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CYTOLOGICAL CHARACTERS, TRANSFORMATION, AND ONTOGENESIS OF DERMAL HISTIOCYTES AND FIBROBLASTS OF RATS

Cell morphology of histiocytes and fibroblasts was observed light‐ and electron microscopically in the subcutaneous connective tissue of unstimulated adult rats. Striking ultrastructural differences between them were noted in the cell surface characters, extent and development of endoplasmic reticula, especially of rough‐surfaced ones, and volumes of lysosomal components. Rosetting assays revealed that histiocytes were positive for EA‐ and EAC‐rosette formation and capable of engaging in immunophagocytosis. However, fibroblasts were negative for both rosettings. In the experiments of vital staining with lithium carmine, Fesin phagocytosis, carrageenin granuloma formation, in vivo culture by intraperitoneal implantation of difision chambers, and subculture, cytological changes of histiocytes or macrophages and fibroblasts, as well as their transformation, were investigated. Although fibroblasts often simulated histiocytes under certain stimulated conditions, no transformation of fibroblasts into histiocytes was confirmed. Ontogenetically, histiocytes already existed in the subepidermal mesenchyme of rat fetuses before differentiation of undifferentiated mesenchymal cells into fibroblasts and showed no relationship to the flbroblasts. Ultrastructural cell morphology and cultural characteristics of the fetal histiocytes were presented and their possible origin was discussed briefly.