Ultrastructure of reorganising visual projections in half tecta of carp kept in constant light

In carp with the caudal half of the tectum cut off the projection of the whole contralateral visual field, mapped electrophysiologically, becomes compressed on to the remaining half tectum during the succeeding months. Ultrastructural study of tecta in various stages of reorganisation shows initially a profuse and widespread sprouting of unmyelinated axons in the optic nerve and external grey layers. This occurs before any compression of the projection is detectable electrophysiologically. After physiological compression is completed the bundles of unmyelinated fibres are greatly reduced and many myelinated axons are present. We conclude that compression initially involves the widespread growth of axon sprouts with the retention and myelination of a selected few when new connections are formed. Contrary to previous reports, exposure to constant light did not always delay or prevent the reorganisation of the visual projection. The widespread occurrence of optic terminal degeneration in both operated and normal tecta of fish kept in constant light suggests that light-induced retinal degeneration may have over-ridden the effect of constant light by providing vacant synaptic sites in the tectum for growing axonal sprouts.     

A comparative physiological and morphological study of periodontal ligament mechanoreceptors represented in the trigeminal ganglion and the mesencephalic nucleus of the cat

A correlative morphological study was carried out on two electrophysiologically identified and located periodontal ligament mechanoreceptors in anaesthetised cats. One mechanoreceptor had its cell body in the mesencephalic nucleus and the other had its cell body in the trigeminal ganglion. Physiological recordings were made from each of their cell bodies. The two receptors were located by punctate and electrical stimuli in the labial aspect of the periodontal ligament of the left mandibular canine tooth. Both receptors had similar positions relative to the tooth apex and fulcrum and were situated in the labial part of the ligament in each tooth. The receptor loci were marked, and these regions were studied in a series of semi-thin and ultra-thin sections. Only Ruffini nerve endings were observed under each ink mark. Both Ruffini nerve endings branched, were unencapsulated and were incompletely surrounded by terminal Schwann cells with extensions projecting towards collagen bundles. The results indicate that periodontal ligament mechanoreceptors with cell bodies in the mesencephalic nucleus and those with their cell bodies in the trigeminal ganglion can both be Ruffini nerve endings. Furthermore, there was no apparent morphological difference between the two periodontal ligament mechanoreceptors.     

Contacts between regenerating axons and the Schwann cells of sciatic nerve segments grafted to the optic nerve of adult rats

Summary The relation between Schwann cells, basal laminae and axons during retinal ganglion cell regeneration was studied by using cellular, acellular and partially acellular sciatic nerve autografts into the optic nerve. Acellular grafts were achieved by temporary compression which eliminates living Schwann cells and axons. The compressed sciatic nerve together with the intact portion was used as a partially acellular graft. The compressed portion was anastomosed to the optic nerve and the intact portion was situated distally. After 3–21 days post-operation, the grafts were studied by thin sectioning and freeze-fracture. Axons were seen to regenerate into cellular grafts in contact with Schwann cells after one week, but not into acellular grafts for the entire period. In the partially acellular grafts, regenerating axons were first observed after two weeks and were always in contact with Schwann cells migrating from the intact portion. Moreover, membrane specializations, fuzzy materials in the space between apposed membranes, and putative tight junctions, were found between regenerated axons including growth cone and Schwann cells, and between adjoining Schwann cells. An extensive meshwork of putative tight junctions was displayed between reforming perineurial cells surrounding the groups of Schwann cells and associated axons. Gap junctions were seen between adjoining Schwann cells, and between reforming perineurial cells. These results suggest that the axonal contact with Schwann cell surfaces plays an important role in retinal ganglion cell regeneration.     

Morphology of Purkinje cell axon terminals in intracerebellar nuclei following inferior olive lesion

We have examined the ultrastructural changes of axons and synaptic boutons in the intracerebellar nuclei of the rat at 3 days to one year after inferior olive lesion performed by means of electrocoagulation or 3-acetylpyridine injection. A large number of preterminal segments and axons terminals undergoes remarkable ultrastructural changes after total or subtotal olivary lesion. Large membrane bound vacuoles and clusters of small synaptic vesicles characterize a good number of these terminals at 3 days up to one month after the lesion. Tightly packed tubules and cisternae of smooth endoplasmic reticulum appear during the first week in an increasing number of axon terminals. Boutons with large whorled bodies formed by smooth membranes increase in number during the second half of the first month and further increase in density until the sixth month. They are still present in large amounts at one year. Immunoreactivity for 3',5'-guanosine-phosphate-dependent protein kinase, which is specific for Purkinje neurons, can be detected in the axons and synaptic terminals displaying the ultrastructural changes described above. These results are discussed in relation to a possible trophic action of the climbing fibers on the Purkinje cells. We suggest that, at least in part, these alterations may be the consequence of the intense Purkinje cell hyperactivity which is present for up to one month from inferior olive lesion.     

The effect of previous transection on quantitative estimates of the preganglionic neurones projecting in the cervical sympathetic trunk of the guinea-pig and the cat made by retrograde labelling of damaged axons by horseradish peroxidase

The numbers of sympathetic preganglionic neurones in the upper thoracic spinal cord retrogradely labelled after application of horseradish peroxidase to the severed axons of the cervical sympathetic trunk have been determined in guinea-pigs and in cats. Neuronal counts were made on both sides of spinal segments C8 to T10 in control animals and in others in which the enzyme was applied 5–8 days after the left cervical sympathetic trunk had been transected. Labelling was not significantly different between sides of unoperated animals, but after previous cervical trunk section labelling was always less, by from 5 to 100% of that on the control side. The results were not significantly different if the application of horseradish peroxidase was made close to the original lesion or 1 cm more proximal along the nerve trunk. Analysis of the dimensions of labelled cells suggested that the deficit in labelling was greatest for small preganglionic neurones.These findings do not support an earlier report¹⁴ that retrograde labelling from regenerating axons is enhanced at this stage after axotomy, and the possible reasons for the discrepancy in results are discussed. It is suggested that labelling of cell bodies may not be restricted if intact axon sprouts are exposed to horseradish peroxidase. Reduced labelling from axons at the time of a second transection might, at least in part, be due to axon atrophy, emphasizing limitations to labelling when horseradish peroxidase is applied to severed axons of fine diameter.The high numbers and reproducibility of our control data enabled estimates to be made of the segmental distribution of the cell bodies of origin of the axons of the cervical sympathetic trunk in both species.     

Up-regulation of GAP-43 and growth of axons in rat spinal cord after compression injury

Summary The growth-associated protein-43 (GAP-43) is an axonal phosphoprotein which is expressed at high levels during development and is reinduced by regeneration in the PNS. Consequently it is believed to be a key molecule in the regulation of axonal growth. However, injury to the CNS does not result in significant regeneration and this has been suggested to correlate with a failure of central neurons to up-regulate GAP-43 after axotomy. We have examined a model of spinal cord injury which is unique in two respects; first dural integrity is maintained by compression of the cord with smooth forceps (thus excluding connective tissue elements) and, secondly, considerable axonal growth has been reported through the resulting lesion. Our previous studies have shown that GAP-43 is extensively distributed in the rat spinal cord (see accompanying paper), but here we have used anti-GAP-43 antiserum at a dilution which did not yield any immunostaining in normal cord. However, supranormal levels of GAP-43 were detected in cell bodies and axons around the lesion within four days of compression injury. Double immunostaining with the RT97 monoclonal antibody indicated that a small subpopulation of neurons local to the site of compression were axotomized and expressed GAP-43 and phosphorylated neurofilament epitopes in their cell bodies. Although damage to long axon tracts was extensive, there was no evidence of regeneration in white matter. On the other hand cavities which formed in grey matter provided an environment for axonal elongation. Immunolabelling with markers for astrocytes and endothelial cells was used to evaluate the interaction of elongating axons with endogenous CNS cell types. Sprouting axons, identified by the presence of elevated levels of GAP-43, did not appear to grow in contact with astrocytes but preliminary evidence suggested that newly formed capillaries provided an appropriate substrate.     

In vivo proliferation,migration and phenotypic changes of Schwann cells in the presence of myelinated fibers

Following injury to a peripheral nerve, changes in the behavior of Schwann cells help to define the subsequent microenvironment for regeneration. Such changes, however, have almost exclusively been considered in the context of Wallerian degeneration distal to an injury, where loss of axonal contact or input is thought to be critical to the changes that occur. This supposition, however, may be incorrect in the proximal stumps where axons are still in contact with their cell bodies. In this work, we studied aspects of in vivo Schwann cell behavior after injury within the microenvironment of proximal stumps of transected rat sciatic nerves, where axons are preserved. In particular we studied this microenvironment proximal to the outgrowth zone, in an area containing intact myelinated fibers and a perineurial layer, by using double immunolabelling of Schwann cell markers and 5-bromo-2'-deoxyuridine (BrdU) labeling of proliferating cells.In normal sciatic nerve, Schwann cells were differentiated, in an orderly fashion, into those associated with unmyelinated fibers that labeled with glial fibrillary acidic protein (GFAP) and those associated with myelinated fibers that could be identified by individual axons and myelin sheaths. After sciatic nerve transection, there was rapid and early expansion in the population of GFAP-labeled cells in proximal stumps that was generated in part, by de novo expression of GFAP in Schwann cells of myelinated fibers. Schwann cells from this population also underwent proliferation, indicated by progressive rises in BrdU and GFAP double labeling. Finally, this Schwann cell pool also developed the property of migration, traveling to the distal outgrowth zone, but also with lateral penetration into the perineurium and epineurium, while in intimate contact with new axons.The findings suggest that other signals, in the injured proximal nerve stumps, beyond actual loss of axons, induce 'mature' Schwann cells of myelinated axons to dedifferentiate into those that up-regulated their GFAP expression, proliferate and migrate with axons.     

Evidence that dorsal locus coeruleus neurons can maintain their spinal cord projection following neonatal transection of the dorsal adrenergic bundle in rats

Summary In adult rats, locus coeruleus neurons which extend axons to the spinal cord are found only at mid-rostrocaudal levels of the nucleus, where they are essentially confined to its ventral, wedge-shaped half (Satoh et al. 1980; Westlund et al. 1983; Loughlin et al. 1986). However, during early postnatal development, coeruleospinal cells are found throughout the locus coeruleus (Cabana and Martin 1984; Chen and Stanfield 1987). This developmental restriction of the distribution of coeruleospinal neurons is due to axonal elimination rather than to cell death, since neurons retrogradely labeled through their spinal axons perinatally are still present in the dorsal portion of the locus coeruleus at survival periods beyond the age at which these cells lose their spinal projection (Chen and Stanfield 1987). I now report that if axons ascending from the locus coeruleus are cut by transecting the dorsal adrenergic bundle on the day of birth, a more widespread distribution of coeruleospinal neurons is retained beyond the perinatal period. These results not only indicate that the absence of the normally maintained collateral of a locus coeruleus neuron is sufficient to prevent the elimination of a collateral which would otherwise be lost, but also may imply that during normal postnatal development the presence of the maintained collateral is somehow causally involved in the elimination of the transient collateral.     

The response of noradrenergic axons to systemically administered DSP-4 in the rat: an immunohistochemical study using antibodies to noradrenaline and dopamine-beta-hydroxylase

The response of noradrenaline (NA) axons to the effects of systemic injections of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) was studied in the rat brain. Antibodies to NA and to dopamine-beta-hydroxylase (DBH) were employed to assess by immunohistochemistry the effects of DSP-4 on NA axons between 6 h and 2 weeks after drug administration. The changes in NA and DBH staining after DSP-4 treatment were restricted to brain regions innervated by the locus coeruleus. In these areas, DSP-4 induced profound loss of both NA and DBH from NA axons, but with a distinctly different time-course. While NA disappeared within hours after drug treatment, DBH staining of NA axons remained unchanged during the first 4 days after DSP-4 treatment. Thereafter, there was an abrupt loss of DBH staining which coincided with the appearance of numerous brightly stained, thick and swollen NA axons. The distribution of these fibres suggests that they represent the distal ends of preterminal NA axons. Two weeks after drug treatment, NA axons could no longer be visualized by either NA or DBH immunohistochemistry in regions affected by DSP-4. During this 2-week time-period, the staining of cell bodies in the locus coeruleus and of ascending NA axons in the dorsal bundle was unaffected. The results suggest two phases in the response of NA axons to DSP-4: an acute phase, marked by loss of transmitter, and a neurodegenerative phase, characterized by loss of DBH and structural disintegration of NA axons.     

Pathogenesis of blood-filled cavities in estrogen-induced anterior pituitary tumors in male Sprague-Dawley rats.

The formation of blood-filled cavities in developing tumors of the anterior pituitary of estrogen-treated male Sprague-Dawley rats was studied in a serial sacrifice experiment. Two treated and 2 control rats were killed at each of 15 time points ranging from 7-272 days after sc implantation of an estradiol-17 beta pellet. The pituitaries were examined using light and electron microscopy. Changes at 7-9 days after implantation included epithelial cell swelling and trabecular arrangement. At 11-13 days, epithelial cells were further enlarged. Arrangement of epithelial cells in islands and endothelial degeneration were first seen at this interval. Also, any sinusoids were distended, whereas some were compressed by swollen epithelial cells. At 16-81 days, scattered necrotic and immature epithelial cells were present, and cell size decreased. Endothelial degeneration and both distended as well as compressed sinusoids were more prominent at this time. Loss of basement membrane was first seen during this interval. At 114-133 days, small hemorrhagic areas partially lined by epithelium were first seen; sinusoidal compression, endothelial necrosis, and loss of basement membrane were more frequent, but there was less sinusoidal distention. Between 150 and 272 days, epithelial cells were increasingly pleomorphic and arranged in nodules, and there was an increase in number and size of the hemorrhagic areas. Sinusoidal compression, endothelial necrosis, and loss of basement membrane were abundant, whereas sinusoidal distention had almost disappeared at this interval. Local compression of sinusoids and perhaps compression of pituitary surface veins due to epithelial cell swelling, were thought to play a primary role in the development of ischemic endothelial damage leading to loss of endothelial lining and basement membrane, and eventually to the formation of blood-filled spaces partially lined by epithelial cells.     

Receptive-field properties of adult cat’s retinal ganglion cells with regenerated axons

Receptive-field properties of retinal ganglion cells (RGCs) that had regenerated their axons were studied by recording single-unit activity from strands teased from peripheral nerve (PN) grafts apposed to the cut optic nerve in adult cats. Of the 286 visually responsive units recorded from PN grafts in 20 cats, 49.7% were classified, according to their receptive-field properties, as Y-cells, 39.5% as X-cells, 6.6% as W-cells, and 4.2% were unclassified. The predominant representation of Y-cells is consistent with a corresponding morphological study (Watanabe et al. 1993a), which identified α-cells as the RGC type with the largest proportion of regenerating axons. Among the X-cells, we only found ON-center types, whereas both ON-center and OFF-center Y-cells were found. As in intact retinas, the receptive-field center sizes of Y-cells and W-cells were larger than those of X-cells at corresponding displacements from the area centralis. Within the 10° surrounding the area centralis, the receptive fields of X-cells with regenerated axons were larger than those in intact retinas, suggesting that some rearrangement of retinal circuitry occurred as a consequence of degeneration and regeneration. Receptive-field center responses of Y-, X-, and W-type units with regenerated axons were similar to those found in intact retinas, but the level of spontaneous activity of Y- and X-type units was, in general, less than that of intact RGCs. Receptive-field surrounds were weak or not detected in more than half of the visually responsive RGCs with regenerated axons. Received: 17 November 1997 / Accepted: 19 June 1998     

Patterns of functional synaptic connections in organized cultures of cerebellum

Organized cultures of mouse cerebellum with separated regions containing cortical, deep nuclear neurons and brain stem neurons from the peduncular zone were used for electrophysiological studies of axonal projections and synaptic interactions. Responses of single neurons of each of the regions were recorded extracellularly and intracellularly during localized electrical stimulation of other parts of the explant, and indicated extensive synaptic interactions. Cortical stimulation inhibited deep nuclear neurons, apparently monosynaptically, and frequently caused antidromic activation of these cells. Synaptic responses of brain stem neurons to cortical stimulation were usually excitatory, but these were often succeeded by inhibitory potentials. Since brain stem cells were often antidromically activated, the excitatory synaptic responses may be mediated by collaterals of antidromically stimulated brain stem axons. Stimulation of the deep nuclear region could evoke inhibitory or excitatory potentials in cortical neurons, the most frequent response being an excitatory postsynaptic potential which was followed in about 2 ms by an inhibitory potential. Most excitatory and some inhibitory postsynaptic potentials followed high frequency stimulation with constant latencies.The results indicate that within these cultures there are rich synaptic interconnections, many of which follow patterns resembling those seen in the intact brain. The monosynaptic inhibitory projection from the cortex to the deep nuclei and collateral inhibition by Purkinje cell axons appear to be features of cerebellar function that are reproduced in this culture model. In addition, a projection from the deep nuclei to the cortex recently described in the intact cerebellum is also present in the cultures and gives postsynaptic potential responses typical of excitatory afferents to the cerebellar cortex. Such cultures appear useful as an experimental model for the study of synaptic mechanisms or the effects of drugs in the mammalian CNS.     

Compartmentalisation of the developing trigeminal ganglion into maxillary and mandibular divisions does not depend on target contact

During development axons contact their target tissues with phenomenal accuracy but the mechanisms that control this homing behaviour remain largely elusive. A prerequisite to the study of the factors involved in hard-wiring the nervous system during neurogenesis is an accurate calendar of developmental events. We have studied the maxillary and mandibular components of the trigeminal system to determine the stages during embryogenesis when a gross somatotopic order is first established within the trigeminal ganglion and the axons projecting to the brainstem. The retrograde transganglionic fluorescent tracers DiO and DiI were injected into the maxillary and mandibular arches or their derivatives in fixed mouse embryos staged between 13 and 40 somites (E9–E11). After 1–4 wk, the distribution of the 2 tracers was determined using confocal laser scanning microscopy. The first maxillary nerve cell bodies and their developing axons were labelled at the 30 somite stage (E10). This was 2 somite stages earlier than the mesencephalic nucleus and the ganglion cell bodies of the mandibular nerve. The gross somatotopic division of cells within the trigeminal ganglion projecting to the maxillary and mandibular targets was established by the 32 somite stage (E10). This arrangement was evident as 2 groups of cell bodies occupying adjacent but separate regions of the trigeminal ganglion. The central branches of the maxillary and mandibular cell bodies entered the metencephalon as 2 distinct bundles at the same stage. The trigeminal motor nucleus was first detected at the 38 somite stage (E10.5).
Gross somatotopy in the major divisions of the trigeminal ganglion is established before outgrowing axons have contacted their peripheral target tissue at E10.5. This suggests that target tissues do not induce somatotopy.     

The ultrastructure of adrenergic neurons in sympathetic ganglia of the newborn rabbit after treatment with 6-hydroxydopamine

The effects of 6-hydroxydopamine, an analog of dopamine which produces degeneration of peripheral adrenergic nerve terminals in adult animals, on adrenergic neurons in the sympathetic ganglia of newborn rabbits were studied with the electron microscope. Animals were treated with a 50 mg/kg dose of 6-hydroxydopamine within six hours after birth and subsequently given daily doses until seven injections had been administered. The results of this study indicate that the adrenergic neurons underwent a “reaction” to 6-hydroxydopamine which was first manifested by an increase in filaments and smooth membranous structures in the cell bodies. In the axons of these neurons a similar increase in filaments and membranous elements, as well as accumulations of dark-core vesicles and mitchondria, were evident. Microtubules, although plentiful in normal cells and axons, were not conspicuous in these cells and axons. This initial reaction was later followed by degeneration of the cell bodies and axons and their eventual disappearance, leading to a reduction in cell population and size of the ganglia. It is suggested that the initial effect of 6-hydroxydopamine on adrenergic neurons (in newborn animals) may be an interference with their axoplasmic transport mechanism which leads to cell degeneration and death. This study also has shown that some of the small “granule-containing” cells in sympathetic ganglia may be sensitive to 6-hydroxydopamine, at least in newborn rabbits, as is indicated by their degeneration.     

An experimental animal model of spinal root compression syndrome: an analysis of morphological changes of myelinated axons during compression radiculopathy and after decompression

The treatment of radicular pain is mainly empirical because there are only few experimental studies dealing with morphological changes during compression radiculopathy. The goal of the study was to investigate changes in the morphology of myelinated axons during spinal root compression and the influence of decompression in a new rat model. The number of myelinated axons and their diameter were measured at 1, 2, 5, and 8 weeks during compression of the dorsal spinal root. The same approach was applied for 1-week compression followed by decompression for 1 or 2 weeks and compression for 5 weeks followed by 3-week decompression. A decrease in the number of myelinated axons (particularly those of large diameters) occurred after compression for 1 week. Continued compression for up to 8 weeks resulted in centripetal increase in the number of myelinated axons and the persistence of a small fraction of large myelinated axons at the site of compression. After that time, a decreased number of axons and a reduced fraction of large myelinated axons occurred again. Decompression after 1-week compression caused a rapid increase in the number of both small and large myelinated axons within the spinal root including the site of compression. A small fraction of regenerated axons was found after 5-week compression followed by 3-week decompression. Finally, we investigated the time course of the temporary increase in the number of regenerated myelinated axons during dorsal root compression for up to 8 weeks. The efficacy of decompression was superior when applied one week after compression or after regress of the acute phase of aseptic inflammation associated with fragility of spinal root. The results of the study verify the need for early surgical decompression to prevent irreversible damage of the spinal roots.     

Projections of intestinal neurons showing immunoreactivity for vasoactive intestinal polypeptide are consistent with these neurons being the enteric inhibitory neurons.

Experiments were performed to determine if the distribution of vasoactive intestinal peptide(VIP)-like immunoreactivity in nerve cell bodies and axons of the myenteric plexus and circular muscle of the small intestine is consistent with VIP being the transmitter of enteric inhibitory neurons. Immunoreactivity for VIP was found in nerve cell bodies of the myenteric plexus and in axons within the myenteric plexus and circular muscle. When the axons in the myenteric plexus were interrupted, there was accumulation of material showing reactivity for VIP on the oral side, indicating that the neurons project in an anal direction. The VIP-like immunoreactivity in axons which supply the circular muscle disappeared after a myectomy in which the overlying myenteric plexus was removed, but remained intact when extrinsic nerves were served. The projections of VIP neurons from the myenteric plexus to the circular muscle correspond to the expected projections of enteric inhibitory neurons determined by functional studies.     

Distribution of sacral afferent axons in cat urinary bladder

Terminal axon density was examined in five selected regions of the urinary bladder in five cats. The trigone region had more terminal axons than other bladder regions. Extra-terminal axons in the trigone were outside muscle fascicles and thought to be afferent axons related to the hypogastric nerve. In four of the cats, afferent axons of the pelvic nerve were identified by degeneration following sacral spinal ganglionectomy. The afferent axons were distributed equally to all regions of the bladder, implying that micturition sensitivity is not preferentially organized in the bladder. One-third of the sacral afferent axons crossed to the contralateral side of the bladder. This bilateral redundancy constitutes a safety feature. Afferent terminal axons were more numerous outside than inside muscle fascicles. Morphologically, afferent terminations outside muscle fascicles appeared to be tension receptors, while terminations inside the fascicles are candidates for volume receptors. The greatest number of degenerating terminal axons was found 14 days after ganglionectomy. Thirteen percent of degenerating axons contained agranular vesicles, and these were presumed to be autonomic postganglionic neurons with cell bodies in spinal ganglia.     

Alveolar type II cell responses to chronic inhalation of chrysotile asbestos in rats

The effects of chronic exposure to chrysotile asbestos on alveolar type II cells were examined in the lungs of Fischer 344 rats. Morphometric and three-dimensional analyses were used to characterize the alveolar type II cell and to determine the relationship of asbestos fiber localization to ultrastructural change in these cells. During the 2-yr period of study, type II cell number and volume increased to values more than 4 times those seen in controls. Ultrastructurally, cisternal dilations of the rough endoplasmic reticulum (RER) composed 12% of the total cell volume after 12 mo of exposure to asbestos and was still 15% of the total cell volume 1 yr after fiber exposure had ended compared to less than 1% in control cells. Asbestos fiber density surrounding these cells was directly proportional to the degree of cisternal dilatation in the cell; however, lamellar body volume and number in these cells were not different from that found in control type II cells. The incidence of a subset of type II cells with large lamellated inclusions was 10-fold greater in regions near bronchiolar-alveolar duct junctions, compared to more distal gas exchange regions of the lungs. Normal-sized lamellar bodies were fused to these large lamellated inclusions. These cells also contained significantly greater numbers of lamellar bodies and multivesicular bodies than those type II cells in more distal lung regions. These ultrastructural changes observed in type II cells may be a simple dose response to inhaled asbestos or the manifestation of two distinct populations of cells in the lungs that respond to asbestos in different ways. Asbestos fiber dose, cellular microenvironment, and aberrations of the cell plasma membrane and/or cell cytoskeleton (i.e., microtubules and filaments) are discussed as potential factors in the changes noted in type II cells.     

The association of octopamine with specific neurones along lobster nerve trunks.

Octapamine and its synthetic enzyme, tyramine beta-hydroxylase (TBH), are found in high concentrations at two points along second thoracic nerve roots in lobsters. The first is in the proximal section of the second root between the ventral nerve cord and the bifurcation of the root into medial (to flexor muscles) and lateral (to extensors) branches. The second region of high concentration is within a well known crustacean neurosecretory system, the pericardial organ, located close to the ends of the lateral branches of the roots. 2. With several different staining procedures, small clusters of nerve cell bodies are found within the connective tissue sheath in the proximal regions of the second roots. No cell bodies are seen in the pericardial organ regions. Cell bodies are variable in number and position between corresponding roots in the same animal and homologous roots among different animals. The average numbers of cell bodies, however, correlate well with TBH and octopamine content, and with the synthesis of octopamine in these same regions of roots. 3. Small clusters of root cell bodies dissected from preparations have greater than 500-fold higher activities of TBH than isolated efferent excitatory and inhibitory or afferent sensory axons. 4. Along with octopamine, the preferential synthesis of acetylcholine and serotonin is also seen in proximal segments of roots. Acetylcholine synthesis in these regions may represent transmitter synthesized in the nerve terminals innervating the root cells. The role of serotonin in these regions is not understood at this time but the amounts of endogenous serotonin found are only a tenth of the amounts of octopamine present. 5. Dopamine is not synthesized from tyrosine in second thoracic roots. However, if DOPA or dopamine are used as precursor compounds, then noradrenaline, which is usually not found in lobsters, can be accumulated in proximal segments of roots. 6. Phenolamines are converted to two further metabolites by lobster tissues. The compounds are unidentified and are named fast and slow product on the basis of their migration on electrophoresis at acid pH. Some partial characterization of slow product reveals that it is a mixture of compounds that can be converted on mild acid hydrolysis to fast product and the parent phenolamine. 7. The several lines of evidence presented suggest that nerve cells found in the proximal segments of the second thoracic roots contain and can synthesize octopamine. Since not all the cells in any single root have been analysed for octopamine or TBH, however, the possibility that one or more of the cells contain physiologically interesting substances other than octopamine is not eliminated.