Intermediate-sized filaments in leukemia cells.

Electron microscopic studies on human acute leukemias have shown that leukemic populations contain spherical and polarized cells in various proportions. As recorded by time-lapse cinematography, the two cell configurations represent different functional states: resting cells are completely spherical, locomotive cells are polarized with a conspicuous extension posteriorly. In 9 out of 12 cases of acute myeloid leukemia the two cell configurations were found to coincide with a different pattern of intermediate-sized filaments (ISF). Most spherical myeloblasts possessed large bundles of ISF (a minority had small bundles), whereas polarized myeloblasts showed small groups or single filaments. A similar correlation between cell shape and arrangement of ISF was observed in a transplantable undifferentiated rat leukemia. Two concepts can be distinguished with regard to the role of fibrillar structures in leukemic myeloblasts: thick bundles of ISF either represent a pathological state or have a functional significance. A tentative interpretation of our own results provides some arguments in favor of a disaggregation-reaggregation cycle of thick ISF bundles, whereas a pathological ("end stage") nature of these structures appears less likely.