High yields of mouse macrophage-melanocyte heterokaryons and macrophage-macrophage homokaryons were obtained through the virus-induced fusion of cells spread on a glass surface. After fusion there was a striking reorganization of cellular architecture by means of a colcemid-sensitive process. Heterokaryons were isolated through the use of differential trypsinization and many underwent division to form melanocyte-like hybrids. The selective uptake of dextran sulfate by macrophages served as a useful cytoplasmic marker in identifying hybrids.

Many characteristic macrophage properties were altered in the heterokaryons. Within an hour of fusion macrophage nuclei became swollen, nucleoli were more prominent, and increased nuclear RNA synthesis occurred. 3 hr after fusion, a wave of DNA synthesis took place in the previously dormant macrophage nuclei.

The fate of typical macrophage markers was examined in both heterokaryons and homokaryons. Macrophage homokaryons continued to exhibit active phagocytosis of sensitized erythrocytes, whereas this capacity was lost irreversibly in heterokaryons. The loss of phagocytic activity of heterokaryons occurred at an exponential rate and was accelerated by high concentrations of calf serum.

Another macrophage surface marker, a divalent cation-dependent adenosine triphosphatase (ATPase), could be demonstrated histochemically on heterokaryons. Shortly after fusion, it was present in discrete regions, but it became more diffuse and disappeared within a day.

Acid phosphatase-positive secondary lysosomes and retractile lipid droplets disappeared from heterokaryons but continued to accumulate in macrophage homokaryons.

These observations indicate that typical macrophage properties cease to be expressed in heterokaryons, and melanocyte functions presumably predominate in heterokaryons and hybrids.

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