Regeneration 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.

Innervation of Meissner's corpuscles and Merkel -cells by transplantation of embryonic dorsal root ganglion cells after peripheral nerve section in rats

Transplantation of embryonic motor neurons has been shown to improve motor neuron survival and innervation of neuromuscular junctions in peripheral nerves. However, there have been no reports regarding transplantation of sensory neurons and innervation of sensory receptors. Therefore, we hypothesized that the transplantation of embryonic sensory neurons may improve sensory neurons in the skin and innervate Merkel cells and Meissner's corpuscles. We obtained sensory neurons from dorsal root ganglia of 14-day rat embryos. We generated a rat model of Wallerian-degeneration by performing sciatic nerve transection and waiting for one week after. Six months after cell transplantation, we performed histological and electrophysiological examinations in naïve control, surgical control, and cell transplantation groups. The number of nerve fibers in the papillary dermis and epidermal-dermal interface was significantly greater in the cell transplantation than in the surgical control group. The percent of Merkel cells with nerve terminals, as well as the average number of Meissner corpuscles with nerve terminals, were higher in the cell transplantation than in the surgical control group, but differences were not significant between the two groups. Moreover, the amplitude and latency of sensory conduction velocity were evoked in rats of the cell transplantation group. We demonstrated that the transplantation of embryonic dorsal root ganglion cells improved sensory nerve fiber number and innervation of Merkel cells and Meissner's corpuscles in peripheral nerves.

Origin and regenerative potential of vertebrate mechanoreceptor-associated stem cells

Meissner corpuscles and Merkel cell neurite complexes are highly specialized mechanoreceptors present in the hairy and glabrous skin, as well as in different types of mucosa. Several reports suggest that after injury, such as after nerve crush, freeze injury, or dissection of the nerve, they are able to regenerate, particularly including reinnervation and repopulation of the mechanoreceptors by Schwann cells. However, little is known about mammalian cells responsible for these regenerative processes. Here we review cellular origin of this plasticity in the light of newly described adult neural crest-derived stem cell populations. We also discuss further potential multipotent stem cell populations with the ability to regenerate disrupted innervation and to functionally recover the mechanoreceptors. These capabilities are discussed as in context to cellularly reprogrammed Schwann cells and tissue resident adult mesenchymal stem cells.

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.

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.

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.

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).

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.

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.

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)

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.

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)