Complex nerve terminal formation in the phasic muscles of frogs

Synapses with complex nerve terminals consisting of several terminal arbors of a single axon divided by myelin segments were investigated using histological and electrophysiological techniques during experiments on the cutaneous-pectotoralis muscles of different aged frogs. Numbers of synapses with complex nerve terminals were shown to increase during the postnatal developmental process. The relationship between the complexity of nerve terminals, summated length of terminals, and size of muscle fiber is described. Some terminal arborizations at complex nerve terminals originate from nodes of Ranvier; these are marked by low quantal secretion and a distinctive pattern of sodium current decay along the path of the terminals. The causes and mechanisms governing increased complexity of nerve endings in phasic muscles are discussed, together with transmitter release patterns at these endings. It is postulated that growth and myelination processes occur in parallel at the nerve terminal.A. A. Ukhtomskii Physiological Institute, Leningrad State University; S. A. Kurashov Medical Institute, RSFSR Ministry of Health, Kazan'. V. I. Ul'yanov State University, Kazan'. Translated from Neirofiziologiya, Vol. 22, No. 1, pp. 99–107, January–February, 1990.     

Activity-driven sharpening of the retinotectal projection in goldfish: Development under stroboscopic illumination prevents sharpening

Blocking or synchronizing activity during regeneration of the retinotectal projection prevents both the sharpening of the retinotopic map recorded on tectum and the refinement of the structure of individual arbors within the plane of the map, and this refinement is triggered by N-methyl-d-aspartate (NMDA) receptors. We tested whether activity-driven refinement also occurs during development of the projection in larval and young adult goldfish. Shortly after hatching, larval goldfish were placed into tanks within light-tight chambers illuminated by a xenon strobe at 1 Hz for 14 h of each daily cycle. Fish were reared for 1.5–2 years, until large enough to record in our retinotectal mapping apparatus (6 cm length). Age- and size-matched controls had normal maps with multiunit receptive fields (MURFs) recorded at each tectal point of 10.8° (0.16 S.E.M., n = 5), whereas the strobe-reared fish had only roughly retino-topic maps with much enlarged MURFs averaging 26.7° (1.41 S.E.M., n = 5). This enlargement represents an abnormal convergence onto each tectal point, as the maps failed to sharpen during development. The arbors of individual retinal axons were stained with horseradish peroxidase (HRP) in larval fish and in adult strobereared and control fish. They were drawn with camera lucida from tectal whole mounts, and analyzed for spatial extent in the plane of the retinotopic map, order of branching, number of branch endings, depth of termination, and caliber of the parent axon. Arbors from larval fish (1–2 weeks) were small (approximately 50 × 40 μm) with less than 10 branches, occupied a single strata, and could not be separated into different classes by caliber of axon. The 87 arbors stained in control adult fish (6 cm long) were much like previously examined adult arbors, with those from fine, medium, and coarse axons averaging 115, 166, and 194 μm in extent, respectively, and having 17–24 branch endings. The 110 arbors from 12 strobe-reared fish were often abnormal. Although the fasciculation was normal, the extrafascicular routes were abnormal with reversing turns. The axons often had branches along their course, and these branches were scattered across a wider extent, rather than forming a distinct cluster. In contrast, neither the number of branches nor the depths of termination was significantly changed in any group. The coarse caliber arbors were most abnormal, being 64% longer and 30% wider than controls. The fine caliber arbors were also significantly larger by about 20%, but the medium caliber arbors were not enlarged. The enlarged arbors partially account for the unsharpened electrophysiological maps. Together the results show that during development, as well as during regeneration, the retinotectal map is subject to an activity-driven sharpening process. © 1993 John Wiley & Sons, Inc.     

The cellular organization and nervous supply of the basilar papilla in the lizard,Calotes versicolor

Summary The basilar papilla of the lizard Calotes versicolor contains about 225 sensory cells. These are of two types: the short-haired type A cells in the ventral (apical) part of the organ, and the type B cells with long hair bundles, in the dorsal (basal) part of the organ. The type A cells are unidirectionally oriented and are covered by a tectorial membrane while the type B cells lack a covering structure and their hair bundles are oriented bidirectionally. Apart from those differences, the type A and type B cells are similar. They are columnar, and display the features common to most sensory cells in inner ear epithelia. The sensory cells are separated by supporting cells, which have long slender processes that keep the sensory cells apart. Close to the surface of the basilar papilla a terminal bar of specialized junctions interlocks adjacent cells. Below this, adjacent supporting cells are linked by an occluding junction.The cochlear nerve enters from the medial (neural) aspect. The fibres of the nerve lose their myelin sheaths as they enter the basilar papilla. Each sensory cell is associated with several nerve endings. All the nerves identified were afferent. Marked variations were seen between nerve endings in the basilar papilla, but no morphological equivalents of any functional differences were observed.This work is supported by grant no. B76-12X-00720-11A from the Swedish Medical Research Council, and by funds from the Karolinska Institute, Stockholm, Sweden.     

The lorenzinian ampullae of Polyodon spathula

Summary Light and electron microscopic observations on the ampullary organs of Polyodon spathula (Chondrostei, Osteichthyes) reveal a sensory epithelium similar to that found in the Lorenzinian ampulla, an electroreceptor found in marine Elasmobranchs.The sensory cells have a very small luminal part provided with a cilium. They are innervated by many nerve endings. Each nerve fibre apparently makes synaptic contact with several sensory cells. The synaptic structure in the sensory cell is composed of a flat sheet, the outermost part of which is surrounded by 3 or 4 annuli of densely staining material. The sheet extends into a protrusion of the sensory cell, and there is a corresponding invagination in the nerve terminal.The conclusion that these organs are electroreceptors, is supported by the finding that the fish responds to the introduction of an iron tube in the aquarium, whereas a wooden rod introduced in the same way causes no response.     

STRUCTURE OF THE MACULA UTRICULI WITH SPECIAL REFERENCE TO DIRECTIONAL INTERPLAY OF SENSORY RESPONSES AS REVEALED BY MORPHOLOGICAL POLARIZATION

The anatomy of the labyrinth and the structure of the macula utriculi of the teleost fish (burbot) Lota vulgaris was studied by dissection, phase contrast, and electron microscopy. The innervating nerve fibers end at the bottom of the sensory cells where two types of nerve endings are found, granulated and non-granulated. The ultrastructure and organization of the sensory hair bundles are described, and the finding that the receptor cells are morphologically polarized by the presence of an asymmetrically located kinocilium in the sensory hair bundle is discussed in terms of directional sensitivity. The pattern of orientation of the hair cells in the macula utriculi was determined, revealing a complicated morphological polarization of the sensory epithelium. The findings suggest that the interplay of sensory responses is intimately related to the directional sensitivity of the receptor cells as revealed by their morphological polarization. The problem of efferent innervation is discussed, and it is concluded that the positional information signaled by the nerve fibers innervating the vestibular organs comprises an intricate pattern of interacting afferent and efferent impulses     

Supravital Methylene Blue Staining of Piloneural Complexes of Common Fur Hair Follicles in the Rat

Light microscopic observations employing supravital methylene blue staining are presented for piloneural complexes of common fur hairs in the mystacial pad of the rat snout. The investigation revealed anatomical details of piloneural complexes belonging to follicles of both vellus and guard hairs. In the methylene blue stained preparations, different types of palisade-like lanceolate nerve fiber endings could be discriminated. The thicker vellus and thinner guard hairs (hair diameter: 15-25 μm) exhibited a different innervation pattern compared to the thicker guard hairs, and two subtypes of piloneural complexes could be distinguished. Both subtypes were characterized by slightly stained lanceolate endings and the absence of a circular nerve fiber plexus. One subtype, however, showed strongly stained spines originating from the lanceolate endings. A few spines of adjacent lanceolate endings appeared in contact with each other. In the second subtype, these spines were replaced by anastomoses suggesting a delicate terminal nerve fiber network. The moderately stained lanceolate endings located primarily at the follicles of thicker guard hairs (hair diameter: 30-40 μm) showed smooth outlines, but were characterized by the occurrence of an intensely stained additional circular nerve fiber plexus. The differences in the morphology of piloneural complexes associated with the follicles of common fur hairs suggest differences regarding their mechanoreceptive tasks.     

金鱼再生的视神经在顶盖投射精确化的DiI顺行标记研究

本文用微量显微注射法,在金鱼视网膜的背侧用亲脂类荧光染料DiI标记少量神经节细胞,通过顺行标记研究了视神经再生过程中视网膜顶盖投射的精确化过程。在损伤视神经后的不同时期观察了再生视神经纤维在顶盖整装片上的分布。在再生早期它们以超出正常的途径由背腹两侧进入顶盖,广泛分布。但其中大部分仍分布于顶盖腹侧的靶区。在再生晚期通过精确化,重建如正常鱼一样精确的视网膜顶盖投射。这个精确化过程表现在以下三方面:(1)再生于顶盖错误区域的再生视神经纤维的消失;(2)再生早期视神经纤维主干上生长的侧部分支的消失;(3)到达靶区的再生视神经纤维形成重迭的终末分支。由以上结果推测,顶盖中可能存在两类不同的因子:一类是普通诱向因子,存在于整个顶盖中,它在再生早期引导再生的视神经纤维长入顶盖。另一类是神经营养因子,它具区域特异性,在再生晚期引导视神经纤维到达顶盖靶区,形成精确的视网膜顶盖投射。     

Structure of the sensory innervation of the anal canal in the pig. A light- and electron-microscopical study

The sensory innervation of the anal canal of the pig was investigated by light and electron microscopy. The distribution of the different types of sensory nerve endings correlates with the histology of different zones: (1) After the rectal mucosa there was a zone lined with nonkeratinized stratified squamous epithelium. (2) A middle zone was lined with keratinized stratified squamous epithelium. Here the dermis already showed a papillary and reticular layer. (3) The last zone showed hairy skin with a high hair density. The following nerve endings were found: Free nerve endings reached the stratum superficiale in nonkeratinized squamous epithelium and the stratum granulosum in the keratinized squamous epithelium. Dermal free nerve endings were found in all zones near the epithelium and two different types were identified as those derived from C-fibers and those from A-delta-fibers. Merkel nerve endings showed different features depending on their location. Few Merkel-like cells were found in the epithelium of the anal crypts. Typical Merkel Tastscheiben were located at the base of epithelial ridges or pegs in zones 2 and 3. The number of Merkel cells varied up to 200. The myelinated afferent fiber supplied 10-15 Merkel cells. Merkel cells were also found regularly in the outermost layer of the external rooth sheath of hair follicles at about the same level as perifollicular nerve endings. Lamellated corpuscles were found in the dermis of all zones except the cranial part of zone 1, where the anal crypts are located. Generally they consisted of a central nerve terminal which may be branched. Each terminal was surrounded by an inner core of concentrically arranged lamellae of the terminal Schwann cell and one or several inner cores were included in a capsule of perineural cells. The size of the corpuscle, the regularity of the inner core and the number of capsular layers depended on the location of the corpuscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Supravital Methylene Blue Staining of Piloneural Complexes of Common Fur Hair Follicles in the Rat

Light microscopic observations employing supravital methylene blue staining are presented for piloneural complexes of common fur hairs in the mystacial pad of the rat snout. The investigation revealed anatomical details of piloneural complexes belonging to follicles of both vellus and guard hairs. In the methylene blue stained preparations, different types of palisade-like lanceolate nerve fiber endings could be discriminated. The thicker vellus and thinner guard hairs (hair diameter: 15-25 μm) exhibited a different innervation pattern compared to the thicker guard hairs, and two subtypes of piloneural complexes could be distinguished. Both subtypes were characterized by slightly stained lanceolate endings and the absence of a circular nerve fiber plexus. One subtype, however, showed strongly stained spines originating from the lanceolate endings. A few spines of adjacent lanceolate endings appeared in contact with each other. In the second subtype, these spines were replaced by anastomoses suggesting a delicate terminal nerve fiber network. The moderately stained lanceolate endings located primarily at the follicles of thicker guard hairs (hair diameter: 30-40 μm) showed smooth outlines, but were characterized by the occurrence of an intensely stained additional circular nerve fiber plexus. The differences in the morphology of piloneural complexes associated with the follicles of common fur hairs suggest differences regarding their mechanoreceptive tasks.     

Histology and ultrastructure of the olfactory organ of the freshwater pulmonate Helisoma trivolvis

Summary The olfactory organ of Helisoma trivolvis is located on the surface of the body at the base of the cephalic tentacles. An evagination of skin, the olfactory plica, at the base of the tentacle extends over the olfactory organ dorsally. The epithelium of the olfactory organs contains unspecialized epithelial cells, ciliated epithelial cells, basal cells, mucous secretory cells, and sensory dendrites. The surface of the epithelium has a complex brush border of thick plasmatic processes, which branch to form several terminal microvillar twigs. Long slender cytoplasmic processes form a dense spongy layer among the plasmatic processes beneath the level of the terminal twigs. Bipolar primary sensory neurons clustered beneath the epithelium of the olfactory organ send dendrites through the epithelium to the free surface. Some sensory endings have a few short cilia, but most bear only microvilli. Cilia of sensory endings and epithelial cells extend beyond the brush border of the epithelium. Small axons arise from the perikarya of the sensory neurons and enter a branch of the olfactory nerve. HRP tracing indicates that the axons pass to the cerebral ganglion without interruption. Histochemical tests indicate that the sensory neurons are neither aminergic nor cholinergic.     

Demonstration and localization of synapsin I in vestibular receptors in the cat: immunocytochemical study

The presence and localization of synapsin I, a neuron-specific phosphoprotein, was investigated in the cat vestibular epithelium, using a rabbit antisynapsin I anti-serum. The staining was performed by immunofluorescence or by a peroxidase-antiperoxidase (PAP) technique. A strong immunoreactivity was observed with both methods. This immunoreactivity appeared as spherical patches distributed in the lower part of the epithelium. This distribution pattern is very similar to that of the efferent synaptic endings which form axodendritic synapses with the afferent nerve chalice of type I hair cells, or axosomatic synapses with type II hair cells. Some of the nerve chalices were also labelled; in this case, the immunoreactivity was more evident with PAP staining. These results thus suggest the presence of large amounts of synapsin I in the vestibular efferent nerve endings. These endings are known to be filled with numerous synaptic vesicles. This localization of synapsin I is well correlated with previous work that report a close association between synapsin I and small synaptic vesicles. The presence of synapsin I in sensory endings such as the afferent nerve chalices was unexpected and is under investigation.     

Early postmortem changes in the Organ of Corti (guinea pig)

Summary Early post mortem changes in the Organ of Corti are described. 15 minutes after death, when kept at room temperature, 20° C (63° F), an oedematous swelling is observed within the cytoplasm of hair cells and nerve endings, the latter being more severe affected. After 30 minutes post mortem the mitochondria of the hair cells have also become significantly swollen. Three hours post mortem the general character of the hair cells is still recognizable, but most of the nerve endings have been completely destroyed. Acknowledgement. We wish to express our appreciation of the skilful technical assistance of Mrs. B. Flock, Miss A.-M. Lundberg, Miss Sonja Löfvenius, Mr. G. Bornholm and Mr. Rune Ragnefjell.This work was supported by grants from the Swedish Medical Research Council, the National Institute of Health grant (no NB 03956-02), the Therese and Johan Anderssons minne, the Gustav and Tyra Svenssons fund and the King Gustav V Research Fund.     

On a Possible Hair Cell Turn-Over in the Inner Ear of the Caecilian Ichthyophis glutinosus (Amphibia: Gymnophiona)

The inner ear of the caecilian Ichthyophis glutinosus is briefly described. An analysis of the fine structure of the utricular macula show the presence of several varieties of hair cells. At least one type seems to be degenerating, another developing. A slow hair cell turnover is proposed. The number of afferent nerve endings in contact with a few hair cells were found to be 6–13 as reconstructed from electron microscopical serial sectioning.     

Proliferating hair cell precursors in the ear of a postembryonic fish are replaced after elimination by cytosine arabinoside

Identified, proliferating S-phase cells in the postembryonic fish ear are known to be the precursors to new hair cells. It is not known, however, whether the ability to proliferate is restricted to a small population of cells. The ability of cells that are not normally in the cell cycle to enter S-phase was examined using the antimitotic drug cytosine arabinoside (ara-C). The normal population of S-phase cells in the saccule was destroyed by a single large dose of ara-C. Two weeks later, the prsence of S-phase cells was evaluated using the S-phase marker bromodeoxyuridine. The results strikingly demonstrate that S-phase cells are replaced, since S-phase cells returned to the saccule in the same number as found in normal fish. The data are interpreted to suggest that a large number of nonsensory support cells are capable of entering the cell cycle and that some mechanism must regulate which of these are actually cycling at any given time. © 1995 John Wiley & Sons, Inc.     

Expression of neurotrophins and Trk receptors in the developing,adult, and regenerating avian cochlea

We studied the expression of neurotrophins and their Trk receptors in the chicken cochlea. Based on in situ hybridization, brain-derived neurotrophic factor (BDNF) is the major neurotrophin there, in contrast to the mammalian cochlea, where neurotrophin-3 (NT-3) predominates. NT-3 mRNA labeling was weak and found only during a short time period in the early cochles. During embryogenesis, BDNF mRNA was first seen in early differentiating hair cells. Afferent cochlear neurons expressed trkB mRNA from the early stages of gangliogenesis onward. In accordance, in vitro, BDNF promoted survival of dissociated neurons and stimulated neuritogenesis from ganglionic explants. High levels of BDNF mRNA in hair cells and trkB mRNA in cochlear neurons persisted in the mature cochlea. In addition, mRNA for the truncated TrkB receptor was expressed in nonneuronal cells, specifically in supporting cells, located adjacent to the site of BDNF synthesis and nerve endings. Following acoustic trauma, regenerated hair cells acquired BDNF mRNA expression at early stages of differentiation. Truncated trkB mRNA was lost from supporting cells that regenerated into hair cells. High levels of BDNF mRNA persisted in surviving hair cells and trkB mRNA in cochlear neurons after noise exposure. These results suggest that in the avian cochlea, peripheral target-derived BDNF contributes to the onset and maintenance of hearing function by supporting neuronal survival and regulating the (re)innervation process. Truncated TrkB receptors may regulate the BDNF concentration available to neurites, and they might have an important role during reinnervation. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 1019–1033, 1997     

The effects of transplantation and regeneration of sensory neurons on a somatotopic map in the cricket central nervous system

The restoration of the cercal afferent projection of crickets was examined after severing the cercal nerve or amputating the cercus and reimplanting it. After either maneuver the sensory neurons regenerated arborizations in the central nervous system (CNS) within about 1 month. In order to assess the role of the pathway taken to the CNS in controlling the growth of the terminal arborization, we transplantated left cerci to the right side of the host. The operation mismatched the mediolateral axes of host and graft tissues. In one-third of the neurons examined, the axon trajectories of the regenerated neurons were altered. The terminal arborizations in these cases were unusual; for example, one neuron arborized in an abnormal area as well as in its normal area. In rare instances this neuron arborized only in incorrect areas of the CNS. Thus, it appears that axon pathway can have an effect on the central structure of sensory neurons. However, in most cases after the surgery, the neurons were able to reach their proper target areas even by circuitous routes. The proximodistal coordinate of the map is isomorphic with sensory neuron age, because the most distal receptors are produced early in postembryonic development and new ones are added proximally at each molt. We tested the possibility that the order of differentiation was critical for generating the afferent projection with two experiments. First, the distal cercus including the distal members of the clavate array was amputated. The specimen regenerated an entire distal cercus including distal clavate receptors. When newly generated, distal neurons were stained, the terminal arbors were identical to the amputated neurons they replaced. In this case, both age and order of arrival were reversed from normal yet the topographic projection pattern was not altered. Second, we transplanted young cerci onto older specimens and then examined the regenerated arbors of the transplanted sensory neuron. The immature neuron arborized in the adult nervous system exactly as the mature homolog. Thus the age of a sensory neuron did not appear to be a controlling variable in the elaboration of a terminal arborization. The significance of these results is discussed in the context of two models for development of orderly neuronal connections.     

SYNAPTIC STRUCTURES IN THE LATERAL LINE CANAL ORGAN OF THE TELEOST FISH LOTA VULGARIS

This paper is a preliminary report on some of the results of electron microscopic studies on the lateral line canal organ of the teleost fish Lota vulgaris. It deals with the ultrastructure of the synaptic area on the hair cells of the sensory epithelium and describes the nerve endings as well as a complicated system of foldings of the hair cell plasma membranes enclosing portions of the hair cell cytoplasm in the synaptic area. These findings are discussed in the light of present knowledge of the ultrastructure of other receptoneuronal synapses.     

Hair cell regeneration in the chick inner ear following acoustic trauma: ultrastructural and immunohistochemical studies

The regeneration of hair cells in the chick inner ear following acoustic trauma was examined using transmission electron microscopy. In addition, the localization of proliferation cell nuclear antigen (PCNA) and basic fibroblast growth factor (b-FGF) was demonstrated immunohistochemically. The auditory sensory epithelium of the normal chick consists of short and tall hair cells and supporting cells. Immediately after noise exposure to a 1500-Hz pure tone at a sound pressure level of 120 decibels for 48 h, all the short hair cells disappeared in the middle region of the auditory epithelium. Twelve hours to 1 day after exposure, mitotic cells, binucleate cells and PCNA-positive supporting cells were observed, and b-FGF immunoreactivity was shown in the supporting cells and glial cells near the habenula perforata. Spindle-shaped hair cells with immature stereocilia and a kinocilium appeared 3 days after exposure; these cells had synaptic connections with the newly developed nerve endings. The spindle-shaped hair cell is considered to be a transitional cell in the lineage of the supporting cell to the mature short hair cell. These results indicate that, after acoustic trauma, the supporting cells divide and differentiate into new short hair cells via spindle-shaped hair cells. Furthermore, it is suggested that b-FGF is related to the proliferation of the supporting cells and the extension of the nerve fibers.