Electron microscopy of actin rods and bundles in Dictyostelium discoideum by high-pressure freezing.

A new type of actin rods comprising actin tubules appears in dormant spores of Dictyostelium discoideum. Occasionally, the rods in the nucleus were observed in an amorphous state using a combination of high-pressure freezing and freeze-substitutions. Also in the case of actin bundles formed in the nudeus of vegetative cells exposed to dimethyl sulphoxide, actin filaments seemed to be embedded in matrices. The karyoplasm of spores fixed by the above method appeared to be denser than that obtained by other methods. As soluble materials may be efficiently retained in the nucleus, actin tubules or actin filaments embedded in those materials may result in hazy images of actin rods and bundles.