Cytoarchitectonic analysis of the bullfrog (Rana catesbeiana) spinal cord by means of electron microscopy with special reference to distribution of microneurons

Cellular and spatial changes in the anuran superior olive across metamorphosis

In many vertebrate species, the superior olive in the auditory brainstem plays an essential role in sound source localization. Little is known, however, about the structural and functional changes in this nucleus during development when alterations in head size and shape as well as in inner ear projections are expected to affect the perception of binaural cues. Using stereological techniques, we investigated the changes in several cellular and spatial features of the bullfrog superior olive across metamorphosis, the time period during which the animal transforms from a totally aquatic larva to a semiterrestrial adult. The total number of cells shows a strongly linear increase from hatchling through late larval stages. The number of neurons decreases during metamorphic climax stages, and recovers to pre-metamorphic climax levels in the early post-metamorphic froglet stage. The number of glial cells increases during the early larval period, and remains relatively stable, with no systematic variation, from late larval to froglet stages. The volume of the superior olive increases rapidly in early larval stages, followed by a much-attenuated rate of growth between late larval and froglet stages. These morphological changes may provide a substrate for the functional restructuring of the bullfrog superior olive, shortly before the switch from aquatic to mostly atmospheric hearing.

Membrane currents and morphological properties of neurons and glial cells in the spinal cord and filum terminale of the frog

Using the patch-clamp technique in the whole-cell configuration combined with intracellular dialysis of the fluorescent dye Lucifer yellow (LY), the membrane properties of cells in slices of the lumbar portion of the frog spinal cord (n=64) and the filum terminale (FT, n=48) have been characterized and correlated with their morphology. Four types of cells were found in lumbar spinal cord and FT with membrane and morphological properties similar to those of cells that were previously identified in the rat spinal cord (Chvátal, A., Pastor, A., Mauch, M., Syková, E., Kettenmann, H., 1995. Distinct populations of identified glial cells in the developing rat spinal cord: Ion channel properties and cell morphology. Eur. J. Neurosci. 7, 129-142). Neurons, in response to a series of symmetrical voltage steps, displayed large repetitive voltage-dependent Na(+) inward currents and K(+) delayed rectifying outward currents. Three distinct types of non-neuronal cells were found. First, cells that exhibited passive symmetrical non-decaying currents were identified as astrocytes. These cells immunostained for GFAP and typically had at least one thick process and a number of fine processes. Second, cells with the characteristic properties of rat spinal cord oligodendrocytes, with passive symmetrical decaying currents and large tail currents after the end of the voltage step. These cells exhibited either long parallel or short hairy processes. Third, cells that expressed small brief inward currents in response to depolarizing steps, delayed rectifier outward currents and small sustained inward currents identical to rat glial precursor cells. Morphologically, they were characterized by round cell bodies with a number of finely branched processes. LY dye-coupling in the frog spinal cord gray matter and FT was observed in neurons and in all glial populations. All four cell types were found in both the spinal cord gray matter and FT. The glia/neuron ratio in the spinal cord was 0.78, while in FT it was 2.0. Moreover, the overall cell density was less in the FT than in the spinal cord. The present study shows that the membrane and morphological properties of glial cells in the frog and rat spinal cords are similar. Such striking phylogenetic similarity suggests a significant contribution from distinct glial cell populations to various spinal cord functions, particularly ionic and volume homeostasis in both mammals and amphibians.

Late-generated cells in the lateral motor columns of developing frog spinal cord

Spinal motor neurons of larval frogs continue to be generated for the lateral motor columns (LMCs) even after their peak numbers have been reached and during the major period of normal neuronal loss. This study elucidates the extent to which relatively late generated motor neurons survive to the end of larval development. Bromodeoxyuridine was injected into Rana pipiens tadpoles at mid-larval stages twice a day for 5 days in order to label LMC neurons arising during the period of greatest neuron loss. Counts of labeled neurons visualized by immunocytochemical methods revealed that substantial numbers of late generated motor neurons survive to the end of larval development and may be retained in greater proportion than early generated motor neurons (prior to the onset of normally occurring neuron loss). Tadpoles that had been exposed to exogenous thyroxine so as to increase proliferative activity and thereby label more motor neurons during the injection period, exhibited an increase in labeled LMC cells although the total number of neurons remained comparable to controls. Protracted neurogenesis concurrent with cell loss during the later phases of development is a major determinant of the final population of the LMC.

Primary afferent activity, putative excitatory transmitters and extracellular potassium levels in frog spinal cord

1. Changes in extracellular K+ activity were measured with ion-selective microelectrodes in the grey matter of the isolated hemisected frog spinal cord. The magnitude of the elevation of [K+]o (delta[K+]o) produced by repetitive stimulation (25 Hz, 10 s) of afferent fibres in the sciatic nerve was monotonically related to the strength of the electrical stimuli applied to the sciatic nerve. Repetitive stimulation of the largest diameter A alpha and A beta fibres, which were found histologically to comprise only 11% of the afferent axons in the dorsal root, elevated [K+]o to approximately 60% of the maximum level seen when all afferent fibres were stimulated. 2. Addition of Mg2+ (20 mM) to Ringer solution devoid of Mg2+ reduced delta[K+]o by over 85% suggesting that about 15% of delta[K+]o results from action potentials in presynaptic primary afferents. When 20 mM-Mg2+ was added to spinal cords bathed in Ringer solution containing a physiological (i.e. 1.0 mM) concentration of Mg2+, delta[K+]o was reduced by ca. 65-75% indicating that in spinal cords bathed in medium containing 'physiological' concentrations of Mg2+ about 25-35% of the K+ is released from primary afferent fibres. 3. Application of excitatory amino acids and agonists increased [K+]o with the following potency pattern: quisqualate greater than kainate greater than NMDA (N-methyl-D-aspartate) greater than glutamate greater than aspartate. 4. D(-)-2-Amino-5-phosphonovalerate (APV), an NMDA antagonist, reduced [K+]o by only about 50%, but kynurenate, an NMDA and non-NMDA antagonist, reduced [K+]o by approximately 85%; i.e. the same levels observed when synaptic transmission was blocked with 20 mM-Mg2+. These findings support the idea that synaptic release of excitatory amino acids such as L-glutamate and/or L-aspartate and subsequent activation of specific receptors by these putative transmitters are necessary for the postsynaptic component of delta[K+]o. 5. Addition of tachykinins elevated [K+]o but the effect appeared to require the participation of excitatory amino acids because it was blocked by APV and by kynurenate. 6. The finding that tetrodotoxin substantially reduced the ability of excitatory amino acid agonists and tachykinins to elevate [K+]o suggests that discharges in interneurones as a result of excitatory amino acid receptor activation are responsible for the postsynaptic component of delta[K+]o.

Motoneuron number in the lumbar lateral motor column of larval and adult bullfrogs

Motoneuron number in the lumbar lateral motor column of the bullfrog, Rana catesbeiana, was investigated through the course of premetamorphic development and in postmetamorphic frogs. Motoneurons were distinguished on the basis of histological characteristics into two classes, type L (less differentiated) and type M (more differentiated). The number of type L motoneurons on each side showed a precipitous decline between stages V and VI (6,300 to 2,500) and a slower rate of loss until stage XI (to 550). Type M motoneurons increased in number between stages V and VII (560 to 2,775) and declined precipitously between stages VII and VIII to a value similar to that of juvenile frogs (1,100). These changes in motoneuron number do not correspond to the formation of myotubes or to the appearance of contractile properties in hindlimb muscles. The development of myotubes in the hindlimb occurs only after total motoneuron number has declined by 35%. Similarly, hindlimb muscle contraction develops after the early decline in type L motoneuron number and is restricted to proximal thigh at the peak of type M motoneuron number. In postmetamorphic frogs, a weak (r = 0.44) but statistically significant correlation was found between type M motoneuron number and body length. In the largest frogs (greater than 15 cm body length), 1262 +/- 157 (mean +/- s.d.) motoneurons were present, whereas the smallest frogs (less than 5 cm body length) had 1099 +/- 98 motoneurons. These results are not consistent with previous findings that the variance of motoneuron number among small frogs is greater than that among larger frogs. The present results are thus inconsistent with explanations of size-related differences in motoneuron number that are based on selection of small frogs with greater number of motoneurons for survival. The increase in motoneuron number may be due to a slow addition of newly born motoneurons to the LMC or to the differentiation of existing motoneurons. The latter possibility is supported by the finding that the number of presumptive type L profiles is less in larger frogs.

Morphological analysis of astrocytes in the bullfrog (Rana catesbeiana) spinal cord with special reference to the site of attachment of their processes

An attempt was made to elucidate the morphological features of astrocytes in the bullfrog spinal cord by means of a combination of electron microscopy, the Golgi method, and the intravascular dye injection method. Astrocytic somata are densely concentrated both in the wall of the central canal and in its proximity, and diffusely distributed in the intermediate and the ventral part of the gray matter. The most complicated and densest vascular network is found in the dorsal part of the gray matter. There is little correlation between the density of the distribution of astrocytic somata and that of the vascular network. Each astrocyte emits one process and ramifies by repeated bifurcations as it approaches the white matter or enters it. All these branching processes reach the pial surface of the spinal cord (the principal processes). Total rostrocaudal extent of their ramification is within 400 micron. A great number of small lateral offshoots (the secondary processes) arise both from the somata and the principal processes. Electron microscopy of vessel walls and the pial surface revealed that the principal processes attach to the subpial basement membrane with a specialized structure, an electron-dense layer, while the secondary processes merely surround blood vessels in a mode of juxtaposition. Comparison between amphibian and mammalian astrocytes is made regarding the site of attachment of their processes.

Coronate cells: displaced amacrines of the rabbit retina?

The rabbit retinal ganglion cell layer contains a soma population which is morphologically distinct from the demonstrated ganglion cells. These "coronate cells" (Vaney, '80) have many features in common with classical neurons and are qualitatively different from typical glial cells. Detailed investigation by specific staining, ribonuclease treatment, and electron microscopy indicates that coronate cells are microneurons. The coronate cells contain somatic subsurface cisternae typical of rabbit amacrine cells, and are similar in appearance to some neurons of the amacrine cell layer. It is suggested that they represent, at least in part, a population of "displaced" amacrine cells.

Fine structural changes in nerve cell bodies of the adult rabbit dorsal motor vagal nucleus during axon reaction

Counts of neuronal nucleoli were made in the dorsal motor vagal nucleus (DMV) of the adult rabbit 10, 18, 70 and 90 days following unilateral cervical vagotomy. The structural characteristics of nerve cell bodies in the DMV were studied electron microscopically 2--90 days after cervical vagotomy. The nucleolar counts indicated that 20% of the large DMV neurones had disappeared ipsilateral to the operation 10 days postoperatively (p.o.), 65% 18 days p.o. and 70% 70 and 90 days p.o. No loss of small neurones was found. Large neurones ipsilateral to the operation showed nuclear displacement, infoldings of the nuclear membrane and disappearance of granular endoplasmic reticulum beginning 4 days p.o. and being prominent 6--18 days p.o. At the peak of the response, 10--18 days p.o., reacting neurones showed nucleolar condensation and vacuolation, the appearance of intranuclear electron-dense particles, extensive accumulation of intracytoplasmic lipid droplets, increased numbers of microtubules and neurofilaments, focal mitochondrial aggregates, and widespread mitochondrial degeneration. Ten to 21 days p.o. degenerating neurones were observed. After 30 days p.o. survival a partial recovery of surviving large DMV neurones seemed to have taken place. The findings are interpreted as indications of distubed protein metabolism, oxidative metabolism and intraneuronal transport in the axotomized DMV neurones. The unique response of these neurones compared to previously studied peripherally projecting neurones is emphasized.