Early forms of terminal degeneration in the spinal trigeminal nucleus following rhizotomy

Glutamate and AMPA receptor immunoreactivity in Ia synapses with motoneurons and neurons of the central cervical nucleus

Axonal tracing and high resolution immunocytochemistry were used to identify transmitter content and postsynaptic receptors in synapses between Ia primary afferents and motoneurons and in neurons of the central cervical nucleus (CCN), respectively, in the rat. The terminals, as well as the target neurons, were identified by postembedding immunogold detection of transganglionically or retrogradely, respectively, transported cholera toxin B subunit (CTB), and in adjacent sections postembedding immunogold was employed to demonstrate glutamate and AMPA receptors in the same synapses. A total of 390 CTB-labelled Ia boutons in apposition to CTB-labelled motoneurons, CCN neurons or unlabelled dendrites in the surrounding neuropil were traced in section series from two animals. A third animal was used as a control. In the motor nucleus, a majority of the synapses were with medium-sized dendrites, whereas in the CCN the distribution was skewed towards fine-calibre dendrites. In both nuclei, somatic and juxtasomatic synapses were quite infrequent (<10%). All of the CTB-labelled Ia boutons recovered in the sections incubated for glutamate (n=323) were enriched with glutamate immunoreactivity. One hundred and fifty of these disclosed synaptic contact in at least two ultrathin sections. In this sample, 50% (33-59%) appeared immunoreactive to receptor sub-units GluR1-4 in at least two ultrathin sections, whereas 35% were labelled in one section only. Distribution of gold particles relative to presynaptic and postsynaptic membrane profiles (n=23) revealed a close correlation between AMPA immunoreactivity and the postsynaptic membrane of the synapse. Finally, immunogold particles signalling GluR1 were observed much less frequently than particles signalling GluR2/3 or GluR4. Our results provide additional strong evidence that chemical transmission at Ia synapses is mediated by glutamate and identify GluR2/3 and GluR4 as important postsynaptic receptors.

High-intensity focused ultrasound selectively disrupts the blood-brain barrier in vivo

High-intensity focused ultrasound (HIFU) has been shown to generate lesions that destroy brain tissue while disrupting the blood-brain barrier (BBB) in the periphery of the lesion. BBB opening, however, has not been shown without damage, and the mechanisms by which HIFU induces BBB disruption remain unknown. We show that HIFU is capable of reversible, nondestructive, BBB disruption in a targeted region-of-interest (ROI) (29 of 55 applications; 26 of 55 applications showed no effect); this opening reverses after 72 h. Light microscopy demonstrates that HIFU either entirely preserves brain architecture while opening the BBB (18 of 29 applications), or generates tissue damage in a small volume within the region of BBB opening (11 of 29 applications). Electron microscopy supports these observations and suggests that HIFU disrupts the BBB by opening capillary endothelial cell tight junctions, an isolated ultrastructural effect that is different from the mechanisms through which other (untargeted) modalities, such as hyperosmotic solutions, hyperthermia and percussive injury disrupt the BBB.

Unipolar brush cell axons form a large system of intrinsic mossy fibers in the postnatal vestibulocerebellum

The unipolar brush cells (UBCs), a class of neurons recently identified in the granular layer of the vestibulocerebellum, receive excitatory synaptic input from mossy fibers (MFs) in the form of a giant glutamatergic synapse. UBCs are provided with axons that bear synaptic endings situated at the center of glomeruli, similar to cerebellar MF afferents. A single MF stimulus evokes a prolonged train of action potentials in the UBC (Rossi et al., 1995), which is presumably distributed to postsynaptic targets. Knowledge of the synaptic connections of UBC axons is essential to define the role of these cells in the integration of vestibular signals in the cerebellar circuitry. To evaluate these connections, the nodulus (folium X) was isolated from vermal slices of postnatal day 8 mice, cultured for 2-4 or 15-30 days in vitro, and studied by electron and fluorescence microscopy. The peak of degeneration of extrinsic MF terminals, which have been severed from the parent cell bodies, was observed at 2 days in vitro (DIV). Quantification of degenerating and nondegenerating (e.g., intrinsic) MF terminals indicated that about half of the MF terminals were provided by local UBC axons synapsing on dendrites of granule cells and other UBCs. The proportion of nondegenerating vs. degenerating MF terminals terminating on UBCs also indicated that approximately two-thirds of the intrinsic MFs are involved in UBC-UBC connections. In long-term cultures, the granular layer appeared well preserved and the UBC axons formed an extensive system of MF collaterals. It is suggested that UBCs may act by spatially amplifying vestibular inputs carried by extrinsic MFs.

Ultrastructural detection of neuronally transported choleragenoid by postembedding immunocytochemistry in freeze-substituted Lowicryl HM20 embedded tissue

Choleragenoid (cholera toxin B-fragment; CTB) is an anterograde, retrograde and transganglionic neuronal tracer. We describe a method for detecting CTB-labeled neuronal cell bodies, neurites and boutons at the ultrastructural level, using postembedding immunogold techniques on freeze-substituted Lowicryl HM20 embedded nervous tissue. Primary afferents and motoneurons were labeled by injection of CTB in the dorsal ramus of the C2 spinal nerve of the rat. Following fixation with paraformaldehyde (4%) and glutaraldehyde (0.25%), tissue sections from the spinal cord C2 segment were freeze-substituted and embedded in Lowicryl HM20 and subsequently processed with postembedding immunocytochemistry for CTB and glutamate. Immunogold particles indicating CTB immunoreactivity were found over primary afferents and motoneurons. In primary afferents in the central cervical nucleus (CCN) and motor nuclei, immunogold labeling was seen in boutons over vesicle-containing axoplasm and to a lesser extent over axoplasm devoid of vesicles, but not over mitochondria or axolemma. In motoneurons, immunogold particles were seen over the Golgi apparatus in the soma and over lysosomes in both soma and dendrites. Quantification of glutamate-like immunoreactivity in 20 CTB-labeled and 20 CTB-negative boutons in the neuropil was found similar, indicating that CTB does not interfere with the immunocytochemical detection of neuronal epitopes such as the transmitter substance glutamate.

Experimental trigeminal nerve injury

The successful reinnervation of peripheral targets after injury varies with the axonal population of the nerve that is injured and the extent of the dislocation of its central component from the peripheral endoneurial tube. Larger-diameter axons such as those supplying mechanoreceptors recover more readily than narrower axons such as those supplying taste. A complex, bi-directional interaction between lingual epithelium and sprouting nerve results in the redifferentiation of taste buds after denervation. Dentin and the dental pulp provide a strong attraction to sprouting nerves and will become reinnervated from collateral sources if recovery of the original innervation is blocked. The most effective repair technique for transected lingual nerves is one which brings the cut ends together rather than one that provides a temporary bridge. Injuries can result in cell death in the trigeminal ganglion but only if the injury is severe and recovery is prevented. Lesser damage results in chromatolysis and the increased expression of neuropeptides. All nerve injuries bring about changes in the trigeminal nucleus. These occur as changes in receptive field and the incidence of spontaneously active neurons, effects which are consistent with the unmasking of existing afferents. These functional changes are short-lived and reversible. Morphologically, nerve injury results in terminal degeneration in the nuclei and an increased expression of the c-Fos gene and some neuropeptides. Only a chronic constriction injury induces behavioral changes. The adult trigeminal system retains considerable plasticity that permits it to respond successfully to nerve injury. Much remains to be learned about this response, particularly of the trophic factors that control peripheral recovery and the central response to more severe injuries.

Electron microscopy of immunoreactivity patterns for glutamate and gamma-aminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (Subnucleus Caudalis)

We studied the ultrastructure of the synaptic organization in the feline spinal trigeminal nucleus, emphasizing specific neurotransmitter patterns within lamina II of the pars caudalis/medullary dorsal horn. Normal adults were perfused, and Vibratome sections from pars caudalis were processed for electron microscopy. Ultrathin sections were reacted with antibodies for the excitatory neurotransmitter glutamate (Glu) and for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by using postembedding immunogold techniques. Both single- and double-labeled preparations were examined. Results with single labeling show that Glu-immunoreactive terminals have round synaptic vesicles and form asymmetric synaptic contacts onto dendrites. GABA-immunoreactive axon terminals and vesicle-containing dendrites have pleomorphic vesicles, and the axon terminals form symmetric contacts onto dendrites and other axons. Double labeling on a single section shows glomeruli with central Glu-immunoreactive terminals that are presynaptic to dendrites, including GABA+ vesicle-containing dendrites. These Glu+ terminals are also postsynaptic to GABA+ axon terminals, and these GABA-immunoreactive terminals may also be presynaptic to the GABA+ vesicle-containing dendrites. Quantitative analyses confirm the specificity of the Glu and GABA immunoreactivities seen in the various glomerular profiles. The results suggest that a subpopulation of Glu-immunoreactive primary afferents (excitatory) may be under the direct synaptic influence of a GABA-immunoreactive intrinsic pathway (inhibitory) by both presynaptic and postsynaptic mechanisms.

Electron microscopic localization of nerve growth factor receptor (p75)-immunoreactivity in pars caudalis/medullary dorsal horn of the cat

Previous studies have demonstrated the presence of nerve growth factor receptor [NGFr(p75)]-immunoreactivity (IR) in the spinal trigeminal nucleus of both 8-10 week-old kittens and mature cats. Most of the NGFr(p75)-IR is lost following retrogasserian rhizotomy, indicating that the majority of the NGFr(p75)-IR within the spinal trigeminal nucleus is of trigeminal primary afferent origin. Here, we examined the ultrastructural localization of NGFr(p75)-IR within lamina II outer of pars caudalis/medullary dorsal horn in the mature cat. Lamina II outer represents a location where dense NGFr(p75)-IR is seen with the light microscope. The NGFr(p75)-IR identified with the electron microscope was located within small thinly myelinated and unmyelinated axons and within axon terminals. The terminals with NGFr(p75)-IR typically formed asymmetric synaptic specializations onto dendritic profiles and at times were postsynaptic to other axon terminals at symmetric synaptic specializations. The terminals with NGFr(p75)-IR were either simple (associated with a single profile) or more complex, such as those that typically formed the central element in synaptic glomeruli. The NGFr(p75)-IR in terminals was especially prominent on microtubules and the plasmalemma and these findings are consistent with proposed roles for NGFr(p75) in axoplasmic/neuronal transport and as a membrane protein, respectively. The profiles with NGFr(p75)-IR seen with the electron microscope indicate a primary afferent origin and show some similarities when compared to other markers of primary afferent fibers such as calcitonin gene-related peptide. In addition, a possible role for NGFr(p75) in the transmission of nociceptive stimuli is also discussed.

Effects of peripheral axotomy on presynaptic axon terminals with GABA-like immunoreactivity

The facial nerve was unilaterally crushed at its exit from the stylomastoid foramen in three 3-month old male rats. After 10 days survival, before the regenerating axons had reinnervated their target muscles, the facial nucleus was examined to determine central patterns of response in material prepared to demonstrate the presence of GABA-like immunoreactivity with postembedding procedures using gold-labeled secondary antibody. The uninjured nucleus served as a control. In both control and injured nuclei, the GABAergic terminals synapse with all parts of the motor neurons, except the axon, and exhibit diverse morphologies. GABAergic axon terminals vary in their size and in the electron density of their axoplasm and the majority of the terminals contain pleomorphic vesicle profiles that display a range in their packing density and size. In both control and injured facial nuclei, only approximately 40% of the axon terminal profiles with pleomorphic vesicles exhibit GABA immunoreactivity. A morphometric analysis of the synaptic vesicle profiles in the GABA-positive terminals reveals that following axotomy there is no change in the mean number of synaptic vesicle profiles per GABAergic terminal profile. However, the mean size of the synaptic vesicle profiles in these terminals shows an axotomy-induced 50% increase, without change in the shapes of the enlarged vesicle profiles. Also, the numerical density of gold particles associated with the GABA-positive terminals is consistently greater in the injured than the control axon terminals. In the control animals quantitative analysis of the relative distribution of all axon terminal profiles in the neuropil categorized by the shape of their vesicle profiles as round, pleomorphic, or flat is 57:37:6. Ten days after axotomy the ratio of these categories in the injured nucleus has shifted to 35:60:5. This study demonstrates that the functional state of a postsynaptic target can influence the morphology of vesicle profiles in presynaptic elements as well as patterns of its afferent input.

Sequential events of degeneration and synaptic remodelling in the viper optic tectum following retinal ablation. A degeneration, radioautographic and immunocytochemical study

The ultrastructural changes taking place in the retino-recipient layers of the viper optic tectum were examined between 5 and 122 days after retinal ablation. The initial degeneration of retinotectal terminals proceeds at widely different rates and is characterized by a marked degree of polymorphism in which a number of different patterns can be discerned. In the final stages of degeneration, either both the degenerating bouton and the distal portion of the postsynaptic element are engulfed by reactive glia, or, more frequently, only the degenerating terminal is eliminated and the postsynaptic differentiation remains. The free postsynaptic differentiations are reoccupied predominantly by boutons containing pleiomorphic vesicles and which are for the most part gamma-aminobutyric acid (GABA)ergic, thus forming heterologous synapses; less frequently these sites are occupied by boutons of the ipsilateral visual contingent to form homologous synapses. These two processes, both of which depend on terminal axonal sprouting, take place within the first 3 postoperative months. They are followed by a decrease in the number of heterologous synapses and a concurrent increase in the number of homologous synapses newly formed by optic boutons generated by collateral preterminal sprouting of ipsilateral retinotectal fibres. The data suggest that partial deafferentation of the optic tectum induces a transitory GABAergic innervation of free postsynaptic sites prior to the restoration of new retinal synaptic contacts.

Contralateral degeneration in the cat spinal trigeminal nucleus following unilateral retrogasserian trigeminal rhizotomy

Retrogasserian trigeminal rhizotomy was used to study the central projections and patterns of degeneration in the spinal trigeminal nucleus (STN). At survival times of 3-20 days, reduced silver stains show extensive degeneration throughout the ipsilateral STN and in addition, well delineated degeneration was identified in the periobex region of the contralateral STN that varied with survival time. The results suggest that primary afferents may contribute to this contralateral projection.

Changes to synaptic ultrastructure in field CA1 of the rat hippocampus following intracerebroventricular injection of kainic acid

To assess the nature and extent of ultrastructural damage due to low unilateral intracerebroventricular doses of kainic acid, treated rats were killed at survival times from 8 h to 14 weeks. Degenerative changes in field CA1 of the hippocampus included dark profiles (often presynaptic), lucent areas enveloping axonic or dendritic elements, damaged myelin sheaths, and enlarged glial profiles. The effect of kainic acid ipsilaterally was maximal at three days but also apparent up to 14 weeks. Contralateral CA1 showed similar though less extensive abnormalities. These observations suggest that, despite rapid synaptic replacement (Nadler et al., Brain Res. 191, 387-403, 1980), long-term electrophysiological abnormalities (Cornish and Wheal, Neuroscience 28, 563-571, 1989) may stem not only from inappropriate reactive synaptogenesis but also from a continuing state of neuronal degeneration.

Analysis of neuronal networks: a review of techniques for labeling axonal projections

In order to analyze connections between neurons in the vertebrate central nervous system, methods have been developed to label a given population of axons of known origin so that they can be differentiated from other, non-labeled structures. Three such methods are reviewed here: experimentally induced orthograde (Wallerian) degeneration, axon transport of radioactive proteins demonstrated by autoradiography, and axon transport of macromolecules that can be reacted histochemically to yield a visible reaction product. Each of the methods has particular strengths and weaknesses. Degeneration methods may differentiate between different functional classes of axons which have different fiber diameters. However, degeneration distorts the morphology of axon terminals, making them more difficult to interpret, and degenerating terminals may be removed rapidly by phagocytosis. Autoradiography of radioactive terminals preserves normal fine structure, but the necessary exposure times extend the method by weeks or months, and care must be exercised to distinguish labeled axons from other structures exhibiting background or transneuronal radioactivity. Histochemical methods, such as those used to demonstrate horseradish peroxidase conjugated to wheat germ lectin (WGA-HRP), are sensitive and rapid, but the injection site must be carefully characterized, and the presence of transneuronal label may make interpretation of the results difficult. Experimental methods of axonal labeling have been invaluable in studying neuronal networks. Each of the methods described here may be of particular value, given the nature of the system to be analyzed.

Ultrastructural changes of the central scalloped (C1) primary afferent endings of synaptic glomeruli in the substantia gelatinosa Rolandi of the rat after peripheral neurotomy

Fine structural changes were observed in the dark scalloped central C1 terminals of type I synaptic glomeruli in spinal cord segments C6-C7 of the rat 3 days after cutting the three main forelimb nerves. Twenty-six per cent of the C1 terminals occurring on the ipsilateral side showed a lighter appearance due to a decrease in the number of synaptic vesicles. The number of synaptic vesicles per unit section area was only 42% of that present in normal C1 terminals on the contralateral side. The number of synaptic contacts of C1 terminals with the profiles surrounding them in each glomerulus was diminished and glial envelopment was increased to 15% of C1 terminal contour. Up to day 12, vesicle and synaptic losses were gradually aggravated and glial apposition was increased, but no obvious signs of glial engulfment were observed. From day 3 to day 12, altered C1 terminals increased in number, while those that appeared normal decreased. The latter had disappeared at day 12 and the altered ones at day 15, and from this stage type I glomeruli were no longer present on the treated side. The lack of electron-dense degenerative bouton changes characteristic of Wallerian degeneration offers an explanation for the lack of or minimal amount of argyrophilic structures which has been found consistently in the substantia gelatinosa during transganglionic degeneration. The gradual decay of the C1 terminals raises the question of their fate. Future studies with the use of a stable marker might provide an answer.

Electron-lucent degenerating geniculate terminals in cat striate cortex

Degenerating geniculate axon terminals in cat striate cortex have been previously described as electron-dense. After electrolytic lesion of the lateral geniculate nucleus, we observed degenerating terminals in layer 4 of striate cortex which were electron-lucent. The lucent terminals --which co-exist with the dense terminals--are characterized by a pale matrix, large size, distorted mitochondria, and a paucity of synaptic vesicles. They preferentially (82.5%) contact dendritic spines. Lucent terminals were common in layer 4, rare in layer 6, and absent from layers 1 through 3 and layer 5. This distribution is consistent with the projection of the lateral geniculate nucleus to the striate cortex. Thus, geniculate terminals undergo both the electron-lucent and electron-dense degeneration reactions in cat striate cortex, and the lucent terminals make a significant contribution to the amount of degeneration present. The relationship of lucent degeneration to other forms of degeneration is discussed.

Spino-olivary termination on spines in cat medial accessory olive

Three functional regions of the inferior olive, the caudal medial accessory olive (cMAO) and the caudal and rostral dorsal accessory olive (DAO) receive input from the spinal cord. The present study determined how spinal inputs to cMAO interact with olivary neurons. These inputs were labeled by injections in cat lumbosacral of wheat germ agglutinin conjugated to horseradish peroxidase. The tracer was visualized with tetramethylbenzidine. The morphology of the labeled spino-olivary terminals and the relationship between these terminals and postsynaptic elements were determined. Spino-olivary terminals in cMAO displayed the morphological characteristics classically associated with excitatory synapses. Almost three quarters synapsed on spines, most of which contacted other spines, forming spine clusters. The majority of postsynaptic spines also received convergent input from apparently excitatory, nonlumbosacral afferents. This postsynaptic organization provides several possible benefits for the putative role of cMAO in the control of posture. An earlier study demonstrated that in DAO, almost three quarters of lumbosacral, spino-olivary terminals synapse on dendrites (Molinari: Neuroscience 27:425-435, 1988). Thus, lumbosacral afferents appear to differ fundamentally in the way in which they interact with neurons in cMAO and DAO. These results suggest that the way information is processed may be as important in determining the functional differences between olivary regions as what information is received.

A longitudinal study of the effects of retinal ablation on the organization of the retinal target lamina of the optic tectum in the teleost, Rutilus rutilus

The optic tecta of 55 Rutilus rutilus were examined at intervals varying from 2 days to 4 years after unilateral retinal ablation. Qualitative ultrastructural examination of the retinal target lamina of the optic tectum (stratum fibrosum et griseum superficiale, SFGS) revealed that an initial period of degeneration and glial reaction, each of which could take one of a variety of forms and which lasted for 1-3 months after ablation, was followed by the temporary formation of heterologous synapses which persisted for a further 1-12 months. This in turn was followed by the degeneration of these synapses during the second year after ablation. Quantitative analysis at the level of the light microscope revealed a shrinkage of the SFGS throughout the level of the light microscope revealed a shrinkage of the SFGS throughout the first 14 postoperative months with no further reduction taking place thereafter. Analysis at the ultrastructural level revealed that this shrinkage was due to the disappearance, and not to the reduction in size, of pre- and postsynaptic profiles accompanied by glial reaction. No signs of collateral sprouting were seen throughout the survival period. Thus, partial deafferentation of the SFGS leads in the long run to a marked impoverishment of its neuronal network, without any apparent compensation.

Ultrastructure of degenerative changes following ricin application to feline dental pulps

The ultrastructure of degenerative changes within the ipsilateral trigeminal ganglion, and partes caudalis and interpolaris of the spinal trigeminal nucleus in the cat is described following the application of the potent toxin ricin to the tooth pulps of unilateral maxillary and mandibular posterior teeth, including the cuspids. Survival times ranged from 6 to 10 days. Typical changes identified within the ipsilateral trigeminal ganglion included myelin fragmentation and 'compartmentalization' of the axoplasm of medium-sized myelinated axons, while small myelinated and unmyelinated axons underwent a more variable response ranging from electron-lucent to electron-dense changes. The affected cell body was characterized by the presence of swollen, electron-lucent mitochondria, a reduction of cytoplasmic ribosomes and a filamentous hyperplasia. Other changes often included an eccentric nucleus and satellite cell proliferation. Degenerative changes often occurred in isolated elements surrounded by normal profiles, suggesting specificity of ricin within the trigeminal ganglion. Changes within brainstem axons showed both an electron-dense and a lucent, fragmenting type of axonal alteration. Terminal changes ranged from electron-dense to lucent and also included filamentous hyperplasia and 'hyperglycogenesis'. The altered axonal knobs contained round synaptic vesicles that were presynaptic to dendritic profiles and postsynaptic to terminals containing flattened synaptic vesicles. The above brainstem alterations were identified specifically in the following areas: ventrolateral, medial and dorsomedial pars interpolaris; the ventrolateral and mid-dorsal to dorsomedial areas of the marginalis and outer substantia gelatinosa layers of pars caudalis; and in ventral pockets corresponding to lamina V of the medullary dorsal horn. Dense alterations within terminals containing flattened synaptic vesicles that are typically presynaptic to primary afferents in these areas were rare findings, but along with vacuolization of dendritic profiles suggest a trans-synaptic effect possibly due to the exocytosis of ricin. The results are discussed in relation to different reports of dental projections and with regards to patterns of transganglionic degeneration.

Ultrastructural identification and localization of climbing fiber terminals in the fastigial nucleus of the cat

Following injections of 3H-leucine and 35S-methionine in the caudal half of the medial accessory olive, labeled climbing fibers were found contralateral to the injection site in the sagittal A-zone of the cerebellar vermis and in the fastigial nucleus. Labeling in the fastigial nucleus was analyzed with ultrastructural autoradiography. Labeled boutons of climbing fibers were found in the neuropil but never on somata. They contain spherical vesicles and occasionally some dense core vesicles in an electron-lucent matrix. The terminals of climbing fiber collaterals in the fastigial nucleus resemble climbing fiber terminals in the molecular layer with respect to their internal ultrastructural characteristics.

Transganglionic degeneration in vibrissae innervating primary sensory neurons of the rat: a light and electron microscopic study

Previous studies have shown that transection of peripheral branches of primary sensory neurons leads to light microscopical degeneration argyrophilia and ultrastructural changes in the central termination areas of these neurons. This type of degeneration has been termed transganglionic degeneration (TGD). In the present experiments TGD has been studied specifically in neurons innervating the rat vibrissae at the light and electron microscopic levels. Light microscopically, small amounts of degeneration argyrophilia are observed in the magnocellular zone of the trigeminal subnucleus caudalis at 8-14 days survival. At longer survival times there are substantial amounts of degeneration in this area. At the ultrastructural level the first signs of TGD are observed at 6 days survival, when some terminals show a small increase in electron density, loss of synaptic vesicles, and mitochondrial disintegration. Terminals showing a more advanced increase in electron density become common at 8 days survival, but few of them are still left at 14 days survival. Neurofilamentous terminals appear in small numbers 8-14 days postoperatively. Various forms of degeneration in myelinated axons are observed from 8 days survival and are common also at 80 days survival. Electron-dense axons are rather unfrequent, but more or less disrupted myelin sheaths containing disintegrated axoplasmic remnants and empty areas are common as well as extremely expanded myelin sheaths. Glial cells containing axonal and myelin debris are seen from 8 days survival and become a more common finding at longer survivals. A most striking finding 8-10 days postoperatively is a complex relationship between glial cells and less darkened terminals, indicating phagocytosis before reaching an entirely darkened state. The findings clearly show that peripheral nerve transection leads to severe central alterations in a population of mechanoreceptor neurons innervating the vibrissae of the adult rat.

A morphometric study of the effects of maturation and aging on synaptic patterns in the spinal trigeminal nucleus of the cat

Morphometric methods have been used to study the synaptic and terminal patterns in cat trigeminal nucleus, pars interpolaris, during development and aging. Ages 1, 3, 6, 11, 16, 21, 27, 110, 600 days and 8 and 11 years were studied. Both proportions and densities (number per unit area) of certain terminals and synapses showed significant changes with age. Axoaxonic synapses especially showed two major periods of increase (3-6 days and 21-27 days). The values of most parameters increased in the 21-27 day period to peak levels and then decreased gradually with age. The results indicate two separate critical synaptogenic periods of development and a loss of synaptic elements in aging. Factors contributing to these changes are discussed as is the potential for plasticity in the different afferents at each period.

Ultrastructure of transganglionic degeneration in brain stem trigeminal nuclei during normal primary tooth exfoliation and permanent tooth eruption in the cat

Electron microscopy is used to study changes in the axons and terminals in the cat brain stem trigeminal nuclei, main sensory, and partes interpolaris and caudalis, during the process of natural tooth shedding. Areas previously showing light optical argyrophilic degeneration products and adjacent areas lacking this degeneration are included. Various types of alteration occur early during tooth loss, including increased presumed glycogen, increased cytoplasmic density, flocculence, lucency, and neurofilamentous hyperplasia. By the stage of maximum exfoliation, terminals and axons of marked density become prominent in areas showing argyrophilia, whereas nondense forms occur elsewhere. By late eruption ages, all forms of degenerated terminals and axons are rare, but phagocytes are heavily laden with similar forms of debris. The sequence of ultrastructural events is discussed in light of recent studies of transganglionic degeneration, their correlation with light microscopic findings, and the potential implications for central plasticity in this system.

Effects of dorsal rhizotomy on the several types of primary afferent terminals in laminae I-III of the rat spinal cord. An electron microscope study

After cervical dorsal rhizotomy, small dark central terminals (C1) of glomeruli underwent electron dense changes at 8 h and were all degenerated at 36 h; their number persisted, though slightly diminished, up to 15 days, glial engulfment being negligible. Light large central terminals without neurofilaments (CIIa) showed electron-lucent or electron-dense degeneration from 14 to 36 h, while those with neurofilaments (CIIb) exhibited increased neurofilamentous areas, with depletion and presynaptic concentration of synaptic vesicles as in the electron-lucent change, at the 8-36 h postrhizotomy periods. Both CII-varieties were all degenerated at 36 h and became electron dense at 48 h; glial phagocytosis was intense and no terminals were present after 4 days. It is concluded that in the rat the 3 types of central glomerular terminals are primary axons, and that each type undergoes a different pattern of degeneration which points to a separate primary afferent origin. Numerous nonglomerular axodendritic endings began showing electron-dense degeneration at 8 h which rapidly masked their normal structure, although most appeared to contain round agranular vesicles, and some of them dense-cored vesicles (in lamina I). A few endings exhibited electron-lucent degeneration. Labeling methods seem preferable for studying the primary origin of nonglomerular terminals, due to the difficulty in recognizing the normal predegenerative structure of these profiles.

Age-related fine structural changes in axons and synapses during deafferentation of the rat pyriform cortex: a possible basis for plasticity

The purpose of this study was to compare the sequence of axonal and synaptic alterations following deafferentating lesions at selected postnatal ages and relate these changes to synaptic organization in the olfactory cortex. Rats received unilateral olfactory bulb ablation at 2 1/2, 6, 9 and 13 days of age and were studied at survivals of 12 h to 30 days. At least three clearly different forms of acute degeneration were seen; flocculent, granular and dense with the granular form an intermediate form. The proportion of granular and especially dense degeneration increases after six days of age as does the presence of glia. The denser the type of degeneration, the greater the retention of remnants of this form of synaptic degeneration at deafferented postsynaptic sites. This as well as the increased presence of glia after six days may be important factors in the limitation of plastic reorganization or reinnervation in more mature individuals. The youngest operated animals show rapid vacating of the receptor site, relative absence of glia and striking evidence of competitive reoccupation of deafferented sites.

Cerebral ischemia in the rat: ultrastructural and morphometric analysis of synapses in stratum radiatum of the hippocampal CA-1 region

A 10-min long ischemic insult followed by up to 60-min survival results in several changes of the synaptic ultrastructure in the hippocampal CA-1 region. The alternations consist of gradual change of synaptic curvature from neutral to positive, cleavage and decrease in thickness of the postsynaptic densities and, in the case of many terminals, wrinkling of their profiles. The most striking form of damage are membrane discontinuities which begin to appear in very small numbers after 20 min of blood reflow and become much more pronounced after 60 min. The development of those modifications seems to be time-related, whereas decrease in the number of synaptic vesicles, as shown by the morphometric analysis, occurs after 10 min and does not progress any further after 20 and 60 min. This decrease is most pronounced in the immediate vicinity of the presynaptic membrane. Although the observed signs of ultrastructural alternations of synapses in the postischemic period appear to conform to the general pattern of synaptic degeneration observed under other conditions, the severity of ischemia is underlined with the rate at which those changes develop, thus pointing toward grossly disturbed metabolism of postischemic neurons. Recently, a number of theories have been advanced, discussing significance of ischemic destruction of membrane phospholipids. These theories are discussed in the context of membrane discontinuities reported in this investigation.

The distribution of visceral primary afferents from the pelvic nerve to Lissauer's tract and the spinal gray matter and its relationship to the sacral parasympathetic nucleus

The central distribution of visceral primary afferent fibers from the pelvic nerve of the cast and the relationship of these fibers to preganglionic neurons of the sacral parasympathetic neurons (SPN) have been studied. Horseradish peroxidase (HRP) applied to the cut pelvic nerve was detected ipsilaterally in preganglionic neurons and dorsal root ganglion cells (segments S1-S3), and in central afferent projections to Lissauer's tract (LT), the dorsal columns, the dorsolateral funiculus, and spinal gray matter. The afferent projections were strongest in the region of the SPN (S1-S3) but extended far beyond its limits (e.g., LT was labeled from L4 to Cx7). In the transverse plane, collateral fiber bundles formed a thin shell around the dorsal horn predominantly within lamina I and expanded into terminal fields in the gray matter. The more prominent lateral collateral projection (LCP) extended into laminae V and VI, whereas the medial one (MCP) ended in the dorsal commissure. In longitudinal planes these projections exhibited a periodicity with an interval of approximately 200 micrometer. The distribution of afferent collateral projections overlaps the regions where many preganglionic neurons and their dendritic extensions are located, and also areas known to contain interneurons involved in visceral pathways. A differential distribution of afferents within the SPN was noted where a higher intensity was observed in proximity to those neurons located in laminae V and VI, which innervate the colon, and a lower intensity near neurons located in Lamina VII which innervate the bladder. This is consistent with the known spinal control of colon reflexes and the supraspinal control of bladder reflexes. The widespread rostrocaudal extent of the pelvic primary afferent projection is consistent with the necessity for the integration of somatic and autonomic elements from various levels of the lumbo-sacral-coccygeal spinal cord in the performance of pelvic visceral functions.

An electron microscopic study of the afferent connections of the lateral reticular nucleus of the cat

The mode and pattern of termination of the afferents to the lateral reticular nucleus (LRN) of the cat were examined at the cellular level through the ultrastructural localization of induced degeneration. Examination of the LRN following hemicordotomy at the fifth and sixth cervical levels revealed that most of the degenerating terminals were in contact with intermediate and distal dendrites, and that most of these degenerating terminals were small and contained round vesicles. Fewer degenerating terminals were observed on the somata and proximal dendrites after spinal hemisection, and most of these terminals were large and contained round vesicles. Following lesions of the pericruciate cortex, small degenerating terminals were occasionally observed making contact onto intermediate and distal dendrites. Degenerating rubral terminals were observed synapsing on somata, somatic and dendritic spines, proximal dendrites and most commonly on intermediate and distal dendrites following lesioning of the red nucleus. The degenerating axosomatic rubro-LRN terminals belonged to the large, round-vesicle terminal population, while those degenerating terminals contacting intermediate and distal dendrites belonged to the small, round-vesicle population. Small, degenerating terminals were occasionally seen following lesions of the fastigial nucleus, and they made synaptic contact mainly onto intermediate and distal dendrites and dendritic spines. The present ultrastructural observations taken together with the convergence pattern of LRN afferents and the available electrophysiological data on inputs to the LRN suggest an extensive integration of converging impulses from two or more afferent sources to the rostral LRN neurons. The results of this study therefore support the veiw that the rostral LRN functions as a comparator of command signals from the motor cortex and red nucleus and feedback signals from the spinal cord and cerebellum during ongoing movement.

Electron microscopic demonstration of cerebello-oculomotor fibres in cat

Few synaptic buttons with the typical changes of "electron-dense degeneration" were found in the nucleus of the oculomotor nerve of cats, which underwent complete bilateral ablation of the cerebellum. The degenerating buttons were mainly found in the neuropil, contacting small dendritic profiles. For the scanty number of degenerating buttons observed and for the particular location of its terminal buttons, the direct cerebello-oculomotor connection appears to be an ancillary route in the cerebello-oculomotor linkage, which involves mainly polysynaptic routes.

Degeneration of hippocampal CA3 pyramidal cells induced by intraventricular kainic acid

Degeneration of hippocampal CA3 pyramidal cells was investigated by light and electron microscopy after intraventricular injection of the potent convulsant, kainic acid. Electron microscopy revealed evidence of pyramidal cell degeneration within one hour. The earliest degenerative changes were confined to the cell body and proximal dendritic shafts. These included an increased incidence of lysosomal structures, deformation of the perikaryal and nuclear outlines, some increase in background electron density, and dilation of the cisternae of the endoplasmic reticulum accompanied by detachment of polyribosomes. Within the next few hours the pyramidal cells atrophied and became electron dense. Then these cells became electron lucent once more as ribosomes disappeared and their membranes and organelles broke up and disintegrated. Light microscopic changes correlated with these ultrastructural observations. The dendritic spines and the initial portion of the dendritic shaft became electron dense within four hours and degenerated rapidly, whereas the intermediate segment of the dendrites swelled moderately and became more electron lucent. No degenerative changes were evident in pyramidal cell axons and boutons until one day after kainic acid treatment. Less than one hour after kainic acid administration, astrocytes in the CA3 area swelled, initially in the vicinity of the cell body and mossy fiber layers. It is suggested that the paroxysmal discharges initiated in CA3 pyramidal cells by kainic acid served as the stimulus for this response. Phagocytosis commenced between one and three days after kainic acid administration, but remained incomplete at survival times of 6-8 weeks. Astrocytes, microglia, and probably oligodendroglia phagocytized the degenerating material. These results point to the pyramidal cell body and possibly also the dendritic spines as primary targets of kainic acid neurotoxicity. In conjunction with other data, they support the view that lesions made by intraventricular kainic acid can serve as models of epileptic brain damage.

Alterations in axons and synapses of olfactory cortex following olfactory bulb lesions in newborn rats

The olfactory cortex of rats is being studied at various survival times following deafferentating olfactory bulb ablation on the day of birth. The neonatal axons and synaptic terminals undergo rapid, flocculent degeneration and fragmentation. Most are not electron-dense and therefore probably not argyrophilic at this particular age of the lesion. The degeneration and removal of debris is far more rapid than in adults, yielding a markedly enlarged extracellular space with a relative absence of glia at the vacated postsynaptic "thickenings". Denervated postsynaptic "thickenings" become occupied by neuronal and nonneuronal profiles and profiles of uncertain origin, singly or in various combinations, or the sites may remain partially vacant. One or more axons with synaptic vesicles often aggregated at the site are commonly involved. Certain terminals form contacts on progressively greater lengths of the "thickening" until typical synaptic contacts predominate by 14 days survival. The results suggest a competitive reinnervation process and provide a fine structural explanation for the events leading to alterations in this pathway following neonatal deafferentation.

Loss and reacquisition of hippocampal synapses after selective destruction of CA3-CA4 afferents with kainic acid

Intraventricular injections of kainic acid were used to destroy the hippocampal CA3-CA4 cells bilaterally in rats, thus denervating the inner third of the molecular layer of the fascia dentata and stratum radiatum of area CA1. Electron microscopic studies showed that this lesion reduced the synaptic density of the CA1 stratum radiatum by an average of 86%. The synaptic density of the inner third of the dorsal dentate molecular layer declined by two-thirds and the corresponding zone of the ventral dentate molecular layer by about half. Within 6-8 weeks the synaptic density of these laminae had been restored to the control value or nearly so. In the CA1 stratum radiatum about 72% of the synaptic contacts destroyed by the lesion were replaced, the inner third of the ventral dentate molecular layer recovered 75% of its lost synapses and the inner third of the dorsal dentate molecular layer apparently recovered virtually all of them. The newly formed synapses did not differ noticeably from those normally present. A kainic acid lesion reduced the synaptic density of the outer two-thirds of the dentate molecular layer by 30% within 3-5 days, despite a virtual absence of presynaptic degeneration in that zone. This result implies a substantial disconnection of perforant path synapses. It did not appear to depend on the extent of denervation of the inner zone. The loss of perforant path synapses was completely reversible. We suggest that the dentate granule cells shed a portion of their synapses in response to a substantial loss of neurons to which they project and regained them when their axons had formed new synaptic connections.

Pallidosubthalamic projection in the cat. Electron microscopic study

The degenerative changes within the cat's subthalamic nucleus (Sth) following lesions of the external pallidum were studied by electron microscopy. Four to five days following pallidal lesions a great number of terminals undergoing degenerative changes were encountered in the ipsilateral Sth. The contralateral Sth was free of degeneration. The degenerating terminals show predominantly the light degenerative type, less frequently the dark degenerative pattern, and occasionally exhibit signs of filamentous hyperplasia. The degenerated boutons usually insert on perikarya of the large Sth neurons, on proximal dendrites, and more rarely contact dendritic spines. They were observed neither to perform synaptic contacts with the perikarya of the small Sth neurons nor with other vesicle-containing profiles. On the basis of the ultrastructural aspect of the degenerating terminals, they were identified as F1 terminals, discriminated in a previous study (Romansky et al., 1978). The normal appearance, the synaptic relationships, and the degenerative features of the F1 terminials in the Sth closely resemble the entopeduncular terminals in the thalamus described by Rinvik and Grofová (1974a), and Grofová and Rinvik (1974). The possible contribution of the interrupted passing fibers to the observed degeneration is discussed. The present findings corroborate the relevant morphological, physiological, neurochemical, and neuropharmacological data in the literature.

Neurogenesis and neuron regeneration in the olfactory system of mammals. III. Deafferentation and reinnervation of the olfactory bulb following section of the fila olfactoria in rat

Axotomy at the level of the lamina cribrosa in rat induces rapid degeneration of the olfactory sensory axons in the bulb. The phenomenon, which is limited to the layer of olfactory fibres and to the glomeruli of the bulb, can be observed as early as 15-24 h after surgery, and peaks at 3-4 days. The glomeruli located in the rostro-ventral portion of the bulb are affected first, and the process extends to the dorso-caudal portion with a delay of 12-24 h. Reactive hypertrophy of the glia coincides with removal of the degenerating terminals, and is maximal 48 h after axotomy. Axotomy does not preclude reinnervation of the bulb by axons originating from new, reconstituted neurons in the olfactory neuroepithelium. These new axons begin to reach the periphery of the bulk approximately at the 20th day post-operative and then reinnervate the glomeruli. The rostro-ventral portion of the bulb is the first to be reinvaded by the new axons. The glomeruli reacquire a morphological pattern similar to controls between 20 to 30 days.

Ultrastructure of supraspinal dorsal root projections in the toad. II. The cerebellar granular layer

Following section of the left dorsal roots, degenerating fibres and boutons were observed in the granular layer of the ipsilateral cerebellum. The degenerating terminals were identified as large en passant varicosities of mossy fibres contacting the dendrites of presumptive granule cells. They contained round synaptic vesicles and neurofilaments and established Gray type I contacts. The terminals initially underwent filamentous degeneration with neurofilamentous hypertrophy, swollen mitochondria and loss of synaptic vesicles. At later survival times (6--30 days) they acquired an electron-dense appearance due to an increase and clumping of the filamentous component. After injection of horseradish peroxidase into the left cerebellum, all ipsilateral spinal ganglia showed a few (2--3%) labelled cells, indicating that a primary afferent contribution to this pathway originated from each segment of the spinal cord.

Synaptic fine structure and the termination of corticospinal fibers in the lateral basal region of the cat spinal cord

The lateral basal region (LBR) of the spinal cord gray matter (Rexed's laminae IV-VII) by physiologic and anatomic criteria is the major terminal zone for the corticospinal (CS) tract in the cat. The neurons in this area are medium-sized with abundant spines on their dendrites. Axon terminals on the dendrites and somata of these neurons form synapses easily classified as asymmetric with spheroid vesicles and symmetric with flattened vesicles. There are rare exceptions to this. In a systematic count of terminals, 82% have spheroid and 18% flattened vesicles. The majority of all terminals are on dendrites (84.9%) and a minority on somata (14.1%). Less than 1% are axoaxonic. Degeneration of the corticospinal tract was produced by transecting one hemisphere of our experimental cats. Its termination in the lower cervical cord was studied for 17 hours to 7 days after surgery. Vesicle depletion and clumping and dense polymembranous inclusions were the most common forms of degeneration. Filamentous proliferation in the terminals was also prominent; dark degeneration, however, was infrequent. The percent of degenerating CS terminals in the LBR was the following: 17 hours - 2.2%, 36 hours - 4.19%, 2 days - 10.3%, 3 days - 8.4%, 4 days - 6.9%, 7 days - 10.25%; 84.8% of degenerating CS terminals were axodendritic and 15.2% axosomatic.

An ultrastructural study of transganglionic degeneration in the main sensory trigeminal nucleus of the rat

In adult rats subjected to unilateral transection of the infraorbital nerve, the main sensory trigeminal nucleus was studied by electron microscopy. Post-operative survival times varied between 2 and 60 days. A variety of ultrastructural alterations was observed from the sixth post-operative day onwards. These changes were in many respects similar to those seen in the course of Wallerian degeneration. Neurofilamentous boutons and axons were found 6-30 days post-operatively. Various types of dark boutons were observed between 7 and 30 days and axonal changes indicative of degeneration between 7 and 60 days post-operatively. Astrocytes and microglial cells contained degenerating structures 7-60 days post-operatively. The alterations observed in the present study are interpreted as related, at least in part, to the nerve cell degeneration and the nerve cell death previously shown to occur in the trigeminal ganglion after infraorbital nerve transection.

Terminal proliferation and synaptogenesis following partial deafferentation: the reinnervation of the inner molecular layer of the dentate gyrus following removal of its commissural afferents

The inner one-third of the dendritic region of the dentate gyrus granule cells in adult rats receives projections primarily from the commissural fibers of the contralateral hippocampus and the associational fibers of the ipsilateral hippocampus. At two to four days following the complete removal of the contralateral hippocampus, approximately 25% of the terminals in the inner molecular layer are observed degenerating. This provides an excellent model system to investigate possible terminal proliferation induced by deafferentation since (1) the experimental lesion is easily reproducible, (2) no retrograde reactions occur in the granule cells as a direct result of the lesion, (3) no shrinkage is detected in this region following commissural deafferentation, (4) the same dendritic region can be relocated precisely in each animal, and (5) the synaptic counts are highly consistent between animals. Results from this study and from previous investigations demonstrate that the commissural projection is contained within a 0-80 mu zone directly above the granule cell layer; Complete photomontages of this zone were taken, but only the 40-80 mu zone was quantified for neuronal and glial changes in three normal, five 2- to 4-day, and five 50- to 75-day postlesion animals. The average synaptic count dropped to 64% of control values by 2 to 4 days, returned to 97% by 50- to 75 days postlesion, The number of terminals showing multiple synaptic contacts increased slightly in the long-term animals. Measurements of average terminal area showed no change between the short- and long-term survival groups. These results indicate that this dendritic region is reinnervated following partial deafferentation and that the reinnervation is due primarily to the formation of new terminals rather than the expansion of pre-existing terminals.

Patterns of degeneration in the external cuneate nucleus after multiple dorsal rhizotomies

Unilateral, intradural dorsal rhizotomies (C3-Cs) were performed on adult rats to study the patterns of synaptic organization of ascending dorsal root fibers in the external cuneate nucleus (ECN). Animals were permitted to survive for periods of time ranging from 3 hours to 12 days. Sham-operated animals presented a morphology indistinguishable from that of normal, unoperated animals. In rhizotomized animals, degeneration was observed ipsilaterally at all survival periods. After postoperative survivals of 3 to 14 hours some terminal boutons displayed clumping and diminution in numbers of synaptic vesicles and, in addition, degeneration myelinated axons were observed at this time. There was considerable degeneration in the neuropil between 24 and 48 hours postoperative. Two forms of degeneration occurred in axons and terminal boutons with comparable frequency: electron lucent degeneration and electron opaque degeneration. Reactive phagocytic glial cells contained degenerated masses, lipoid droplets, lysosome-like structures and myelin fragments. After postoperative survivals of four to six days, lucent and opaque degenerating terminals were less numerous. Neurofilamentous degeneration was observed only occasionally. Unaltered synaptic membrane specializations were present and were usually abutted by glia. At 12 days postoperative, synaptic glomeruli and serial synapses were not seen. Invaginating dendritic spines were rarely seen. Bouton populations that remained unualtered were: small (0.3-3.0 micron) boutons that contact dendritic shafts and somata, nodal synaptic boutons and boutons containing granular vesicles (80-100 nm).

An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. I. Magnitude and time course of degeneration

Synapses in the rat dentate gyrus are rapidly lost after removal of the primary input from the entorhinal cortex. In this paper we describe the extent and time course of degeneration and in the subsequent paper the nature of the reinnervation processes. They synapses of entorhinal afferents are remarkably concentrated in their zone of termination. Unilateral removal of the rat entorhinal cortex results in the loss of about 86% of all synapses in the outer three-fourths of the molecular layer of the epsilateral dentate gyrus. Entorhinal synapses are all asymmetric (Gray type I) and terminate on dendritic spines. Analysis of the degeneration reaction provides a means to examine the characteristics of the loss of a relatively homogeneous afferent on a single cell type. The morphological characteristics of the the degenerating terminals showed some heterogeneity; both the electron lucent and electron dense types of degenerating terminals were identified. The electron lucent type was observed only at short survival times. The time course of the loss of degenerating terminals was resolvable into two components, each of which followed first order decay kinetics. Thus degenerating entorhinal terminals behaved as a population which disappeared randomly at a rate dependent on the fraction of terminals present at any time. The loss of degenerating terminals was accompanied by the loss of postsynaptic sites. At short survival times the majority of postsynaptic sites (defined by the presence of a postsynaptic density) had disappeared. There was also a loss of complex spines and some shrinkage of the molecular layer.

Acetylcholinesterase activity of synaptic structures in the spinal trigeminal nucleus

The electron microscope has been used to study the localization of acetylcholinesterase (AChE) activity in the spinal trigeminal nucleus of normal cats with special emphasis on the distribution near synaptic structures. Reaction product is found around both round and flattened synaptic vesicle-containing axon terminals, particularly in synaptic clefts and often specifically associated with the presynaptic, or less frequently the postsynaptic membrane. The presence of reaction product at these specific sites suggests that these are areas of high AChE activity and that acetylcholine may be important in neurotransmission in these regions.

Early terminal degeneration of cerebellar climbing fibers after destruction of the inferior olive in the rat. Synaptic relationships in the molecular layer

The cerebellar molecular layer in adult rats has been studied with the electron microscope at several early and consecutive survival times following 3-acetylpyridine intoxication. Climbing fiber (CF) terminals underwent a fast process of electron-dense degeneration which became apparent from 16 hours onwards. A small proportion of degenerating terminals were depleted of vesicles and filled with a dark flocculent and granular homogeneous matrix. Microtubular changes in degenerating CF tendrils were observed. CF terminals were found in relation with every Purkinje cell in normal animals and completely disappeared within 72 hours after the treatment. CF synapses were found on Purkinje dendritic and somatic thorns, sometimes also on the dendritic shafts or even on the Purkinje soma. Convincing evidencd of synaptic contacts of CF varicosities on either basket or stellate cells could not be obtained. CF synapses with Golgi II cell dendrites in the molecular layer were described. Decrease in the number of post-synaptic dendritic thorns normally assigned for CF synapses was observed consequential to CF anterograde degeneration. The observations are consistent with previous conclusions drawn from light microscopic studies that the clearing up of CF debris in the molecular layer is completed within the short time of three days, and that the inferior olive seems to be the only source of CFs.

The projection of the lateral geniculate nucleus to area 17 of the rat cerebral cortex. I. General description

Lesions were made in the lateral geniculate nucleus of the rat and the consequent degeneration in area 17 of the cerebral cortex was studied by light and electron microscopy. These lesions produced prominent degeneration of axon terminals in layer IV extending into layer III and a much lesser amount in layers I and VI. The darkened degenerating axon terminals forming asymmetric synaptic junctions and were frequently surrounded by hypertrophied astrocytic processes. These terminals appeared to be disposed randomly, forming no discernible patterns. In layer IV 83% of the synapsing, degenerating terminals formed junctions with dendritic spines, 15% with dendritic shafts, and 2% with neuronal perikarya. The dendritic shafts and neuronal perikarya appeared to belong to spine-free stellate cells. The dendrites giving rise to the spines receiving degenerating axon terminals could not be identified, for most of the spines appeared as isolated profiles that could not be traced back to their dendritic shafts. One example of a degenerating axon terminal synapsing with an axon initial segment was encountered. Small, degenerating myelinated axons were prevalent in layers VI, V and IV, but were only infrequent in the supragranular layers. These results are compared with those obtained in other studies of thalamocortical projections.