Ultrastructure of schwann and perineural sheaths at the mouse neuromuscular junction

Abstract

Abstract Intramuscular nerve branches are covered by a complex of epithelial sheaths which decrease in thickness and complexity as the nerves branch to reach ultimate termination of neuromuscular junctions. The outermost layer (epineurium) of intramuscular nerves is composed of thin processes of fibrocytes lacking basement lamina and associated with parallel bundles of collagen. Internal to this layer are two or three layers of interdigitating perineural cells covered with basement lamina. The innermost layer is frequently incompletely covered by basement lamina within the perineural sheath. Schwann cells covered with basement lamina enclose myelinated and unmyelinated axons. In the region of neuromuscular junctions, the basement lamina of Schwann cell processes merge with the myofiber basement lamina. Thin perineural cell processes form a bell‐shaped covering which does not reach the muscle basement lamina. The relationships of myelin to the axon as the terminal axon emerges from the Schwann cell closely resembles the relationships of these structures in nodes of Ranvier.