contracting epithelioid cell granuloma and ICR mice, a resistant strain.　　

In very early lesions at one week following initial prototheca inoculation, cellular infiltration with varying numbers of polymorphonuclear leukocytes, lymphocytes and some macrophages was observed throughout the dermis and subcutaneous fat tissue. In early lesions at one to two months after inoculation, focal granulomas composed of histiocytic cells and/or macrophages were observed. Mast cells were occasionally present among the histiocytic cell infiltrates. In the granulomatous lesions at two to three months, scattered eosinophils and some lymphocytes were seen. Central necrosis, with numerous neutrophils and many endospores surrounded by the granuloma, was often observed. In late stage lesions at six months, massive lymphocyte and plasma cell infiltration surrounding and/or intervening between vacuolated epithelioid cell clusters was evident. Histological reactions in epithelioid cell granuloma and the ultimate course of this disease can be staged from the histological point of view as follows: 1) diffuse inflammation, 2) cell proliferation leading to epithelioid cell formation, 3) hypertrophy of epithelioid cells with consequent formation of cell aggregates and/or organized granuloma and 4) degeneration of granuloma. Histological reactions were found in this study to be essentially the same in ICR and BALB/c mice, though a time lag was noted in the course of epithelioid cell granuloma formation.