Lymphocytes from Sprague-Dawley rats have been cultured on monolayers of embryo-derived fibroblasts from the same outbred strain. Under these conditions the lymphocytes form aggregates, and transformation of small lymphocytes to lymphoid blast cells occurs within these aggregates. Transformation is characterized cytologically by enlargement of the nucleus, dispersion of nuclear chromatin, and the appearance of a prominent nucleolus. The principal cytoplasmic changes are an increase in cytoplasmic volume, a marked increase in number of ribosomes, and a clustering of ribosomes. These changes parallel those seen in the transformation of lymphocytes caused by a variety of treatments. One apparent difference is the paucity of lysosomes and lipid inclusions in the lymphocytes that transform on the monolayer.

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