Surface morphology of mitogen-activated human lymphocytes and their derivatives in vitro

Human peripheral blood lymphocytes were cultured with phytohaemagglutinin or pokeweed mitogen for various intervals up to 17 days and studied by scanning electron microscopy. Activated lymphocytes in 3 day cultures were large irregular cells characterised mainly by an abundance of very fine microvilli, which were much thinner, shorter and more densely packed than the microvilli on uncultured lymphocytes. Cells intermediate in size and surface morphology between these and unstimulated lymphocytes were numerous in 1 day cultures. Some motile cells and large cells with finger-like or conical microvilli were also present. Cell counts showed that after 6 days in phytohaemagglutinin culture small villous cells resembling normal healthy lymphocytes were progressively more numerous, suggesting that most of the activated cells reverted to small lymphocytes. Very large cells were also present and displayed a heterogeneous surface morphology of villi, ridges, blebs and ruffles. A few of these cells had typical monocytoid features but others were predominantly villous. In pokeweed mitogen cultures there was extensive cellular degeneration affecting all cell types after 6 days. In unstimulated cultures the lymphocytes had fewer microvilli or were smooth-surfaced. There was extensive lymphocyte degeneration after 3 days and eventually typical monocytes were the predominant cells. Large, villous cells, which could be activated lymphocytes, were occasionally encountered in unstimulated cultures.