Actin microfilaments and fibroin secretion in the silkgland cells of Bombyx mori. Effects of cytochalasin B.

Bombyx mori posterior silkgland cells exhibit an impressive microfilament apparatus located at the cellular apex. It consists of bundles of packed, long microfilaments of 50-70 A diameter running along circumferences delimiting the lumen of the gland, perpendicularly to the flow of luminal silk. Microfilaments are closely associated with microtubules of the cytoplasmic 'radial microtubule system'. Immunolabelling with purified antihuman actin antibodies was used to demonstrate their actin-like nature. Apical microfilaments are sensitive to cytochalasin B (CB) which selectively inhibits the secretion of fibroin. Following the removal of the drug, microfilaments recover their normal morphology and secretion resumes. The possible implication of contraction of microfilaments in the process of secretion is discussed.