Activation of satellite glial cells in trigeminal ganglion following dental injury and inflammation.

Satellite glial cells (SGCs), a peripheral neuroglial cell, surround neurons and form a complete envelope around individual sensory neurons in the trigeminal ganglia (TG), which may be involved in modulating neurons in inflammation. The purpose of this study was to determine the effect of dental injury and inflammation on SGCs in the TG. Pulp exposure (PX) was performed on the first maxillary molar of 28 rats. The neurons innervating injured tooth in TG were labeled by the retrograde transport of fluoro-gold (FG). Specimens were collected at 1, 3, 7, 14, 21 and 28 days after PX and stained immunohistochemically for glial fibrillary acid protein (GFAP), a marker of SGCs activation, in the TG. We observed that GFAP-immunoreactivity (IR) SGCs enclosed FG-labeled neurons increased in a time-dependent manner after PX. The neurons surrounded by GFAP-IR SGCs were mainly small and medium in size. The GFAP-IR SGCs encircled neurons increased significantly in the maxillary nerve region of the TG at 7-28 days following PX. The results show that dental injury and inflammation induced SGCs activation in the TG. It indicates that activation of SGCs might be implicated in the peripheral mechanisms of pain following dental injury and inflammation.

Mandible measurements and dental midline deviation after alveolar nerve transection in growing rabbits / Medidas mandibulares y desviación de la línea mediana dental luego de la transección del nervio alveolar inferior en conejos en crecimiento

The relationship between sensitive innervation and normal mandibular bone development has been described in the literature. Therefore, neural damage is a potential cause of osseous deformities, particularly in growing subjects. The aim of this project is to present the mandible measurements obtained after the transection of the inferior alveolar nerve of growing rabbits. A specific surgical protocol was designed to carry out the unilateral nerve transection by avoiding musculoskeletal injuries. Twenty New Zealand White rabbits one week post-weaning were used, 12 as an experimental group and 8 as a control group (Sham operated). The animals were sacrificed 90 days postoperatory, and the mandibles carefully dissected. Dental midline deviation data were obtained under anesthesia, previous to sacrifice. All measurements were obtained with a micron digital caliper. For this study, only anterior-posterior measurements were obtained from five points specifically determined on the rabbit mandible. Each measurement was made three times by the same examiner and the average value was considered. Regarding the anterior-posterior measurements, the molar and incisive regions of the denervated hemimandible were significantly shorter than the corresponding regions in the non-denervated side. The control group did not show these differences. A dental midline deviation was observed, but not always directed on the operated side. However, the deviation values were greater when oriented to the denervated side. These changes did not cause evident deformity or dysfunction in the masticatory system of the animals. They were fed normally and their weight was considered within normal parameters while growing. Despite the biological relationship between sensory inervation and bone morphology, the effect of sensory denervation in early stages of bone growth appears to generate only small alterations on the mandible morphology. However, these alterations do not lead to functional proble...

Se ha descrito una importante relación entre la inervación sensitiva y aspectos biológicos mandibulares. Consecuentemente, el daño a la estructura nerviosa es una causa potencial de alteraciones en el desarrollo mandibular, sobre todo en etapas de crecimiento. El objetivo de este trabajo es presentar mediciones mandibulares realizadas posterior a una lesión del nervio sensitivo en una etapa temprana de crecimiento. Se diseñó un protocolo quirúrgico para realizar la transección del nervio alveolar inferior sin lesionar estructuras musculoesqueléticas. Veinte conejos Neo zelandeses blancos fueron utilizados, una semana post-destete, 12 como grupo experimental y 8 como grupo control. Los animales fueron sacrificados 90 días después y las mandíbulas cuidadosamente disecadas. La información de la desviación de la línea media se obtuvo bajo anestesia, previo al sacrificio. Todas las mediciones se obtuvieron con un calibrador digital. Las medidas antero-posteriores se obtuvieron a partir de cinco puntos específicamente determinados en la mandíbula del conejo. En cuanto a las medidas antero-posteriores, las región molar e incisiva de las hemimandíbulas denervadas fueron significativamente menores que la correspondiente del lado no denervado. El grupo control no mostró esta diferencia. La línea media dental siempre se observó desviada, pero no siempre hacia el lado intervenido. Sin embargo, la media de desviación fue mayor cuando ésta se orientó hacia el lado denervado. Estos cambios no causaron deformidad evidente o disfunción en el sistema masticatorio de los animales, los cuales se alimentaron normalmente. El peso de los mismos fue considerado dentro de los parámetros normales. A pesar de la relación biológica entre la inervación sensitiva y la morfología del hueso, el efecto de la denervación sensitiva en las primeras etapas de crecimiento parece generar sólo pequeñas alteraciones en la morfología mandibular. Sin embargo, éstas no conducen a problemas funcionales...

Hypoglossal nucleus projections to the rat masseter muscle.

We investigated in the rat whether hypoglossal innervation extended to facial muscles other than the extrinsic musculature of the mystacial pad. Results showed that hypoglossal neurons also innervate the masseter muscle. Dil injected into the XII nucleus showed hypoglossal axons in the ipsilateral main trunk of the trigeminal nerve. After Gasser's ganglion crossing, the axons entered into the infraorbital division of the trigeminal nerve and targeted the extrinsic muscles of the mystacial pad. They also spread into the masseter branch of the trigeminal nerve to target the polar portions of the masseter muscle spindles. Retrograde double labelling, performed by injecting Dil into the pad and True Blue into the ipsilateral masseter muscle, showed labelled hypoglossal neurons in the medio-dorsal portion of the XII nucleus. The majority of these neurons were small (15-20 microm diameter), showed fluorescence for Dil and projected to the mystacial pad. Other medium-size neurons (25 microm diameter) were instead labelled with True Blue and projected to the masseter muscle. Finally, in the same area, other small hypoglossal neurons showed double labelling and projected to both structures. Functional hypotheses on the role of these hypoglossal projections have been discussed.

Differential expression of HNK-1 and p75(NTR) in adult canine Schwann cells and olfactory ensheathing cells in situ but not in vitro.

Olfactory ensheathing cells (OECs) are promising candidates for autologous cell transplantation therapies of nervous system injury and disease. Large animal models are relevant for transferring experimental data into clinical practice. In vivo studies have suggested that adult canine OECs may display similar regenerating capacities as their rodent counterpart. However, data on their molecular phenotype required for generating pure cell preparations are still scarce. In the present study, we comparatively analyzed expression of the carbohydrate HNK-1 epitope and the neurotrophin receptor p75(NTR) in adult canine Schwann cells and olfactory ensheathing cells in situ and in vitro. Myelinating and nonmyelinating Schwann cells in situ exclusively expressed HNK-1 and p75(NTR), respectively, whereas OECs were negative for both markers. In vitro, OECs and Schwann cells shared cell surface expression of p75(NTR) but not of HNK-1, which could be detected transiently in intracellular vesicles. This suggests that Schwann cells and OECs in vitro phagozytose HNK-1+ cellular debris. The cultivation-induced downregulation of HNK-1 expression in Schwann cells and upregulation of p75(NTR) in OECs argues for the possibility that axonal signals control the expression of both markers in situ. Whereas HNK-1 expression in Schwann cells is most likely controlled by signals inducing myelination, e.g., neuregulin, the mechanisms that may suppress p75(NTR) expression in OECs in situ remain to be elucidated. Interestingly, HNK-1 expression in the adult dog was found in both sensory and motor nerve myelinating Schwann cells. This is reminiscent of humans and differs from rodents; it also underscores the importance of large animal models for translational research.

Orbital passage of pterygopalatine ganglion efferents to paranasal sinuses and nasal mucosa in man.

Parasympathetic nerves of pterygopalatine ganglion origin are considered to enter the orbit and distribute to the nasal mucosa with the anterior ethmoidal nerve. As their distribution has never been demonstrated the present study was undertaken to seek evidence of their passage and to identify their relationship with the ethmoidal nerves. The soft tissues of the pterygopalatine fossa and orbit from sixteen sides of twelve cadavers were removed in one piece and either dissected or cut coronally into slabs and prepared histologically using montages of thin resin-embedded sections at intervals suitable for nerve path tracing. Several of the rami orbitales passing mediodorsally from the ganglion enter the orbit apically, branch and enter the posterior ethmoidal foramen terminating in the lining of the paranasal sinuses and others advance to enter the anterior ethmoidal canal to reach the nasal mucosa. No junctions were made with ethmoidal nerves within the orbit or the canal. Failure of surgical lesions of the anterior ethmoidal nerve as a treatment for vasomotor rhinitis may be attributed to the sparing of the separate parasympathetic nerves. Appropriate chemical lesions, on the other hand, could ensure destruction of isolated parasympathetic nerves while limiting damage to the larger anterior ethmoidal nerve.

Morphology, topography and cytoarchitectonics of the pterygopalatine ganglion in Egyptian spiny mouse (Acomys cahirinus, Desmarest).

Using the thiocholine method of Koelle and Friedenwald and histological techniques the pterygopalatine ganglion in Egyptian spiny mouse (Acomys cahirinus, Desmarest) was studied. The ganglion was found to be a single irregular cluster of neurocytes, situated on the medial surface of the maxillary nerve. The ganglion is composed of oval, elliptical and sometimes fusiform ganglionic neurones in compact arrangement without a thick connective-tissue capsule.

Immunohistochemical analysis of neuronal nitric oxide synthase in the trigeminal ganglia of the crotaline snake Trimeresurus flavoviridis.

The expression of neuronal nitric oxide synthase (nNOS) in the trigeminal ganglia (TG) of the infrared-sensitive crotaline snake Trimeresurus flavoviridis was studied immunohistochemically. The percentage of nNOS-positive (+) neurons in the TG was significantly higher (about 3.5-fold, P<0.001) in the mandibular division than in the infrared-sensory processing area (the maxillary division and ophthalmic ganglion). nNOS was found in varying sizes of TG neurons. However, nNOS (+) neurons were more abundant in small and large neurons than in medium-sized neurons, which include most of the infrared-sensitive neurons of the TG. These findings suggest that nNOS may be involved in normal physiological functions, such as the transmission of tactile, vibrotactile, and nociceptive sensations in the TG, rather than in infrared sensory processing in this species.

Neither peripheral nerve input nor cortical NMDA receptor activity are necessary for recovery of a disrupted barrel pattern in rat somatosensory cortex.

Elevating cortical serotonin (5-HT) in rats from postnatal day (P-) 0 to P-6 by administering the monoamine oxidase (MAO(A)) inhibitor, clorgyline, produces a dose-dependent spectrum of effects on rat somatosensory organization, ranging from enlarged with indistinct septa to a complete lack of vibrissae-related patterns. However, if clorgyline treatment is stopped on P-6, a qualitatively and quantitatively normal vibrissae-related pattern of thalamocortical afferents appears in somatosensory cortex (S-I) on P-10. We employed high performance liquid chromatography (HPLC), infraorbital nerve (ION) transection, N-methyl-D-aspartate (NMDA) receptor blockade, 1,1'-dioctadecyl-3,3,3"3'-tetramethylindocarbocyanine perchlorate (DiI) labeling of thalamic afferents, and CO histochemistry to determine whether peripheral nerve input and/or cortical NMDA receptor activity were required for the recovery of vibrissae-related patterns in clorgyline-treated animals. Clorgyline administration from P-0 to P-6 produced a 1589.4+/-53.3% increase in cortical 5-HT over control animals on P-6 and a 268.8+/-6.3% elevation over controls at P-10. Postnatal day 6 pups had significantly altered vibrissae-related patterns in S-I following 6 days of clorgyline treatment but by P-10, the characteristic vibrissae-related patterns were restored. Neither transection of the ION nor application of the NMDA antagonist, DL-2-amino-5-phosphonovaleric acid (APV), to the cortices of P-6 pups that were treated with clorgyline from birth had any significant effect on the recovery of the vibrissae-related patterns by P-10. These results indicate that neither peripheral nerve input nor cortical NMDA receptor activity are necessary for the restoration of cortical vibrissae-related patterns in rats that have sustained transient elevations of 5-HT.

Innervation of the rat pineal gland by PACAP-immunoreactive nerve fibers originating in the trigeminal ganglion: a degeneration study.

In order to establish that the pineal gland is innervated by pituitary adenylate cyclase-activating polypeptide (PACAP)-immunoreactive nerve fibers originating in the trigeminal ganglion, ophthalmic and maxillary nerves were transected by using a subtemporal fossa approach. The number of PACAP-immunoreactive nerve fibers in the pineal gland of rats with a total transection of the nerve was compared with that of rats without surgery. In the operated rat, PACAP-immunoreactive nerve fibers in the superficial pineal decreased remarkably, indicating that the trigeminal ganglion was the origin of these nerve fibers. This research provides evidence supporting the hypothesis that PACAP-immunoreactive nerves regulate the synthesis and/or secretion of melatonin in the pineal gland.

Deafferentation-induced apoptosis of neurons in thalamic somatosensory nuclei of the newborn rat: critical period and rescue from cell death by peripherally applied neurotrophins.

This study shows that unilateral transection of the infraorbital nerve (ION) in newborn (P0) rats induces apoptosis in the contralateral ventrobasal thalamic (VB) complex, as evidenced by terminal transferase-mediated deoxyuridine triphosphate-biotin nick end labelling (TUNEL) and electron miscroscopy. Double-labelling experiments using retrograde transport of labelled microspheres injected into the barrel cortex, followed by TUNEL staining, show that TUNEL-positive cells are thalamocortical neurons. The number of TUNEL-positive cells had begun to increase by 24 h postlesion, increased further 48 h after nerve section, and decreased to control levels after 120 h. Lesion-induced apoptosis in the VB complex is less pronounced if ION section is performed at P4, and disappears if the lesion is performed at P7. This time course closely matches the critical period of lesion-induced plasticity in the barrel cortex. Nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF), applied on the ION stump alone or in combination, are able to partially rescue thalamic neurons from apoptosis. Total cell counts in the VB complex of P7 animals that underwent ION section at P0 confirm the rescuing effect of BDNF and NGF. Blockade of axonal transport in the ION mimics the effect of ION section. These data suggest that survival-promoting signals from the periphery, maybe neurotrophins, are required for the survival of higher-order neurons in the somatosensory system during the period of fine-tuning of neuronal connections. We also propose that anterograde transneuronal degeneration in the neonatal rat trigeminal system may represent a new animal model for studying the pathways of programmed cell death in vivo.

An in vitro electrophysiological and Co2+-uptake study on the effect of infraorbital nerve transection on the cortical and thalamic neuronal activity.

Changes of neuronal membrane characteristics in somatosensory barrel cortex and barreloid thalamus were investigated in rats following unilateral transection of the infraorbital nerve. Kainate induced Co2+-uptake method and image analysis were used to assess the Ca2+ permeability of non-NMDA (N-methyl-D-aspartate) glutamate receptors. Changes in some biophysical parameters of the affected cortical neurons were also investigated by intracellular recording in slice experiments. The altered neuronal activity was measured on days 1, 5 and 14 after surgery. Kainate induced Co2+ uptake increased markedly reflecting enhanced Ca2+ permeability of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate/kainate (AMPA/KAIN)-type receptors. Changes were more pronounced in the cortex than in the thalamus and peaked on the first day following nerve transection. After that, parameters gradually returned to the normal level. However, a small enhancement was still detectable in the cortex at the end of the 2-week-long observation period. In parallel with the increased Co2+-uptake, moderate membrane potential changes, stronger spiking activity and enhanced excitability were characteristic for cortical neurons. The observed alterations in neuronal characteristics underlie the reorganization and regeneration processes following injuries or surgeries. We can conclude that immediate change of the receptive field in the barrel cortex following unilateral nerve transection is based on changes in biophysical parameters of the neurons. Altered peripheral activation evokes changes in the neuronal activity, thus providing opportunity for a quick synaptic rearrangement. AMPA/KAIN-type glutamate receptors have a decisive role in the regulation of these processes. This kind of synaptic plasticity is more significant in the cortex than in the thalamus.

A longitudinal study of the effects of prenatal ethanol exposure on neuronal acquisition and death in the principal sensory nucleus of the trigeminal nerve: interaction with changes induced by transection of the infraorbital nerve.

The present study determines (1) whether ethanol-induced microencephaly results from reductions in neuronal acquisition (i.e., cell proliferation and neuronal migration) and/or increases in neuronal death and (2) whether ethanol exacerbates death by the same mode as that for naturally occurring or lesion-induced neuronal death. Pregnant rats were exposed to a diet containing 6.7% (v/v) ethanol or an isocaloric control diet during the last two weeks of gestation. At birth, the right infraorbital nerves of the pups were transected. The numbers of neurons in the principal sensory nucleus of the trigeminal nerve (PSN) on both sides of the pons were examined at various prenatal and early postnatal timepoints. The numbers of pyknotic and argyrophilic PSN cells were also counted. Ethanol delayed and reduced (19.9%) the prenatal acquisition of PSN neurons. The postnatal decline in neuronal number (indicative of neuronal death) was significantly increased (10.6%) by ethanol. Likewise, the numbers of pyknotic and silver-stained cells were significantly higher in ethanol-treated rats. Lesion of the infraorbital nerve induced significant transsynaptic neuronal death in the control rats. Ethanol increased the amount of death caused by the lesion; however, it altered neither the timing of the neuronal loss nor the incidence of pyknosis or silver-staining. Therefore, ethanol affects both neuronal acquisition and survival; the greater effect being on neuronal acquisition. The timing and morphology of dying cells indicate that regardless of the cause (natural processes, ethanol-induced, or lesion-induced), neurons die in the developing PSN by the same mode.

Central projections of nerves innervating the rabbit maxillary sinus localized using wheat germ agglutinin-horseradish peroxidase or choleragenoid-horseradish peroxidase.

Central projections of nerves innervating the rabbit maxillary sinus were localized by using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or choleragenoid-horseradish peroxidase (B-HRP). Tracer was placed into the left maxillary sinus; rabbits were killed 3 or 5 days later, and histochemical localization of transported WGA-HRP or B-HRP was performed. Labeled cell bodies (437-545/animal) were seen in the ipsilateral trigeminal ganglion. Very few labeled cell bodies (zero to three/animal) were observed in the contralateral ganglion. The area of cell bodies labeled by WGA-HRP appeared similar to the area of cell bodies labeled by B-HRP. Transganglionic projections from either tracer were localized to lamina II of the ipsilateral subnucleus caudalis. In addition, WGA-HRP labeling was occasionally observed in lamina I. No labeling was present in other areas of the brainstem. In contrast to the above results, other studies have demonstrated that B-HRP produces terminal-like labeling in deeper layers of the gray matter. We injected B-HRP into the infraorbital nerve and sciatic nerve, which are known to contain projections to deep layers of the gray matter. Labeling was observed in the deep layers of the medullary or spinal dorsal horn 5 days later, suggesting that nerves innervating the sinus only project to superficial laminae. These results suggest that neurons in superficial laminae of the subnucleus caudalis may be important for the reflex initiation of the increased glandular secretions in the maxillary sinus during sinusitis.

Histometric study of myelinated fibers in the human trigeminal nerve.

The trigeminal ganglion, roots and the initial portion of the ophthalmic, maxillary and mandibular nerves were dissected in 3 cadavers, to study the number, area and composition of the fascicles, and the density and diameter spectra of myelinated fibers. The total number of fibers (x 1000) was 26 in the ophthalmic, 50 in the maxillary, and 78 in the mandibular division, 7.7 in the motor root and 170 in the sensory root. In all nerves, the histograms of fiber diameter had a bimodal distribution. Cutaneous and muscle nerve fascicles clearly differed in the fiber density and diameter. The ophthalmic and maxillary nerves (cutaneous) had similar fascicles, and their maximum fiber diameter averaged 14.5 microns. Most fascicles of the mandibular nerve (probably cutaneous fascicles) closely resembled those of the ophthalmic and maxillary nerves, but in some fascicles (probably muscle nerves) the fibers were larger, with a maximum diameter of 19.3 microns. The findings in the three peripheral divisions agree with electrophysiological data about sensory and motor conduction in human trigeminal nerves. The observation that the ophthalmic and maxillary nerves have similar fiber spectra indicates that a special fiber composition does not account for the sparing of the ophthalmic division in trigeminal neuralgia. The absence of very large (A alpha) fibers in the sensory root does not support the view that impulses from muscle spindles are conducted along this root.

Neonatal transection alters the percentage of substance-P-positive trigeminal ganglion cells that contribute axons to the regenerate infraorbital nerve.

Neonatal transection results in a marked reduction of the number of trigeminal (V) ganglion cells that contribute axons to the regenerate infraorbital nerve (ION; Jacquin and Rhoades, 1985; Chiaia et al., 1987). Such lesions also produce a profound deafferentation of the V brain stem complex that appears to spare the innervation of layers I and II of subnucleus caudalis (SpC) by substance-P-positive (SP-positive) primary afferents (Jacquin and Rhoades, 1985; Rhoades et al., 1988). In the present study, we combined retrograde tracing with immunocytochemistry to determine whether neonatal transection of the ION alters the percentage of SP-positive V ganglion cells that contribute axons to this V branch upon regeneration. In V ganglia ipsilateral to the intact ION (n = 8), 11.6% +/- 3.2% of the cells labeled after application of true blue (TB) to the ION were also SP-positive. In ganglia ipsilateral to the neonatally damaged nerve (n = 8), 18.6% +/- 4.7% of the cells labeled after application of TB to the regenerate ION were also SP-positive (p less than 0.001). We also compared the SP content of intact ganglia (n = 10) with that of ganglia ipsilateral to the damaged nerve (n = 10) by means of radioimmunoassay. The normal V ganglia contained (mean +/- SD) 3496 +/- 774 pg SP/mg protein. The value for the ganglia ipsilateral to the damaged nerve was 5533 +/- 1746 pg SP/mg protein (p less than 0.01). There was no significant difference between SP levels on the control and partially deafferented sides of the brain stem in neonatally nerve-damaged adult rats. In one additional experiment, we injected TB into both vibrissa pads of seven rats on the day of birth prior to transection of the ION. After an 8-hr delay, the nerve on one side was then cut and allowed to regenerate, and both V ganglia were then processed for immunocytochemistry. On the nerve-damage side, 25.8% of the TB-labeled cells were SP-positive. The value for the intact side was 12.0% (p less than 0.000001). This result demonstrated that the lesion-induced change in the percentage of SP-positive ION cells was not the result of either late-growing axons from SP-positive ganglion cells that may have been missed by our nerve cuts or collateral sprouting into the regenerate ION by undamaged SP-positive ganglion cells.

Cation-binding sites in trigeminal ganglia and maxillary nerve: unusual reactivity of perikarya, stem axons and satellite cells.

We have used the cupric/ferrocyanide reaction to study cation-binding in trigeminal ganglia and maxillary nerve of adult rats. Unmyelinated axons did not react, whereas myelinated axons were stained at nodal, paranodal or cleft sites. At 'nodal' sites, metallic deposits were found in the axoplasm, along the axolemma, and at the extracellular interfaces of the paranodal myelin. At 'paranodal' sites, particles were concentrated in the paranodal axoplasm and in the intracellular spaces of the myelin loops. Most maxillary axons examined at successive sites had all nodal or all paranodal staining, but 13 of 51 had a mixture. In trigeminal ganglia there was no staining of perineurial sheath, endoneurial cells or mast cells. Satellite cells and their basal laminae were prominently stained, with those around small neurons more reactive than those of large neurons. Patches of neuronal membrane on cell bodies were stained, more often for small than large neurons. The axon hillock and proximal stem axon were not stained in some cases, but approximately half the neurons had staining of perikaryal cytoplasm at the axon hillock or a dense asymmetric band in the proximal stem axon. Strong intraaxonal staining was found at the junction between unmyelinated proximal and myelinated distal stem axon. In distal stem axons, staining was found at the first myelin segment and at each successively thicker myelin segment; staining was mostly weak and paranodal, with intensity proportional to myelin thickness. The T-junction between stem and main myelinated axon had nodal or paranodal patterns; unmyelinated T-junctions were not stained. The varied cation-binding patterns in trigeminal ganglia show unusual properties of satellite cells and important differences between stem and main axons. The results that the cell membrane of axon hillock and proximal stem regions of many sensory large and small neurons may have numerous sodium channels and could affect signal propagation.

Development of Merkel cell populations with contrasting sensitivities to neonatal deafferentation in the rat whisker pad.

In this study, we used the quinacrine fluorescence technique to investigate the embryonic and early postnatal development of two distinct populations of Merkel cells in the rat whisker pad and the consequences of neonatal deafferentation on their subsequent development. Annular clusters of Merkel cells first appear in the epidermis near the caudal margin of the mystacial region between embryonic days E14 and E15 at dome sites located on horizontal ridges where the primordial vibrissal follicles develop. The development of these cells progresses in a caudorostral sequence across the whisker pad as does the development of the vibrissal follicles. Each cluster eventually forms a conical ridge or collar of about 130 Merkel cells that surrounds the vibrissal hair shaft as it penetrates the overlying pad epidermis. In the vibrissae, which develop as downgrowths from the horizontal ridges at the dome sites, Merkel cells first appear (caudally) between E16 and E17 and form a cylindrical cuff within the outer root sheath; cells are added progressively until about the end of the first postnatal week when a plateau level of about 750-800 cells is reached. Following unilateral transection of the infraorbital nerve at 24-36 hr after birth, these vibrissal Merkel cells continued to develop along a time course that was indistinguishable from normal, at least over the first 2 weeks of postnatal life. In contrast, all or most of the Merkel cells that normally develop within collars or annular clusters in the pad epidermis (around both the vibrissal and intervibrissal or pelage hairs) either disappeared within a few days or failed to develop. Other light and electron microscopic procedures supported the main findings and confirmed that the denervation was successful. Thus, the vibrissal Merkel cells, like those in the glabrous hindpaw, behaved as a distinct class which develops postnatally and is maintained (at least over a 2-week period) without the presence of sensory nerves. Since both the mystacial vibrissae and glabrous hindpaw have specialized cortical representations, a possible relationship between these findings and the organization of the somatosensory cortex during development is discussed.

Mesencephalic trigeminal sensory neurons of cat: axon pathways and structure of mechanoreceptive endings in periodontal ligament.

We injected 3H-proline into cat brainstem in order to label the entire mesencephalic trigeminal nucleus (Mes-V) for autoradiographic analysis of the size and pathways of Mes-V sensory axons and for microscopic study of Mes-V receptor structure in dental tissue. Labeled sensory axons were found in the trigeminal motor and sensory tracts and roots; approximately equal numbers of axons were found in both roots. The sensory root and all three divisions of the trigeminal nerve contained larger Mes-V axons than the motor root. Labeled Mes-V axons were found at the ganglion in the dorsomedial (infratrochlear) branch of the ophthalmic nerve but not in the ventrolateral branch. The mean diameter of Mes-V axons in periodontal ligament was 4.0 +/- 1.9 micron compared to 7.3 +/- 2.1 micron in maxillary and mandibular nerve, suggesting axonal arborization prior to innervation of ligament. Mes-V receptors in dental tissue were confined to ipsilateral periodontal ligament close to the root apex, with greater innervation on the posterior side. Receptor incidence was moderate for most teeth; however, maxillary first and second incisors and maxillary and mandibular canines had focal areas with remarkably dense innervation. No labeled axons were found in pulp of any ipsilateral teeth, and none was found in any contralateral dental tissue. EM-autoradiography demonstrated that Mes-V axons form unencapsulated Ruffini-like mechanoreceptors in periodontal ligament. The preterminal axons were small and myelinated. Neighboring bundles of unmyelinated axons and rare encapsulated endings were not labeled. The labeled mechanoreceptors branched to varying degrees among the ligament fibers; they contained numerous mitochondria and glycogen particles, as well as some vesicles and rare multivesicular bodies. They were surrounded by special Schwann cells that formed one or several layers around the ending. The endings were exposed to the basal lamina at numerous sites and occasionally extended fingers beyond the lamellar Schwann cells to contact ligament collagen.

Structure-function relationships in rat medullary and cervical dorsal horns. I. Trigeminal primary afferents.

Intracellular recording and horseradish peroxidase (HRP) labeling were used to examine structure-function relationships in the medullary dorsal horn (MDH) and rostral cervical dorsal horn. In Nembutal-anesthetized rats, 78 trigeminal (V) primary afferent fibers were physiologically characterized and injected with HRP. Axons were sufficiently well stained to reconstruct all of their collaterals in the MDH. Many also extended into the cervical dorsal horn. Except for four axons, which responded best to noxious stimuli, all responded at short (mean = 0.50 ms) latencies to V ganglion shocks and to innocuous stimulation. Forty-five of our recovered fibers were associated with facial vibrissae and responded in either a rapidly adapting, slowly adapting type I, slowly adapting type IIa, or slowly adapting type IIb fashion. The adequate stimuli consisted of either slow deflection, high-velocity deflection, or a noxious pinch of the vibrissa follicle. The collaterals of all of the above-described mystacial vibrissa primary afferents proceeded directly to their region of arborization in a plane perpendicular to the lateral border of the medulla to collectively form a largely continuous, circumscribed terminal column. This longitudinally oriented column of terminal and en passant boutons angled from lamina V rostrally to lamina III caudally. In the magnocellular laminae of the MDH, all mystacial vibrissa primary afferents gave rise to similarly shaped arbors, regardless of their functional classification. While morphological variability was observed both within and between individual axons, variance between functional classes was no greater than that within a class. Moreover, number of collaterals, number of boutons, or bouton size did not distinguish functional classes. Nonmystacial vibrissa afferent arbors, with more caudal peripheral fields, had their primary arbor focus in C1 and C2 dorsal horn. These arbors had relatively little rostrocaudal overlap with mystacial vibrissa afferents, though they exhibited the same lamina V-to-III shift as they descended through the cervical cord. Unlike mystacial vibrissa afferents in the MDH, their collaterals followed a tortuous course and often occupied laminae II-V in one transverse section. The relative location of each vibrissa afferent's terminal field could be predicted by the particular vibrissa innervated. Dorsal vibrissae afferents had ventrolateral terminations and ventral vibrissae afferents terminated dorsomedially. Rostral vibrissae were represented in the rostral MDH, whereas caudal vibrissae were represented in the caudal MDH and rostral cervical dorsal horn.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure-function relationships in rat medullary and cervical dorsal horns. II. Medullary dorsal horn cells.

In Nembutal-anesthetized rats, 31 physiologically identified medullary dorsal horn (MDH) cells were labeled with horseradish peroxidase (HRP). Ten responded only to deflection of one or more vibrissae. Six cells were activated by guard hair movement only, six by deflection of guard hairs or vibrissa(e), and seven by pinch of facial skin with serrated forceps. Different classes of low-threshold cells could not be distinguished on the basis of their somadendritic morphologies or laminar distribution. Neurons activated by multiple vibrissae were unique, however, in that one sent its axon into the medial lemniscus, and three projected into the trigeminal spinal tract. None of the guard hair-only or vibrissae-plus-guard hair neurons had such projections. Cells that responded best to noxious stimulation were located mainly in laminae I, II, and deep V, while neurons activated by vibrissa(e) and/or guard hair deflection were located in layers III, IV, and superficial V. Low-threshold neurons generally had fairly thick dendrites with few spines, whereas high-threshold cells tended to have thinner dendrites with numerous spines. Moreover, the dendritic arbors of low-threshold cells were, for the most part, denser than those of the noxious cells. Neurons with mandibular receptive fields were located in the dorsomedial portion of the MDH; cells with ophthalmic fields were found in the ventrolateral MDH, and maxillary cells were interposed. Cells sensitive to deflection of dorsal mystacial vibrissae and/or guard hairs were located ventral to those activated by more ventral hairs. Neurons with rostral receptive fields were found in the rostral MDH, while cells activated by hairs of the caudal mystacial pad, periauricular, and periorbital regions were located in the caudal MDH. Receptive-field types were encountered that have not been reported for trigeminal primary afferent neurons: multiple vibrissae; vibrissae plus guard hairs; and wide dynamic range. The latter two can be explained by the convergence of different primary afferent types onto individual neurons. Our failure to find a significant relationship between dendritic area (in the transverse plane) and the number of vibrissae suggests that primary afferent convergence may not be responsible for the synthesis of the multiple vibrissae receptive field. Excitatory connections between MDH neurons may, therefore, account for multiple vibrissae receptive fields in the MDH.