Correlative histochemical and morphological study on the maturation of sensory ganglion cells in the rat

B-Afferents: A fundamental division of the nervous system mediating homeostasis?

The peripheral nervous system (PNS) has classically been separated into a somatic division composed of both afferent and efferent pathways and an autonomic division containing only efferents. J. N. Langley, who codified this asymmetrical plan at the beginning of the twentieth century, considered different afferents, including visceral ones, as candidates for inclusion in his concept of the “autonomic nervous system” (ANS), but he finally excluded all candidates for lack of any distinguishing histological markers. Langley's classification has been enormously influential in shaping modern ideas about both the structure and the function of the PNS. We survey recent information about the PNS and argue that many of the sensory neurons designated as “visceral” and “somatic” are in fact part of a histologically distinct group of afferents concerned primarily autonomic function. These afferents have traditionally been known as “small dark” neurons or B-neurons. In this target article we outline an association between autonomic and B-neurons based on ontogeny, cell phenotype, and functional relations, grouping them together as part of a common reflex system involved in homeostasis. This more parsimonious classification of the PNS, made possible by the identification of a group of afferents associated primarily with the ANS, avoids a number of confusions produced by the classical orientation. It may also have practical implications for an understanding of nociception, homeostatic reflexes, and the evolution of the nervous system.

A method for peripheral chromatolysis in neurons of trigeminal and dorsal root ganglia, produced in rats by lithium

In classical chromatolysis, Nissl bodies are initially lost centrally with subsequent progression to the periphery of the neuron. Peripheral chromatolysis has the opposite pattern; it is less common and more difficult to produce. We describe a new method for producing peripheral chromatolysis in neurons of trigeminal ganglia and dorsal root ganglia that requires only injection of large doses of lithium chloride (LiCl) for two, three or four consecutive daily doses. This method may be useful for elucidating the intraneuronal mechanisms that control the location and structure of the Nissl bodies.

Ontogeny of calretinin expression in rat dorsal root ganglia

The relationship between the expression of calretinin and the maturation level of peripheral sensory neurons was investigated by means of immunohistochemistry and immunoblot analysis. Our immunohistochemical results show that calretinin is expressed during two different developmental phases in rat dorsal root ganglia. The early phase lasts between embryonic days 11 and 14, when calretinin is detectable in the majority (75%) of the cells. A second phase starts at embryonic day 17 and lasts throughout the whole postnatal life, when calretinin is expressed only in a small proportion of the neurons (less than 8%). Between these two periods no calretinin is found in the ganglia. These changes in calretinin expression during embryonic development were confirmed by Western blot analysis. The early expression of calretinin in dorsal root ganglion cells suggests that calretinin may act as a calcium regulator until neurotrophins take over the precise tuning of intracellular calcium concentration.

Chemoattraction of sensory neuron growth cones by diffusible concentration gradients of acetylcholine

Axon guidance cues are critical for the development and repair of both the central and peripheral nervous systems. These cues serve to help select the pathways taken by axon growth cones, by attracting or repulsing them. During development and following injury to the adult peripheral nervous system, neurons must extend processes, often over long distances, through a variety of cellular environments composed of innervated and uninnervated cells, to find, recognize, and synapse on their appropriate targets. The responsibility for recognizing and responding to the extensive number of cues that are encountered as axons elongate falls on the growth cones at the tip of the elongating axons. Cajal (1928) proposed that denervated target cells release diffusible factors that assist in orienting the direction of out-growth of peripheral axons. However, it is only relatively recently that experiments to identify the molecules responsible for serving this function, and the molecular mechanisms by which they function, have begun to bear fruit. Gradients of both substrate-bound and diffusible factors have now been shown to play critical roles in directing axon outgrowth. The present experiments were aimed at determining whether the neurotransmitter acetylcholine (ACh) can act as a chemoattractant for adult sensory neuron growth cones.

Inverse correlation of acetylcholinesterase (AChE) activity with the presence of neurofilament inclusions in dorsal root ganglion neurons cultured in the presence of a reversible inhibitor of AChE

We have previously shown that treatment of cultured dorsal root ganglion neurons (DRGN) with a highly specific, reversible acetylcholinesterase (AChE) inhibitor, BW284c51, retards neuritic outgrowth in a dose dependent manner and is accompanied by the presence of abnormal, perikaryal neurofilament (NF) inclusions in approximately 40% of the cells. Since subpopulations of DRGN have been classified according to their levels of AChE activity, we have combined immunocytochemical and enzyme histochemical techniques to investigate a possible correlation between AChE activity and the presence of NF inclusion formation. Our results show that after inhibitor treatment, cells with low levels of AChE activity have a greater percentage of inclusions, with nearly 75% of cells with undetectable levels of AChE activity containing inclusions. In contrast, inclusions were present in only 3.2% of cells with high levels of AChE activity. This inverse relationship between AChE activity and the presence of NF inclusions supports our previous observations that this enzyme may have extra-synaptic functions which could affect neuronal development and regeneration.

Fixation and occurrence of dark and light cells in taste buds of fungiform papillae

The fine structure of taste buds of the fungiform papillae of the Old World monkey Cynomolgus (Macaca fascicularis) was found to vary with the primary fixative solution used. Standard primary fixative solutions of glutaraldehyde as well as fixatives containing both formaldehyde and glutaraldehyde gave non-reproducible and usually poor results. Osmium tetroxide in distilled water gave reproducible fixation of acceptable quality. Further, when fixed in osmium tetroxide in distilled water, taste buds of the fungiform papillae did not show the dark and light cells well known from other studies. It is therefore suggested that the terms dark and light cells not be used as synonyms for receptor or support cells, since the appearance of taste bud cells is affected by fixation.

Comparative dynamics of retrograde transport of nerve growth factor and horseradish peroxidase in rat lumbar dorsal root ganglia

The dynamics of the retrograde transport of [125I] nerve growth factor (NGF) and horseradish peroxidase (HRP) in dorsal root ganglion (DRG) neurons were studied in rats. After injection of [125I]NGF or HRP into crushed sciatic nerve, labelling was examined in spinal nerves, dorsal root ganglia, dorsal roots and spinal cord. Retrograde transport of either [125I]NGF or HRP was first observed in DRG neurons 6 h after injection: The maximal rate of transport (7 mm h-1) was similar for both proteins. Significant differences in the sizes of DRG neurons labelled by [125I]NGF were observed and were dependent upon survival time. No such difference was seen in HRP-injected animals. At 6 h after injection, 60% of all the HRP-labelled cells had a diameter of more than 25 micron, whereas 90% of all the [125I]NGF-labelled neurons had a diameter of less than 25 micron. With increasing survival times there was a gradual shift in the size of [125I]NGF-labelled neurons towards larger diameters. Thus, 24 h after the [125I]NGF injection, 83% of the labelled cells had a diameter greater than 25 micron. The data suggest that small diameter neurons retrogradely transport and turnover NGF faster than larger diameter neurons. There was a preferential accumulation of silver grains in small DRG neurons (mean diameter 25 micron) at early survival times (4 and 8 h); at the later survival time (24 h) the reverse was observed, i.e. larger neurons (mean diameter 42 micron) were labelled. In contrast, the mean diameter of HRP-labelled neurons remained constant (30 micron) at all times after injection. The total number of neurons ultimately labelled (approximately 80-85%) appeared to be the same with both tracers. In addition, the lack of transganglionic transport of NGF into the spinal cord and the short time span of the observable accumulated radioactivity in DRG neurons suggest the rate of degradation of transported NGF seems to be faster than HRP. As a practical matter, these data indicate that observing cells within DRG which accumulate retrogradely transported [125I]NGF at any one time gives an inaccurate picture of the size properties of cells capable of transporting the ligand.

Ultrastructural features of six types of neurons in rat dorsal root ganglia

Sections of glutaraldehyde-fixed lumbar dorsal root ganglia of the rat were examined in the electron microscope following impregnation with the uranyl-lead-copper technique or postfixation in potassium ferrocyanide-reduced osmium. Three types of ganglion cells (A, B, C) were identified on the basis of their size and the distribution of their organelles. They were further subdivided into six subtypes according to the arrangement and three-dimensional organization of the Nissl bodies and Golgi apparatus in the perikarya. Type A1 cells were large, clear neurons in which Nissl bodies, separated from each other by pale narrow strands of cytoplasm containing small stacks of Golgi saccules and rod-like mitochondria, were evenly distributed throughout the perikaryon. In type A2, the Nissl bodies assumed a similar distribution but were separated by much wider strands of cytoplasm. Type A3, the smallest of the type A category, displayed densely packed Nissl bodies and long stacks of Golgi saccules which formed a perinuclear ring in the midportion of the perikaryon. Type B cells were smaller and showed a concentric zonation of their organelles. In type B1, large Nissl bodies located in an outer cytoplasmic zone were made of long piles of parallel cisternae interrupted by curved Golgi stacks. Type B2 was characterized by a ring-like Golgi apparatus separating the perikaryon in a cortical zone composed mainly of Nissl substance and a juxtanuclear zone containing mitochondria and smooth endoplasmic reticulum. Type C cells were the smallest of the ganglion cells and contained small, poorly demarcated Nissl bodies and a juxtanuclear Golgi apparatus.

Neurogenesis in the trigeminal ganglion of the albino rat: a quantitative autoradiographic study

The time of neuron origin in the trigeminal ganglion was examined in autoradiograms of 60-day-old rats that were exposed to a single pulse of 3H-thymidine on day 11, 12, 13, 14, or 15 of gestation. Heavily labeled neurons, representing cells in or near their last mitotic division at the time of the pulse label, were present in animals injected between embryonic days 11 and 13 with a peak on day 12. Within this time period, larger neurons were generated prior to smaller neurons with a peak for larger cells on day 12 and for smaller cells on day 13. Thus, the majority of trigeminal ganglion neurons are generated over a three-day period just after the midpoint of gestation. Neuron number, size, type, and cytoarchitectural organization were also examined in the ganglion. The mean neuron count per ganglion was 52,372. The size distribution of these cells ranged continuously from 7-61 microns (mean diameter) with no evidence for clearly defined subpopulations. The staining intensity and distribution patterns of the Nissl substance varied greatly from cell to cell precluding the classification of cells as light or dark. Little correspondence between these Nissl features and cell size was found. Among the clusters and rows of neurons in the ganglion, we did not see consistent cytoarchitectonic patterns which might reflect specific sensory receptive fields.

Histochemical study of the cholinesterase activity in cat myenteric ganglia during postnatal development

The changes in the cholinesterase activity of cat myenteric ganglia during the postnatal development are studied histochemically. During early postnatal development, the increase in enzyme activity is particularly pronounced in the perikarya of the myenteric neurones. Intensification of the reaction in the neuropile is characteristic after the 14th postnatal day and also persists in adult animals. The localization and intensity of the histochemical reaction in the individual neurones of a single ganglion is variable. The changes in enzyme activity are not identical in dynamics and localization in the myenteric ganglia in the areas of the alimentary tract examined, this difference being observed throughout the entire postnatal development.

Somatotopic and functional organization of the avian trigeminal ganglion: an HRP analysis in the hatchling chick

While the somatotopic organization of many central systems is well characterized, that of peripheral sensory neurons has not been adequately defined. This is especially true for the trigeminal ganglion. By applying HRP subcutaneously at each of 14 sites and also intramuscularly, it is possible to determine whether the location of sensory neurons within the ganglion reflects their peripheral projections. There is no discernible somatotopic organization of neurons in the ophthalmic lobe. However, the location of maxillary neurons in the maxillo-mandibular lobe is organized with the most posterior cells projecting to sites closest to the ganglion and with neurons located more anteriorly projecting to progressively more distant sites. There is a less well defined organization in the superior-inferior axis of the ganglion, and none along its proximal (root) to distal axis. Mandibular exteroceptive neurons are found primarily in the anterior region of the maxillo-mandibular lobe, while mandibular proprioceptive cells are located in the proximo-central part of this lobe. In most cases there is a considerable scattering of horseradish peroxidase (HRP)-filled neurons. Projections to contralateral ganglia, the trigeminal motor nucleus, and the trigeminal mesencephalic nucleus were also examined. Cytologically, the hatchling trigeminal consists of two interspersed types of neurons: large, lightly staining and smaller, darkly staining cells. Previous experiments have proved that these two cell types do not correspond to each of the embryonic precursors of trigeminal neurons, the neural crest and placodal cells. In this study HRP was found localized in both classes of neurons following injection at all sites, including jaw-closing muscles. This indicates that the dual cytology is not correlated with either distribution of peripheral fibers or exteroceptive vs. proprioceptive functions. The possibilities that the two types of neurons may have different central projections and/or may be related to visceral vs. somatic afferent functions are discussed.

The postnatal development of large light and small dark neurons in mouse dorsal root ganglia: a statistical analysis of cell numbers and size

A method is described for the analysis of cell types in mouse dorsal root ganglia using the distribution of cell cross-sectional areas measured at the level of the nucleolus in 1.5 micron Epon sections. Using a computer program it was possible to demonstrate the existence of two normally distributed sub-populations of neurons in all the 3rd lumbar segment ganglia (17 in number) measured at various ages from birth to 70 days. The two populations appeared to correspond with large light cells and small dark cells. The large light cell bodies increased in size until about 20 days postnatal, subsequently their size decreased whereas the mean size of the small dark cells reached a plateau by about day 10. The relationship of both nuclear volume and surface area to the surface area of the perikaryon differed between light and dark cells. The number of neurons in L3 remained virtually constant at about 6000 throughout the period examined. Since the proportion of neurons in each population was not shown to change with age there was no evidence that cells could change from one type into the other.

Pre- and postnatal development of catecholamine-containing and cholinesterase-positive nerves of the rat cornea and iris

Glyoxylic acid-induced fluorescence technique and a copper thiocholine method were used to investigate the ontogenesis of the catecholamine-containing and cholinesterase-positive nerves of the rat iris and cornea. First fluorescent nerve fibres appeared in the iris on the 18th gestation day and in the cornea on the 19th day. A rapid increase in the density of the adrenergic nerve fibres of the iris continued to the age of three weeks, while the number of such fibres were small in the cornea. Acetylcholinesterase-positive fibres appeared both in the cornea and in the iris on the 19th gestation day. Their density increased more rapidly in the iris, especially in the sphincter muscle, than in the cornea. Non-specific cholinesterase activity was localized in the Schwann cells and the reaction was more intense during development than in the nerves of the cornea of adult rats.

Gustatory pathways in the bullhead catfish. 1. Connections of the anterior ganglion

The central projections of the external gustatory system in bullhead catfish were examined using orthograde degeneration and retrograde transport of horseradish peroxidase (HRP) techniques. Both large and small cells were observed in the anterior ganglion which contains a mixture of elements from the trigeminal, facial and anterior lateral line nerves. Some of the large cells on the lateral margin of the ganglion were found to belong to the lateral line system. No separation of trigeminal and facial nerve connections could be made. Using HRP, the relation between the barbels and specific ganglion regions was determined. The dorsalmost portion of the ganglion received recurrens nerve inputs (from taste buds on the trunk); the rostromedial portion of the ganglion, from the maxillary and mandibular barbel nerves. The facial lobe (similar to part of the n. solitarius) was found to be divided into lobules by fascicles of nerve fibers. The lateral lobule received input only from the dorsal-most part of the ganglion (recurrens nerve: trunk receptors); the intermediate lobule from the rostro-lateral part of the ganglion (nasal barbel); and the medial lobule from the ventral areas of the ganglion (maxillary and mandibular barbels). Thus a topographical relationship exists between the different taste receptor groups and their locus of representation in the facial lobe. The trunk receptors connect to the lateral lobule; the nasal barbel receptors to the intermediate lobule, and the maxillo-mandibular receptors to the medial lobule.

Specificity of retrograde transport of nerve growth factor (NGF) in sensory neurons: a biochemical and morphological study

In previous studies it has been shown that nerve growth factor (NGF) is taken up with a high selectivity by adrenergic nerve terminals and is transported retrogradely to the perikaryon11,22. It was the aim of the present experiments to investigate whether the sensory neurons exhibit the same high degree of selectivity for retrograde transport throughout the whole life cycle, although it is known that their dramatic response to NGF is confined to a short period of ontogenetic development. Unilateral injection of [125I]NGF into the forepaw of adult rats was followed by a preferential accumulation of radioactivity in the sensory ganglia (C6-C7) of the injected side. However, this preferential accumulation was not detectable earlier than 6 h after injection and reached a maximum (ratio between injected and non-injected side, 5:1) after 11-16 h. Transection of the plexus brachialis abolished and local administration of colchicine prior to that of [125I]NGF greatly reduced the preferential accumulation of radioactivity in the ganglia of the injected side. The rate of retrograde transport of NGF in sensory neurons was calculated to be 13 mm/h which is about 5 times faster than that in adrenergic neurons. The selectivity of this retrograde transport was demonstrated by the fact that injection of 125I-labeled bovine serum albumin and cytochrome c did not result in a preferential accumulation of radioactivity in the sensory ganglia of the injected side. Light microscopic autoradiography revealed heavily labeled cells in the sensory ganglia (C6-C7) of the injected side after administration of [125I]NGF into the forepaw. Only cells belonging to the large cell type were labeled. Prolonged (7 mug/g/day over 5 days) injection of NGF into the forepaw of 10-day-old rats did not result in a hypertropic response of the sensory neurons as far as can be judged from morphometric studies at the light microscopic level.