Survival of Pacinian corpuscles after denervation in adult rats

Origin, identity, and function of terminal Schwann cells

The highly specialized nonmyelinating glial cells present at somatic peripheral nerve endings, known collectively as terminal Schwann cells (TSCs), play critical roles in the development, function and repair of their motor and sensory axon terminals and innervating tissue. Over the past decades, research efforts across various vertebrate species have revealed that while TSCs are a diverse group of cells, they share a number of features among them. In this review, we summarize the state-of-knowledge about each TSC type and explore the opportunities that TSCs provide to treat conditions that afflict peripheral axon terminals.

Interosseous Membrane Stimulation: A Treatment for Painful Peripheral Neuropathy

Background

Painful peripheral neuropathy is a condition that may be associated with diabetes as well as other causes of neuropathy. Common treatments for the pain include topical application of capsaicin as well as using oral medications, typically gabapentin. The results are variable and rarely provide substantial lasting relief.

Cases

This report describes how a simple and easy to perform acupuncture technique-interosseous membrane stimulation-was used to treat painful neuropathy in 3 patients: 1 with painful diabetic neuropathy; 1 with idiopathic painful neuropathy; and 1 with painful neuropathy caused by exposure to Agent Orange while serving in Vietnam.

Results

The 3 patients had much relief from the pain associated with their neuropathy for several weeks at a time. With regular treatments, sustained relief was obtained any without the addition of new medication.

Conclusions

Interosseous membrane stimulation is safe, simple, and effective for treatment of painful neuropathy. This treatment should be considered for patients who are suffering with painful neuropathy.

Peripheral nerves in the tibial subchondral bone : the role of pain and homeostasis in osteoarthritis

Osteoarthritis (OA) is a highly prevalent degenerative joint disorder characterized by joint pain and physical disability. Aberrant subchondral bone induces pathological changes and is a major source of pain in OA. In the subchondral bone, which is highly innervated, nerves have dual roles in pain sensation and bone homeostasis regulation. The interaction between peripheral nerves and target cells in the subchondral bone, and the interplay between the sensory and sympathetic nervous systems, allow peripheral nerves to regulate subchondral bone homeostasis. Alterations in peripheral innervation and local transmitters are closely related to changes in nociception and subchondral bone homeostasis, and affect the progression of OA. Recent literature has substantially expanded our understanding of the physiological and pathological distribution and function of specific subtypes of neurones in bone. This review summarizes the types and distribution of nerves detected in the tibial subchondral bone, their cellular and molecular interactions with bone cells that regulate subchondral bone homeostasis, and their role in OA pain. A comprehensive understanding and further investigation of the functions of peripheral innervation in the subchondral bone will help to develop novel therapeutic approaches to effectively prevent OA, and alleviate OA pain.

Cite this article: Bone Joint Res 2022;11(7):439–452.

Demise of nociceptive Schwann cells causes nerve retraction and pain hyperalgesia

ABSTRACT

Recent findings indicate that nociceptive nerves are not "free", but similar to touch and pressure sensitive nerves, terminate in an end-organ in mice. This sensory structure consists of the nociceptive nerves and specialized nociceptive Schwann cells forming a mesh-like organ in subepidermis with pain transduction initiated at both these cellular constituents. The intimate relation of nociceptive nerves with nociceptive Schwann cells in mice raises the question whether defects in nociceptive Schwann cells can by itself contribute to pain hyperalgesia, nerve retraction, and peripheral neuropathy. We therefore examined the existence of nociceptive Schwann cells in human skin and their possible contribution to neuropathy and pain hyperalgesia in mouse models. Similar to mouse, human skin contains SOX10+/S100B+/AQP1+ Schwann cells in the subepidermal border that have extensive processes, which are intimately associated with nociceptive nerves projecting into epidermis. The ablation of nociceptive Schwann cells in mice resulted in nerve retraction and mechanical, cold, and heat hyperalgesia. Conversely, ablating the nociceptive nerves led to a retraction of epidermal Schwann cell processes, changes in nociceptive Schwann cell soma morphology, heat analgesia, and mechanical hyperalgesia. Our results provide evidence for a nociceptive sensory end-organ in the human skin and using animal models highlight the interdependence of the nerve and the nociceptive Schwann cell. Finally, we show that demise of nociceptive Schwann cells is sufficient to cause neuropathic-like pain in the mouse.

A Neuroskeletal Atlas: Spatial Mapping and Contextualization of Axon Subtypes Innervating the Long Bones of C3H and B6 Mice

Nerves in bone play well-established roles in pain and vasoregulation and have been associated with progression of skeletal disorders, including osteoporosis, fracture, arthritis, and tumor metastasis. However, isolation of the region-specific mechanisms underlying these relationships is limited by our lack of quantitative methods for neuroskeletal analysis and precise maps of skeletal innervation. To overcome these limitations, we developed an optimized workflow for imaging and quantitative analysis of axons in and around the bone, including validation of Baf53b-Cre in concert with R26R-tdTomato (Ai9) as a robust pan-neuronal reporter system for use in musculoskeletal tissues. In addition, we created comprehensive maps of sympathetic adrenergic and sensory peptidergic axons within and around the full length of the femur and tibia in two strains of mice (B6 and C3H). In the periosteum, these maps were related to the surrounding musculature, including entheses and myotendinous attachments to bone. Three distinct patterns of periosteal innervation (termed type I, II, III) were defined at sites that are important for bone pain, bone repair, and skeletal homeostasis. For the first time, our results establish a gradient of bone marrow axon density that increases from proximal to distal along the length of the tibia and define key regions of interest for neuroskeletal studies. Lastly, this information was related to major nerve branches and local maps of specialized mechanoreceptors. This detailed mapping and contextualization of the axonal subtypes innervating the skeleton is intended to serve as a guide during the design, implementation, and interpretation of future neuroskeletal studies and was compiled as a resource for the field as part of the NIH SPARC consortium. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR)..

Nerves in Bone: Evolving Concepts in Pain and Anabolism

The innervation of bone has been described for centuries, and our understanding of its function has rapidly evolved over the past several decades to encompass roles of subtype-specific neurons in skeletal homeostasis. Current research has been largely focused on the distribution and function of specific neuronal populations within bone, as well as their cellular and molecular relationships with target cells in the bone microenvironment. This review provides a historical perspective of the field of skeletal neurobiology that highlights the diverse yet interconnected nature of nerves and skeletal health, particularly in the context of bone anabolism and pain. We explore what is known regarding the neuronal subtypes found in the skeleton, their distribution within bone compartments, and their central projection pathways. This neuroskeletal map then serves as a foundation for a comprehensive discussion of the neural control of skeletal development, homeostasis, repair, and bone pain. Active synthesis of this research recently led to the first biotherapeutic success story in the field. Specifically, the ongoing clinical trials of anti-nerve growth factor therapeutics have been optimized to titrated doses that effectively alleviate pain while maintaining bone and joint health. Continued collaborations between neuroscientists and bone biologists are needed to build on this progress, leading to a more complete understanding of neural regulation of the skeleton and development of novel therapeutics. © 2019 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals, Inc.

Computational Parametric Analysis of the Mechanical Response of Structurally Varying Pacinian Corpuscles

The Pacinian corpuscle (PC) is a cutaneous mechanoreceptor that senses low-amplitude, high-frequency vibrations. The PC contains a nerve fiber surrounded by alternating layers of solid lamellae and interlamellar fluid, and this structure is hypothesized to contribute to the PC's role as a band-pass filter for vibrations. In this study, we sought to evaluate the relationship between the PC's material and geometric parameters and its response to vibration. We used a spherical finite element mechanical model based on shell theory and lubrication theory to model the PC's outer core. Specifically, we analyzed the effect of the following structural properties on the PC's frequency sensitivity: lamellar modulus (E), lamellar thickness (h), fluid viscosity (μ), PC outer radius (Ro), and number of lamellae (N). The frequency of peak strain amplification (henceforth "peak frequency") and frequency range over which strain amplification occurred (henceforth "bandwidth") increased with lamellar modulus or lamellar thickness and decreased with an increase in fluid viscosity or radius. All five structural parameters were combined into expressions for the relationship between the parameters and peak frequency, ωpeak=1.605×10-6N3.475(Eh/μRo), or bandwidth, B=1.747×10-6N3.951(Eh/μRo). Although further work is needed to understand how mechanical variability contributes to functional variability in PCs and how factors such as PC eccentricity also affect PC behavior, this study provides two simple expressions that can be used to predict the impact of structural or material changes with aging or disease on the frequency response of the PC.

A RET-ER81-NRG1 Signaling Pathway Drives the Development of Pacinian Corpuscles

Axon-Schwann cell interactions are crucial for the development, function, and repair of the peripheral nervous system, but mechanisms underlying communication between axons and nonmyelinating Schwann cells are unclear. Here, we show that ER81 is functionally required in a subset of mouse RET⁺ mechanosensory neurons for formation of Pacinian corpuscles, which are composed of a single myelinated axon and multiple layers of nonmyelinating Schwann cells, and Ret is required for the maintenance of Er81 expression. Interestingly, Er81 mutants have normal myelination but exhibit deficient interactions between axons and corpuscle-forming nonmyelinating Schwann cells. Finally, ablating Neuregulin-1 (Nrg1) in mechanosensory neurons results in no Pacinian corpuscles, and an Nrg1 isoform not required for communication with myelinating Schwann cells is specifically decreased in Er81-null somatosensory neurons. Collectively, our results suggest that a RET-ER81-NRG1 signaling pathway promotes axon communication with nonmyelinating Schwann cells, and that neurons use distinct mechanisms to interact with different types of Schwann cells.

SIGNIFICANCE STATEMENT

Communication between neurons and Schwann cells is critical for development, normal function, and regeneration of the peripheral nervous system. Despite many studies about axonal communication with myelinating Schwann cells, mostly via a specific isoform of Neuregulin1, the molecular nature of axonal communication with nonmyelinating Schwann cells is poorly understood. Here, we described a RET-ER81-Neuregulin1 signaling pathway in neurons innervating Pacinian corpuscle somatosensory end organs, which is essential for communication between the innervating axon and the end organ nonmyelinating Schwann cells. We also showed that this signaling pathway uses isoforms of Neuregulin1 that are not involved in myelination, providing evidence that neurons use different isoforms of Neuregulin1 to interact with different types of Schwann cells.

A morphologic and quantitative comparison of mechanoreceptors in the tibial remnants of the ruptured human anterior cruciate ligament

Reconstruction of the ruptured anterior cruciate ligament (ACL) does not always result in expected successful outcome. A satisfactory outcome depends not only on the tightness or strength of the graft but also on the quality of proprioceptive restoration. Mechanoreceptors of ACL are supposed to play considerable roles in the proprioceptive feedback system of knee. This study aimed to observe the condition and number of the surviving mechanoreceptors in the tibial remnant of ruptured ACL in human knees.From April 2009 to January 2012, 60 patients with existing free tibial remnants who had undergone arthroscopic ACL reconstruction were enrolled and divided into 4 groups according to the time duration of injury to surgery (Group I: no more than 3 months; Group II: 3 to 6 months; Group III, 6 months to 1 year; Group IV: more than 1 year). Six normal ACL specimens were taken as controls. Specimens were obtained from ACL tibial remnant and stained by the immunohistochemical staining method. The type, size, and quantity of mechanoreceptors were observed under the light microscope. A total of 92 Ruffini-like corpuscles, 9 Pacini-like corpuscles, 5 unclassified neural endings, and free nerve endings were identified via immunohistochemical staining.There were no significant differences in the number of mechanoreceptors in the 5 groups (P = 0.238). Some degenerative changes were observed in Group IV. The results suggest that the residual mechanoreceptors in the ruptured ACL exhibit long-term survival and showed no obvious signs of withering within 1 year.Residual mechanoreceptors do exist in the tibial remnants of ruptured anterior cruciate ligament in human knees and identified clearly by using immunohistochemistry staining. No significant difference was found regarding quantitative variation of the residual mechanoreceptors about the injury duration.

Sensory reanimation of the hand by transfer of the superficial branch of the radial nerve to the median and ulnar nerve

BACKGROUND

It remains a surgical challenge to treat high-grade nerve injuries of the upper extremity. Extra-anatomic reconstructions through the transfer of peripheral nerves have gained clinical importance over the past decades. This contribution outlines the anatomic and histomorphometric basis for the transfer of the superficial branch of the radial nerve (SBRN) to the median nerve (MN) and the superficial branch of the ulnar nerve (SBUN).

METHODS

The SBRN, MN, and SBUN were identified in 15 specimens and the nerve transfer performed. A favorable site for coaptation was chosen and its location described using relevant anatomical landmarks. Histomorphometric characteristics of donor and target were compared to evaluate the chances of a clinical success.

RESULTS

A suitable location for dissecting the SBRN was identified prior to its first bifurcation. Coaptations were possible near the pronator quadratus muscle, approximately 22 cm distal to the lateral epicondyle of the humerus. The MN and SBUN had to be dissected interfasciculary over 82 ± 5.7 mm and 49 ± 5.5 mm, respectively. Histomorphometric analysis revealed sufficient donor-to-recipient axon ratios for both transfers and identified the SBRN as a suitable donor with high axon density.

CONCLUSION

Our anatomic and histomorphometric results indicate that the SBRN is a suitable donor for the MN and SBUN at wrist level. The measurements show feasibility of this procedure and shall help in planning this sensory nerve transfer. High axon density in the SBRN identifies it or its branches an ideal candidate for sensory reanimation of fingers and thumbs.

Distal nerve transfers: a biology-based rationale

Peripheral nerve injuries can result in devastating numbness and paralysis. Surgical repair strategies have historically focused on restoring the original anatomy with interposition grafts. Distal nerve transfers are becoming a more common strategy in the repair of nerve deficits as these interventions can restore function in months as opposed to more than a year with nerve grafts. The changes that take place over time in the cell body, distal nerve, and target organ after axotomy can compromise the results of traditional graft placement and may at times be better addressed with the use of distal nerve transfers. A carefully devised nerve transfer offers restoration of function with minimal (if any) detectable deficits at the donor site. A new understanding of cortical plasticity along with patient reeducation allow for good return of strength and function after nerve transfer.

Solitary chemoreceptor cell survival is independent of intact trigeminal innervation

Nasal solitary chemoreceptor cells (SCCs) are a population of specialized chemosensory epithelial cells presumed to broaden trigeminal chemoreceptivity in mammals (Finger et al. [2003] Proc Natl Acad Sci USA 100:8981-8986). SCCs are innervated by peptidergic trigeminal nerve fibers (Finger et al. [2003]) but it is currently unknown if intact innervation is necessary for SCC development or survival. We tested the dependence of SCCs on innervation by eliminating trigeminal nerve fibers during development with neurogenin-1 knockout mice, during early postnatal development with capsaicin desensitization, and during adulthood with trigeminal lesioning. Our results demonstrate that elimination of innervation at any of these times does not result in decreased SCC numbers. In conclusion, neither SCC development nor mature cell maintenance is dependent on intact trigeminal innervation.

Glial growth factor rescues Schwann cells of mechanoreceptors from denervation-induced apoptosis

Golgi tendon organs and Pacinian corpuscles are peripheral mechanoreceptors that disappear after denervation during a critical period in early postnatal development. Even if regeneration is allowed to occur, Golgi tendon organs do not reform, and the reformation of Pacinian corpuscles is greatly impaired. The sensory nerve terminals of both types of mechanoreceptors are closely associated with Schwann cells. Here we investigate the changes in the Schwann cells found in Golgi tendon organs and Pacinian corpuscles after nerve resection in the early neonatal period. We report that denervation induces the apoptotic death of these Schwann cells and that this apoptosis can be prevented by administration of a soluble form of neuregulin, glial growth factor 2. Schwann cells associated with these mechanoreceptors are immunoreactive for the neuregulin receptors erbB2, erbB3, and erbB4, and the sensory nerve terminals are immunoreactive for neuregulin. Our results suggest that Schwann cells in developing sensory end organs are trophically dependent on sensory axon terminals and that an axon-derived neuregulin mediates this trophic interaction. The denervation-induced death of mechanoreceptor Schwann cells is correlated with deficiencies in the re-establishment of these sensory end organs by regenerating axons.

Reinnervation of rat Pacinian corpuscles after nerve crush during the postcritical period of development

The ultrastructure of crural Pacinian corpuscles was examined after sciatic nerve crush performed in 7- to 20-day-old rats, i.e. during the postcritical period of development when the corpuscles no longer degenerate after axotomy but cease growing. The aim of our study was to assess the innervation pattern and structural changes of the corpuscles following transient denervation and subsequent reinnervation during their maturation and growth. Reinnervated corpuscles were examined by electron microscopy from 2.5 months after nerve crush onwards. After sciatic nerve crush at 7 days of age, the corpuscles are mostly reinnervated with multiple axon terminals, each of them enclosed within a newly formed inner core. The axial multiple cores are in part covered by a layer of concentric inner core lamellae and surrounded by a capsule, both structures having survived from the original corpuscle. After nerve crush at 10 days of age, reinnervated Pacinian corpuscles usually contain, in their axial region, a denervated remainder of the original core together with a few regenerated axon terminals enclosed within new inner cores. These axial structures are surrounded by a layer of concentric lamellae of the original core which may accommodate some regenerated terminals. Additional axon terminals with their small inner cores may be found at the outer aspect of the composite core beneath the capsule. When the nerve is crushed in 15-day-old rats, the inner core which is already well developed remains preserved by the time of reinnervation, and regenerating axons grow in between the original lamellae inducing only moderate neoformation of 2-3 lamellar layers which enclose the terminals. After crushing the sciatic nerve in 20-day-old rats, formation of new inner core lamellae is minimal and regenerated terminals become accommodated between the original lamellar of the core as is the case in adult animals. Regeneration of new inner cores and reinnervation of the preserved lamellar structure thus characterize the recovery of Pacinian corpuscles following reinnervation after nerve crush during the postcritical period of their development.

Peripheral target reinnervation following orthotopic grafting of fetal allogeneic and xenogeneic dorsal root ganglia

The sensory reinnervation of dermal papillae and epidermis of glabrous skin, interosseal Pacinian corpuscles, and muscle spindles of the soleus and extensor digitorum longus muscles has been examined 1, 3, and 8 months (allografts) or 3 and 5 weeks (xenografts) following orthotopic grafting of fetal allogeneic or xenogeneic (mouse) dorsal root ganglia (DRG) into ganglionectomized adult rats. Sensory axons in target tissues were identified immunohistochemically by monoclonal antibodies against growth-associated peptide (GAP-43), heavy neurofilament protein (RT-97), anti-mouse-specific membrane glycoprotein Thy-1.2, and polyclonal antibody to calcitonin gene-related peptide (CGRP). Absence of axonal marker staining in target structures of control animals 10 days or 3 months following ipsilateral enucleation of the L3-L6 DRG without grafting indicated an elimination of host normal (intact), regenerating, or collaterally sprouting nerve fibers. The consistent finding of immunolabeled axons ending free and in encapsulated structures in the target tissues of both allo- and xenografted rats indicates that grafted primary sensory neurons can survive and send axonal processes down the full length of the hind limb, to terminate in host target tissues. Axons of xenografted fetal mouse sensory neurons grow in adult rat hosts for distances of 4 cm or more, attaining lengths far greater than called for by their normal developmental programs.

Pulmonary neuroepithelial bodies in neonatal and adult dogs: histochemistry, ultrastructure, and effects of unilateral hilar lung denervation

In neonatal dogs, neuroepithelial bodies (NEB) are located in the distal lung. They consist of closely packed and granulated epithelial cells showing a positive immune reaction to serotonin and carrying well-developed apical microvilli. They make close contact with capillaries and form morphologically afferent synaptic junctions with intracorpuscular nerve endings. Since most nerve endings degenerate after hilar lung denervation, they are carried by extrinsic, most likely vagal, sensory nerve fibers. We conclude that pulmonary NEB probably are receptor organs, sampling the inspired air and secreting bioactive substances. These might have a local vaso- or bronchoactive regulatory effect, or could be carried to other body parts via the blood vessels. In addition, NEB might induce integrative reflexes via the central nervous system. The NEB intracorpuscular nerve endings also show spontaneous degeneration. This, in addition to the scarcity of NEB in the distal lungs of adult dogs, strongly suggests that the pulmonary NEB are particularly important during the perinatal period of life.

Sensory endings in skin grafts and scars after extensive burns

Fifteen patients who underwent a split thickness skin graft operation for full thickness burns and six patients with postburn scars were biopsied after a standard aesthesiological examination completed with Weber and Dellon tests. A semiquantitative evaluation was performed on immunohistochemically stained sections to determine the presence or absence of PGP 9.5 immunoreactive intraepithelial fibres, complex sensory receptors, nerve fibres in the dermal papillae, vessel-innervating fibres, gland-innervating fibres, and nerve trunks in the deep dermis. The reinnervation pattern was similar in grafts and scars. With regard to sensory receptors, free nerve endings and Merkel-neurite complexes were observed. Statistical analysis suggested a significant correlation between sensibility and the amount of regenerated nerve structures (particularly in the epidermis and dermal papillae).

Reinnervation of Pacinian corpuscles by CNS axons after transplantation to the dorsal column: incidence and ultrastructure

We have investigated the capacity of injured axons in the spinal dorsal columns of young adult rats to reinnervate grafted Pacinian corpuscles. A branch of the hindlimb interosseous nerve with a group of crural Pacinian corpuscles attached to it was autotransplanted to the surface of the spinal cord and the nerve stump was implanted into the dorsal column. Two to three months later 16 grafts were removed for examination by light and electron microscopy. By 3 months after transplantation almost all Schwann cell columns of the grafted nerve branch were occupied by regenerated myelinated and unmyelinated axons. Of 41 corpuscles examined by electron microscopy 24 were reinnervated by 1-3 myelinated fibres which gave rise to multiple terminals in the inner core. The remaining corpuscles appeared to be denervated. Only two of the reinnervated corpuscles contained regenerated endings which reiterated the distinct ultrastructure of normal presynaptic terminals of CNS axons, characterized by clusters of lucent vesicles and paramembranous densities. All other corpuscles were reinnervated by terminals which resembled peripheral mechanosensory endings as they contained mitochondria and very few vesicles. One such corpuscle was coinnervated by small terminals filled with large dense cored vesicles. We assume that the majority of grafted Pacinian corpuscles have been reinnervated by dorsal column axons and that the regenerated terminals with the ultrastructure of peripheral mechanosensory endings derive from central axons of primary sensory neurons, which are apparently capable of constructing mechanosensory-like terminals in response to signals from the Pacinian corpuscles. The vesicle-filled endings are probably formed by second order sensory neurons, corticospinal neurons and small peptidergic neurons unable to adjust their terminals to the new target.

Reinnervation of transplanted pacinian corpuscles by ventral root axons: ultrastructure of the regenerated nerve terminals

This study addresses two questions. Can mature, denervated and transplanted Pacinian corpuscles accept innervation from motor axons? If so, does the alien target influence the structural characteristics of the regenerated motor axon terminals? Pacinian corpuscles from the hind leg of young rats, together with a segment of the nerve branch through which they receive their sensory innervation, were autotransplanted to the surface of the spinal cord and the nerve stump anastomosed to the central stump of a transected lumbar ventral root. Between 4 and 5 months later the grafts were studied by electron microscopy. Ventral root axons regenerated through the endoneurial tubes of the grafted nerve to reach the corpuscles, most of which became reinnervated by one to three myelinated fibres. The fibres lost their myelin sheaths before entering the inner core, branched, and gave rise to multiple terminals in the inner core. The regenerated terminals were packed with spherical synaptic vesicles and closely resembled normal motor nerve terminals. Thus motor axons are able to reinnervate Pacinian corpuscles but the structural characteristics of the terminals are apparently not modified by the alien target tissue. This finding contrasts with previous studies, in which it was found that terminals of the central axons of large dorsal root ganglion cells, induced to reinnervate Pacinian corpuscles, displayed the structural characteristics of peripheral sensory endings rather than those of dorsal root terminals in the spinal cord.

Recovery of non-specific cholinesterase activity in sensory corpuscles of mouse toe skin after irreversible inhibition of this enzyme and cold injury

Mouse digital corpuscles, located in the dermal papillae of toe pad skin, consist of the sensory axon terminals enveloped by the cytoplasmic processes of Schwann-derived cells forming the so-called inner core. The inner core cells are capable to synthetize nCHE molecules which are released into the interlamellar spaces filled by the basal lamina, collagenous microfibrils, and amorphous matrix. In the present study, the histochemical detection of the nCHE activity was investigated in the sensory corpuscles after sciatic and saphenous nerve transections and subsequent application of irreversible nCHE inhibitor (iso-OMPA) or cryo-treatment of toe pad skin. The recovery of the nCHE reaction product in both intact and denervated corpuscles revealed the resynthesis of the nCHE molecules by the inner core cells without assistance of sensory terminals, as well. The cellular constituents of corpuscles were degraded while extracellular matrix appeared to be undamaged after freezing injury. The molecules of nCHE attached to the extracellular matrix components disappeared in coincidence with the disintegration of Schwann-derived cells. After about 5 d of survival, the Schwann cells exhibiting the nCHE reactivity migrated through the basal lamina tubes as guidance of regrowing axons or alone. After 7 d from the treatment, immature Schwann cells marked by the nCHE reaction product occupied the scaffolds of old damaged sensory corpuscles. During further days of surviving, the Schwann cells entering the extracellular matrix of degraded corpuscles were differentiated to the inner core cells. The re-differentiation of the Schwann cells into the inner core cells was observed not only in the presence but also in the absence of sensory terminals. These findings suggest certain trophic independence of inner core cells upon sensory terminals in the sensory corpuscles of adult animals.

Immunocytochemical localization of the neural cell adhesion molecules L1, N-CAM, and J1 in Pacinian corpuscles of the mouse during development, in the adult and during regeneration

The immunocytochemical localization of the neural cell adhesion molecules L1, N-CAM and J1/tenascin was investigated by light and electron microscopical techniques in murine Pacinian corpuscles during development, in the adult and in the regenerating state. In adult corpuscles, L1 was present only at contact sites between the sensory axon and inner core lamellae. From birth, the earliest stage tested, until day 7, L1 was additionally expressed on lamellar processes of the inner core cells. N-CAM was expressed in developing and adult corpuscles on lamellae and somata of the inner and outer core cells at their contact sites but was hardly detectable at contact sites between axolemma and inner core lamellae. J1/tenascin was found only in association with the extracellular material of the inner core, especially with the two radial clefts and the boundary space between inner and outer core. In developing corpuscles, J1/tenascin became detectable on extracellular material with the onset of inner core differentiation at approximately day 2. After transection or crush of the sciatic nerve, L1 disappeared from the corpuscles but reappeared with regrowing axons at contact sites between axonal membranes and inner core cells. At any regenerative stage inner core cells remained L1-negative. In denervated and reinnervated corpuscles the expression pattern of N-CAM and J1/tenascin did not differ from the normal adult. These observations suggest that a sensory organ, the Pacinian corpuscle, differs from the sciatic nerve and the neuromuscular junction in that its expression of adhesion molecules remains the same in the denervated state as in the innervated adult. Furthermore, in the denervated Pacinian corpuscle, adhesion molecule expression does not resemble that of any developmental stage tested. Thus, other cures than regulation of adhesion molecule expression patterns might be involved in the successful reinnervation of sensory corpuscles.

The neural dependency of Merkel cell development in the rat: the touch domes and foot pads contrasted

We have used the quinacrine labeling technique and electron microscopy to study the development of the Merkel cell population in the skin of the rat and how this is affected by denervation produced at birth and at various times thereafter. An unexpected difference was found between the Merkel cells of glabrous and hairy skin. In the paw pads of rats aged 1 day or older the Merkel cells differentiated normally and survived quantitatively in the absence of their nerves. In the touch domes however, denervation at 1-4 days prevented the differentiation of the normal Merkel cell population and led to the disappearance of all or most of the Merkel cells that were already present. The Merkel cells in touch domes of the lower leg were affected by denervation like those of the back skin, differing strikingly from the Merkel cells of the footpads, even though the hairy skin of the leg and the glabrous skin of the foot are innervated by the same anatomical nerve. In adult rats, axons regenerating to denervated paws reinnervated epidermal Merkel cells of the pads and restored essentially normal mechanosensitivity to them; thus the Merkel cells of mammalian glabrous skin, like their counterparts in the wholly glabrous skin of lower vertebrates (S. A. Scott, E. Cooper, and J. Diamond, 1981, Proc. R. Soc. London B211, 455-470; K. M. Mearow and J. Diamond, 1988, Neuroscience 26, 695-708), can act as targets for ingrowing nerves. However, even though the differentiation of Merkel cells in hairy skin is nerve dependent, they probably have in common with the Merkel cells of glabrous skin the role of acting as final targets for nerves during development and regeneration.

Painful dysaesthesias following peripheral nerve injury: a clinical and electrophysiological study

Thirty-three patients with complete median, ulnar or digital nerve transections were studied 4 months to 13 years subsequent to suture or nerve grafting. In all cases, sensory disturbances, in terms of paraesthesia or hypaesthesia, were encountered. Painful or unpleasant symptoms, allodynia or hyperpathia, were observed most frequently in patients with poor recovery. The clinical findings and the patients' subjective complaints were correlated to microneurographic single fibre recordings of regenerated cutaneous mechanoreceptors. In more than 80% of the recordings, discharge properties of regenerated receptors, thresholds and a variety of other electrophysiological data were similar or equal to normal controls. Less than 20% of the receptors exhibited atypical properties suggesting defective steady-state regeneration. The ratio of rapidly adapting (RA-units) to slowly adapting mechanoreceptors (SA-units) was inverse in relation to normals. The density of regenerated RA-receptors was higher in the proximal than in the distal part of the reinnervated area. This paralleled the clinical finding of reduced sensory discrimination in these cases and suggests that SA-units may regenerate preferentially. In painful conditions no single fibres could be recorded, reflecting the relative paucity of fibres and probably the atrophy of the nerve. The results of the microstimulation experiments, although less reliable, revealed some evidence that the central processing of regenerated units is abnormal. Clinical and electrophysiological data supported this concept of central changes underlying some of the phenomena observed during peripheral nerve regeneration.

Basal laminae and Meissner corpuscle regeneration

Murine Meissner corpuscles (mouse digital corpuscles), located in pad skin at the toe tip, consist of lamellar cells with long cellular processes (lamellae) surrounding axon terminals in an onion-skin fashion. Lamellar cell bodies and processes were provided with a basal lamina. The present study was made to examine whether these lamellar cell basal laminae have any specific role in the differentiation of regenerating axons and Schwann cells into specialized axon terminals and lamellar cells, respectively. Pad skin at the toe tip was treated 3-5 X by freezing and thawing. By this treatment, cellular constituents of the corpuscles die and disintegrate into cell debris, leaving in situ basal laminae of the lamellar cells in stacked hollow loops, reminiscent of the original configuration of lamellae. Schwann cells and axons of the ordinary nerve fibers in the pad skin were similarly damaged, and basal laminae of the Schwann cells remained as basal lamina tubes. Three days after treatment, regenerating axons were seen extending through the basal lamina tubes of Schwann cells deep in the toe pad skin. However, no regenerating axons were found in the vicinity of the old corpuscles. Five days after treatment, regenerating axons, some of which were accompanied by migrating Schwann cells and others which were still naked, were noted at the subepidermal region, and began to enter the hollow basal lamina loops of the old corpuscles. Eight-15 days after treatment, regenerating axons which entered the basal lamina loops successively gave rise to branches, and at the same time, accompanying Schwann cells emanated cellular processes through well-preserved basal lamina loops. Fifteen-25 days after treatment, regenerating axons seemed to be morphologically specialized as axon terminals, and accompanying Schwann cells differentiated into definite lamellar cells which surrounded the axon terminals in the same manner as in the normal murine Meissner corpuscles. Although the incidence of good regeneration of the corpuscle was relatively low, these findings suggested that basal laminae of lamellar cells might have some specific properties which could be responsible for the differentiation as well as maintenance of lamellar cells and axon terminals in the Meissner corpuscles.

Degeneration and regeneration of peripheral nerve in the rat trigeminal system. II. Response to nerve lesions

The course of vibrissa sensory receptor denervation and subsequent reinnervation was studied following transection or crush of the rat infraorbital nerve. Eighteen hours after nerve lesion, the large-diameter myelinated nerves supplying the lanceolate receptors of the intermediary zone and the Merkel cells of the stratum basale contained areas of focal axoplasmic abnormalities, and some of the nerve terminals exhibited vacuolization, mitochondrial swelling, and disruption of the neurofilament pattern. The Merkel cells and lanceolate receptors of the intermediary zone were completely deafferented by 24 hours after the nerve injuries. The Ruffini complex, free nerve endings and lanceolate receptors of the inner conical body, as well as the free nerve endings and lanceolate receptors of the connective tissue below the Ringwulst, were completely normal 24 hours after crush or transection of the nerve. These receptors underwent progressive degeneration from days 2 through 6 and the vibrissa was totally denervated by day 7. Regenerating axons were first seen entering the vibrissae 2 weeks after the crush lesion and 1 month following nerve transection. Except for a slight decrease in the percentage of Merkel cells innervated, vibrissae from post-crush animals were virtually indistinguishable from normal by 3 months. In contrast, vibrissae from rats subjected to the transection lesion exhibited evidence of misdirected axons and abnormally reinnervated receptors throughout the course of regeneration. Axons entering the hairs with the main vibrissal nerve were observed contributing to the innervation of the inner conical body, an area normally supplied exclusively by the conus nerve. Many of the lanceolate receptors contained multiple unmyelinated axons, and the usually highly ordered circular innervation of the inner conical body was markedly abnormal. It is suggested that these results may help explain the faulty sensory localization and abnormal sensations reported by patients suffering a peripheral nerve injury.

Transplantation of pacinian corpuscles of the rat into the brain

In adult inbred rats of the AVN strain, branches of the crural interosseous nerve were dissected out from donors and transplanted into the brain of recipients, together with a cluster of Pacinian corpuscles, (either into a suction cavity or the cerebral cortex) into a slit 1-2 mm deep. The grafts were fixed and processed for electron microscopy 10 days to 6 months after the operation, and their ultrastructure was examined. Sporadic axons of small diameter grew into the nerve branches of some of the grafts from 11 days onward, and became myelinated during the 2nd month after the operation, but none of the transplanted Pacinian corpuscles became reinnervated. The corpuscles, however, survived denervation and grafting. Most of them retained a well-preserved inner core and an intact capsule, consisting of a normal complement of 29.2 +/- 1.0 (mean +/- SE) capsular layers (n = 8), as did the corpuscles previously examined after denervation in situ. Some of the corpuscles underwent degenerative changes, presumably due to a delayed or restricted revascularization. In this group of corpuscles, the inner core underwent disintegration and was gradually replaced by collagen fibrils, whereas the capsule remained preserved but the number of its layers eventually reduced by 40%. It is assumed that the lack of reinnervation of the grafted Pacinian corpuscles was due to the paucity of regenerating axons, and their failure to form correct projections along those Schwann cell columns connected with the corpuscles.

Multiple axon terminals in reinnervated Pacinian corpuscles of adult rat

The ultrastructure of Pacinian corpuscles localized beneath the crural interosseous membrane was examined two weeks to 18 months after crushing the sciatic nerve in adult rats. The Pacinian inner core and capsule remained preserved during the transient period of denervation. Regenerating axons reached Pacinian corpuscles approximately three weeks after nerve crush. Up to 15 axonal sprouts entered a single corpuscle at the initial stage of reinnervation, but only 1-3 axons increased in size, myelinated and formed axon terminals in the inner core, the excess sprouts being eliminated. Most corpuscles of the crural group were reinnervated by the end of the first month. Three to 19 months after nerve crush, 10% of corpuscles examined were found to be monoaxonal and monoterminal as before the operation; 74% contained multiple terminals; 16% remained denervated. Over half the multiterminal corpuscles were supplied with a single myelinated axon that branched inside the corpuscles; the rest received two or three myelinated axons which formed several terminals. The terminals were distributed at random, usually in the axial region between the lamellae of the inner core. They were cylindrical, with an oval profile; the larger terminals were filled with mitochondria and microtubules at their circumference and contained a core of neurofilaments. Lateral processes of the terminals were filled with vesicles and had membrane specializations as in normal corpuscles. The mean number of terminals in reinnervated corpuscles was 4.07 +/- 0.37 (S.E.M.) at three months, and 3.26 +/- 0.49 (S.E.M.) 6-18 months after nerve crush. This small decrease was apparently the result of degeneration occasionally observed in some axon terminals at later stages of reinnervation. These experiments thus demonstrate that most rat Pacinian corpuscles become reinnervated with multiple terminals after nerve injury and maintain multiterminal innervation permanently.