The Substructure of Human Platelet Microtubules

The substructure of the marginal bundle of microtubules supporting the discoid shape of platelets has been examined in negatively stained cells. Platelets were swollen first by exposure to distilled water, spread by surface tension, and mounted on carbon stabilized formvar coated grids. The findings differ from the work of previous investigators in the following ways:

1. Usually one microtubule remains in the circumferential bundle after pretreatment with distilled water.

2. The microtubules are composed of 12-15 subfilaments in parallel association. Each subfilament is composed of beads stacked in such a manner as to suggest a helical wind.

3. A diagonal pattern with a periodicity of 40 A° cuts across the long axis of the parallel subfilaments. This pattern suggests that globular subunits are 35-40 A° by 40 A° in size, and that beads on adjacent subfilaments are not in exact parallel register. Thus, the entire microtubule appears to be composed of subfilaments associated in a helical array.

4. At sites of partial fracture in microtubules, subfilaments lose their parallel association and spray out as free elements. The free subfilaments are essentially identical in appearance to hyaloplasmic microfilaments.

The relationship between subfilaments of microtubules, hyaloplasmic microfilaments, and the contractile mechanism of the platelet is discussed.