Immunogold and freeze etch electron microscopic studies of merosin localization in basal lamina of human skeletal muscle fibers

Abstract

Merosin is a basement-membrane-associated protein found in striated muscle, peripheral nerve and placenta, the deficiency of which causes the muscle wasting condition in C57BL/6J-dy/dy, so-called dy/dy mouse. Moreover, merosin is the binding protein of 156 kDa α-dystroglycan which binds dystrophin by way of 43 kDa β-dystroglycan. Therefore, merosin is an important component of the basal lamina of normal skeletal myofibers. We investigated the ultrastructural localization of merosin antibody in normal human skeletal myofibers by using immunogold electron microscopy and freeze etch electron microscopy. The ultrastructure of the basal lamina showed the presence of the lamina lucida, lamina densa and lamina reticularis. The lamina lucida appeared electron translucent with the exception of fuzzy fibrils. The immunogold electron microscopy disclosed that the merosin was present at the innermost layer (lamina lucida) of the basal lamina of normal human skeletal myofibers. With freeze etch replica electron microscopy, short cross-bridge fine fibrils were noted in the lamina lucida, connecting the basal lamina to the outer leaflet of the muscle plasma membrane. They measured 3–13 nm in diameter, 20–90 nm in length and were distributed with a spacing of 30–40 nm. The immunogold particles showing the presence of the merosin epitope were associated with these connecting structures.