The fine structure of the peripheral nerve root sheath in the subarachnoid space in the rat and other laboratory animals

Abstract

Spinal nerve roots from several laboratory animals (rat, mouse, dog, chinchilla) were fixed by perfusion with buffered aldehydes and prepared for electron microscopy. The fine structure of the root sheath possesses a common histologic organization wherever found. Its cellular and fibrous coverings are arranged in several lamellae which can be divided into two basic layers. The cytology of the outer layers resembles the characteristics of cells which line the subarachnoid space elsewhere (pia, arachnoid, arachnoid trabeculae, blood vessel adventitia). These cells adjacent to the subarachnoid space are arranged loosely and are in close association with extracellular connective tissues. The junctions between them appear to be intermittent, which would allow communication between the subarachnoid space and the intercellular spaces. The inner layers (adjacent to the endoneurium) are regularly arranged and are close to each other. Facing the endoneurium a boundary (basement) membrane is almost invariably associated with the basal cell layer. The inner cell layers are more nearly comparable to perineurium but, because of structural variations, are not readily classified. However, in its totality, the root sheath does not correspond histologically to pia, arachnoid or perineurium as they are found in their usual locations.