An ultrastructural study of the development of the chicken perineurial sheath.

Three phases of perineurial development were recognised in the chicken sciatic nerve: (1) an early primitive phase during which the embryonic perineurium appeared to organise from the surrounding mesenchyme; (2) an intermediate phase of differentiation with the formation of a multilayered cellular network around the Schwann cell-axon complexes, and (3) a final phase of maturation during which the perineurial sheath showed features correlating with those of a functional barrier. Our observations support a mesenchymal origin for the perineurium rather than a schwannian derivation with clear separation of these 2 cell populations during all stages of development. Although the embryonic perineurial cells initially showed a fibroblast-like appearance, distinctive features of perineurial differentiation were present as early as 5 1/2 days (d) (Hamburger & Hamilton stage 28 embryo). Perineurial differentiation appeared to be intimately associated with developmental events in the Schwann cell-axon complexes, particularly during the period of most active Schwann cell proliferation. It is proposed that factors released by the Schwann cell-axon complexes during this period may be responsible for perineurial differentiation and organisation from the surrounding mesenchyme. The presence of endoneurial blood vessels following the appearance of perineurial basal lamina at 17 d (stage 43) suggested the emergence of a perineurial barrier. A perineurial architecture and cytological fine structure comparable to that of the adult animal was, however, only observed in the 10 d-old-chick.