Morphometrical evaluation of acute leukemic cells by electron microscopy. Discrepancy between morphological characteristics in FAB classification and electron microscopic morphometry.

Leukemic cells from 39 patients with acute leukemia (20 lymphocytic and 19 myelogenous) were examined by transmission electron microscopy and the nucleus and cytoplasm were measured on the micrographs with a computer-controlled image analyzer. The ratios between the areas of the nucleus and whole cell profile (nucleus/cell ratio), heterochromatin and euchromatin, the nucleolus and nucleus, and the degree of irregularity of the nucleus were compared between the two major types of leukemia studied. Acute lymphocytic leukemia (ALL) cells had a relatively larger nucleus and relatively less cytoplasm than acute myelogenous leukemia (AML) cells, and a greater proportion of the area of the nucleus was occupied by heterochromatin in ALL cells than in AML cells. According to the FAB classification, L1 cells are characterized by narrow, and L2 cells by wide cytoplasm based on light microscopic observation of smeared cells, and we confirmed these features by morphometry of May-Giemsa-stained blood smears. However, by electron microscopy there was no difference in the nucleus/cell ratio between L1 and L2 cells, this constituting a discrepancy between the results obtained by electron and light microscopic morphometry. In addition there was no difference in the degree of nuclear irregularity between L1 and L2 cells. Among AML subtypes, significant differences were observed only in the nucleus/cell ratio between M3 and M1, M2 or M4 cells, and in the heterochromatin/euchromatin ratio between M5 and M1, M2 or M3 cells. In conclusion, electron microscopic morphometry revealed marked differences between ALL and AML, but the differences among their subtypes defined by the FAB classification based on nonmorphometric light microscopy were less evident by electron microscopic morphometry.