Morphological aspects of T cell subpopulations in human blood: characterization of the cerebriform mononuclear cells in healthy individuals.

Cerebriform mononuclear cells (CMC) constitute a morphologically distinct subpopulation of T cells in healthy individuals. They are characterized ultrastructurally by a highly indented nucleus, a high nucleus-to-cytoplasm ratio, condensed chromatin along the nuclear membrane and a scanty cytoplasm. In order to characterize the peripheral blood CMC by enzyme-histochemistry and membrane characteristics, lymphocyte fractions enriched for T cells, T mu cells, T gamma cells and T cells without Fc mu and Fc gamma receptors (T0 cell fraction), or depleted of T cells, were investigated for the presence of alpha-naphthylacetate esterase and acid phosphatase at light and electron microscopic levels. CMC were found exclusively in the T mu-enriched and T0 cell fraction, indicating that these cells have either an Fc mu receptor or no Fc receptor at all. Except for their nuclear indentation, both the CMC in the T mu-enriched fractions and the CMC in the T0 fractions ultrastructurally resembled the characteristic cell in the T mu fraction (TM-type cell), but differed from the characteristic cell in the T gamma fraction (TG-type cell). Moreover, like the TM-type cells all CMC showed paranuclear dots of alpha-naphthylacetate esterase and acid phosphatase activity in their cytoplasm. From these observations it was concluded that CMC without Fc mu receptors were either stimulated T mu cells or precursor T mu cells. Thus CMC in healthy individuals constitute a distinct subpopulation of T cells not only morphologically but also histochemically and immunologically.