Incomplete Microtubules Observed in Mammalian Blood Platelets during Microtubule Polymerization

and 0°C. In both cases the process was reversible (1), 1 the microtubules reappearing when the cold treatment was discontinued. This work deals with the cytoplasmic microtubules in mammalian blood platelets. Unlike the microtubules of the axial filament complex of cilia and flagella, these microtubules are properly preserved only by glutaraldehyde fixation. In platelets the microtubules are arranged in a marginal bundle (3 5) which is thought to be responsible, at least in part, for the maintenance of the lenticular shape of the circulating blood platelet. The report presents some observations on cytoplasmic microtubules of the marginal bundle of mammalian blood platelets subjected to changes in environmental temperature, and shows that, like cytoplasmic microtubules in other cells, platelet microtubules disappear when platelets are placed in the cold, and that they reappear when the platelets are warmed again. Observations are presented which give indications of how microtubules are formed. Changes in the shape of platelets concomitant with changes in the marginal bundle of microtubules will be dealt with in detail in a later report.