Electron microscopic observations of an unusual cytoplasmic inclusion in the calcium-starved white mutant (LU887 × LU897) of Physarum polycephalum

Abstract Electron micrographs of Physarum polycephalum microplasmodia (LU887 × LU897) reveal cytoplasmic inclusions that appear “striated” at low magnifications; at higher magnifications these exhibit a structure that we have interpreted as microtubule bundles. The light and dark regions in the inclusions are due to the affinity of some microtubules for osmic acid; these appear to have dense regions while other microtubules remain electron lucent. The diameters of the microtubules are about 32–33nm; the subunits forming the tubule walls measure about 8–9nm in diameter. The diameter measurements are slightly larger than the dimensions assigned to vertebrate microtubules (28nm); however, the diameter of the subunits in the microtubule wall measures about 8–9nm which is essentially the same measurement reported for vertebrate tubulin dimers.