Intermediate filaments in the giant muscle cells of the nematode Ascaris lumbricoides; abundance and three-dimensional complexity of arrangements.

The body muscle cells of the nematode Ascaris lumbricoides are characterized by massive amounts of intermediate filaments (IF). These occur in all three regions of this giant cell type. They traverse the cytoplasm of the balloon-like belly, which houses the nucleus, and occur as bundles in the arm-like extensions to the nerve. The organization of IF in the third region, the contractile fiber, was analyzed further by serial sections and three-dimensional reconstruction. IF bundles traverse the glycogen-rich lumina of the fiber and reach as baskets around the sarcomeres. Together with numerous dense bodies they form the Z-band-like arrangements. IF bundles reach the plasma membrane at hemidesmosome-like specializations often situated at deep membrane invaginations filled with a fibrillar component of the extracellular matrix. The ultrastructural appearance of IF bundles is connected to the contractional state of the sarcomeres. They appear straight in extended muscle but coil up upon contraction. In the pharynx massive IF bundles are oriented longitudinally. A second type of IF bundles follows the radially oriented sarcomeres. These reveal pronounced Z-band type structures with massive disks. IF surround the sarcomeres and seem to terminate at these disks. We discuss possible functions of the complex IF organization in body muscle and pharynx.