Lymphocytes with parallel tubular structures: morphologically a distinctive subpopulation.

Peripheral blood lymphocytes were fractionated in T, B and Null cell enriched subsets by means of sheep red blood cell rosette (ESRBC) sedimentation and nylon wool adherence. The ultrastructural features of these subpopulation were investigated. The T cell fraction in which the sheep erythrocytes were removed from the ESRBC rosette-forming cells (ESRBC-RFC) by lysis with ammonium chloride, consisted mainly of two morphologically distinctive subsets. The majority of the cells (80%) displayed a smooth surface membrane and had a high nuclear to cytoplasmic ratio with few cytoplasmic organelles. The other cell type (18%) had a relatively rough surface membrane, a low nuclear to cytoplasmic ratio, often an indented nucleus and numerous cytoplasmic organelles such as characteristic amorphous granules and sometimes parallel tubular structures (p.t.s.). If the T cells were obtained after mechanical vibration of the ESRBC-RFC, the majority of these cells appeared morphologically identical to this latter cell type. Cells with p.t.s. and amorphous granules were also demonstrated within the Null and B cell enriched fractions (50% and 25% respectively), though in the B cell enriched fraction this cell type is probably due to a contamination of Null cells. Previous observations had already demonstrated that these cells in the three fractions represent the Fc gamma receptor-bearing lymphocytes. The similarities suggest that the Fc gamma receptor-bearing and p.t.s. containing lymphocytes form a morphologically distinct subpopulation.