Degeneration and regeneration of some mechanoreceptors. An ultrastructural study. II. Ultrastructure of denervated Grandry corpuscles

The ultrastructure of denervated Grandry corpuscles reveals the retrograde character of degenerative changes in the receptor nerve fibre. The denervation affects also the specific cell elements: the Schwann receptor cells and specialized cells. The both cell types pass through the phase of activation followed by degeneration and elimination. The process is significantly prolonged by the specialized cells. The Schwann receptor cells appear more dynamic and sensitive elements in the process of degeneration. After the full elimination of the specialized cells the denervated receptors cannot be established.     

THE INTRAEPIDERMAL INNERVATION OF THE SNOUT SKIN OF THE OPOSSUM : A Light and Electron Microscope Study, with Observations on the Nature of Merkel's Tastzellen

The intraepidermal innervation of the snout skin of the opossum has been studied with the light and electron microscope. Numerous large nerve fibers loose their myelin sheath in the superficial dermis and pass into the epidermis. The basement membranes of the epidermis and Schwann cell become continuous at the point of entry of the neurite into the epidermis. Within the epidermis, the neurite is associated with a specialized secretory epidermal cell, termed a Merkel cell. This cell has many secretory granules apposed to the neurite. The Merkel cells are epidermal cells since they have desmosomes between them and adjacent epidermal cells. The neurite in the stratum spinosum is enveloped by Schwann cells in a manner analogous to the Schwann cell investment of unmyelinated neurites. In the upper stratum spinosum the nerve fiber evidences changes which can be interpreted as degenerative. The Merkel cell-neurite complex is interpreted as representing a sensory receptor unit.     

Volumetric and horseradish peroxidase tracing analysis of rat olfactory bulb following reversible olfactory nerve lesions

Olfactory receptor neurons can regenerate from basal stem cells. Receptor neuron lesion causes degenerative changes in the olfactory bulb followed by regeneration as new olfactory receptor axons innervate the olfactory bulb. To our knowledge, parametric analyses of morphometric changes in the olfactory bulb during degeneration and regeneration do not exist except in abstract form. To better characterize olfactory bulb response, we performed morphometric analysis in rats following reversible olfactory nerve lesion with diethyldithiocarbamate. We also performed anterograde tracing of the olfactory nerve with wheatgerm agglutinin linked to horseradish peroxidase. Results of morphometry and tracing were complementary. The glomerular layer and external plexiform layer showed shrinkage of 45 and 26%, respectively, at 9 days. No significant shrinkage occurred in any other layer. Individual glomeruli shrank by 40-50% at 3 and 9 days following lesion. These data show that degenerative changes occur both in the glomeruli and transneuronally in the external plexiform layer. Olfactory nerve regeneration (identified by WGA-HRP transport) paralleled volumetric recovery. Recovery occurred first in ventral and lateral glomeruli between 9 and 16 days followed by recovery in medial and dorsal glomeruli. These data indicate substantial transynaptic degeneration in the olfactory bulb and a heretofore unrecognized gradient in olfactory nerve regeneration that can be used to systematically study recovery of a cortical structure.     

RAPID DEGENERATION OF AMPULLARY ELECTRORECEPTOR ORGANS AFTER DENERVATION

Electroreceptors (ampullary organs) of the transparent catfish (Kryptopterus bicirrhus) lie in the epidermis, and contain spherical receptor cells that receive purely afferent innervation from the lateral line nerve. Section of this nerve causes rapid degenerative changes to occur in the receptors. Fine structural alterations occur in the receptor cell synapses and nerve fiber 6–12 h postoperatively. Disruption of the receptor cells begins by 18 h and most are lost by 48 h. By 72 h supporting cells and secretory cells also show marked degeneration, and by 96 h they may be totally lost. The rapid degeneration of the electroreceptor organs of Kryptopterus should make them a useful preparation for analysis of neurotrophic functions.     

The effects of normal aging on myelin and nerve fibers: A review

It was believed that the cause of the cognitive decline exhibited by human and non-human primates during normal aging was a loss of cortical neurons. It is now known that significant numbers of cortical neurons are not lost and other bases for the cognitive decline have been sought. One contributing factor may be changes in nerve fibers. With age some myelin sheaths exhibit degenerative changes, such as the formation of splits containing electron dense cytoplasm, and the formation on myelin balloons. It is suggested that such degenerative changes lead to cognitive decline because they cause changes in conduction velocity, resulting in a disruption of the normal timing in neuronal circuits. Yet as degeneration occurs, other changes, such as the formation of redundant myelin and increasing thickness suggest of sheaths, suggest some myelin formation is continuing during aging. Another indication of this is that oligodendrocytes increase in number withage. In addition to the myelin changes, stereological studies have shown a loss of nerve fibers from the white matter of the cerebral hemispheres of humans, while other studies have shown a loss of nerve fibers from the optic nerves and anterior commissure in monkeys. It is likely that such nerve fiber loss also contributes to cognitive decline, because of the consequent decrease in connections between neurons. Degeneration of myelin itself does not seem to result in microglial cells undertaking phagocytosis. These cells are probably only activated when large numbers of nerve fibers are lost, as can occur in the optic nerve.     

Lactic dehydrogenase activity in degenerative neoplastic glial cells after actinomycin application in vitro.

Glial tumors of glioblastoma type, cultured in vitro, have been exposed between the 7th and 14th day of growth to actinomycin C and K in a concentration of 5 x 10(-6) M. The developing degenerative changes in the neoplastic cells were observed after 6, 8, 12 and 24 hours after the addition of actinomycin to the culture of the tumor. It has been found, that the developing degenerative changes in the tumor cells are paralleled by a growing activity of the enzyme tested. The degenerative changes were described in the neoplastic cells, beginning from the accumulation of the enzyme activity in small granules of the cell processes up to very high activity of the enzyme in fragments of the breaking down cells. It is suggested that LDH activity is a good marker of cell form degenerative changes.     

Degeneration and regeneration of some mechanoreceptors. An ultrastructural study. III. Ultrastructure of reinnervated Herbst corpuscles

The ultrastructure of reinnervated Herbst corpuscles shows that the regenerating nerve branches appear in the inner zone of the receptors at the end of the first month after nerve crush. The nerve branches are accompanied by the Schwann receptor cells. Two periods of regeneration can be established. During the first period the changes reflect mainly the quantitative relations between the regenerating nerve branches and the Schwann receptor cells, whereas during the second period the intracytoplasmic and intraaxoplasmic renewal of the organelles take place. The final regeneration of the receptors finishes at the end of the fifth month after nerve crush and one month later after nerve transection. Also, after transection the number of reinnervated receptors is less encountered.     

Effect of food deprivation on histomorphology and cholinesterase activity of taste buds in ratst

The histomorphology and cholinesterase activity of the taste buds and the gustatory nerve fibre sin well-fed, in protein and protein-calorie deficient rats have been investigated. The nerve fibre arborisation in the taste buds is predominantly nonmyelinated and shows degenerative changes ranging from initial swelling to disintegration, fragmentation and finally complete disappearance with the increasing degree and duration of food deprivation. Coincident with these changes in the nerve fibre, the taste bud also shows various stages of degeneration. By contrast, the cholinesterase activity in the gustatory papillae shows an initial increase during the first week followed by a decline in the activity during the succeeding weeks; a second peak of cholinesterase activity appears during 4–6 weeks. The cholinesterase activity is barely detectable after the 8th week. In the more severely protein calorie restricted groups, the cholinesterase changes are more pronounced and abrupt in onset and show a total disappearance by 4–5 weeks.     

Degeneration and regeneration of some mechanoreceptors. An ultrastructural study. IV. Ultrastructure of reinnervated Pacinian corpuscles

The first signs of reinnervation of the Pacinian corpuscles have been established at the middle of the second month after nerve crush. The regenerative process pass through two periods. During the first period the progressive increase of the Schwann receptor cells has been observed parallel to the reduction of the regenerating nerve branches. During the second period the reorganization and renewal of the regenerated organelles takes place. Some organelles as dense core vesicles, coated vesicles and microtubules of the receptor nerve fibre show noticeable dynamics. The regeneration has been established only in the preexisting after denervation capsulated remnants of the receptors. After nerve transection the regeneration is prolonged one month later and the less of quantity reinnervated receptors have been observed.     

Ultrastructural characteristics of the natural heteromorphic growth of Clostridium septicum

Periodic cultures of C. septicum strain No. 59, growing in Pope's broth, have been studied by means of transmission electron microscopy on ultrathin sections. During the lagphase of growth the inoculated bacilliary cells are sequentially converted into giant filamentous multinucleate forms. These changes, reflecting the reaction of phenotypical adaptation to new environmental conditions, are followed by the restoration of the initial phenotype via the fragmentation of the giant cells. At the same time heteromorphic growth becomes, also spontaneously, atypical in some of the multinucleate cells. This atypical heteromorphic growth results in pronounced degenerative changes leading to bacteriolysis and the death of microbial cells due to disturbances in the coordination of the formation of structural and functional complexes in these cells at the period of their phenotypical adaptation. Such pathology of microbial cells leads to elimination of individual cells with developmental defects, which is finally conducive to the sanitation of the population.     

Reciprocal interactions between olfactory receptor axons and olfactory nerve glia cultured from the developing moth Manduca sexta

In olfactory systems, neuron-glia interactions have been implicated in the growth and guidance of olfactory receptor axons. In the moth Manduca sexta, developing olfactory receptor axons encounter several types of glia as they grow into the brain. Antennal nerve glia are born in the periphery and enwrap bundles of olfactory receptor axons in the antennal nerve. Although their peripheral origin and relationship with axon bundles suggest that they share features with mammalian olfactory ensheathing cells, the developmental roles of antennal nerve glia remain elusive. When cocultured with antennal nerve glial cells, olfactory receptor growth cones readily advance along glial processes without displaying prolonged changes in morphology. In turn, olfactory receptor axons induce antennal nerve glial cells to form multicellular arrays through proliferation and process extension. In contrast to antennal nerve glia, centrally derived glial cells from the axon sorting zone and antennal lobe never form arrays in vitro, and growth-cone glial-cell encounters with these cells halt axon elongation and cause permanent elaborations in growth cone morphology. We propose that antennal nerve glia play roles similar to olfactory ensheathing cells in supporting axon elongation, yet differ in their capacity to influence axon guidance, sorting, and targeting, roles that could be played by central olfactory glia in Manduca.     

SUBMICROSCOPIC CHANGES OF THE SYNAPSE AFTER NERVE SECTION IN THE ACOUSTIC GANGLION OF THE GUINEA PIG. AN ELECTRON MICROSCOPE STUDY

The degenerative changes of the synaptic regions after nerve section have been studied with the electron microscope in the interneuronal synapse of the ventral ganglion of the acoustic nerve of the guinea pig. Fixation with buffered osmic tetroxide was carried out 22, 44, and 48 hours after destruction of the cochlea on one side; the contralateral ganglion being used as control. The submicroscopic organization of normal axosomatic and axodendritic synapses is described. In the synaptic ending four morphological components are recognized: the membrane, the mitochondria, the synaptic vesicles (19, 20), and the cytoplasmic matrix. The intimate contact of glial processes with the endings and with the surface of the nerve cell is described. At the level of the synaptic junction there is a direct contact of the limiting membranes of the ending and of the cell body or dendrite. Both contacting membranes constitute the synaptic one with a total thickness of about 250 A. This membrane has regions of higher electron density where the synaptic vesicles come into intimate contact and fuse with it. Definite degenerative submicroscopic changes in the nerve endings were observed after 22 hours of destruction of the cochlea and were much more conspicuous after 44 and 48 hours. After 22 hours there is swelling of the ending and decreased electron density of the matrix. Most synaptic vesicles have disappeared or seem to undergo a process of clumping and dissolution. Some mitochondria also show signs of degeneration. After 44 hours the synaptic vesicles have practically disappeared; mitochondria are in different stages of lysis; the membrane of the ending becomes irregular in shape, and there is shrinkage and in some cases detachment of the ending. No changes in the postsynaptic cytoplasm were observed. These observations and particularly the rapid lysis of the synaptic vesicles are discussed in correlation with data from the literature indicating the early alteration of synaptic function and the biochemical changes occurring after section of the afferent nerve. The hypothesis that the synaptic vesicles may be carriers of acetylcholine or other active substances (19, 20) and that they may act as biochemical units in synaptic transmission is also discussed.²     

Nerve disperses preexisting acetylcholine receptor clusters prior to induction of receptor accumulation in Xenopus muscle cultures

We have investigated the sequential changes of acetylcholine receptor (AChR) distribution on identified Xenopus laevis muscle cells in culture before and after innervation. AChRs on muscle cells were stained with tetramethylrhodamine-conjugated alpha-bungarotoxin and the distribution of AChR clusters was examined on a fluorescence microscope using an image intensifier. Large receptor clusters were identified on muscle cells and their fate was followed afterward. In muscle cells cultured without neural tube cells, about one-half of the identified AChR clusters survived for 2 days. In nerve-muscle cocultures, preexisting AChR clusters survived longer on non-nerve-contacted muscle cells than on muscle cells cultured without nerve. However, in nerve-contacted muscle cells the great majority of preexisting AChR clusters dispersed within 2 days. The dispersal of preexisting AChR clusters preceded receptor accumulation along the path of nerve contact by about 12-16 hr. Therefore, an accelerated dispersal of receptor clusters in innervated muscle cells is not a consequence of receptor accumulation along the nerve. The preexisting AChR clusters located near and far from the nerve contact sites dispersed along a similar time course. Protease inhibitors, trasylol and leupeptin, reduced the nerve-induced dispersal of the preexisting AChR clusters in the period before AChR accumulation at the nerve contact sites but did not do so during the period when AChRs began to accumulate at nerve-muscle contact. The significance of the dispersal of preexisting receptor clusters is discussed with regard to neuromuscular junction formation.     

Growth and elimination of nerve terminals at synaptic sites during polyneuronal innervation of muscle cells: a trophic hypothesis

This paper examines the possibility that the elimination of synapses from cells arises from a competition between the nerve terminals for trophic molecules made available by the cells. This idea is applied to the elimination of synapses that occurs during the polyneuronal innervation of muscle cells which accompanies both the development and reinnervation of muscles. In the proposed model, each motorneuron makes the same amount of receptor in its soma for a trophic molecule provided in limited quantities by each muscle cell; this receptor is then distributed to the collateral terminals of the motorneuron in concentrations proportional to the amount of receptor made in the soma by the motorneuron; the more collateral terminals initially possessed by a motorneuron the less will be their concentration of receptor. The receptors in the several collateral terminals on a muscle cell then compete for the trophic molecule provided by the muscle, and terminal growth is proportional to the number of receptor-trophic-molecule bonds formed. An autocatalytic effect has been introduced whereby the increase in size of a terminal accelerates the rate by which the trophic molecule is made available to that terminal for bonding with its receptors. In addition, the affinity between nerve terminal receptors and muscle molecules can be varied in the model. Finally, motorneuron cell death has been analysed as the elimination of neurons that have insufficient terminal area to take up a growth factor in amounts that will allow for the survival of the neuron.     

CELL DEATH IN THE EMBRYONIC CHICK SPINAL CORD

The events which occur in the death of visceromotor neurons of the cervical region of the chick embryo's spinal cord have been analyzed by electron microscopy. These normal degenerative events are compared with those in the lumbosacral cord where nerve cell death was induced by removal of peripheral organs. The initial set of degenerative changes include a decrease in nuclear size, the clumping of chromatin beneath the nuclear envelope, an increase in electron opacity of the cells, the disappearance of Golgi bodies, and the disaggregation of polysomes. These events are followed by the loss of the nuclear envelope and most of the endoplasmic reticulum, the appearance of bundles of filaments, and the formation of many ribosome crystals. Ribosome crystals are seen only in the dying cells. Their abundance may indicate a drastic reduction in RNA synthesis as one of the initial events which lead to the death of these neurons. The neurons are finally subdivided and engulfed by cells of the normal glial population, and further breakdown of the cell fragments occurs in large phagocytic vesicles of the gliocytes.     

Axonal sprouts of the hypoglossal nerve implanted in the superior cervical ganglion of adult rats establish synaptic contacts under long-lasting GABA effect. An experimental degeneration study

Experimental degeneration was used in this study to determine if the hypoglossal nerve implanted already in the superior cervical ganglion of adult rat under GABA treatment has established morphologically-identifiable synapses with the dendrites of principal ganglion cells. The implanted hypoglossal nerve trunk was cut in a re-operation, and the ganglionic samples were studied by electron microscopy after 0, 6, 12, 24 and 48 h survival times. First signs of degenerative changes were found in the myelinated and non-myelinated axons alike, 6 h after axotomy. The fine-structural signs of degeneration resembled those of the preganglionic nerve fibres. Degenerating nerve terminals establishing synaptic contacts with the dendrites of the principal ganglion cells were also seen, indicating that the axonal sprouts of the implanted hypoglossal nerve established synaptic contacts with the ganglion cells. It remained, however, to be elucidated whether or not these synapses of the hypoglossal nerve are functionally active contacts while the preganglionic innervation is also present within the ganglion.     

Changes in the intervertebral disks in disorders of segmental blood supply of the spine

In 50 mature Chinchilla rabbits a model of chronic insufficiency of blood supply in the lumbar vertebral bodies has been disturbed as a result of unilateral sectioning of the segmentary arteries and veins. By means of light and transmissive electron microscopy the dynamics of structural changes has been followed in tissue of the intervertebral discs for 3 months after the operative intervention. Under hypoxia in the ground substance of the pulposus++ nucleus even proteoglycans granular-filamentous network gradually develops and floccular material and transverse striated filamentous aggregates are accumulated. Notochord cells are subjected to certain degenerative changes and die. Simultaneously fibroblastic cells of the pulposus++ nucleus periphery become activated, they produce glycosaminoglycans and collagen. As a result the hydrated tissue of the pulposus++ nucleus is substituted for a newly formed fibrous cartilage. The process of fibroses in the intervertebral disc is completed in 3 months after blood circulation has been disturbed in the vertebral bodies.     

Submicroscopic changes of the synapse after nerve section in the acoustic ganglion of the guinea pig; an electron microscope study

The degenerative changes of the synaptic regions after nerve section have been studied with the electron microscope in the interneuronal synapse of the ventral ganglion of the acoustic nerve of the guinea pig. Fixation with buffered osmic tetroxide was carried out 22, 44, and 48 hours after destruction of the cochlea on one side; the contralateral ganglion being used as control. The submicroscopic organization of normal axosomatic and axodendritic synapses is described. In the synaptic ending four morphological components are recognized: the membrane, the mitochondria, the synaptic vesicles (19, 20), and the cytoplasmic matrix. The intimate contact of glial processes with the endings and with the surface of the nerve cell is described. At the level of the synaptic junction there is a direct contact of the limiting membranes of the ending and of the cell body or dendrite. Both contacting membranes constitute the synaptic one with a total thickness of about 250 A. This membrane has regions of higher electron density where the synaptic vesicles come into intimate contact and fuse with it. Definite degenerative submicroscopic changes in the nerve endings were observed after 22 hours of destruction of the cochlea and were much more conspicuous after 44 and 48 hours. After 22 hours there is swelling of the ending and decreased electron density of the matrix. Most synaptic vesicles have disappeared or seem to undergo a process of clumping and dissolution. Some mitochondria also show signs of degeneration. After 44 hours the synaptic vesicles have practically disappeared; mitochondria are in different stages of lysis; the membrane of the ending becomes irregular in shape, and there is shrinkage and in some cases detachment of the ending. No changes in the postsynaptic cytoplasm were observed. These observations and particularly the rapid lysis of the synaptic vesicles are discussed in correlation with data from the literature indicating the early alteration of synaptic function and the biochemical changes occurring after section of the afferent nerve. The hypothesis that the synaptic vesicles may be carriers of acetylcholine or other active substances (19, 20) and that they may act as biochemical units in synaptic transmission is also discussed.(2)     

Characterization of small lymphocytes in the bone marrow of mice after prolonged treatment with anti-IgM antibodies.

In mice treated with anti-IgM antibodies from birth, small lymphocytes in the bone marrow and spleen have been characterized by their incidence, surface markers, and turnover. Essentially complete elimination of IgM-bearing small lymphocytes followed injections of anti-IgM but this treatment did not deplete the IgM-negative small lymphocytes in the marrow. These IgM-negative cells also lacked other markers of B lymphocytes, such as receptors for Fc and complement; they failed to bind anti-mouse T-lymphocyte serum and they formed a rapidly renewing population within the marrow. This population may represent cells whose normal differentiation has been aborted by the anti-IgM antibodies or, alternatively, they may be “null” cells, distinct from B lymphocytes.     

Adrenocorticotropic (ACTH) cells of rats after head irradiation.

The heads of intact rats and of rats 15 days after adrenalectomy were irradiated with 1200 R. Some of the adrenalectomized animals with irradiated heads were treated with 5 mg of deoxycorticosterone acetate. In adrenocorticotropic cells of adrenalectomized rats 15 and 30 days after irradiation, the decreased amount of the granular endoplasmic reticulum and the Golgi regression imply a considerably decreased protein synthesis. The finding of an increased number of granules and of granules larger than normal suggests that they are the result of slowed down transport and secretion of the products of these cells' activities. After head irradiation an increased number of degenerative cells was evident in the pituitary of both intact and adrenalectomized rats. The degree of the degenerative changes and the number of degenerative cells was higher 30 than 15 days after irradiation and both were decreased if the adrenalectomized rats were treated with deoxycorticosterone acetate.