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We carried out experiments in rabbits to determine whether treating nerve transplants with gradually increasing concentrations of ethanol, ether and ficin would inhibit the graft-host immune reaction to the allograft. After treatment with ethanol, the basal laminar scaffold of the Schwann cell remained intact and there was satisfactory axonal regeneration. The results after additional treatment with ether or ficin did not achieve such good results. Preservation of the basal lamina is considered to be the essential factor in allowing neural regeneration in these circumstances.

To examine whether the 3-4-cm-long allogeneic basal lamina tubes of Schwann cells serve as conduits for regenerating axons in rabbits, allogeneic saphenous nerve, which had been predenervated and pretreated by freezing, were transplanted from Japanese White rabbits (JW) to New Zealand White rabbits (NW). Animals were killed 1, 2, 6, 8, and 14 weeks after transplantation, and the cytology at the mid-portion of the grafts was examined by electron microscopy. The distal portion of the host saphenous nerves was also examined 14 weeks after grafting. Myelin sheath debris was phagocytosed by macrophages, while the basal lamina of Schwann cells were left intact in the form of tubes. Regenerating axons were first found in such basal lamina tubes 2 weeks after grafting, and gradually increased in number. Host Schwann cells accompanied the regenerating axons behind their growing tips, separating them into individual fibers and forming thin myelin sheaths on thick axons by 6 weeks after grafting. Regenerating nerves were divided into small compartments by new perineurial cells. Newly formed blood vessels were situated outside the compartment 8 weeks after grafting. The percentage of myelinated fibers in the regenerating nerves was roughly 10% at 8 weeks and 30% at 14 weeks after grafting. The diameter of the regenerating axons, both myelinated and unmyelinated, was less than that of normal axons at all the stages examined. Numerous regenerating axons, some of which were fully myelinated, were found at the site 10 mm distal to the distal end of the graft 14 weeks after grafting.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The localization of laminin and fibronectin was examined on the basal laminae of Schwann cells. Basal laminae from sciatic nerves were isolated by sonication, and the localization of laminin and fibronectin on such isolated basal laminae was studied by immunoferritin histochemistry. Laminin was localized mainly on the cellular side (i.e. the side originally facing the Schwann cell plasma membrane) of the basal laminae. On the other hand, fibronectin was found to be present as aggregates only on the interstitial side (i.e., the side originally facing the endoneurial connective tissue) of the basal laminae. Thus, the locations of laminin and fibronectin were distinctly different. It is presumed that laminin might be involved in the attachment of axons and Schwann cells to the basal laminae, while fibronectin mediates the adhesion of the basal laminae to connective tissue elements, including the collagen fibrils. These findings are discussed from a standpoint of nerve regeneration through the basal laminae scaffolds of Schwann cells.

The distribution of carbonic anhydrase (CA) activity was studied by electron microscopic histochemistry in rat Pacinian corpuscles, Meissner corpuscles and Merkel cell-neurite complexes using the cobalt bicarbonate method. The distribution of CA activity in these axon terminals was compared to the activity in sciatic nerve axons. An intense enzymatic CA activity was demonstrated in axon terminals of both Pacinian and Meissner corpuscles, while a weak activity was found within the axoplasm of terminals abutting Merkel cells. Some large- and medium-sized axons in sciatic nerves exhibited an intense activity. These findings indicate that large- or medium-diameter sensory axons innervating corpuscular endings have an intense CA activity extending from their somata to their sensory terminals. Axons to Merkel-neurite complexes differ in CA activity from those innervating Meissner and Pacinian corpuscular endings.

The presence of macrophages in the outer bulb region of mouse, monkey and human Pacinian corpuscles was demonstrated by light and electron microscopy. In the normal, nontreated, Pacinian corpuscles, a few particular cells were located in the spaces between lamellae of the outer bulb. These cells contained numerous vesicles and vacuoles, and various cytoplasmic processes. When horseradish peroxidase (HRP) was injected locally or systemically, many HRP-positive cells, which were considered to be similar to the particular cells described above, were found in the outer bulb region of the corpuscles. Electron microscopy revealed that these cells contained HRP in vesicles and vacuoles, suggesting that they were macrophages vigorously taking up exogenous HRP. Macrophages in the Pacinian corpuscles are considered to work as scavengers to keep the inner environment of the corpuscles clear and constant with regard to its macromolecular content.

During the course of the studies on non-traumatized Pacinian corpuscles from normal human adults, we have frequently encountered corpuscles which have an 'apparently multiple' innervation in both light and electron microscopic preparations. On closer inspection of serial sections for both light and electron microscopy, these 'apparently multiple' axon terminals have been found in fact to be branches of the main central axon within the inner core of the corpuscle. Sprouting occurred at the trunk or at the extreme tip of the main axon, and such sprouts extended in various directions from the central axon throughout the inner core, producing tortuous and complex patterns of this 'multiple' innervation. These axonal sprouts do not have separate inner cores separated from one another, but rather are embedded in a common inner core. The presence of a common inner core thus differentiates normal axonal sprouts from the experimentally or pathologically produced multiple innervation that results form regeneration of axons in a previously denervated corpuscle. We conclude that the inner core of Pacinian corpuscles is a unique micro-environment promoting sprouting of sensory axon in the normal human adult as well as juvenile Pacinian corpuscles.

Cytological changes in mouse decidual cells during the early stages from 4.5th to 11th day of pregnancy were examined by electron microscopy with special reference to gap junctions. The most noticeable feature of decidual cells was that there was an abundance of various-shaped gap junctions such as flat, undulated, omega-shaped, and annular-shaped gap junctions. Serial sectioning revealed that annular-shaped gap junctions were separated from the decidual cell plasma membrane. Intramembranous particles (IMPs) of the gap junctions as seen by freeze-fracture displayed a well-packed arrangement and a homogeneous size. Degenerative changes first appeared in decidual cells directly contiguous with trophoblasts on the 7th day, and spread to the deeper area as clearly seen on the 8th day. Along with degeneration of decidual cells, annular-shaped gap junctions were enfolded and gradually degraded by lysosomes. IMPs of such degenerative gap junctions were irregularly arranged and heterogenous in size. The decidual tissue became thin with elongated decidual cells as the placental disc grew on the 10th day. Extremely long undulated gap junctions were present in decidual cells, and a small number of annular-shaped gap junctions were also found in the cytoplasm. The presence of abundant gap junctions as demonstrated in the present study suggests that decidual cells will be able to develop as well as to function synchronously. In addition, it is suggested that redundant gap junctions become annular-shaped to be finally degraded by lysosomes.

The endoneurium of the mouse sciatic nerve was studied by scanning and transmission electron microscopy to elucidate their three-dimensional architecture. The endoneurium consisted of collagen fibrils, and occasional fibroblasts and blood vessels. Collagen fibrils surrounded individual nerve fibers, forming two distinct layers of connective tissue sheath: the outer one was composed of bundles of longitudinally oriented collagen fibrils and the inner one was of a delicate network of interwoven thin collagen fibrils. These outer and inner layers of collagen fibrils correspond to the two fibrous sheaths known as the sheath of Key and Retzius and the sheath of Plenk and Laidlaw, respectively, revealed on nerve fibers by silver impregnation. Some of the finest collagen fibrils forming the inner network are closely attached to the basal lamina of Schwann cells, suggesting that these fibrils are concerned with the connection between the basal lamina and the inner collagen network. These two layers of collagen fibrils were found on all the nerve fibers, suggesting that they represent a general structure ensheathing the peripheral nerve fibers. Although these layers occur also on unmyelinated nerves, they are most developed on the largest myelinated fibers.

The development of the lamellar cells of mouse digital corpuscles (Meissner corpuscle) was studied by light and electron microscopic histochemistry for cholinesterase (ChE) The materials used were the hind limbs taken from fetuses at 14, 17 and 20 days of gestation, and from young mice at 1, 5, 7, 15 and 20 days after birth. Embryonal Schwann cells had non-specific ChE activity in the cisternae of the rough endoplasmic reticulum and the nuclear envelope, suggesting that they had the ability of synthesizing the enzyme. After birth, such ability gradually decreased and by five days of age non-specific ChE activity was no longer demonstrable in Schwann cells at the time when myelin sheath formation began. However, Schwann cells which were associated with the axonal tips penetrating into the epidermis still had an intense non-specific ChE activity. Such Schwann cells surrounded the axons in gradually increasing numbers of cytoplasmic processes, which later became the lamellae around the axon terminals; thus by 20 days after birth they had differentiated into mature lamellar cells of Meissner corpuscles. These lamellar cells had, as in the embryonal Schwann cells, an intense ChE activity in the cytoplasm. These findings indicate that the lamellar cell is a specialized form of Schwann cell which still retains the embryonal characteristics for synthesizing non-specific ChE.

Part of the dorsal funiculus of the adult male rat (Wistar) spinal cord was treated for 1 h at the thoracolumbar level by running hot water, at approximately 48-50 degrees C, through a polyethylene tube 2 mm in diameter in contact with the dura. Animals were fixed 1 day to 4 weeks later and the spinal cords were examined by light and electron microscopy. The affected area in the dorsal funiculus was approximately 1 mm long and less than 1 mm wide at the dorsal surface, and varied from 0.4 to 0.7 mm in depth. Within 3 days after treatment, almost all the myelin sheaths in the affected area were degraded, leaving the axons denuded, and at the same time astrocyte endfeet at the glial limiting membrane were swollen and partly destroyed. Almost all the denuded axons remained intact, exhibiting no noticeable morphological changes. There was evidence of a moderate vasogenic oedema, but minimal signs of haemorrhage in the lesion. Seven days after treatment, many immature Schwann cells but no oligodendrocytes were found between the denuded axons. By 2 weeks many of the denuded axons were remyelinated, and by 4 weeks almost all of those axons located near the pial and perivascular surfaces had been remyelinated by Schwann cells, while most of those located in the deep and marginal zones bordering the adjoining intact areas were remyelinated by oligodendrocytes. Longitudinal sections revealed that at nodes of Ranvier PNS-type myelin sheaths were apposed by either intact or newly formed CNS-type myelin sheaths. A typical glial limiting membrane was not reformed beneath the pial surface, but an inconspicuous one was found between the PNS- and CNS-type fibre areas.

The three-dimensional architecture of the autonomic nerve terminals in the rat exocrine pancreas was investigated by scanning electron microscopy using the HCl-digestion method as well as by transmission electron microscopy. Unmyelinated nerves presumed to be autonomic in nature were found in networks covering the outer layers of arterioles and their capillary extensions, with nerve fibers often leaving the capillaries to surround acini in the interacinar spaces. Schwann cells formed scaffolds for axons of the networks. No other distinct type of cells such as the so-called interstitial cells of Cajal were found to be associated with the formation of the networks. Although nerve fibers of the networks were locally in close association with the walls of the blood vessels or with the bases of acinar cells, no specialized axonal contacts with these tissues were found. However, local swellings, presumably varicosities, were observed by scanning electron microscopy on the surface of nerves. These findings suggest that the networks of unmyelinated nerves represent terminal apparatuses of the autonomic nerves in the pancreas. Schwann cells in the terminal networks were characterized by an unusual abundance of cell organelles. These "terminal Schwann cells" well correspond in location and reticular extension to the "interstitial cells of Cajal" as demonstrated by silver impregnation and vital methylene blue staining. The occurrence of well developed Golgi apparatuses, rough endoplasmic reticulum, and numerous ribosomes suggests that the cells are specialized Schwann cells which most likely require high levels of cellular activity in order to maintain their elaborate cytoplasmic processes extending along the terminal networks, and also to sustain the specific functions of axon terminals.

Cryo-treated nerves whose Schwann cells had been killed by repeated freezing and thawing were xenogenically grafted into sciatic nerves from rats (Wistar, as donor) to mice (ddy strain, as recipient) to examine whether Schwann cell basal lamina tubes of cryo-treated xenogeneic grafts were effective conduits for regenerating axons. For comparison and evaluation of the effectiveness of this technique, experiments using grafts without the cryo-treatment were carried out. Cells in cryo-treated xenografts degraded into cell debris immediately after grafting and then were phagocytized by macrophages. After the cellular components had been removed from the graft, Schwann cell basal laminae remained intact in situ, serving as conduits for the regenerating axons. The process of nerve regeneration was almost the same as that observed in cryo-treated auto- and allografts, except that the regeneration was slightly delayed in the xenogeneic graft. In contrast, an extensive cell infiltration occurred in the non-treated grafts. It appeared that the donors Schwann cells in the graft deteriorated due to immunological reactions and were finally eliminated by macrophages, leaving their basal laminae undamaged in situ. The initiation of nerve regeneration including perineurial sheath formation in non-treated grafts was, therefore, significantly delayed, but once begun, it proceeded in the same manner as in the cryo-treated grafts. These findings strongly indicate that Schwann cell basal laminae can serve as effective pathways for regenerating axons even in the xenograft. Moreover, cryo-treated xenogeneic grafts are more desirable than non-treated ones, since dead Schwann cells in the former can be removed in the early period (4-14 days) from the graft without causing any immunological reaction, thus resulting in the facilitation of nerve regeneration.

Pacinian corpuscles of cat mesentery were studied with freeze-fracture and thin sectioning methods after chemical fixation. Intramembranous particles (IMPs) exhibit differences in both density and pattern of distribution between the axolemma of the smooth short axis (x-axis) region and that of the axonal spine region of the long axis (y-axis) of the axon terminal. The axolemma of the x-axis has IMPs at a density of 2687 +/- 581 per micron2 (mean +/- S.E.M.), and these particles are 9.0 +/- 1.7 nm (mean +/- S.D.) in diameter. In contrast, the axolemma of the y-axis has a higher density of IMPs (3607 +/- 612 per micron2) which are larger (diameter, 10.0 +/- 1.7 nm). The particle distribution is not homogeneous in x-axis membranes as there are small patchy areas devoid of particles scattered throughout the entire surface. The E-face of the axolemma has a low density of IMPs (approximately 200 per micron2 in both x- and y-axes). However, IMPs in the E-face are smaller (approximately 9 nm) in the x-axis than in the y-axis (approximately 10 nm). The inner core lamellar cells have IMPs at a density of 3276 +/- 739 per micron2 and 553 +/- 169 per micron2 in the P- and E-faces, respectively. The particles are about 10 nm in diameter in both faces. Many gap junctions occur between lamellar cells especially near the clefts, suggesting that hemilamellae of each inner core half are kept at the same electrotonic potential. The outer core lamellar cells have IMPs at a density of 2239 +/- 403 per micron2 and 536 +/- 123 per micron2 in their P- and E-faces, respectively. The particles are approximately 10 nm in diameter in both faces. A noteworthy finding is that tight junctions are prominent at cell-to-cell appositions within individual lamellae, especially in the first and second (or sometimes third) innermost lamellae of the outer core. These tight junctions are considered to be a barrier to the leakage of fluid and/or ions between interlamellar spaces as well as between inner and outer cores. An intermediate cell layer is identified between the inner and outer cores. The connective tissue space of this cell layer corresponds to the endoneurium, indicating that intermediate layer cells are comparable to endoneurial fibroblasts.(ABSTRACT TRUNCATED AT 400 WORDS)

The three-dimensional morphology of the surface of myelinated nerve fibres in the mouse sciatic nerve was studied by scanning electron microscopy after combined potassium hydroxide treatment and collagenase digestion (to remove the surrounding collagen fibrils and basal laminae from nerve fibres) as well as by transmission electron microscopy. The myelinated nerve fibre appeared as a long cylinder with sporadic annular constrictions corresponding to the nodes of Ranvier. Slight swellings of the surface due to Schwann cell nuclei were usually found at the middle of each internode. The surface of the nerve fibre clearly exhibited a network of bulges, which consisted of longitudinal bands extending from the nuclear swelling to the nodes of Ranvier through the internode, and transverse trabeculae bridging between these longitudinal bands. These bulges on the surface of nerve fibres were the site of the retained Schwann cell cytoplasm external to the myelin lamellae. These cytoplasmic networks on myelinated fibres presumably corresponded to the networks described by Cajal following silver impregnation. In addition, other thin elevations and focal round swellings were also found associated with these longitudinal bands and transverse trabeculae. These networks of Schwann cell cytoplasm are considered to be cytoplasmic channels for nutrition. The two apposing paranodal bulbs of nodes of Ranvier were often asymmetrical in their structure. The networks of the paranodal region were more complicated than those in the internode. The networks of Schwann cell cytoplasm converged into a continuous circumferential collar toward the node, which in turn gave rise to finger-like projections into the nodal gap.

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.

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)

Mechanoreceptive sensory corpuscles (murine Meissner corpuscles) in the toe pad skin of mice, consisting of axon terminals and lamellar cells, were studied following freeze-substitution in order to clarify the plasma membrane relationships between axon terminals and lamellar cells. Tissue preservation of corpuscles was excellent when the corpuscle was located within 10 microns from the contact surface with the precooled metal block. The axolemmata appeared more electron-opaque than did plasma membranes of lamellar cells. The inner leaflet of the unit membranes was thicker than the outer leaflet in the axolemma, and the contour of cell plasma membranes was relatively smooth and straight. Characteristic focal or regional approximations of plasma membranes were noted between the axon and abutting lamellae. Such membrane appositions resembled gap junctions, although no gap junctions were found between the axon and lamellae in chemically fixed materials. Similar gap junction-like close appositions of plasma membranes also were found between neighboring lamellae. These approximations occurred more frequently than typical gap junctions seen in chemically fixed materials. These findings indicate that there may be a relationship of the plasma membranes in the axon terminals and in abutting lamellae as well as between neighboring lamellae that have not been identified as yet in conventional chemically fixed material. Another striking finding was that basal laminae on lamellar cells exhibited the same electron opacity as the surrounding connective tissue matrix and thus the two are indistinguishable from one another. Furthermore, the lamina lucida was not evident, and basal lamina material was directly contiguous with the plasma membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

The ultrastructure of cat mesenteric Pacinian corpuscles in cross and longitudinal sections has been examined. The terminal ends of lamellar cells of the inner core have been identified in longitudinal sections through the proximal portion of the inner core. These terminal bulbous expansions contain characteristics concentric membranes of rough endoplasmic reticulum and in some cases masses of oval membranous inclusions. The central axon as seen in cross section is oval in profile, having X-(short) and Y-(long) axes, and each axonal face is characterized by specializations of the axolemma. At the X-axis, the inner lamellae of the inner core tightly abut a smooth axolemma, with no intervening connective tissue matrix, in a manner reminiscent of a neuroepithelium. The axolemma of the Y-axis has numerous axonal spines (microspikes) that project into the cleft in the inner core. The extent of the axolemma having axonal spines can only be appreciated in longitudinal sections. The clefts contain a specialized connective tissue with elastic and collagen fibrils. The connective tissue compartment of fibers and matrix separating individual inner core lamellae is unique, in that it contains extremely thin collagen fibrils measuring approximately 15 nm in diameter. The diameter of collagen fibrils increases as the cleft is approached. Here the fibrils resemble typical endoneural collagen.

In order to examine nerve regeneration under conditions in which the basal laminae of the glial limiting membranes (GLM) and blood vessels were preserved intact, the intraorbital segment of adult rat optic nerve was frozen locally. During the next 3 months, degenerative and regenerative changes in axons and glial cells were observed by light and electron microscopy. On the day after treatment, all the myelinated and unmyelinated axons in the central zone of the lesion were damaged. The astrocyte endfeet of the GLM as well as the blood vessels were extensively disrupted, while their basal laminae were preserved apparently intact as a continuous sheet. Three days after treatment, regenerating axons appeared in the central zone of the lesion. They contained various numbers of clear and dense-cored vesicles as well as some smooth endoplasmic reticulum. The regenerating axons gradually increased in number, especially beneath the pial and perivascular surfaces of the lesion, where an abundance of regenerating axons was found 3 months after treatment. A few of these axons were abnormally remyelinated by oligodendrocytes. In addition to this axonal regeneration through the intraoptic nerve compartment, fine regenerating axons were seen growing out through GLM into the pial connective tissue 3 weeks after treatment. Astrocyte endfeet of the GLM became irregular in contour, protruding in a fern-leaf fashion into the pial connective tissue. Fine naked axons grew out through these protrusions and subsequently increased in number, vigorously growing in large bundles both proximally and distally along blood vessels in the pial connective tissue. Bundles of regenerating axons extended as much as 1.5 mm from the site of the lesion 3 months after surgery. These bundles were covered by thin processes of pial or arachnoidal non-neuronal cells, and the regenerating axons remained unmyelinated. The above findings indicate that under well-nourished conditions, adult mammalian optic nerve exhibits considerable regenerative ability.

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.

The pacinian corpuscle is composed of an inner and an outer core or bulb. The former is formed by tightly packed and multi-layered thin cellular processes (lamellae) of lamellar cells which surround a centrally located axon terminal, and the latter, also called the capsule, is made up of very loosely piled layers of thin cells which encircle the inner core. Lamellar cells of the inner core are considered to be specialized Schwann cells, and the outer core cells are modified perineurial cells. In the present study, the matrix filling the extracellular spaces of the inner core consisted of basal lamina-like amorphous materials, sparce fine collagen fibrils, and the ground substance embedding these structural components. No definite basal laminae were found on the inner core lamellae except on the peripherally located ones which had distinct basal laminae. Outer-core cells were invested along the entire contour by distinct basal laminae. The interspace between the inner and outer cores was a continuation of the nerve endoneurium. The purpose of this investigation was to determine whether the extracellular matrix of the pacinian corpuscle, especially that of the inner core, has the ability to cause corpuscle regeneration, i.e. to make the regenerating axons and Schwann cells differentiate into corpuscular axon terminals and inner core cells, respectively. Pacinian corpuscles in the periosteum at the distal end of the fibula of mice were repeatedly frozen (3-5 times) in situ with forceps cooled with liquid nitrogen. Within 2-3 days, all the cellular constituents of the corpuscle had degraded, while the extracellular matrices of the inner and outer cores apparently remained undamaged. After 5-7 days, regenerating axons and accompanying immature Schwann cells entered these extracellular matrices of the inner cores. A remarkable finding was that these immature Schwann cells were detached from the axon, and sent thin cellular processes around the axon in a characteristic fashion, basically forming the same pattern as lamellae in a normal corpuscle. The regeneration of the inner core was completed by about 40 days after the freezing treatment. In the outer core, perineurial cells proliferated and extended through the basal lamina tubes of the old cells, becoming new outer core cells. These findings indicate that the extracellular matrix of the pacinian corpuscle has a specific property to cause the regeneration of the corpuscle.

Changes in the diameter of collagen fibrils were observed with an electron microscope in the endo- and epineurium of sciatic nerves of mice during development from 12 days of gestation to 5 months after birth. It was noted that endoneurial collagen fibrils appeared in embryonic mice at 15 days of gestation, and at the same time, basal laminae began to appear sporadically on the Schwann cell plasmalemma. No fibroblasts were seen at this developmental stage. Collagen fibrils in the endoneurium remained as thin as they were when they first appeared, being in the narrow range of 250-300 A in diameter, while those in the epineurium became much thicker (400-450 A, 5 months after birth) as is also the case in dermal connective tissues. The present study shows that the endoneurial collagen fibrils were different in their developmental pattern from those of the epineurial or of other connective tissues, lending support to the concept that the endoneurial collagen fibrils are particular in nature, being so-called histological reticular fibers.

The purpose of this study was to examine whether the basal laminae of Schwann cells in allografts could survive immunological rejection and serve as a conduit for regenerating nerves, as in the case of autogenic nerve grafts. Allografts of nerves were carried out using sciatic nerves of mice after the grafts had been repetitively frozen to kill their Schwann cells. Two mouse strains, C57BL/6N and C3H/HeN, were used, as they are known to differ in major histocompatibility complex. The mid-portion of the grafted nerve segments was examined by electron microscopy. In addition, the toe pad skin and lumbrical muscles were examined for determining whether regenerating nerves reinnervate sensory end organs and motor endplates. The process of nerve regeneration in the allograft was the same as that seen in the autograft. Cells in the graft disintegrated into cell debris and were phagocytized by macrophages, whereas the basal laminae of Schwann cells were not removed by macrophages, remaining in the form of tubes or scaffolds. Regenerating nerve fibers grew out through such basal lamina scaffolds, keeping in contact with the inner surface. Digital sensory corpuscles and motor endplates of the operated side were well reinnervated. The results indicate that the basal laminae of Schwann cells of the allograft may survive and serve as a conduit for regenerating axons in the same way as in the case of an autograft.

Nerve segments approximately 6-7 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 place, being in contact with the proximal stump of the sciatic nerve. The animals were sacrificed 2, 3, 5, 7 and 10 days, 2, 3, 5 and 8 weeks after the grafting. The grafts were examined at the middle level, i.e., about 3-4 mm distal to the proximal end of the graft, by light and electron microscopy. Within 2-3 days after the grafting, the dead Schwann cells were disintegrated into fragments and gradually phagocytized by macrophages. However, the basal laminae of the Schwann cells remained as empty tubes (basal lamina scaffolds). The notable finding was that the regenerating axons always grew through these basal lamina scaffolds. New Schwann cells seemed to migrate along these axons from the proximal stumps. The number of axons growing through the basal lamina scaffolds gradually increased with time. These axons were surrounded in a bundle by Schwann cells. About 1 week after the grafting, axons began to be segregated into smaller bundles by Schwann cells. Axons with a relatively large diameter (about 2 microns) tended to be sorted out and surrounded by their own Schwann cells. The myelination began about 2 weeks after the grafting on such large diameter axons. The basal lamina scaffolds, through which the regenerating axons had grown, were gradually disintegrated into fragments by the expansive forces due to the increase in number and volume of the regenerating axons and Schwann cells. Groups of axons, which had been derived from the same basal lamina scaffolds, were enclosed with the cells resembling perineurial epithelial cells. These perineurial epithelial cells proliferated and further separated groups of axons into smaller ones or even into single axons. The number of myelinated axons increased with the advancement of regeneration. These results show that the basal lamina scaffolds of Schwann cells serve as efficient conduits for the elongation, maintenance and maturation of regenerating axons.

Peculiar myelin figures were abundantly found in some basal-granulated cells including S, D1 and I cells in human Brunner's glands. Intense acid phosphatase activity was found in the periphery of the myelin figures, indicating that they were secondary lysosomes or residual bodies. The acid phosphatase activity was also found in some secretory granules. There were some secretory granules which were partly membranous in content, suggesting the initial stage of their degradation into myelin figures. There were also features indicating the fusion of secretory granules with the myelin figures. All these findings suggest that the myelin figures are the products of lysosomal degradation of secretory granules. The rate of occurrence of basal-granulated cells containing myelin figures in Brunner's glands tended to be higher in subjects with duodenal ulcer than in cases of gastric cancer or ulcer.

The pattern of nerve regeneration in the grafted rabbit cornea was investigated by electron microscopy. Grafted corneas were excised 2, 7, 14 and 28 days after grafting, and processed for observation by conventional electron microscopy. In the normal, unoperated cornea Schwann cell basal laminae are, unlike those of ordinary peripheral nerves, discontinuous and fragmentary on the fibers coursing through the corneal stroma. In the early stage of regeneration, while numbers of regenerating axons extended through the Schwann cell columns of regenerating axons extended through the Schwann cell columns in the grafts, many other regenerating axons elongated as single fibers through the corneal stroma outside the Schwann cell columns. These single naked axons were later enveloped by Schwann cell cytoplasm, contributing to the overall dense irregular pattern of regenerated nerves in the grafted cornea. It was thought that the regenerating axons can extend throughout the stroma without the guidance of basal lamina tubes, making the corneal stroma a favorable environment for nerve regeneration.

The present report is the second of two studies re-evaluating the cytological characteristics of Pacinian corpuscles. The extreme tip of the axon of a Pacinian corpuscle has been identified and is quite different from the previously described ultraterminal region. The latter is the site where the inner core lamellae begin to terminate and is characterized by a smooth axolemma. The extreme tip lacks inner core lamellae directly abutting the axolemma and is instead characterized by the presence of many axonal spines projecting into a matrix of basal lamina-like material. The extreme tip of the axon thus resembles the organization of the axolemma facing the clefts of the inner core. The axonal spines at the cleft and extreme tip are proposed as a site of restricted current flow due to the tight apposition of inner core lamellae to the axolemma of X-axis. The hemi-inner cores thus could restrict current flow to the cleft. These anatomical specializations could represent both a source and a sink for K+ ions during mechano-electric transduction and account in part for the exquisite sensitivity of Pacinian corpuscles to complex pressure waves.

To distinguish axons from Schwann cell processes in the denervated (Büngner's bands) and reinnervated peripheral nerves, the nearest-neighbor distance of intermediate filaments (NND) was measured in axons and Schwann cells from denervated and subsequent regenerating peripheral nerves. It was revealed that the NND was much larger in regenerating axons (41.9 +/- 14.1 nm) than in Schwann cell processes (23.1 +/- 7.1 nm in regeneration and 19.7 +/- 5.8 nm in denervation). In addition, the NND was also measured in the normal adult and developing peripheral nerves, and it became clear that in all cases the NND in axons (29.0-41.9 nm) was larger than in Schwann cells (19.7-23.1 nm). Thus, it can be generally considered that the NND is larger in axons than in Schwann cells. This fact can be used for the distinction between axons and Schwann cell processes, when the latter have a profile similar to that of the former as in Büngner's bands and in the regenerating nerves.

Morphological specializations of smooth muscles from the middle part of the cat ureter have been examined by electron microscopy using thin sections and freeze-fracture replicas. It is notable that there were many gap junctions which were characteristic in being formed on the surface of blunt cytoplasmic projections protruded from each neighboring cells. In addition to gap junctions, there were two other different junctions, i.e, intermediate junctions and "focal attachments". The latter represents the cell attachment in which two apposing plasma membranes are attached to each other without the intervension of basal lamina in the narrow intercellular space. Caveolae were lined in rows which were intervened by dense band zones parallel to the long axis of the muscle fibre as seen by freeze-fracture replica. Gap junctions were noted to be localized within or along the caveolar zones, being elliptical in shape with the long axis parallel to muscle fibres. As observed in thin sections, gap junctions in freeze-fracture replicas were noted on the top of cytoplasmic protrusions. No membrane specializations were observed in freeze-fracture replicas which corresponded to the "focal attachment" or intermediate junction as seen in thin sections. As to the innervation, the autonomic nerve terminals were in abundance, closely apposed to the smooth muscles. In addition to autonomic nerve terminals, sensory nerve terminals were observed between muscle fibres, consisting of axon terminals and lamellar cells, the same structure as mechanoreceptive lamellar corpuscles known in other organs.

To follow the subsequent history of grafted tissue in experiments designed to study regulation and commitment in the amphibian neural plate, previous workers have relied on graft scars, vital dyes applied externally to cells, or xenoplastic grafts. Each of these methods has been criticized on the grounds that they do not indicate unambiguously the origins of individual cells within the operated host. To overcome these difficulties, homoplastic, genetically marked embryonic grafts were taken from the prospective spinal neuroectoderm of triploid and tetraploid Xenopus laevis frogs and transplanted to presumptive eye and prosencephalic regions of the neural plate of diploid X. laevis embryos. Orthotopic presumptive eye grafts also were done. Marked cells were scored in section either by nucleolar number or computerized nuclear size analysis. Of 28 heterotopically grafted embryos that survived to stage 41, when the retina has differentiated, prospective spinal cord neuroectoderm in eight animals gave rise to cell types unique to the eye. The remaining 20 survivors appeared to be mosaic. These results substantiate claims of regulation in the neural plate and extend these observations to the level of individual cell types, a level of resolution not previously obtained in other studies.

The microvascular architecture of the human corpus cavernosum penis was studied by scanning electron microscopy of vascular corrosion casts. The corpus cavernosum was supplied by the penile deep artery. It gave off branches to become either arteries distributed within the corpus cavernosum or those directly supplying the corpus spongiosum urethrae. The former arteries further divided into small arteries which fell into two categories: 1) arteries breaking up into capillaries, and 2) arteries draining directly into the cavernous sinuses. The capillaries were collected into venular networks just beneath the tunica albuginea (the subalbugineal venular plexus), while the cavernous sinuses were collected into venules at the periphery of the corpus cavernosum. These postcavernous venules also received venules from the subalbugineal venular plexus, and left the corpus cavernosum. Thus, two circulatory routes are evident within the corpus cavernosum. These findings suggested that the penile erectile cycle is controlled by hemodynamic changes between these two routes within the corpus cavernosum.

Four monocytoid cell lines, JOSK-I, -S, -M, and -K, were newly established successfully from peripheral blood of two cases of acute monocytic leukemia and one case each of acute myelomonocytic leukemia and chronic myelogenous leukemia in myelomonocytic blast crisis. In order to establish permanent cell lines, cultures of leukemic blasts were initiated in 96-well microtiter plates. Each cell line grew in a suspension culture with a doubling time of 24-32 h and has been serially maintained for over 20 mo. Each line had immature monocytic properties as judged from the results of cytological, immunochemical, and functional analyses. The cells showed a positive reaction for alpha-naphthyl butyrate esterase which was completely inhibited by sodium fluoride and exhibited immature monocytic features on electron microscopic observation. They also had surface markers specific for the monocyte-macrophage lineage. Chromosome analyses showed that each line had a variety of marker chromosomes; furthermore, these established lines exhibited high potentialities involving morphological and functional differentiation into more mature monocytic cells when induced by several chemical inducers. We also found that two of the established cell lines produced much interleukin 1 activity without any stimuli. These new lines might be valuable for studying the regulation of monocyte-macrophage differentiation and host defense mechanisms.

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.

Nerve endings of the trigeminal nerve were examined in the lip, nostril, upper jaw and supraorbital skin of cat by the horseradish peroxidase (HRP) tracing method. Wheatgerm agglutinin-HRP (WGA-HRP) as injected into the spinal trigeminal nucleus was transported transganglionically to nerve endings. The present study deals with presumptive baroreceptive nerve endings found on arteriovenous anastomoses and on some other large arteries. By light microscopy HRP-labeled, complexly arborized nerve endings were found on the walls of arteries located deep in the subcutaneous tissue of the lip and nostril. These arteries were identified in serial sections to be an arterial segment of an arteriovenous anastomosis. By electron microscopy characteristic nerve endings were located in the adventitia and partly extended into the adjacent connective tissue of the arterial wall. These terminals had no connective tissue capsule. Axon terminals were somewhat enlarged (1-2 microns in diameter) and extended through bundles of collagen fibrils. The terminals contained an abundance of mitochondria and some vesicles. Axon terminals were typically covered by thin Schwann cell processes, but parts of the axolemma were sometimes devoid of such Schwann cell coverings, being invested only by basal laminae. Cell bodies of Schwann cells were located apart from axons. These light and electron microscopic features of the endings resembled those of other well-defined baroreceptors reported in the carotid sinus, aortic arch and endocardium, as well as of Ruffini terminals and Golgi tendon organs, suggesting that they would be baroreceptors of arteriovenous anastomosis. In addition HRP-labeled single nerve fibers with varicosities were found in the walls of some large arteries in the facial skin. By electron microscopy, such a HRP-positive nerve fiber contained some mitochondria and vesicles in varicosities and coursed with a bundle of HRP-negative fine fibers in the adventitia of arteries. These HRP-labeled single fibers were considered to be sensory derived from trigeminal nerves.

In an attempt to identify regenerating axons in the central nervous system, a partial transection of the dorsal funiculus in the rat spinal cord was carried out with a pair of microdissection scissors, and a nylon thread loop was inserted into the lesion to demarcate the severed tissue. Nerve regeneration through the demarcated lesion was observed 4-20 days after the operation by light and electron microscopy. In the early stage, many naked axons appeared from the caudal part of the lesion, and some of these further extended into the demarcated space. They contained an accumulation of mitochondria, smooth-surfaced endoplasmic reticulum and vesicles in the axoplasm; this axoplasmic feature indicated that they were regenerating axons. They gradually increased in number, and took highly irregular courses exhibiting various fluctuations in diameter throughout their lengths. Immature Schwann cells as well as glial cells including oligodendrocytes and astrocytes appeared in close association with these regenerating axons. Oligodendrocytes eventually formed thin myelin sheaths. On the other hand, naked axons were present deflecting outside the thread loop; they showed no axoplasmic characteristics as described above. These axons could be regarded as uninjured ones merely undergoing demyelination due to the surgery. Thus, regenerating axons were clearly distinguished from merely demyelinated ones, and some of them were shown to grow through the traumatic lesion in the dorsal funiculus of the rat spinal cord.

Basal laminae (BL) were separated from Schwann cells of rat sciatic nerves by means of weak sonication, and the anionic sites of the BL were demonstrated by using cationized ferritin (CF) or ruthenium red (RR). CF particles were deposited in clusters at intervals of 100-150 nm on the interstitial side of the BL facing the connective tissue, while the cellular side facing the Schwann cell plasmalemma showed only an occasional deposition of CF particles. RR-positive sites were found only on the interstitial side with a pattern of distribution comparable to that of CF-binding sites. These results indicate that the patterns of anionic site distribution are different between the inner and outer surfaces of the Schwann cell BL.

In order to study the pathogenesis of juvenile-type chronic myelocytic leukemia (CML), we examined the colony-forming capacity and colony composition in the bone marrow (BM) and peripheral blood (PB) of three children with juvenile-type CML. Large numbers of granulocytes and macrophage colonies were formed by BM and PB cells. Whole agar culture staining revealed that especially macrophage colonies increased in comparison with normal controls. After removal of carbonyl iron-laden cells with a magnet or deprivation of cells adherent to glass from BM cells, the number of macrophage colonies markedly reduced in comparison with the number of colonies formed by untreated BM cells, suggesting that some of the macrophage colony-forming cells (M-CFC) may have phagocytic and/or adherent activity. Radiation sensitivity and thymidine suicide rate of these M-CFC were not different from those of granulocyte colony-forming cells (G-CFC). The predominance of M-CFC in juvenile-type CML may be one of the reflections of fetal-type myelopoiesis since M-CFC are predominant in cord blood and PB in the neonatal period. Moreover, considerable numbers of erythroid-colony-forming units (CFU-E) were present in PB of all patients. It may be concluded that juvenile-type CML is a panmyelopathy with the predominance of M-CFC.

An experimental study was performed in order to evaluate the effects of veno-arterial counterpulsation (VACP) on ischemic-injured myocardium. Following ischemic episodes in 9 dogs, reperfusion by VACP was performed under normal temperature for 45 min, while reperfusion by partial cardiopulmonary bypass (PCPB) was performed in another 9 dogs in order to observe cardiac function, myocardial ultrastructure and cytochemical changes. The results thus obtained can be summarized as follows: Cardiac output recovered to 100% of the pre-ischemic value 60 min after weaning from cardiopulmonary bypass in the VACP group; however, only 70% at most could be recovered in the PCPB group. Glycogen granules were favorably maintained during cardiopulmonary bypass or even after the weaning in the VACP group as compared to the PCPB group. Sarcomere length after removal of cardiopulmonary bypass was almost similar to the control value in the VACP group, although this was longer than the control value. Succinate dehydrogenase activity in mitochondria was better maintained in the VACP group than in the PCPB group. From the evidence mentioned above, it was suggested that VACP is useful for the recovery of aerobic metabolism and myocardial contractility in ischemic-injured myocardium.

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)

The freeze-fracture replication technique was used to study the mechanoreceptive digital corpuscles in toe pads of mice. The axon terminal plasmalemma had intramembranous particles (IMPs) at a density of 2367 +/- 517 microns-2 (mean +/- S.E.M.) in the P-face and 84 +/- 4 microns-2 in the E-face. Particles were 10 +/- 1.8 nm in diameter in the P-face and 10 +/- 1.5 nm (mean +/- S.D.) in the E-face. Particle-rich and particle-free areas were noted in the P-face. The lamellar cell plasmalemma had IMPs at a density of 3359 +/- 224 microns-2 in the P-face and 265 +/- 95 microns-2 in the E-face. Particles were 10 +/- 1.4 nm in diameter in the P-face and 10 +/- 1.6 nm in the E-face. Non-terminal unmyelinated fibres in the connective tissue compartment of toe pads were also examined: the P-faces of the axolemma and Schwann cell plasmalemma had IMPs at a density of 1356 +/- 283 microns-2 and 1514 +/- 514 microns-2, respectively, while the E-face of these membranes had only a few particles. Particles were 9 +/- 1.2 nm and 10 +/- 1.6 nm in diameter in the P-faces of axon and Schwann cell plasmalemmata, respectively. The results show that the IMPs in terminal axolemma and in lamellar cell plasmalemma have a much higher density than those of non-terminal axons or Schwann cells in myelinated and unmyelinated fibres. In addition, IMPs in the terminal axolemma are larger than those in non-terminal axolemma except for the nodal axolemma. It can be said that plasmalemmata of both the axon terminals and lamellar cells of digital corpuscles are specialized in terms of IMPs, suggesting that they have specific physiological properties in mechanoreceptive functions including mechano-electric transduction.

The present report reviews the physiological and morphological specializations of Pacinian corpuscles and other mechanoreceptors that are present in the skin and connective tissues of the body as well as the cochlea. The remarkable sensitivity of Pacinian corpuscles is such that the only form of mechanical energy that could be perceived by a Pacinian corpuscle is a sound wave. In fact the human finger as demonstrated by Munger and Ide (1987) can perceive sound waves when water is the coupling agent. The structural specializations are equally remarkable with extensive membrane specializations of both the inner core and inner portion of the outer core. The halves of the inner core are each coupled with gap junctions and the inner portion of the outer core joined with numerous tight junctions. The cleft regions have specializations involving the axolemma that consist of numerous axonal spines containing bundles of filaments projecting into the cleft of the inner core. These structural specializations are thought to represent specializations for mechano-electric transduction analogous in many respects to the hair cells of the cochlea. A hypothesis for mechano-electric transduction is presented that may account in part for the extreme sensitivity of Pacinian corpuscles and other mechanoreceptors.

Certain parts of the peripheral nervous system were observed by a modification of the KOH-collagenase method by Miller et al. (1982). The sciatic nerve of the mouse, and dorsal root ganglion, lingual muscle and jejunum of the rat were fixed over one day at room temperature in a fixative containing paraformaldehyde and glutaraldehyde. After fixation, specimens were treated with 5N KOH solution for 5-10 min at 60 degrees C, immersed in a collagenase solution for 3-5 h at 37 degrees C and processed for scanning electron microscopy (SEM). In adequately treated specimens, connective tissue matrices and basal laminae were completely removed without causing any severe tissue damage. The three-dimensional visualization of cellular elements of peripheral nerves was enabled with the following results: 1) Individual nerve fibers of the sciatic nerve were clearly exposed. Myelinated fibers were non-branching cords with local annular constrictions at the node of Ranvier, while unmyelinated ones branched and anastomosed with one another to form loose networks. The sites of accumulation of the Schwann cell cytoplasm swelled on the surface of the myelinated fibers. Mesaxons were visualized as longitudinal furrows. 2) Ganglion cells covered by satellite cells were observed in the dorsal root ganglion. The ganglion cells were covered by their own convoluted dendro-axonal processes, thus forming the initial glomeruli of Cajal. Satellite cells at the glomeruli extended many finger-like projections surrounding the dendro-axonal processes. 3) Motor endplates were observed in lingual muscles. Terminal Schwann cells (teloglia) at the endplate were clearly visualized: the round perikaryon extended cytoplasmic processes along the axonal branches within the endplate. The processes issued fine finger-like projections from their margins. 4) Vascular autonomic plexuses were clearly demonstrated in lingual muscles. Unmyelinated nerves branched and anastomosed to form elaborate nerve networks around the vessels. Neuronal processes and associated Schwann cells were identifiable at high magnification. 5) Submucous nerve plexuses in the jejunum consisted of numbers of ganglia and interconnecting strands of fibers which formed very complicated networks. These observations indicate that this modified KOH-digestion method is useful for the SEM study of the three-dimensional cellular organization of nervous elements in various tissues.