Prolymphocytic leukaemia: an ultrastructural study of 22 cases.

Twenty-two cases of prolymphocytic leukaemia (PLL) have been studied by transmission electron microscopy (TEM); 17 had B-cell surface markers (B-PLL) and five had T-cell characteristics (T-PLL). The predominant cell, the prolymphocyte, has distinct features which were common to all cases: it is a relatively large lymphoid cell with a prominent nucleolus, well condensed peripheral nuclear chromatin and a variable amount of heterochromatin in intranuclear clumps. Eight cases of PLL (seven B-PLL, one T-PLL) showed minor morphological deviations from typical PLL which may present diagnostic difficulties when studied only at light microscopy: the cells in four of these had some features in common with those of chronic lymphocytic leukaemia (CLL), namely round regular nuclei and heavy chromatin condensation, but the nucleoli were larger in the PLL cases. In four other cases nuclear clefting was a prominent feature at TEM. B-PLL can be distinguished by ultrastructural features from other B-cell malignancies, e.g. follicular lymphoma, B-CLL and B-acute lymphoblastic leukaemia.