Degeneration of mouse digital corpuscles

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.

Spinal cord injury transiently alters Meissner's corpuscle density in the digit pads of macaque monkeys

Meissner's corpuscles (MCs) are cutaneous mechanoreceptors found in glabrous skin and are exquisitely sensitive to light touch. Along with other receptors, they provide continuous sensory feedback that informs the execution of fine manual behaviors. Following cervical spinal deafferentation injuries, hand use can be initially severely impaired, but substantial recovery occurs over many weeks, even when ~95% of the original input is permanently lost. While most SCI research focuses on central neural pathway responses, little is known about the role of peripheral receptors in facilitating recovery. We begin to address this by asking the following: (1) What is the normal pattern of MCs in the distal pads of all five digits in the macaque monkey (with hands similar to humans)? (2) What happens to these receptors 4-5 months following either a dorsal column lesion (DCL) or a combined dorsal root/dorsal column lesion (DRL/DCL), when functional recovery is largely complete? (3) What happens chronically, 12-14 months later? Our findings show that in normal monkeys, MCs are densest in the distal pads of the opposing thumb and index finger, with the greatest concentration on the thumb. This reflects a close functional relationship between receptor density and precision grip. At 4-5 months post-injury, there was a (~30%) loss of MCs on the deafferented digits of the injured hand compared with the contralateral side. However, 12-14 months after a DRL/DCL, receptor densities had returned to normal levels. Our findings indicate a complex peripheral response and highlight the importance of the periphery in shaping central changes.

Peripheral choline acetyltransferase in rat skin demonstrated by immunohistochemistry

Conventional choline acetyltransferase immunohistochemistry has been used widely for visualizing central cholinergic neurons and fibers but not often for labeling peripheral structures, probably because of their poor staining. The recent identification of the peripheral type of choline acetyltransferase (pChAT) has enabled the clear immunohistochemical detection of many known peripheral cholinergic elements. Here, we report the presence of pChAT-immunoreactive nerve fibers in rat skin. Intensely stained nerve fibers were distributed in association with eccrine sweat glands, blood vessels, hair follicles and portions just beneath the epidermis. These results suggest that pChAT-positive nerves participate in the sympathetic cholinergic innervation of eccrine sweat glands. Moreover, pChAT also appears to play a role in cutaneous sensory nerve endings. These findings are supported by the presence of many pChAT-positive neuronal cells in the sympathetic ganglion and dorsal root ganglion. Thus, pChAT immunohistochemistry should provide a novel and unique tool for studying cholinergic nerves in the skin.

Ultrastructures of mechanoreceptors in the oral mucosa

The present review describes the fine structures of lamellated mechanoreceptive corpuscles, Merkel cell-neurite complexes and free nerve endings in the oral mucosae of mammals, with special attention to axon terminals and lamellar cells. The mechanoreceptive nerve endings of the oral mucosa were studied using histochemistry, immunohistochemistry and transmission electron microscopy techniques. The organized mechanoreceptive corpuscles are present in the mucosae of gingiva, cheek, tongue and soft and hard palate. They are elongated or globular in shape, being located in the connective tissue papillae. The capsule is composed of several layers of cytoplasmic extensions of perineural cells. Numerous bundles of collagen fibers are noted at the periphery of the corpuscle. The lamellated corpuscles are surrounded by several layers of superimposed flattened capsular cell processes. The interlamellar spaces are 0.2-0.4 micron in width and filled with thin fibrillar collagen fibers embedded in the amorphous substance. The lamellar cells contain rich microtubules and are characterized by the presence of caveolae on the surface plasma membrane. The terminal axon contains an abundance of mitochondria and small clear vesicles (20-50 nm in diameter). There are neurofilaments in the center of the axon terminal. Intermediate-type junctions are seen between the adjacent lamellar cells and between the axon and adjacent lamellae. The free nerve endings are found in the subepithelial regions, very close to the basal laminae of mucosal epithelium. They are surrounded by a thin cytoplasm of Schwann cells. Sometimes Schwann cell basal larinae become multilayered. Merkel cells are present within the basal layer of mucosal epithelium and contain characteristic electron-dense granules that are located almost exclusively at the side of cytoplasm in contact with axon terminals. Intermediate-type junctions are noted between axon terminals and Merkel cells.

Developmental dependency of Meissner corpuscles on trkB but not trkA or trkC

The distribution of S100-immunoreactive (ir) corpuscular endings was examined in the palate of wildtype and knockout mice for trkA, trkB or trkC. In wildtype mice, S100-ir corpuscular endings were abundant at the top of palatal rugae. The endings contained 2-4 parallel arrays of S100-ir neurites. The distribution of S100-ir nerve endings in trkA and trkC knockout mice was similar to that in wildtype mice; S100-ir corpuscular endings were abundant in palates of the mutant mice. In trkB knockout mice, the palate was devoid of corpuscular endings, An immunoelectron microscopic method indicated that S100-ir corpuscular endings were identical to Meissner corpuscles. The normal development of Meissner corpuscles is probably dependent on trkB but not trkA or trkC.

Degeneration and regeneration of cutaneous sensory nerve formations

Auxiliary structures of the cutaneous sensory nerve formations (SNF) are dependent on sensory innervation during their critical period of development. Denervation of mature cutaneous corpuscles results in survival of the terminal Schwann cells and the capsular structures which are probably responsible for successful reinnervation of the cutaneous SNF. In addition, the basal lamina tubes of Schwann cells are connected with the terminal Schwann cells and play an important role in the guidance of regrowing axons to their original targets. Long-lasting denervation causes atrophic changes of the terminal Schwann cells and alterations of their molecular equipment. These atrophic changes in the terminal Schwann cells may be responsible for erroneous reinnervation of cutaneous SNF. A population of the cutaneous Merkel cells surviving denervation may also serve as targets for regrowing sensory axons. The basal laminae of terminal Schwann cells are produced under control of the sensory terminals during maturation of cutaneous SNF. In adult animals, the basal laminae are capable of stimulating differentiation of migrated Schwann cells to the terminal Schwann cells without the presence of the sensory terminals. Nonspecific cholinesterase (nChE) is secreted by the terminal Schwann cells and is attached to their extracellular matrix. The synthesis of these molecules in adult animals is not influenced by the sensory terminals. However, the presence of nChE molecules is associated with living terminal Schwann cells. Fetal orthotopically grafted dorsal root ganglion (DRG) neurons have the ability to reinnervate cutaneous SNF of adult hosts. When cutaneous areas are denervated, axons from adjacent sensory nerves may extend collateral branches into this area. The capacity for such extension is dependent on: (1) type of sensory nerve ending, C and A delta fibers having significantly greater capacity than sensory axons of larger caliber; (2) age of the animal, immature animals generally showing a greater capacity for collateral sprouting; (3) the state of the adjacent axons, those already in a growth mode being more capable than "resting" ones; and (4) the regional and mechanical conditions at the site of denervation, hindpaw skin being much less extensively reinnervated by collateral fibers than that of the trunk.

Enzyme histochemistry of cutaneous sensory nerve formations

The cutaneous sensory nerve formations belong to the structures which are studied intensely by the enzyme activity histochemistry since the early history of this technique. The histochemical localization of the activities of nonspecific cholinesterase, alkaline phosphatases, acid phosphatase, adenosine tri- and diphosphatases, adenylate cyclase, and dipeptidylpeptidase-IV in the cutaneous sensory nerve formations, mainly sensory corpuscles, is reviewed. The histochemical approach has brought new knowledge of both morphological building of these unique structures and their biochemical constituents. Taken together, the present results of enzyme histochemistry provide insight into the function of enzymes, and disclose new relationships between the sensory terminals and auxiliary structures in the cutaneous sensory nerve formations.

Effect of age on the structure of Meissner corpuscles in murine digital pads

A light and electron microscopic study was performed to determine age changes in Meissner corpuscles. In forepaw digital pads of mice aged to their maximum life expectancy, corpuscles were found to increase in size and complexity until middle age, and then to become smaller, disorganized and lobulated with more advanced age. Nerve terminals at more advanced ages became attenuated with a loss of axonal processes, increased density of the axoplasm, and disordered arrangement of the organelles. Degeneration of axonal mitochondria accelerated with age. Lamellar cell processes investing the axons often become dense and attenuated with fewer plasmalemma-associated vesicles. Basal laminae remained where lamellar processes had disintegrated. Lipofuscin was seen in the lamellar cells only at extremely old age. Extracellular material composed of fine basal lamina substance and collagen fibrils increased remarkably with age. Increased growth and complexity of corpuscles until middle age perhaps compensated for age-associated loss of corpuscles and primary sensory neurons. Changes predominating at older ages are attributed to distal axonopathy and atrophy of the sensory neurons. The probable effect of these age changes on cutaneous sensitivity is considered in relation to current theory of mechanoelectric transduction.

Peripheral nerve regeneration

Peripheral nerve regeneration comprises the formation of axonal sprouts, their outgrowth as regenerating axons and the reinnervation of original targets. This review focuses on the morphological features of axonal sprouts at the node of Ranvier and their subsequent outgrowth guided by Schwann cells or by Schwann cell basal laminae. Adhesion molecules such as N-CAM, L1 and N-cadherin are involved in the axon-to-axon and axon-to-Schwann cell attachment, and it is suggested that integrins such as alpha 1 beta 1 and alpha 6 beta 1 mediate the attachment between axons and Schwann cell basal laminae. The presence of synaptic vesicle-associated proteins such as synaptophysin, synaptotagmin and synapsin I in the growth cones of regenerating axons indicates the possibility that exocytotic fusion of vesicles with the surface axolemma supplies the membranous components for the extension of regenerating axons. Almost all the subtypes of protein kinase C have been localized in growth cones both in vivo and in vitro. Protein kinase C and GAP-43 are implicated to be involved in at least some part of the adhesion of growth cones to the substrate and their growth activity. The significance of tyrosine kinase in growth cones is emphasized. Tyrosine kinase plays an important role in intracellular signal transduction of the growth of regenerating axons mediated by both nerve trophic factors and adhesion molecules. Growth factors such as NGF, BDNF, CNTF and bFGF are also discussed mainly in terms of the influence of Schwann cells on regenerating axons.

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).

Ultrastructure and innervation of neuroepithelial bodies in the lungs of newborn cats

Neuroepithelial bodies (NEB) occur throughout the airway mucosa and alveolar parenchyma of kitten lungs. In the bronchi, they are often situated on top of a cartilage plate. They form compact corpuscles containing 10-20 corpuscular cells and appear covered with a layer of flattened Clara cells. Kitten NEB are occasionally observed to display mitosis of the corpuscular epithelial cells. A prominent blood capillary lies at their basal pole. The corpuscular cells contain numerous dense core vesicles (DCV), whose number and diameter remain unchanged with age. Kitten NEB are innervated by nerve fibres that "loop" through the corpuscle and form morphologically afferent as well as efferent nerve endings. The nerve endings display afferent synaptic junctions with the corpuscular cells and sometimes run in clusters, so that they contact each other. Many nerve endings undergo spontaneous degeneration. We conclude that kitten NEB are well adapted to function as chemoreceptors and as endocrine or paracrine organs. Their chemoreceptor activity could be modulated by axon reflexes since their afferent nerve endings are often continuous with the efferent ones, as well as by interneural modulation since nerve endings often form clusters. In addition, kitten NEB innervation appears to involute rapidly soon after birth. This may indicate that their chemoreceptor function is only of primary importance during gestation and at birth. However, the secretory function of kitten NEB, as evidenced by the unchanged numbers and dimensions of their DCV, seems to remain steady throughout life.

Combination of non-specific cholinesterase histochemistry and immunofluorescence staining for the study of the sensory innervation of skin and muscle

In the present study we describe the application of the non-specific cholinesterase (nChE) histochemical method for the detection of encapsulated sensory nerve endings prior to immunofluorescence staining of the sensory nerve fibres. The nChE staining of Schwann-derived structures surrounding sensory terminals allowed us to identify unequivocally the sensory corpuscles in the skin and the muscle proprioceptors (muscle spindles and Golgi tendon organs) in longitudinal sections of muscle tissue. The nChE staining of sensory nerve endings and immunofluorescence-labelled nerve fibres and their terminals could be viewed and photographed in the same section using appropriate filters. Since nChE activity persists in terminal Schwann cells for a long time after loss of the sensory axons, this combined enzyme- and immunohistochemical approach is also useful for experimental studies involving denervation and re-innervation of sensory nerve endings.

Effacement and regeneration of tactile lamellar corpuscles of rat after postnatal nerve crush

The development of Meissner-like lamellar corpuscles was studied in rat toe pads under normal conditions and after crushing the sciatic nerve in 1- to 15-day-old animals. During normal development, rat lamellar corpuscles begin to differentiate first by postnatal day 8. By this time, sensory axons have grown up to the apex of dermal papillae and form axon terminals beneath epidermis. The terminals are ensheathed by lamellar cells derived from Schwann cells. First thin lamellae are formed around the terminals 8-12 days after birth, and the number of lamellar layers increases until the corpuscles become structurally mature by 20 days after birth. A mature corpuscle consists of two or more terminals, each surrounded by approximately 10 lamellae, all components being enclosed by an incomplete capsule. No lamellar corpuscles develop in toe pads after crushing the sciatic nerve in newborn rats, and only occasional corpuscles regenerate after nerve crush at 5 days of age. The corpuscles fail to develop because dermal papillae remain permanently denervated after crushing the nerve early postnatally. After nerve crush in 10-day-old rats, lamellar corpuscles regenerate by 1 month after the operation, but they remain underdeveloped: their number and size are smaller than normal even 1 year after injury, and their terminals are encircled only by 1-3 lamellar layers. After nerve crush in 15-day-old rats, the corpuscles recover upon reinnervation and their size and lamellation become almost normal.

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.

Degeneration and regeneration of peripheral nerve in the rat trigeminal system: III. Abnormal sensory reinnervation of rat guard hairs following nerve transection and crush

The present study was undertaken in an attempt to better understand the abnormalities of cutaneous sensibility that are present in patients following nerve injury with concomitant cutaneous denervation and subsequent reinnervation. Reinnervated intervibrissal pelage of the rat mystacial pad was studied in silver-impregnated sections 3 and 5 months after transecting and 2 and 5 months after crushing the infraorbital nerve. The sensory terminals on guard and vellus hairs were analyzed in serial paraffin sections and in thick frozen sections. In normal rat mystacial skin, approximately nine/ten of innervated guard hairs have a typical piloneural complex consisting of a palisade of highly regular lanceolate terminals surrounded by circularly arranged Ruffini terminals and free nerve endings (FNEs). The remaining one of ten innervated guard hairs has only circularly arranged presumptive FNEs and Ruffini terminals. Vellus hairs, either singly or in clusters, typically have only circularly arranged terminals that in many cases are simple FNEs. We first recognized abnormalities in innervation of hairs following nerve transection and fully expected nerve terminals to be completely normal following nerve crush. Almost all reinnervated sensory nerve terminals associated with guard hairs were markedly abnormal following nerve transection and quantitatively abnormal following nerve crush. Following nerve transection, lanceolate terminals were almost completely absent, and they were remarkably reduced in number following nerve crush. Vellus hairs when reinnervated typically lacked the complex circular presumptive Ruffini terminals. These findings may be in part the basis for the abnormal cutaneous sensory perceptions (dysasthesias and paresthesias) noted in human subjects following damage to nerves with subsequent sensory reinnervation of the skin.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron microscopical study of non-specific cholinesterase activity in simple lamellar corpuscles of glabrous skin from cat rhinarium: a histochemical evidence for the presence of collagenase-sensitive molecular forms and their secretion

The distribution of nCHE activity was studied histochemically in simple lamellar corpuscles (SLCs) of glabrous skin from cat rhinarium. The Schwann cells forming myelin sheaths in preterminal part of SCLs exhibited no positive reaction for nCHE activity. Prevalent reaction product was localized extracellularly in the inne core enveloping terminal portion of unmyelinated sensory axon. A dot-like shaped reaction product was deposited in the basal lamina of the inner core cells and their cytoplasmic lamellae or was scattered in enlarged interlamellar spaces. Only small amount of fine end product was found to be associated with the plasma membrane of inner core lamellae. Fine reaction product for nCHE activity was consistently localized in perinuclear and rER cisternae and saccules of the Golgi apparatus of inner core cells. Some vesicles around rER and the Golgi apparatus, ones beneath the plasma membrane, and tubular-like cisternal profiles oriented towards the surface contained nCHE end product, as well. The intracellular and extracellular localization of nCHE reaction product suggests that this enzyme behaves in cat SLCs as a secreted rather than as an integral membrane protein. A large amount of dot-like reaction product in the interlamellar spaces disappeared if the skin sections were treated with collagenase before incubation in the medium for histochemical detection of nCHE activity. The decrease of nCHE end product in SLCs of the skin sections after collagenase digestion was corroborated by means of light microdensitometer and electrometrical measurement. The histochemical detection and electrometrical measurement revealed that the majority of the reaction product in the interlamellar spaces of inner core corresponds with the nCHE molecules sensitive to collagenase treatment and they are probably counted among asymmetrical molecular forms.

Fine structural localization of non-specific cholinesterase activity in rat tendon organs

Electron microscopical localization of non-specific cholinesterase (nChE) activity was studied in tendon organs of the rat hindlimb muscles. The comparison between neurotendinous part (with high nChE activity) and purely collagenous compartment(s) (with very low nChE activity) demonstrated that Schwann cells are the fundamental source of this enzyme in rat tendon organs. Although particles of the nChE reaction product were also found in and around fibroblasts in both neurotendinous and purely collagenous compartments, their contribution to the overall nChE was not significant. nChE activity in rat tendon organs displayed heterogeneity along the Ib sensory axon; the highest activity was related to the Schwann cell investment of the unmyelinated part of Ib axon, lower activity to sensory terminals covered only by basement membrane and negligible activity to the myelinated part of sensory axons. Particles of the non-specific cholinesterase reaction product persisted in the basement membrane of Schwann cells 20 d after degeneration of Ib sensory axons and their terminals. The function of non-specific cholinesterase in sensory receptors is still not clear. It is suggested that this enzyme may be involved in the maintenance of the ionic milieu around sensory axon terminals during or after functional activity.

Cholinesterase activity of lamellated sensory corpuscles in the rat lip

Non-specific cholinesterase (ChE) activity was demonstrated in lamellated sensory corpuscles of the rat lip by light and electron microscopy using Karnovsky and Roots' method. ChE activity was present in the interlamellar spaces between neighbouring lamellae as well as in the peri-axonal space between axon terminals and their adjacent lamellae. Reaction products were also deposited in some caveolae of the lamellar cell plasma membrane, and in the cisternae of the rough endoplasmic reticulum as well as in the nuclear envelope of lamellar cell bodies. No reaction products were detected within the axon terminals. The findings show that the lamellated corpuscles in the rat lip, like other mechanoreceptors, have intense ChE activity which is probably synthesized in lamellar cells and released into the intercellular spaces. That ChE activity is particularly strong in mechanoreceptors composed of lamellar cells suggests that this enzyme would play an important role in function and/or maintenance of such mechanoreceptive corpuscles.

Cholinesterase activity of lamellated sensory corpuscles in the rat lip

Non-specific cholinesterase (ChE) activity was demonstrated in lamellated sensory corpuscles of the rat lip by light and electron microscopy using Karnovsky and Root's method. ChE activity was present in the interlamellar spaces between neighbouring lamellae as well as in the periaxonal space between axon terminals and their adjacent lamellae. Reaction products of ChE activity were also deposited in some caveolae of the lamellar cell plasma membrane, and in the cisternae of the rough endoplasmic reticulum as well as in the nuclear envelope of lamellar cell bodies. No definite reaction products were detected within the axon terminals. These findings show that the lamellated corpuscles in the rat lip, like other mechanoreceptors, have an intense ChE activity which is mainly associated with lamellar cells. It can be said that ChE histochemistry is useful to detect mechanoreceptors. The functional significance of ChE in mechanoreceptors is discussed.

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.

Effects of age on Meissner corpuscles: a study of silver-impregnated neurites in mouse digital pads

Investigation focused on finding qualitative and quantitative evidence of age-changes in the quantity of neural surface within Meissner corpuscles. These mechanoreceptors were studied in 53 mice (nine age-groups) ranging from 1.7 to 24 months old. Forepaw digital pads were formalin-fixed and frozen-sectioned parallel to each digit and perpendicular to the skin. Serial sections were then silver-impregnated to allow light microscopic examination of the neurites (axons) in corpuscles. From young (1.7-7 months) to middle age (9-15 months), neurites became more coarse, tortuous, ramified, varicose, and thus more complex. At old age (18-24 months), neurites seemed attenuated and showed more of an irregular winding, twisted, or tangled pattern with less parallel orientation to the skin surface than the regular spiraled, looped, or arched pattern typical at young and middle ages. Corpuscle size appeared greatest at middle age, smallest at young age. Dermal papillae not occupied by corpuscular neurites were most abundant at old age. The number of corpuscles per area and neurites per corpuscle decreased significantly with age, whereas the number of neurite bifurcations per corpuscle increased significantly. Morphometric analysis of neurites projected by a camera lucida onto a planimeter showed that the length of neurites meandering through a fixed interval of tissue increased significantly until age 12 months-evidence of increased tortuosity; the area of neurites measured within the same fixed interval, and the area of neurite terminals changed significantly as inverse parabolic functions of age-evidence of increased volume until middle age, which decreased thereafter. The general trend of these changes implied growth at young age and atrophy at old age.(ABSTRACT TRUNCATED AT 250 WORDS)

Schwann cell basal lamina and nerve regeneration

Nerve segments approximately 7 mm long were excised from the predegenerated sciatic nerves of mice, and treated 5 times by repetitive freezing and thawing to kill the Schwann cells. Such treated nerve segments were grafted into the original places so as to be in contact with the proximal stumps. The animals were sacrificed 1, 2, 3, 5, 7 and 10 days after the grafting. The grafts were examined by electron microscopy in the middle part of the graft, i.e. 3-4 mm distal to the proximal end and/or near the proximal and distal ends of the graft. In other instances, the predegenerated nerve segments were minced with a razor blade after repetitive freezing and thawing. Such minced nerves were placed in contact with the proximal stumps of the same nerves. The animals were sacrificed 10 days after the grafting. Within 1-2 days after grafting, the dead Schwann cells had disintegrated into fragments. They were then gradually phagocytosed by macrophages. The basal laminae of Schwann cells, which were not attacked by macrophages, remained as empty tubes (basal lamina scaffolds). In the grafts we examined, no Schwann cells survived the freezing and thawing process. The regenerating axons always grew out through such basal lamina scaffolds, being in contact with the inner surface of the basal lamina (i.e. the side originally facing the Schwann cell plasma membrane). No axons were found outside of the scaffolds. One to two days after grafting, the regenerating axons were not associated with Schwann cells, but after 5-7 days they were accompanied by Schwann cells which were presumed to be migrating along axons from the proximal stumps. Ten days after grafting, proliferating Schwann cells observed in the middle part of the grafts had begun to sort out axons. In the grafts of minced nerves, the fragmented basal laminae of the Schwann cells re-arranged themselves into thicker strands or small aggregations of basal laminae. The regenerating axons, without exception, attached to one side of such modified basal laminae. Collagen fibrils were in contact with the other side, indicating that these modified basal laminae had the same polarity in terms of cell attachment as seen in the ordinary basal laminae of the scaffolds.(ABSTRACT TRUNCATED AT 400 WORDS)

Regeneration of mouse digital corpuscles

Changes in morphology and nonspecific cholinesterase (ChE) activity during the reinnervation of digital corpuscles of mouse toe pads after transection of the sciatic nerve were examined by light and electron microscopy. Reinnervation occurred about 30 days after nerve section. A small-diameter axon was present near the shrunken atrophic lamellae of the previously denervated corpuscle. From 40 to 60 days after nerve transection, the ingrowing axons branched; and the enlarged tip of each branch contained numerous mitochondria. Lamellar cells and cytoplasmic lamellae at this stage were still smaller than normal. Lamellar cells had normal structure by 4 months after nerve section. Following reinnervation, evidence of ChE activity in the interlamellar spaces at first remained unchanged in intensity and distribution from that of the denervated corpuscle. About 10 days after reinnervation, enzyme activity increased in the perineural spaces and was evident, as in the normal corpuscle, in the cisternae of the rough endoplasmic reticulum and nuclear envelope of the lamellar cell body. By 4 months after nerve division, the intensity and distribution of the ChE activity were almost the same as in the normal corpuscle. From the above findings the following conclusions could be drawn: (1) The shrunken atrophic lamellar cells remaining after denervation are utilized are utilized by the ingrowing axons to regenerate normal digital corpuscles. (2) The lamellar cell is dependent on the well-developed axon terminals to maintain its normal morphology and to synthesize ChE.