Formation of Pure Suspensions of Mast Cells in Tissue Culture by Differentiation of Lymphoid Cells From the Mouse Thymus2

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Abstract
During studies on the possible effect of a mammalian leukemia virus (isolated by Moloney) on tissue cultures of thymus cells, a synchronized mass differentiation of lymphoid cells to mast cells was observed. Cell suspensions, prepared from thymuses of normal Swiss mice, 15 to 30 days of age, were seeded and maintained in Eagle's medium with 20 percent horse serum on unirradiated feeder layers of mouse embryo cells. During the first few days there was considerable degeneration of the lymphocytes, followed by a proliferation of primitive lymphoid cells similar to large lymphocytes, which were termed mastoblasts. These primitive cells were found in colonies, scattered singly on the feeder layer, or floating freely in the medium. They exhibited a strong tendency to migration. From the chromo-phobic region located in the vicinity of the nucleus of these cells, a new cytoplasmic region morphologically distinct and rich in vacuoles started to appear, which caused the cytoplasm to increase markedly. In this new region the metachromatic granules were formed. Cells with metachromatic granules appeared in large quantities not before the 12th day in vitro. In the stage characterized by cells termed young mast cells, the metachromatic cells underwent mitosis. The course of differentiation was terminated by a stage in which the cells were rich in metachromasia, lacked nucleoli, and had no mitotic activity. As a result of the mass differentiation, pure suspensions of mature mast cells were obtained during the 2d month in vitro, while other cells from the thymus degenerated, differentiated to mast cells, or joined the feeder layer. These suspensions of mast cells were kept on feeder layers of mouse embryo cells for 80 days from initiation of the cultures. The mass differentiation to mast cells was not found in the absence of the feeder layer. A quantitative difference between virus-inoculated and control cultures was observed in all the experiments. This difference was reflected by the greater number of mast cells in the inoculated cultures. Suspensions of mast cells in vitro were not obtained in cultures from mouse spleens.