The microtubules of the mature erythrocyte of the chicken are confined to a band at the periphery. Whole-mount electron microscopy after extraction reveals that the number of microtubules in each cell is almost the same. All the microtubules can be depolymerized by incubation in the cold, and the marginal band can be quantitatively and qualitatively reformed by return to 39 degrees C. These properties allow the reformation of the marginal band to be treated as an in vivo microtubule assembly reaction. The kinetics of this reaction and the intermediates detected during reformation suggest a mechanism of microtubule organization that is distinct from that observed in other cell types. Apparently only one or two growing microtubule ends are available for assembly--assembly is only detected at the cell periphery, even at early times--and there is no evidence of the participation of a microtubule-organizing center.

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