The fate of India ink particles and polystyrene latex beads injected into the dermis and subcutis of the skin of the auricle and back in mice was observed with the naked eye, light microscopy and electron microscopy.
The tattoo patterns made by injected ink particles remained essentially unchanged for life as observed with the naked eye. India ink particles and latex beads were endocytosed by fibroblasts as well as macrophages in the dermis and subcutis. Numerous ink particles or small latex beads (0.22μm in diameter) were packed into vacuoles 0.1-10.0μm in diameter which occupied a large volume of the cytoplasm of the cell body and processes of fibroblasts, whereas numerous particles and larger beads (0.22 and 2.0μm) were taken up into the cell body of macrophages in the vicinity. Most fibroblasts, characterized by long cell processes and well developed rough endoplasmic reticulum, are easily distinguished from macrophages, the latter being round or oval in shape, and having many lysosomes and numerous irregularly shaped microvillous projections. It is believed that fibroblasts taking up and storing the ink particles or latex beads move poorly and are almost fixed in the connective tissue: the tattoos therefore do not change markedly.
It is emphasized that the uptake and long-term storage of ink particles and latex beads by the dermal and subcutaneous fibroblasts represent a specific non-inflammatory defense mechanism that protects the living body, without immune reactions, against injuries and invasions by non-toxic foreign agencies. The histiocyte, a term proposed by KIYONO (1914) for a fixed macrophage on the basis of his studies using vital dye staining, is considered to include, in addition to true macrophages, fibroblasts showing endocytotic activities for small foreign bodies such as acid dyes, ink particles, and latex beads.