Electron microscopic observations of epithelioid cell granulomas experimentally induced by prototheca in the skin of mice.

Electron microscopic analysis was conducted on epithelioid cell granulomas experimentally induced by prototheca in the skin of mice. In BALB/c mice, epithelioid cell granulomas were easily induced. Each cell had a round nucleus with euchromatin on the margin, abundant cytoplasmic organelles, and particularly abundant rough endoplasmic reticulum (rER). Cells with a plasmacytoid appearance were the most common. Fibroblast-like epithelioid cells were sometimes observed. The whole body and cell membrane of an organism was usually within the cytoplasm of epithelioid cell. In ICR mice, epithelioid cell granulomas were not easily induced. Cells which had ingested whole organisms had spindle shaped and/or oval nuclei with marginated heterochromatin and with abundant dilated rER. Near completion of organism digestion, the cells had irregular and/or indented nuclei with marginated heterochromatin and often one or two distinct nucleoli. The cytoplasm was often replaced with numerous phagocytic vacuoles. At higher magnification, secondary lysosomes and Golgi complexes forming Golgi lamellae could be seen in the cells in which cytoplasmic organelles such as endoplasmic organelles and mitochondria were abundantly present. In contrast to the BALB/c mice, the remnants of digested organisms, split up into many small pieces, could be seen in the cytoplasm. The cells with a fibroblast-like appearance were most frequent, but plasmacytoid cells were sometimes observed. In the present study, the ultrastructural features of epithelioid cells in BALB/c and ICR mice clearly differed. Plasmacytoid cells were the most abundant in BALB/c mice, while fibroblast-like cells were in ICR mice.