Fine structural and immunohistochemical identification of perineurial cells connecting proximal and distal stumps of transected peripheral nerves at early stages of regeneration in silicone tubes.

Perineurial cells are specialized connective tissue cells that form a barrier between endoneurium and epineurium in normal nerves. In the present study, the formation of the perineurium after transection of rat sciatic nerves was investigated. The cord bridging the gap between proximal and distal stumps through silicone tubes was studied 3, 7, 12, 18, and 21 days after surgery using electron microscopy and antibodies against epithelial membrane antigen (EMA), a marker for perineurial cells that has thus far not been applied to the study of differentiating cells in nerve tubulation systems. Initially, a thin cord consisting of fibrin bridged the gap between the stumps. At 7 days, longitudinal cells had migrated from both stumps toward the center of the tubes on the surface of the fibrin cord. These cells were immunoreactive with anti-EMA. At 12 days, ultrastructural features of perineurial cells (desmosomes, tight junctions, actin filaments with dense bodies, tonofilaments) were prominent in these cells. Subsequently, the gap was bridged through the perineurial tube by endothelial cells, pericytes, fibroblasts, Schwann cells, and axons. At 21 days, a single large nerve fascicle ensheathed by a mature perineurium was found between the stumps. Thus, the first cells to connect proximal and distal stumps in the investigated nerve regeneration silicon chamber system are perineurial cells. Through the tube formed by these cells, blood vessels and nerve fibers bridge the gap. Therefore, establishment of a perineurial connection between nerve stumps appears to be important in the sequence of events during nerve regeneration.