Lymphocyte differentiation in the rabbit thymus.

Density separation of rabbit thymus lymphoid cells on a continuous and linear Ficoll-Metrizoate gradient resulted in the appearance of three subpopulations of lymphoid cells with peak fractions at densities of approx. 1.067, 1.077 and 1.084 g/ml. The subpopulation of the lowest density appeared to consist of cortisone-resistant cells of various size classes, all characterized by a relative low nucleo-cytoplasmic ratio. Glycerol-induced redistribution of particles within the plasma membrane (IMP) of the lymphoid cells, as revealed with the freeze-fracture technique, was present in the subpopulation of lowest density as well as in thymus lymphoid cells of cortisone-treated rabbits in a high percentage of the cells, and present in a very small percentage of the cells of highest density. It is concluded that an increase in the mobility of plasma membrane components may be a significant feature of T-cell education in the rabbit thymus.