Role of Contractile Microfilaments in Macrophage Movement and Endocytosis

PHAGOCYTOSIS of bacteria and other large particles and pinocytosis of colloids—two processes collectively termed endocytosis—are among the characteristic properties of macrophages. When mouse peritoneal macrophages in culture are observed by phase contrast microscopy, most small endocytotic vesicles (pinosomes) are seen to be formed in the region of ruffled membrane activity, usually in a pseudopod¹. The phase-lucent pinosomes move rapidly towards the Golgi region where they unite with phase-dense granules to form secondary lysosomes. Although there is evidence that both phagocytosis and pinocytosis in macrophages have a high temperature coefficient and require metabolic energy¹, the mechanism of endocytosis is unknown. Clearly, movement of the plasma membrane and directional movement of pinosomes is involved. During the past few years attention has been drawn to the apparent association in many cells between movement and the presence of contractile microfilaments of about 50 Â diameter^2,3. Some of these are actin-like and can bind heavy meromyosin to give distinctive “arrowhead” structures in electron micrographs⁴. One of us (S. de P., in preparation) has found that the peripheral or cortical cytoplasm of macrophages contains a network of microfilaments, some of which may be inserted into the plasma membrane. These filaments bind heavy meromyosin (Figs. 1 and 2) and details of their structure and disposition will be published later.