3H-Thymidine labeled mast cells in mice treated with 20-methylcholanthrene: proliferation of precursor cells, their transformation into mast cells and migration of the latter.

In an attempt to clarify the kinetics of increase of mast cells, autoradiographic studies were performed on the mice which received a painting of 20-methylcholanthrene on the skin. In the first experiment, mice received 20-methylcholanthrene painting on their back for two, four and eight weeks. A fourty-eight hours' cumulative labeling with 3H-thymidine was performed directly before sacrifice. No labeled mast cells were found in the painted skin, despite a marked increase of mast cells in number. In the second experiment, mice received 20-methylcholanthrene painting for thirty-one days. A twenty-four hours' cumulative labeling was performed three, five, seven, fourteen, twenty-one and thirty-one days before sacrifice. A high labeling index of mast cells, up to 61.29%, was obtained in each group. When the time lapse between the time of cumulative labeling and the sacrifice is long, the labeling index is higher in the subcutaneous tissue, and when the time lapse is short, the labeling index is higher in the subepidermal tissue. From these results it was concluded that; 1) An increase of mast cells is due predominantly to the proliferation of precursor cells and their transformation into mast cells. 2) The proliferation of precursor cells occurs mainly in the subepidermal layer of the skin. 3) After the transformation, mast cells may migrate into the deeper layer of the skin.