Histopathological changes following removal of the perineurium

Gli1 Regulates the Postnatal Acquisition of Peripheral Nerve Architecture

Peripheral nerves are organized into discrete compartments. Axons, Schwann cells (SCs), and endoneurial fibroblasts (EFs) reside within the endoneurium and are surrounded by the perineurium, a cellular sheath comprised of layers of perineurial glia (PNG). SC secretion of Desert Hedgehog (Dhh) regulates this organization. In Dhh nulls, the perineurium is deficient and the endoneurium is subdivided into small compartments termed minifascicles. Human Dhh mutations cause a neuropathy with similar defects. Here we examine the role of Gli1, a canonical transcriptional effector of hedgehog signaling, in regulating peripheral nerve organization in mice of both genders. We identify PNG, EFs, and pericytes as Gli1-expressing cells by genetic fate mapping. Although expression of Dhh by SCs and Gli1 in target cells is coordinately regulated with myelination, Gli1 expression unexpectedly persists in Dhh null EFs. Thus, Gli1 is expressed in EFs noncanonically (i.e., independent of hedgehog signaling). Gli1 and Dhh also have nonredundant activities. Unlike Dhh nulls, Gli1 nulls have a normal perineurium. Like Dhh nulls, Gli1 nulls form minifascicles, which we show likely arise from EFs. Thus, Dhh and Gli1 are independent signals: Gli1 is dispensable for perineurial development but functions cooperatively with Dhh to drive normal endoneurial development. During development, Gli1 also regulates endoneurial extracellular matrix production, nerve vascular organization, and has modest, nonautonomous effects on SC sorting and myelination of axons. Finally, in adult nerves, induced deletion of Gli1 is sufficient to drive minifascicle formation. Thus, Gli1 regulates the development and is required to maintain the endoneurial architecture of peripheral nerves.SIGNIFICANCE STATEMENT Peripheral nerves are organized into distinct cellular/ECM compartments: the epineurium, perineurium, and endoneurium. This organization, with its associated cellular constituents, is critical for the structural and metabolic support of nerves and their response to injury. Here, we show that Gli1, a transcription factor normally expressed downstream of hedgehog signaling, is required for the proper organization of the endoneurium but not the perineurium. Unexpectedly, Gli1 expression by endoneurial cells is independent of, and functions nonredundantly with, Schwann Cell-derived Desert Hedgehog in regulating peripheral nerve architecture. These results further delineate how peripheral nerves acquire their distinctive organization during normal development, and highlight mechanisms that may regulate their reorganization in pathologic settings, including peripheral neuropathies and nerve injury.

Perivascular Hedgehog responsive cells play a critical role in peripheral nerve regeneration via controlling angiogenesis

Hh signaling has been shown to be activated in intact and injured peripheral nerve. However, the role of Hh signaling in peripheral nerve is not fully understood. In the present study, we observed that Hh signaling responsive cells [Gli1(+) cells] in both the perineurium and endoneurium. In the endoneurium, Gli1(+) cells were classified as blood vessel associated or non-associated. After injury, Gli1(+) cells around blood vessels mainly proliferated to then accumulate into the injury site along with endothelial cells. Hh signaling activity was retained in Gli1(+) cells during nerve regeneration. To understand the role of Hedgehog signaling in Gli1(+) cells during nerve regeneration, we examined mice with Gli1(+) cells-specific inactivation of Hh signaling (Smo cKO). After injury, Smo cKO mice showed significantly reduced numbers of accumulated Gli1(+) cells along with disorganized vascularization at an early stage of nerve regeneration, which subsequently led to an abnormal extension of the axon. Thus, Hh signaling in Gli1(+) cells appears to be involved in nerve regeneration through controlling new blood vessel formation at an early stage.

How big of a threat is needlestick-injury-induced complex regional pain syndrome? A "scientific" perspective

Complex regional pain syndrome (CRPS), previously known as reflex sympathetic dystrophy, is attracting more public attention in Japan which is likely a result of the recent upsurge in lawsuits filed against medical institutes. A recent court ruling over a case of injection-needlestick-injury induced CRPS has touched off serious debates among both medical practitioners and legal professionals. Although the court rejected the plaintiff's claims, the high court admitted them in view of the evidence and the entire pleadings and ordered the defendant to pay compensation. As venipuncture is the most frequently conducted and minimally invasive procedure in daily clinical practice, this court decision has attracted tremendous interest throughout the nation, alarming medical practitioners, and encouraging attorneys. The purpose of this article is twofold: to highlight the patient's clinical course in summary based on an unofficial case law report and to provide a scientific perspective on this issue based on recent relevant articles.

Barriers of the peripheral nerve

This review introduces the traditionally defined anatomic compartments of the peripheral nerves based on light and electron microscopic topography and then explores the cellular and the most recent molecular basis of the different barrier functions operative in peripheral nerves. We also elucidate where, and how, the homeostasis of the normal human peripheral nerve is controlled in situ and how claudin-containing tight junctions contribute to the barriers of peripheral nerve. Also, the human timeline of the development of the barriers of the peripheral nerve is depicted. Finally, potential future therapeutic modalities interfering with the barriers of the peripheral nerve are discussed.

Influence of breaching the connective sheaths of the donor nerve on its myelinated sensory axons and on their sprouting into the end-to-side coapted nerve in the rat

The influence of breaching the connective sheaths of the donor sural nerve on axonal sprouting into the end-to-side coapted peroneal nerve was examined in the rat. In parallel, the effect of these procedures on the donor nerve was assessed. The sheaths of the donor nerve at the coaptation site were either left completely intact (group A) or they were breached by epineurial sutures (group B), an epineurial window (group C), or a perineurial window (group D). In group A, the compound action potential (CAP) of sensory axons was detected in ~10% and 40% of the recipient nerves at 4 and 8 weeks, respectively, which was significantly less frequently than in group D at both recovery periods. In addition, the number of myelinated axons in the recipient nerve was significantly larger in group D than in other groups at 4 weeks. At 8 weeks, the number of axons in group A was only ~15% of the axon numbers in other groups (p<0.05). Focal subepineurial degenerative changes in the donor nerves were only seen after 4 weeks, but not later. The average CAP area and the total number of myelinated axons in the donor nerves were not different among the experimental groups. In conclusion, myelinated sensory axons are able to penetrate the epiperineurium of donor nerves after end-to-side nerve coaption; however, their ingrowth into recipient nerves is significantly enhanced by breaching the epiperineurial sheets at the coaptation site. Breaching does not cause permanent injury to the donor nerve.

The blood-nerve barrier: structure and functional significance

The blood-nerve barrier (BNB) defines the physiological space within which the axons, Schwann cells, and other associated cells of a peripheral nerve function. The BNB consists of the endoneurial microvessels within the nerve fascicle and the investing perineurium. The restricted permeability of these two barriers protects the endoneurial microenvironment from drastic concentration changes in the vascular and other extracellular spaces. It is postulated that endoneurial homeostatic mechanisms regulate the milieu intérieur of peripheral axons and associated Schwann cells. These mechanisms are discussed in relation to nerve development, Wallerian degeneration and nerve regeneration, and lead neuropathy. Finally, the putative factors responsible for the cellular and molecular control of BNB permeability are discussed. Given the dynamic nature of the regulation of the permeability of the perineurium and endoneurial capillaries, it is suggested that the term blood-nerve interface (BNI) better reflects the functional significance of these structures in the maintenance of homeostasis within the endoneurial microenvironment.

Expression of tight and gap junctional proteins in the perineurial window model of the rat sciatic nerve

Limited perineurial injury, known as a perineurial window, can lead to neuropathic pain. This article hypothesizes that the recovery of the perineurium is associated with the intercellular junctional proteins. It analyzes the expressions of occludin, ZO-1, and connexin 43 by immunoconfocal microscopy. Seven days after injury, immunoreactivities for occludin and ZO-1 were observed, although there was no connexin 43 detected. Then, 21 days after injury, immunoreactivity for connexin 43 were observed. These results indicate that recovery of the perineurium is associated with the intercellular junctional proteins and that the recovery of gap junctions is delayed compared with that of tight junctions.

The endoneurial response to microsurgically removed epi- and perineurium

The purpose of the study was to examine the response of the endoneurium of the rat sciatic nerve after removal of the epi- and perineurium. For this purpose, segments (4-5 mm long) of the whole epi- and perineurium around the rat sciatic nerve were microsurgically removed (the peel-off area) and the endoneurium was left intact. The post-operative changes were followed up to 5 weeks post-operatively (PO) by histo- and immunohistochemical studies. Additionally, neuromorphometric analyses considering the number of Schwann cells, axons, macrophages and endothelial cells were examined in the peel-off area. The results showed that at the operative area the central part of the endoneurium (65% of the total area of the endoneurium) remained morphologically intact, but the outer part of the endoneurium (35% of the total area) reacted strongly and showed Wallerian type of degeneration. The number of axons and Schwann cells decreased 3 days PO. However, after 2 weeks the number of Schwann cells increased markedly and the highest number was noted 5 weeks PO. A great number of capillaries were observed in the outer part 1 week PO. A rapid invasion of macrophages was noted at the outer part of the endoneurium immediately after the operation. During the regeneration the endoneurial fibroblasts in the peripheral zone started to form minifascicle-like formations, which resulted in a distinct dense outer part of the endoneurium. This dense outer zone was preserved up to 5 weeks PO and participated in the formation of a new perineurium-like structure, but no distinct new perineurium was formed. At the border zone, areas beside the normal epi- and perineurium proliferation of preserved perineurial cells were noted, which fused to the outer part of the dense endoneurium. On focal areas, an attachment of the operated area to the adjoining muscle was observed. This study shows for the first time that despite the microsurgical removal of epi- and perineurium, the inner part of the endoneurium stays intact, but in the outer part of the endoneurium marked reactive changes ensue, probably to protect the injured peripheral nerve.

An experimental study on the perineurial window

Neurological symptoms of herniated nerve fibers resulting from limited perineurial injury from sharp materials such as needles have become a recent topic in clinical practice. However, the mechanism of this disorder, which is known as a perineurial window, has not been clarified. To investigate the mechanism of nerve damage in the perineurial window, we designed small (1-mm length) and large (5-mm length) perineurial windows using tibial nerves of Wistar rats. In the 1-mm group, a marked hernia of the endoneurial contents developed soon and decreased in size with time, but protrusion of nerve fibers was still observed after 12 weeks. Nerve fibers in both the herniated portion and under the edge of the window were damaged. Even after 12 weeks, regeneration of the nerve fibers and the perineurium was incomplete. In contrast, in the 5-mm group, the initial endoneurial edema was remarkable, but herniated nerve fibers were not seen after 12 weeks. Neurological impairment in the 5-mm group was marked in the early stage but rapidly recovered. The repair of the perineurium and nerve fibers in the 1-mm group was slower than in the 5-mm group. Persistent neurological symptoms in the perineurial window appeared to be more closely associated with entrapment of nerve fibers at the window edge rather than with disruption of endoneurial homeostasis.

Role of axon-deprived Schwann cells in perineurial regeneration in the rat sciatic nerve

The role of Schwann cells (SC) in perineurial regeneration after nerve injury has not yet been resolved. It was hypothesized that SC alone are able to induce at least partial morphological restoration of the destroyed orthotopic perineureum (PN). To test the hypothesis, a permanently denervated segment of the rat sciatic nerve was made acellular by freeze-thawing, except in its most proximal part where non-neuronal cells were left intact. Restoration of the frozen segment by these cells was examined by electron microscopy and immunohistochemistry of the SC marker, S-100 protein, 4 and 8 weeks after injury. The PN regenerated from undifferentiated fibroblast-like cells. In the presence of migrant SC without axons, regenerated cells in the place of the former PN were stacked in several layers and, in accordance with the hypothesis, partially expressed typical features of the perineurial cells (PC): pinocytotic vesicles, short fragments of basal lamina and tight junctions. Migrant SC induced formation of pseudo-minifascicles even in the epineurium. In these, SC organized the adjacent fibroblasts into a multilayered circular sheath, and induced their partial differentiation towards perineurial cells. Further experiments demonstrated that regenerating axons are required for complete morphological differentiation of the regenerated perineurial cells either in the orthotopic PN or in minifascicles.

Blood-nerve barrier in the frog during wallerian degeneration: are axons necessary for maintenance of barrier function?

Blood-nerve barrier tissues (endoneurial blood vessels and perineurium) of the frog's sciatic nerve were studied during chronic Wallerian degeneration to determine whether barrier function depends on the presence of intact axons. Sciatic nerves of adult frogs were transected in the abdominal cavity; the ends were tied to prevent regeneration and the distal nerve stumps were examined. Vascular permeabilities to horseradish peroxidase and to [14C]sucrose increased to day 14, returned toward normal levels by 6 weeks, and continued at near normal levels to 9 months. Perineurial permeabilities to the tracers increased by day 10 and remained elevated at 9 months. Proliferation of perineurial, endothelial, and mast cells occurred between 3 days and 6 weeks, resulting in an increased vascular space (measured with [3H]dextran) and number of vascular profiles. The perineurium increased in thickness and the mast cells increased in number. This study indicates that during Wallerian degeneration of the frog's sciatic nerve there is 1) a transitory increase in vascular permeability distal to the lesion, that is related to changes within the endoneurium; 2) an irreversible increase in permeability of the perineurium, which begins later than that seen in the endoneurial blood vessels; and 3) proliferation of non-neuronal components in the absence of regenerating neuronal elements. The results indicate that maintenance of vascular integrity does not require the presence of axons in the frog's peripheral nerve, whereas perineurial integrity and barrier function are affected irreversibly by Wallerian degeneration.

Endoneurial proliferation of perineurial cells in leprosy

In leprous neuropathy the perineurium has very often an abnormal multilayered appearance and is infiltrated by many different types of inflammatory cells. We report here 13 cases characterized by an abnormal endoneurial proliferation of fibroblasts which seems to differentiate in perineurial cells. In several instances there is formation of many intrafascicular microcompartments. Such aspects have been described in various, but infrequent, cases of experimental and human neuropathies. It seems that severe Wallerian degeneration, diffuse endoneurial macrophage infiltration and lesion of the perineurium might lead to such a process.

Invited review: focal nerve injury: guidance factors during axonal regeneration

Regeneration of axons after nerve injury is not random but is often not precise, particularly after injuries that involve nerve transection. The factors that guide regenerating axons toward appropriate terminations are complex, but developments in neurobiology are beginning to indicate the mechanism that are involved. These include neurotrophic and neurite promoting factors, chemotactic influences from peripheral tissues, and the properties of the extracellular matrix. Understanding these mechanisms may indicate means of improving functional recovery after nerve injury.

Localized hypertrophic neuropathy: possible focal perineurial barrier defect

Localized hypertrophic neuropathy (hypertrophic mononeuropathy) is a rare benign condition that generally occurs in people under 40 years of age. Our immunocytochemical (S-100 protein) study of four new cases confirms previous observations that the cells forming the hypertrophic onion bulb are composed of perineurial cells. These observations and previously published illustrations, reveal a curious hyalinization of the outer perineurium of affected fascicles which suggests the absence of a perineurial barrier. Compartmentation (compartmentalization) of the endoneurium in hypertrophic mononeuropathy closely mimics the transient compartmentation which occurs in the distal nerve stumps of axotomized nerves, particularly in nerves in which re-innervation is prevented. Compartmentalization also can be produced by resection of the perineurial sheath. These findings suggest that hypertrophic mononeuropathy may be a reactive condition due to focal damage to the perineurial barrier.

Sensorimotor perineuritis--an autoimmune disease?

The literature contains a single description of sensory perineuritis (Asbury et al 1972). These patients demonstrated a painful, distal, sensory neuropathy, and examination of peripheral nerve biopsies revealed focal thickening and inflammatory infiltrates of the perineurium. We report a patient with sensorimotor peripheral nerve dysfunction, accompanied by progressive slowing of nerve conduction velocity. Examination of a sural nerve biopsy demonstrated focal thickening of the perineurium, inflammatory infiltrates, and necrosis of perineurial cells. Immunohistology revealed a patchy precipitation of IgG and IgM on perineurial cells. Ultrastructurally, mononuclear cells were found adjacent to perineurial cells undergoing necrosis. The patient showed gradual improvement partially coinciding with a course of steroid therapy. We suggest that this neuropathy is caused by damage to the perineurial barrier possibly by an immune-mediated destruction of perineurial cells and subsequent compression of the endoneurial content by perineurial scarring.

Experimental study of the regenerative potential of perineurium at a site of nerve transection

A single fascicle of sciatic nerve was transected in a series of rats. The nerve was repaired by one of three experimental models: 1) perineurial suturing with bulging axons untrimmed. 2) perineurial suturing with bulging axons trimmed, and 3) perineurial suturing to produce a misaligned fascicle. Nerves were excised at 0 to 42 days and were examined in thin, longitudinal section. The complete absence of perineurial regeneration was observed at all time intervals and in all models. Regeneration of axons within the fascicle was disordered. Axonal regeneration extended into the surrounding connective tissue and infiltrated both the proximal and distal perineurium.