Light and electron microscope immunohistochemical methods were used to study the distribution of several proteins in rat skeletal muscle. The aims were to identify components of muscle fiber basement membrane and to compare the small fraction (0.1%) of the basement membrane that extends through the synaptic cleft at the neuromuscular junction with the remaining, extrasynaptic portion. Synaptic basement membrane is functionally specialized and plays important roles in neuromuscular function and regeneration. Laminin, fibronectin, collagen IV, collagen V, and a collagenous protein (high-salt-soluble protein [HSP]) are all present in muscle fiber basement membrane. Laminin and collagen IV are concentrated in basal lamina (the feltlike, inner layer of the basement membrane) and are shared by synaptic and extrasynaptic regions. Fibronectin, also present synaptically and extrasynaptically, is present in basal lamina and in the overlying reticular lamina. Collagen V and HSP are present throughout extrasynaptic basement membrane but are absent from synaptic sites; HSP is concentrated in the reticular lamina and on the outer surface of the basal lamina. These results, together with experiments reported previously (Sanes and Hall, 1979. J. Cell Biol: 83:357--370), provide examples of three classes of components in muscle fiber basement membrane--synaptic, extrasynaptic, and shared.

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