Reinnervation of transplanted Pacinian corpuscles by ventral root axons: Ultrastructure of the regenerated nerve terminals

Summary 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.     

Multiple axon terminals in reinnervated Pacinian corpuscles of adult rat

Summary 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.     

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

Summary 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 lamellae 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.     

Ultrastructural analysis of ectopic synaptic boutons arising from peripherally regenerated primary afferent fibers

The central axons of peripherally regenerated Abeta primary sensory neurons were impaled in the dorsal columns of alpha-chloralose-anesthetized cats 9-12 mo after axotomy. The adequate peripheral stimulus was determined, and the afferent fibers intracellularly stimulated while simultaneously recording the resulting cord dorsum potentials (CDPs). Fibers that successfully had reinnervated the skin responded to light tactile stimulation, and evoked CDPs that suggested dorsally located boutons were stained intracellularly with horseradish peroxidase (HRP). Two HRP-stained regenerated Abeta afferent fibers were recovered that supported large numbers of axon collaterals and swellings in laminae I, IIo, and IIi. Sections containing the ectopic collateral fibers and terminals in the superficial dorsal horn were embedded in plastic. Analyses of serial ultrathin sections revealed that ectopic projections from both regenerated fibers supported numerous synaptic boutons filled with clear round vesicles, a few large dense core vesicles (LDCVs) and several mitochondria (>3). All profiles examined in serial sections (19) formed one to three asymmetric axo-dendritic contacts. Unmyelinated portions of ectopic fibers giving rise to en passant and terminal boutons often contained numerous clear round vesicles. Several boutons (47%) received asymmetric contacts from axon terminals containing pleomorphic vesicles. These results strongly suggest that regenerated Abeta fibers activated by light tactile stimuli support functional connections in the superficial dorsal horn that have distinct ultrastructural features. In addition, the appearance of LDCVs suggests that primary sensory neurons are capable of changing their neurochemical phenotype.     

The ultrastructure of sensory nerve endings in the human knee joint capsule

Summary The ultrastructure of sensory nerve endings in the human knee joint capsule was studied. Three types of nerve endings were found: free nerve endings (FNE), Ruffini corpuscles and Pacini corpuscles.In the joint capsule, FNE are located below the synovial layer and within the fibrous layer near blood vessels. These nerve terminals derive from myelinated A-fibres or from unmyelinated C-fibres. Their structure is almost identical to FNE in human hairy and non-hairy skin.Ruffini corpuscles are present within the fibrous layer and the ligaments of the capsule in three variations: small Ruffini corpuscles without a capsule, small with a connective tissue capsule, and large Ruffini corpuscles with an incomplete perineural capsule. Their afferent axons are myelinated and measure 3–5 m in diameter. Inside the corpuscle, nerve terminals are anchored in the connective tissue belonging to the fibrous layer or to the ligaments respectively. The presence of an incomplete perineural capsule depends on the structure of the surrounding connective tissue. In ligaments with collagenous fibrils oriented in a parallel fashion, the perineural capsule is well-developed and the Ruffini corpuscle resembles a Golgi tendon organ; in areas where the fibrils show no predominant orientation, Ruffini corpuscles lack a capsule.Small Pacini corpuscles are situated within the fibrous layer near the capsular insertion at the meniscus articularis or at the periost. They consist of one or several inner cores and a perineural capsule of 1–2 layers. Larger Pacini corpuscles with one or several inner cores and a perineural capsule consisting of 20–30 layers are found on the outer surface of the fibrous layer.The ultrastructure of these nerve endings is compared with the ultrastructure of articular receptors of various animals and with the ultrastructure of sensory nerve endings in the skin of several mammalian species including man.Supported by the Verein zur Förderung der Erforschung und Bekämpfung rheumatischer Krankheiten e.V. in Bad Bramstedt andby the Deutsche Forschungsgemeinschaft (Sch 587/1-4)     

Multiple innervation of rat pacinian corpuscles regenerated after neonatal axotomy

The regeneration of Pacinian corpuscles was studied on the crural interosseous membrane in rats in which the sciatic nerve has been crushed at birth. The original population of developing Pacinian corpuscles rapidly degenerated after neonatal axotomy. Subsequently, the regenerating axons induced the differentiation of corpuscles de novo.The first regenerating Pacinian corpuscles were found on the interosseous membrane 19 days after neonatal axotomy. Each was, as a rule, supplied by several regenerating axons and contained a rudimentary inner core enclosing the largest axon terminal. By day 40 postnatal, the regenerated corpuscles usually contained several axon terminals enclosed by 2–7 inner cores compressed within a common outer capsule. The majority of corpuscles remained polyaxonal and contained multiple inner cores up to at least 11 months. This is in contrast to normal corpuscles that have one terminal enclosed in an inner core and capsule.The mean number of regenerated Pacinian corpuscles was13.2 ± 1.3 (±s.e.; n = 5) at one to four months after axotomy, i.e. 27.5% of the mean number48.0 ± 1.3 (n = 5) corpuscles found on normal interosseous membranes. The number of regenerated axons of the interosseous nerve supplying the corpuscles was decreased to about 40% of the normal number. The regenerated corpuscles were small and the diameters of the regenerated axons were much reduced.The permanent polyaxonality of newly formed Pacinian corpuscles together with their small size and number is an example of an aberrant course of regeneration in the rat after axotomy performed during the critical perinatal period of development.     

The expression of ENaC and ASIC2 proteins in Pacinian corpuscles is differently regulated by TrkB and its ligands BDNF and NT-4

Pacinian corpuscles are innervated by large myelinated Aα-β axons from the large- and intermediate-sized sensory neurons of dorsal root ganglia. These neurons express different members of the degenerin/epithelial Na⁺ channel (DEG/ENa⁺C) superfamily of proteins with putative mechanosensory properties, whose expression is regulated by the TrkB–BDNF system. Thus, we hypothesized that BDNF and/or NT-4 signalling through activation of TrkB may regulate the expression of molecules supposed to be necessary for the mechanosensory function of Pacinian corpuscles. To test this hypothesis we analyzed the expression and distribution of ENa⁺C subunits and acid-sensing ion channel 2 (ASIC2) in Pacinian corpuscles from 25 days old mice deficient in TrkB, BDNF and NT-4. Pacinian corpuscles in these animals are normal in number, structure, and expression of several immunohistochemical markers. Using immunohistochemistry we observed that the β-ENa⁺C and γ-ENa⁺C subunits, but not the α-ENa⁺C subunit, were expressed in wild-type animals, and they were always found in the central axon. ASIC2 immunoreactivity was found in both the central axon and the inner core cells. The absence of TrkB or BDNF abolished expression of β-ENa⁺C and ASIC2, whereas expression of γ-ENa⁺C did not change. Expression of β-ENa⁺C and γ-ENa⁺C subunits in NT-4 deficient mice was found in the axons but also in the inner core cells whereas levels of expression of ASIC2 were increased in these animals. This study suggests that expression in Pacianian corpuscles of some potential mechanosensory proteins is regulated by BDNF, NT-4 and TrkB.     

Single mechanoreceptors (Pacinian corpuscles) studied by application of colchicine to the nerve

After application of colchicine to the caudal mesenteric nerve of cats, which contains sensory fibers for single mechanoreceptors (Pacinian corpuscles) the axon of the nerve endings degenerates. The character of the ultrastructural changes in the receptor is virtually identical with that observed during degeneration of axons after division of the nerve, but degeneration takes place much more slowly. The ultrastructural, electrophysiological, and biochemical changes taking place in the Pacinian corpuscles are not the result of the direct effect of the alkaloid, but are realized through the nerve, evidently by the blocking of axoplasmic transport. The results are evidence in support of the view that the structures of the receptor are under neurotrophic control by sensory neurons.Department of Histology, Kazan' Medical Institute, (Presented by Academician of the Academy of Medical Sciences of the USSR A. P. Avtsyn.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 86, No. 10, pp. 387–389, October, 1978.     

Sensory nerve endings in the anterior cruciate ligament (Lig. cruciatum anterius) of sheep

This study examines the structure of sensory nerve endings in the sheep anterior cruciate ligament (ACL). Three types of nerve endings are found: free nerve endings (FNE), Ruffini corpuscles, and lamellated corpuscles. The FNE (more than 100) are found subsynovially. The afferent nerve fibres are either thin myelinated axons (Aδ) or C fibres with diameters of 1–2 μm. FNE have been reported to function as thermoreceptors and polymodal nociceptors. In addition, FNE are also seen between fascicles of collagen fibres, often close to blood vessels. Part of this group may be efferent autonomic fibres controlling local blood flow. The corpuscles are seen subsynovially and between fascicles of connective tissue close to the attachment points of the ACL. A ligament contains about 20 Ruffini corpuscles, which are mainly located in the subsynovial connective tissue. They consist of cylinders formed from perineural cells surrounding the afferent myelinated axons (diameters 4–5 μm) with enlarged nerve terminals anchored between collagen fibres. These enter in bundles from the surrounding connective tissue at one open pole, pass through the length of the cylinder, and leave at the other pole. Functionally, Ruffini corpuscles have been described as slowly adapting stretch receptors. Lamellated corpuscles (usually between 5 and 15) are found in the subsynovial connective tissue. The afferent myelinated axon has a diameter of 4–6 μm, and the nerve terminal is located in the centre of numerous layers formed by lamellated terminal glial cells and by a perineural capsule. They are known to function as rapidly adapting pressure receptors. The most important function of the ACL is its mechanical function, but additional sensory functions must be considered triggering reflex mechanisms in case of extreme positioning or overload. Anat Rec 254:13–21, 1999. © 1999 Wiley‐Liss, Inc.     

Distribution of lumbar afferent axons in muscle coat of cat urinary bladder

Lumbar spinal ganglia, L_2,3,4 were ablated in four cats to determine the distribution of degenerating lumbar afferents in the urinary bladder. Bladders were collected at 7, 14, 21 and 35 days following ganglionectomy. Six regions of the urinary bladder were sampled bilaterally and examined ultrastructurally in each cat. Overall, 3,033 terminal axons were counted, of which 2.6% were degenerating; of these, 9% had synaptic vesicles and were interpreted as efferent axons of postganglionic neurons with cell bodies in spinal ganglia. Lumbar afferents were most numerous in the trigone region, followed next by the ventral neck region; regions cranial to the ureters had similar small populations of lumbar afferents. A similar distribution pattern was observed for terminal axons containing granular synaptic vesicles. The relative concentration of lumbar innervation caudal to the ureters seems to account for the increased density of terminal axons observed in this region. Lumbar afferents were distributed bilaterally to the bladder and were numerically similar within and outside muscle fascicles. Ultrastructural evidence supports the position that bladder receptors are free nerve endings except for sparse Pacinian corpuscles.     

The induction of a regenerative propensity in sensory neurons following peripheral axonal injury

Summary Injury of the peripheral axons of primary sensory neurons has been previously shown to increase the probability that the corresponding central axons would grow from the injured spinal cord into a peripheral nerve graft. This phenomenon has been used to investigate the nature of extrinsic cues from injured nerves that trigger an enhanced regenerative propensity within sensory neurons. In 13 groups of rats, a segment of the right sciatic nerve was grafted to the dorsal columns of the spinal cord and the left sciatic nerve was subjected to mechanical injury, injection of colchicine or infusion of nerve growth factor. Subsequently, neurons in lumbar dorsal root ganglia with axons growing from the spinal cord into a graft were identified by retrograde perikaryal labelling and compared for the two sides. The aim was to mimic or modify the inductive effect of nerve transaction by alternative or additional manipulation of the nerve. Growth of central axons was less enhanced by peripheral axonal interruption if the length of the proximal stump was increased or if a distal stump was present to permit rapid regeneration. However, the regenerative response following nerve transection was altered little by crushing the proximal stump or injecting it with colchicine or nerve growth factor. It is suggested that sensory neurons are stimulated to regenerate by peripheral axonal injuries that reduce some normal retrograde regulatory influence of Schwann cells.     

Electron microscopical localization of non-specific cholinesterase activity in three principal parts of cat Pacinian corpuscles

Non-specific cholinesterase (nCHE) activity was demonstrated histochemically on electron microscope level in 3 parts of Pacinian corpuscles from the cat mesenterium. The reaction product was abundant on the plasmic membrane of the inner core lamellae. Schwann cells of a myelinated portion of a sensory axon in Pacinian corpuscles were devoid of positive reaction to nCHE activity. These results are in agreement with those obtained in other sensory nerve endings. Therefore, the cells investing the terminal portion of an unmyelinated sensory axon are considered to be specialized Schwann cells. Furthermore, the amount of end product was lower on the specialized Schwann cells around ultraterminal than terminal portion of a sensory axon. A significantly higher nCHE activity confined to the inner core around the terminal portion of a sensory axon suggests the participation of this enzyme in the maintenance of ionic milieu in periaxonal microenvironment.     

Sensory nerve endings in the beak skin of Japanese quail

Summary This study is concerned with the distribution and ultrastructure of sensory nerve endings in the beak skin of adult Japanese quail (Coturnix coturnix japonica). The following nerve endings were found: free nerve endings, clusters of dermal Merkel nerve endings, Herbst corpuscles and Ruffmi corpuscles. The latter were found only in the dermis of the tip of the upper beak. The remaining endings were present in the skin of all areas of upper and lower beak. Free nerve endings were supplied by either thin myelinated axons or unmyelinated C-fibers and were localized in the dermis close to the basal layer of the epidermis. Merkel cells formed clusters (up to 50) localized below and between the epidermal cones of the beak skin. Disc-shaped thickenings of nerve endings were squeezed between individual Merkel cells. Small Herbst corpuscles were found in the dermis close to the epidermal cones of the beak skin. Large Herbst corpuscles occurred in deep layers of the dermis. The Ruffmi corpuscles were cylindrical in shape (80 m × 400 m) and arranged in groups of up to ten corpuscles. Each corpuscle was surrounded by an incomplete fibrous capsule.     

Immunohistochemical localization of neurocalcin in human sensory neurons and mechanoreceptors

The localization of neurocalcin in the developing and adult human peripheral nervous system (dorsal root and sympathetic ganglia (DRG, SG), and enteric nervous system (ENS)) was investigated using immunohistochemistry. A subpopulation of large-sized neurons in DRG of 9 and 12 weeks old embryos showed immunoreactivity (IR), whereas the sympathetic ganglia or enteric neurons did not. In adults, neurocalcin IR was restricted to a subpopulation of large (13%) and intermediate (15%) sized neurons in DRG. The protein was also found in muscular (67%) and cutaneous (12%) nerve fibers, as well as in the axons supplying muscular (muscle spindles, Golgi's tendon organs, and perimysial Pacinian corpuscles) and cutaneous (Meissner's but not Pacinian corpuscles) mechanoreceptors, as well as motor end-plates. Present results demonstrate that neurocalcin in both developing and adult humans can be used as a specific marker for a subpopulation of sensory neurons coupled to proprioception and touch, and for axons of motoneurons forming motor end-plates.     

Possible Role of the Myelinated Neural Network in the Parietal Peritoneum in Rats as a Mechanoreceptor

A network of myelinated nerve fibers in the peritoneum covers the abdominal wall. We studied the topographic distribution of this network, explored the fibers' destination in the central nervous system, and examined the markers in these fibers in order to identify the nature of the sensation conveyed by the network of nerve fibers in rats. We used Sihler's method, which stains myelinated fibers in whole mount materials, and observed a dense nerve network and endings toward the peritoneal cavity in the peritoneum that covers the abdomen's lateral bulge. We studied the axonal transport of cholera toxin subunit B to investigate the central projections of this network in order to identify its function. After applying the tracer in the peritoneum, we observed many labeled terminals in the medial part of laminae 3–5 of the spinal cord. A small number of labeled terminals was observed in the dorsal nucleus of Clarke and gracile nucleus. Labeled somata were observed in the dorsal root ganglia (DRG). Most (96%) were larger than 35 μm. We performed immunohistochemistry of the abdominal wall, using antiserum against the 200‐kD neurofilament (a marker for mechanosensory neurons). We observed many positive nerve fibers in the peritoneum. Because cell bodies in the DRG were large, their nerve terminals ended in the base of the dorsal horn, which is known to transmit proprioceptive information, and the network possesses the marker for mechanosensitive fibers; therefore, it appears that the myelinated nerve network conveys information about distension and/or contraction of the abdominal wall. Anat Rec, 300:1662–1669, 2017. © 2017 Wiley Periodicals, Inc.     

Oral sensory papillae, chemo- and mechano-receptors, in the snake, Elaphe quadrivirgata. A light and electron microscopic study

The oral sensory papillae of the snake (Elaphe quadrivirgata), comprising a compound sensory system located along the tooth rows, were studied by light microscopy, immunohistochemistry for neuron specific enolase and S 100 protein, and scanning and transmission electron microscopy. Each sensory papilla exhibited a single taste bud and free nerve endings in the epithelium, and Meissner-like corpuscles, branched coiled terminals, and lamellated corpuscles in the connective tissue. The taste buds consisted of four types of cells; the type III cells, exclusively synapsing onto intragemmal nerves, were identified as gustatory in function. The gustatory cells included dense-cored and clear vesicles in the cytoplasm. These vesicles were accumulated both in the presynaptic and infranuclear regions, suggesting dual functions: the synaptocrine and paracrine/endocrine release of signal substances. The free nerve endings constantly contained mitochondria and frequent clear vesicles. The Meissner-like corpuscles were located in the uppermost zone of the connective tissue. These corpuscles consisted of nerve fibers and lamellar cells. The nerve fibers, rich in mitochondria, were folded and layered on each other. The branched coiled terminals were localized in the connective tissue along the side wall of the papillae. Nerve fibers, free from a Schwann-cell covering, swelled up to make terminals which accumulated mitochondria and glycogen particles. The lamellated corpuscles were associated with the nerve-fiber bundles in the connective tissue. Consisting of a central nerve axon and lamellar cells encircling it, these corpuscles resembled mammalian Vater-Pacini corpuscles, except that they lacked a capsule. These findings demonstrated that the snake sensory papilla represents one of the most specialized, compound sensory systems among vertebrates, which may play an important role in receiving chemical and mechanical information on prey.     

Sensory innervation of the human palmar aponeurosis in healthy individuals and patients with palmar fibromatosis

The human palmar aponeurosis is involved in hand proprioception, and it contains different sensory corpuscle morphotypes that serve this role. In palmar fibromatosis (classically referred to as Dupuytren''s disease), the palmar aponeurosis undergoes fibrous structural changes that, presumably, also affect the nervous system, causing altered perception. We analysed the various sensory nerve formation morphotypes in the palmar aponeuroses of healthy subjects and patients with palmar fibromatosis. To do this, we used immunohistochemistry for corpuscular constituents and the putative mechanoproteins PIEZO2 and acid‐sensing ion channel 2. Free nerve endings and Golgi‐Mazzoni, Ruffini, paciniform and Pacinian corpuscles were identified in both the healthy and the pathological conditions. The densities of the free nerve endings and Golgi‐Mazzoni corpuscles were slightly increased in the pathological tissues. Furthermore, the Pacinian corpuscles were enlarged and displayed an altered shape. Finally, there was also morphological and immunohistochemical evidence of occasional denervation of the Pacinian corpuscles, although no increase in their number was observed. Both PIEZO2 and acid‐sensing ion channel 2 were absent from the altered corpuscles. These results indicate that the human palmar aponeurosis is richly innervated, and the free nerve endings and sensory corpuscles within the palmar aponeurosis undergo quantitative and qualitative changes in patients with palmar fibromatosis, which may explain the sensory alterations occasionally reported for this pathology.     

Reactive changes in dorsal roots and dorsal root ganglia after C7 dorsal rhizotomy and ventral root avulsion/replantation in rabbits

Current surgical treatment of spinal root injuries aims at reconnecting ventral roots to the spinal cord while severed dorsal roots are generally left untreated. Reactive changes in dorsal root ganglia (DRGs) and in injured dorsal roots after such complex lesions have not been analysed in detail. We studied dorsal root remnants and lesioned DRGs 6 months after C7 dorsal rhizotomy, ventral root avulsion and immediate ventral root replantation in adult rabbits. Replanted ventral roots were fixed to the spinal cord with fibrin glue only or with glue containing ciliary neurotrophic factor and/or brain-derived neurotrophic factor. Varying degrees of degeneration were observed in the deafferented dorsal spinal cord in all experimental groups. In cases with well-preserved morphology, small myelinated axons extended into central tissue protrusions at the dorsal root entry zone, suggesting sprouting of spinal neuron processes into the central dorsal root remnant. In lesioned DRGs, the density of neurons and myelinated axons was not significantly altered, but a slight decrease in the relative frequency of large neurons and an increase of small myelinated axons was noted (significant for axons). Unexpectedly, differences in the degree of these changes were found between control and neurotrophic factor-treated animals. Central axons of DRG neurons formed dorsal root stumps of considerable length which were attached to fibrous tissue surrounding the replanted ventral root. In cases where gaps were apparent in dorsal root sheaths, a subgroup of dorsal root axons entered this fibrous tissue. Continuity of sensory axons with the spinal cord was never observed. Some axons coursed ventrally in the direction of the spinal nerve. Although the animal model does not fully represent the situation in human plexus injuries, the present findings provide a basis for devising further experimental approaches in the treatment of combined motor/sensory root lesions.     

The structure and function of cutaneous sensory receptors

The present review of cutaneous sensory receptors begins with a consideration of free nerve endings (FNEs) that can be considered as sensory terminals evidencing the least structural specialization of the axon and associated cells. Using the criteria established by Kruger et al (1981), FNEs of both A delta and C fibers can be identified on the basis of ultrastructural characteristics that include an intimate relationship between axons and the associated epithelium, the lack of a complete Schwann cell investment, the accumulation of numerous vesicles and other cytoplasmic organelles, and for A delta terminals a 1:1 relationship between axon and investing Schwann cell. Using these criteria, the so-called genital end bulbs of the human glans penis are merely a skein of FNEs based on the ultrastructural study of Halata and Munger (1986). Hair follicles of most species studied to date (the exception being the rabbit and to some extent the guinea pig) are multiply innervated with lanceolate, Ruffini and FNEs. The lanceolate terminals are the rapidly adapting terminals that are numerous in guard hairs. Ruffini terminals of hairs resemble those of the periodontal ligament or joint capsules and both are remarkably similar to Golgi tendon organs in terms of ultrastructural characteristics. The key ultrastructural characteristic is the encircling of collagen bundles by axons and associated Schwann and connective tissue cells. Axons frequently enter the epidermis either to terminate as FNEs or become associated with Merkel cells in glabrous skin at the base of the papillary ridges or in clusters of Merkel cells in hairy skin in touch domes or Haarscheiben. Merkel cells have clusters of apparent secretory granules polarized toward the axon and the axon is typically a slowly adapting mechanoreceptor. The function of the granules is not known. Pacinian corpuscles are the largest of the corpuscular receptors of the dermis and are characterized by an elaborate inner core of stacks of numerous thin lamellae arranged in a bilaterally symmetrical manner. Based on the fact that the lamellae are coupled with gap junctions and the outer core lamellae isolated by numerous tight junctions, the authors have proposed that the unique ionic environment may be in part responsible for the remarkable sensitivity of Pacinian corpuscles (Munger and Ide, 1987). Meissner corpuscles are a typical corpuscular receptor of murine (Ide, 1976, 1977), marsupial and primate glabrous skin (Munger, 1971). The axons typically weave back and forth between stacks of lamellae.(ABSTRACT TRUNCATED AT 400 WORDS)     

Topography and ultrastructure of sensory nerve endings in the joint capsules of the Kowari (Dasyuroides byrnei), an Australian marsupial

Summary The present investigation is concerned with the topography and ultrastructure of sensory nerve endings in the joint capsules of the Kowari (Dasyuroides byrnei), an Australian marsupial. Material for light and electron microscopy was obtained from shoulder, elbow and knee joint capsules.On the basis of differences in the organization of the connective tissue belonging to the fibrous layer, 3 variants of capsule structure have been distinguished: a rigid, a flaccid and an intermediate type. Whilst the rigid type is characterized by dense connective tissue in the clearly demarcated fibrous layer, the flaccid type shows loose, irregularly arranged connective tissue in the fibrous layer which merges into the synovial layer of the joint capsule. the morphology of the intermediate type corresponds to an intermediate stage between the former two types.In the fibrous layer of the joint capsules three different types of sensory nerve endings were observed: free nerve endings, Ruffini corpuscles and lamellated corpuscles. The free nerve endings are supplied by myelinated afferent axons (1–2 m in diameter); the terminal thickenings of which are incompletely surrounded by a terminal Schwann cell. Ruffini corpuscles are present in three different varieties: 1. small corpuscles without a perineural capsule predominantly within the flaccid part of the capsule; 2. slightly larger corpuscles with an incomplete perineural capsule and 3. large corpuscles resembling Golgi tendon organs which predominantly occur in the rigid parts of the capsule. The afferent myelinated axons measure 2–4 m in diameter. The lamellated corpuscles show two variants: 1. small corpuscles with a 2 to 4-layered perineural capsule in the rigid parts of the joint capsules and 2. large corpuscles with two longitudinal clefts of the inner core in the flaccid parts. Both types are supplied by myelinated axons of 3–5 m in diameter. Thus, in the fibrous layer of the rigid type of joint capsules large Ruffini and small lamellated corpuscles predominate, whereas the fibrous layer of the flaccid type coincides with small Ruffini and large lamellated corpuscles. The present data, therefore, corroborate the concept that the morphology of mechanoreceptors depends upon the texture of the surrounding connective tissue.Supported by the Verein zur Förderung der Erforschung und Bekämpfung rheumatischer Krankheiten e.V. in Bad Bramstedt, and by the Deutsche Forschungsgemeinschaft (Ha 1194/2-1)