The association of microtubules with the plasmalemma in epidermal tendon cells of the river crab

Abstract

The mode of association of microtubules (MTs) with the plasmalemma in epidermal tendon cells of the river crab, Potamon dehaani was studied by thin-section electron microscopy. In the leg muscle, the tendon cells connect striated muscle cells with the cuticle, forming specialized junctions at both ends. At the muscle-tendon cell junction, the apposed plasmalemmas are interdigitated in a zig-zag pattern separated by a uniform space of about 50 nm, where the basal lamina is shared by two cells. At the tendon cell-cuticle junction, the plasmalemma of the tendon cell forms many conical invaginations, into which dense fibrous material extends from the cuticle. Inside the tendon cell, numerous microtubules run parallel to the direction of tension transmission and are arranged into parallel bundles of various sizes. Within such bundles, fine filamentous structures cross-link adjacent MTs. MTs span the entire length of the cell and attach at their both ends to the junctional domains of the plasmalemma. The junctional plasmalemma is characterized by formation of an electron-dense undercoat, through which MTs are connected with the plasmalemma proper. The ultrastructural features of MT association with the plasmalemma are basically the same at both junctions. At the junctions, MTs usually terminate with free ends and are linked laterally to the plasmalemmal undercoat with fine filamentous structures. These observations emphasize the role of the plasmalemmal undercoat as a device of the attachment of MTs to the plasmalemma.