Creating Barriers
Authors: James L. Salzer
Journal: Neuron (1999)

Unlike the central nervous system, which resides within the sanctuary of the calvarium and the vertebral canal, peripheral nerves are subject to stretching and other mechanical stresses imposed by movement and pressure. To protect against such forces, peripheral nerves come equipped with their own structural support in the form of an extensive extracellular matrix and a well-organized connective tissue sheath. This cellular sheath also has the important role of maintaining the integrity of the internal peripheral nerve environment. It does so by creating a physical barrier that, under physiologic conditions, limits the entry of biologically active proteins, infectious agents, and the migration of blood-borne cells into the nerve bundles (9Peters A. Palay S.L. Webster H.D The Fine Structure of the Nervous System. Third Edition. Oxford University Press, New York1991Google Scholar). Cells of the connective tissue sheath that surrounds nerve fascicles, the perineurium, are joined by tight junctions that block diffusion of proteins into the nerve bundles; tight junctions also form between the endothelial cells of capillaries within the nerve bundles themselves and constitute a distinct blood–nerve barrier (7Olsson Y Crit. Rev. Neurobiol. 1990; 5: 265-311PubMed Google Scholar). The signals that regulate the formation of the perineurium, and the development of its diffusion barrier, have been unknown. In a report in this issue of Neuron, 8Parmantier E. Lynn B. Lawson D. Turmaine M. Namini S.S. Chakrabarti L. McMahon A.P. Jessen K.R. Mirsky R Neuron. 1999; 23 (this issue): 713-724Abstract Full Text Full Text PDF PubMed Scopus (258) Google Scholar provide strong evidence that Schwann cells induce the formation of the perineurium, which normally acts as the tissue–nerve diffusion barrier, and that they do so via the signaling molecule Desert hedgehog (Dhh) (8Parmantier E. Lynn B. Lawson D. Turmaine M. Namini S.S. Chakrabarti L. McMahon A.P. Jessen K.R. Mirsky R Neuron. 1999; 23 (this issue): 713-724Abstract Full Text Full Text PDF PubMed Scopus (258) Google Scholar). Before considering their results in more detail, it is useful to briefly consider the components of the connective tissue sheath of peripheral nerves. These include the epineurium, which surrounds the entire nerve, the perineurium, which surrounds the nerve fascicles, and the endoneurium, which is the connective tissue within the perineurium associated with the nerves themselves (see figure). Of particular interest for the barrier function of the connective tissue sheath, and the principal focus of the studies described by 8Parmantier E. Lynn B. Lawson D. Turmaine M. Namini S.S. Chakrabarti L. McMahon A.P. Jessen K.R. Mirsky R Neuron. 1999; 23 (this issue): 713-724Abstract Full Text Full Text PDF PubMed Scopus (258) Google Scholar, is the perineurium. This is a compact, multilayered arrangement of flattened polygonal cells and basal lamina that are concentrically organized around fascicles of nerve fibers. Perineurial cells are of mesenchymal origin (4Bunge M.B. Wood P.M. Tynan L.B. Bates M.L. Sanes J.R Science. 1989; 243: 229-231Crossref PubMed Scopus (157) Google Scholar). They initially form a loose, permeable sheath around axons and Schwann cells, which may recruit them from the surrounding mesenchyme, and from which they are separated by the extracellular matrix (5Du Plessis D.G. Mouton Y.M. Muller C.J. Geiger D.H J. Anat. 1996; 189: 631-641PubMed Google Scholar). These cells later undergo a mesenchymal-to-epithelial transition, forming tight junctions and gap junctions and organizing into the perineurium proper. 8Parmantier E. Lynn B. Lawson D. Turmaine M. Namini S.S. Chakrabarti L. McMahon A.P. Jessen K.R. Mirsky R Neuron. 1999; 23 (this issue): 713-724Abstract Full Text Full Text PDF PubMed Scopus (258) Google Scholar have now addressed the signals that promote mesenchymal cells to form the perineurium. An important starting point for these studies was the observation of 1Bitgood M.J. McMahon A.P Dev. Biol. 1995; 172: 126-138Crossref PubMed Scopus (1175) Google Scholar that Dhh was expressed by Schwann cell precursors in developing peripheral nerves. Dhh is one of three members of the Hedgehog family of intercellular signaling molecules that are expressed in higher vertebrates and that play key roles in patterning of the embryo (6Ingham P.W EMBO J. 1998; 17: 3505-3511Crossref PubMed Scopus (381) Google Scholar). After Hedgehog proteins are secreted, they undergo proteolysis and covalent linkage to cholesterol, which promotes their association with the cell surface and their ability to signal via their cognate receptor, Patched (Ptc), a multipass transmembrane protein. Binding of Hedgehog to Ptc is thought to relieve the normal inhibitory effect of Ptc on the seven-transmembrane protein Smoothened (Smo), resulting in activation of intracellular signaling pathways and transcription factors. 8Parmantier E. Lynn B. Lawson D. Turmaine M. Namini S.S. Chakrabarti L. McMahon A.P. Jessen K.R. Mirsky R Neuron. 1999; 23 (this issue): 713-724Abstract Full Text Full Text PDF PubMed Scopus (258) Google Scholar found that immature Schwann cells express Dhh, whereas perineurial precursors express Ptc, consistent with a paracrine signaling mechanism. Dhh expression was maximal in embryonic Schwann cells but continued to be expressed postnatally by myelinating, rather than ensheathing, Schwann cells. 8Parmantier E. Lynn B. Lawson D. Turmaine M. Namini S.S. Chakrabarti L. McMahon A.P. Jessen K.R. Mirsky R Neuron. 1999; 23 (this issue): 713-724Abstract Full Text Full Text PDF PubMed Scopus (258) Google Scholar next analyzed the peripheral nerves of mice nullizygous for dhh, which have defects in spermatogenesis but otherwise develop normally (2Bitgood M.J. Shen L. McMahon A.P Curr. Biol. 1996; 6: 298-304Abstract Full Text Full Text PDF PubMed Scopus (521) Google Scholar). Strikingly, all components of the connective tissue sheath—i.e., the epi-, peri-, and endoneurium—were grossly defective in these mice, whereas myelinated and ensheathed nerve fibers had normal morphologies. Both the epi- and perineurium were disorganized and poorly compacted, the perineurium had few cell layers, and perineural cells were ectopically located within the endoneurial space and failed to express the gap junction protein connexin 43. Of particular interest, the barrier function of the perineurium was deficient as evidenced by the ability of injected proteins, and neutrophils elicited by local injection of bacterial antigens, to cross the perineurium. Consistent with loss of the diffusion barrier, ultrastructural studies revealed that the tight junctions between perineurial cells were abnormal despite the expression of several tight junction proteins. From these and other studies, Schwann cells have emerged as key regulators of peripheral nerve development, being required for the survival of neurons (10Riethmacher D. Sonnenberg-Riethmacher E. Brinkmann V. Yamaai T. Lewin G.R. Birchmeier C Nature. 1997; 389: 725-730Crossref PubMed Scopus (600) Google Scholar), the distribution of ion channels along nerve fibers (11Salzer J.L Neuron. 1997; 18: 843-846Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar), and, with this report, the development of the connective tissue sheath and the nerve–tissue barrier. Of course, Schwann cells are themselves subject to dramatic regulation by neurons, and these studies raise the intriguing question as to whether neurons regulate Schwann cell expression of Dhh and, thereby, perineurial sheath development. The formation and integrity of the blood–nerve barrier appears to be dependent on axonal signals (3Bouldin T.W. Earnhardt T.S. Goines N.D J. Neuropathol. Exp. Neurol. 1991; 50: 719-728Crossref PubMed Scopus (36) Google Scholar); whether the perineurial diffusion barrier is similarly dependent is as yet unclear (7Olsson Y Crit. Rev. Neurobiol. 1990; 5: 265-311PubMed Google Scholar). These studies will likely spur renewed interest into the physiologic role of the perineurial barrier. dhh null mice may provide a useful model for investigations of the biological function of this barrier, both in physiologic and pathologic settings. They may have an ancillary benefit for investigations of other aspects of peripheral nerve development, since the absence of a perineurial barrier renders the nerves in these mice much more accessible to experimental manipulation. These studies also raise the question of whether Dhh plays a role in the development of tight junctions between endothelial cells within the endoneurium and, by analogy, in the blood–brain barrier. Finally, as the initial recruitment of mesenchymal cells around axon–Schwann cell units in embryonic peripheral nerves was unaffected in the dhh null mice, other signals distinct from Dhh must be involved and remain to be identified. By demonstrating a key role of Schwann cells and Dhh in the formation of the perineurium and the nerve–tissue diffusion barrier, the authors have opened a new and promising window into these and other unresolved questions of peripheral nerve development.