Microtubules in mammalian erythroblasts. Are marginal bands present?

The purpose of this study was to determine if marginal bands, such as those present in mature nucleated red blood cells of other non-mammalian vertebrates and in primitive mammalian erythrocytes are present in definitive mammalian erythroblasts. In a small number of erythroblasts examined from mouse spleen, a bundle of 5-8 microtubules could be seen. These microtubules appeared similar to those previously identified by others as marginal band microtubules in liver and marrow erythroblasts. However, it was difficult to distinguish these bundles from remnants of mitotic spindle microtubules, or bundles of microtubules which extend to the midbody, a structure which is seen quite frequently in sections of erythroid cells. Triton extraction, a process which renders cytoskeletal elements such as microtubules more visible, also failed to confirm the presence of conventional marginal bands in these cells. It is suggested that use of the term "marginal band" be restricted to those cases in which it can be unequivocally demonstrated that a bundle of microtubules encircles the perimeter of the cell.