Cytoplasmic microtubules in mammalian erythropoietic cells

Electron microscopic examination of rabbit erythroblasts revealed the presence of microtubules, 210–270 Å in diameter, composing a marginal band in the cell periphery. This band was composed of 6–8 microtubules arranged more or less parallel to one another. Transverse sections indicated that 16–18 tubules may comprise the band. The latter appeared to be poorly formed or absent in reticulocytes and erythrocytes. Microtubules were also found in the deeper regions of the cytoplasm as well as composing th emitotic spindle of dividing erythroblasts. With the exception of the procentriole and the asters and chromosomes of dividing cells, microtubules were not continuous with other organelles. The function of the marginal band is interpreted as imparting elasticity and resiliency to the cell, a cytoskeleton responsible for maintenance of cell form. “Coated” micropinocytotic vesicles in erythroblasts, hepatocytes, and reticulo‐endothelial cells are also described. Most of the “coated” vesicles in erythropoietic cells did not contain ferritin. It is suggested that, in the “coated” vesicles, ferritin may be in a highly dilute form or, alternatively, that its relative absence may indicate that iron is obtained predominantly through the plasma transferrin system rather than through incorporation of iron in the form of ferritin.