Electron microscopic observations on the slime mold Physarum polycephalum with specific reference to fibrillar structures

Abstract

Microplasmodia, motile plasmodia, and spherules of the slime mold Physarum polycephalum were studied by electron microscopy to further localize and characterize fibrillar structures possibly pertaining to cytoplasmic streaming and related kinetic phenomena. Fibrils consisting of filaments approximately 55 A in diameter are found in or subjacent to the hyaline, organelle-free cortical cytoplasm. They occasionally make contact with the plasma membrane of free surfaces but are more closely associated with the plasma membrane of “invaginations” that divide the nodules of microplasmodia into networks of lobules and strands, and the channel walls of motile plasmodia into compartments that communicate with the central channel. If contractile, the fibrils are so positioned in circular and longitudinal arrays as to bring about changes in diameter and length associated with cytoplasmic streaming and motility. All plasmodia are encapsulated in a fibrillar network. Its possible role in providing restoring forces during streaming and motility is suggested by morphological similarities between it and the internal fibrils.