The epineurium is a layer of connective tissue enclosing the other layers of the nerve. The nerve sheath cells are principally the Schwann cell and the perineurial cell, both characterized by cytoplasmic processes. Schwann cells, the inside layer of the endoneurium, surround the axon and produce myelin, external lamina and collagen. They have interdigitating cytoplasmic processes and continuous external lamina. In myelinated nerves there is one Schwann cell per axon, and in non-myelinated nerves there are several axon segments within a single Schwann cell, but with very few layers of schwannian plasma membrane.
Perineurial cells form a few circumferential layers outside the endoneurium.
A third cell type has been identified within the endoneurium as a slender dendritic spindle cell which displays CD34 positivity, and which appears to be distinct from Schwann, perineurial or endothelial cells. It may be analogous to the dermal dendritic fibroblast. The cytoplasm contains microfibrils, lysosomes, mitochondria. Antoni A areas. Long-spacing collagen in seen in the stroma of benign peripheral nerve sheath tumors and occasionally in malignant ones, but is not specific.