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PubMed2005

Behavior and corpus callosum morphology relationships in velocardiofacial syndrome (22q11.2 deletion syndrome).

Velocardiofacial syndrome (VCFS) is a neurodevelopmental disorder caused by a microdeletion on chromosome 22q11.2 that predisposes affected individuals to learning disabilities and psychiatric conditions. Previous research has indicated that compared with comparison children, children with VCFS have larger corpus callosal areas. Children with VCFS are often diagnosed with comorbid attention deficit hyperactivity disorder (ADHD), and previous research has indicated that children with ADHD often have smaller corpus callosal areas than controls. The present study investigated two hypotheses: children with VCFS would have larger callosal areas than controls, and children with VCFS+ADHD would have smaller callosal areas than children with VCFS. Corpus callosum area was obtained from the mid-sagittal slice and was assessed in children with VCFS (n=60) and age- and gender-matched control participants (n=52). Results indicated that all of the corpus callosum measures were significantly different between the two groups except for the genu. Across all measures, children with VCFS demonstrated a larger corpus callosum area. Within the VCFS sample, children with VCFS+ADHD (n=30) had smaller total callosal, splenium, and genu areas than children with VCFS alone. Although children with VCFS+ADHD had smaller total callosal areas than children with VCFS, relative to control participants, these children had larger total callosal and subregion areas except for the genu. In addition to other anatomic anomalies, corpus callosal abnormalities appear to be another variable to consider when analyzing brain/behavior relations in this population.

Adolescent↗
PubMed1998

Tetrad pollen formation in quartet mutants of Arabidopsis thaliana is associated with persistence of pectic polysaccharides of the pollen mother cell wall.

The quartet (qrt) mutants of Arabidopsis thaliana produce tetrad pollen in which microspores fail to separate during pollen development. Because the amount of callose deposition between microspores is correlated with tetrad pollen formation in other species, and because pectin is implicated as playing a role in cell adhesion, these cell-wall components in wild-type and mutant anthers were visualized by immunofluorescence microscopy at different stages of microsporogenesis. In wild-type, callose was detected around the pollen mother cell at the onset of meiosis and around the microspores during the tetrad stage. Microspores were released into the anther locule at the stage where callose was no longer detected. Deposition and degradation of callose during tetrad pollen formation in qrt1 and qrt2 mutants were indistinguishable from those in wild-type. Enzymatic removal of callose from wild-type microspores at the tetrad stage did not release the microspores, suggesting that callose removal is not sufficient to disperse the microspores in wild-type. Pectic components were detected in the primary wall of the pollen mother cell. This wall surrounded the callosic wall around the pollen mother cell and the microspores during the tetrad stage. In wild-type, pectic components of this wall were no longer detectable at the time of microspore release. However, in qrt1 and qrt2 mutants, pectic components of this wall persisted after callose degradation. This result suggests that failure of pectin degradation in the pollen mother cell wall is associated with tetrad pollen formation in qrt mutants, and indicates that QRT1 and QRT2 may be required for cell type-specific pectin degradation to separate microspores.

Arabidopsis↗
PubMed1989

Hand and sex differences in the isthmus and genu of the human corpus callosum. A postmortem morphological study.

The midsagittal area of the corpus callosum was measured in its entirety and in seven subdivisions in a sample of 50 brains consecutively obtained from autopsies of individuals who had neuropsychological testing before death. A 12-item test of hand preference was used as an index of the pattern of interhemispheric functional asymmetry. Callosal size was analysed for two factors: hand preference, classified as consistent-right-hand preference (CRH) versus non consistent-right-hand preference, and sex. The group of nCRH (n = 18) was found to have a larger overall callosal area, with the greatest difference occurring in the posterior body segments, especially the isthmus. The isthmus probably includes interhemispheric fibres from posterior parietal and superior temporal cortex which involves cortical regions related to functional asymmetry. The results of variation in callosal morphology are discussed as part of a possible substrate of functional asymmetry and due to variation in axonal elimination in early brain development. Sex differences were found in several aspects of callosal anatomy. (1) The difference between hand groups in the posterior body occurred in interaction with sex:handedness was a factor in callosal size in males (n = 15), but not in females (n = 35). This result is consistent with the general hypothesis of females having less clear lateralization than males. (2) Females did not have a larger overall callosum or a larger splenium, either in absolute size or size proportional to brain weight. The latter measure was considered since callosal area correlated with cerebrum weight (r = 0.48). In contrast, female of both hand groups were found to have a larger proportional isthmus compared to CRH males. (3) Of all callosal regions, only the genu and a part of the anterior body were found to be larger in absolute size in males than females. (4) Callosal size decreased with chronological age in males, but not in females.

Adult↗
PubMed2005

Parasagittal asymmetries of the corpus callosum.

Significant relationships have been reported between midsagittal areas of the corpus callosum and the degree of interhemispheric transfer, functional lateralization and structural brain asymmetries. No study, however, has examined whether parasagittal callosal asymmetries (i.e. those close to the midline of the brain), which may be of specific functional consequence, are present in the human brain. Thus, we applied magnetic resonance imaging and novel computational surface-based methods to encode hemispheric differences in callosal thickness at a very high resolution. Discrete callosal areas were also compared between the hemispheres. Furthermore, acknowledging the frequently reported sex differences in callosal morphology, parasagittal callosal asymmetries were examined within each gender. Results showed significant rightward asymmetries of callosal thickness predominantly in the anterior body and anterior third of the callosum, suggesting a more diffuse functional organization of callosal projections in the right hemisphere. Asymmetries were increased in men, supporting the assumption of a sexually dimorphic organization of male and female brains that involves hemispheric relations and is reflected in the organization and distribution of callosal fibers.

Adult↗
PubMed1985

Physiological properties of individual cerebral axons studied in vivo for as long as one year.

The long-term stability of conduction velocity and recovery processes were studied in a fast-conducting (corticotectal) and in a more slowly conducting (visual callosal) axonal system. Chronic microelectrode recording methods were used in conjunction with antidromic activation via electrical stimulation at one or more axonal site. These methods enabled 54 axons to be studied for greater than 20 days and seven of these cells to be studied for 101-448 days. The conduction velocities of corticotectal axons were characteristic of myelinated axons and were very stable over time. The conduction velocities of most callosal axons were characteristic of nonmyelinated axons, and 68% of callosal axons had conduction velocities that were stable over long periods of time. Of the remaining callosal axons, approximately one third showed an increase in conduction velocity (8-14%), whereas two thirds showed a progressive and systematic decrease in conduction velocity (6-81%). These changes in conduction velocity were distributed along the callosal axon, rather than limited to a single segment of axon. Although the refractory period of callosal and corticotectal axons showed considerable variability over time, the minimal interval between two conducted impulses was stable. The stability of this property was remarkable because the minimal interspike intervals of different axons with similar conduction velocities often differed greatly. Callosal axons show a supernormal period of increased conduction velocity following the relative refractory period and a subsequent subnormal period of decreased conduction velocity following a burst of prior impulses. In different callosal axons the magnitude of the velocity changes (percent change) differs greatly, even among axons of the same conduction velocity. For a given axon, however, these properties are very stable over time. These results on axonal properties may be useful in studies requiring the examination of extracellular responses of individual neurons over long periods of time. Antidromic latency provides a useful means of identifying a cell, particularly when conduction times are long. The stability of the minimal interspike interval and the supernormal period within individual axons make them suitable as ancillary criteria in identifying individual neurons. These three measures are independent of spike amplitude and waveform, and together they provide a "signature" by which individual cortical neurons can be identified over periods that represent a significant portion of the lifespan of adult mammals.

Animals↗
PubMed1998

Corpus callosum atrophy is a possible indicator of region- and cell type-specific neuronal degeneration in Alzheimer disease: a magnetic resonance imaging analysis.

BACKGROUND: Pathological studies in Alzheimer disease indicate the specific loss of layer III and V large pyramidal neurons in association cortex. These neurons give rise to long corticocortical connections within and between the cerebral hemispheres. OBJECTIVE: To evaluate the corpus callosum as an in vivo marker for cortical neuronal loss. METHOD: Using a new imaging technique, we measured region-specific corpus callosum atrophy in patients with Alzheimer disease and correlated the changes with neuropsychological functioning. Total cross-sectional area of the corpus callosum and areas of 5 callosal subregions were measured on midsagittal magnetic resonance imaging scans of 14 patients with Alzheimer disease (mean age, 64.4 years; Mini-Mental State Examination score, 11.4) and 22 healthy age- and sex-matched control subjects (mean age, 66.6 years; Mini-Mental State Examination score, 29.8). All subjects had minimal white matter changes. RESULTS: The total callosal area was significantly reduced in the patients with Alzheimer disease, with the greatest changes in the rostrum and splenium and relative sparing of the callosal body. Regional callosal atrophy correlated significantly with cognitive impairment in the patients with Alzheimer disease, but not with age or the white matter hyperintensities score. CONCLUSIONS: Callosal atrophy in patients with Alzheimer disease with only minimal white matter changes may indicate loss of callosal efferent neurons in corresponding regions of the cortex. Because these neurons are a subset of corticocortical projecting neurons, region-specific callosal atrophy may serve as a marker of progressive neocortical disconnection in Alzheimer disease.

Aged↗
PubMed1975

Interhemispheric neocortical connections of the corpus callosum in the normal mouse: a study based on anterograde and retrograde methods.

Interhemispheric neocortical connections are widely distributed through the corpus callosum in the mouse. Callosal connections are present in all cytoarchitectonic fields except field 25. The distal extemity representations of SmI, and MsI the representation of the mystacial vibrissae in SmI, and the more peripheral field representation of VI are relatively acallosal. Dense projections lie in the midline or truncal representations of SmI, MsI, SmII, at the vertical meridian representations bordering field 17, and medial to the AI representation. The radial distribution of terminals is bimodal in most cytoarchitectonic fields. It is unimodal in the supracallosal segment of field 29b and fields 49 and 27, trimodal in fields 13 and 35. The cells of origin of callosal fibers appear to have the same topographic pattern of distribution as the callosal terminals, observing the same steep and gradual density gradients. No cells giving rise to callosal axons are identified in the acallosal regions of fields 2 and 17. Further, superficial focal lesions in cortical areas which receive callosal connections give rise only to homotopic contralateral degeneration. Acallosal areas of 17 and 2 give rise to no callosal connections. The cells of origin of callosal connections are located at all laminar levels of the cortex and include pyramidal and polymorphic cells but not the granule cells of layer IV.

Animals↗
PubMed1979

Commissural columns in the sensory-motor cortex of monkeys.

Callosally projecting cells and the terminal ramifications of their axons were identified in the monkey sensory-motor cortex by retrograde and anterograde labeling techniques, often by double labeling cells and axons in the same animal. Bundles of callosal fibers terminate in small column-like zones 0.5-1 mm wide in the motor cortex (area 4) and in the first (SI) and second (SII) somatic sensory areas. Such columns are aligned in register to form elongated strips extending mediolaterally in the long axes of the pre- and postcentral gyri. Significant portions of area 4, SI and SII, in regions corresponding to the representations of the hand and foot, are not callosally connected. The cells of origin of callosal fibers in SI are largely confined to layer IIIB and form columns and strips corresponding to the above. In connected zones of SI, the callosal connection is reciprocal and precisely point-to point. This and the laminar distribution of the terminal ramifications of callosal fibers (to layers I-IV) suggest that callosal fibers may arise from the terminate upon exactly homotopic, column-like groups of layer IIIB pyramidal cells. Commissurally projecting cells and their terminal ramifications are not limited to particular architectonic fields or particular parts of fields in SI. All architectonic fields of SI project heterotopically to the contralateral SII.

Animals↗
PubMed1983

The relation of corpus callosum connections to architectonic fields and body surface maps in sensorimotor cortex of new and old world monkeys.

Corpus callosum connections of parietal and motor cortex were studied in New World owl monkeys (Aotus trivirgatus) and Old World macaque monkeys (Macaca fascicularis) after multiple injections of 3H-proline and horseradish peroxidase, HRP, into one cerebral hemisphere, and extensive microelectrode mapping of architectonic Areas 3b, 1, and 2 of the other hemisphere. Results were obtained both from parasagittal brain sections cut orthogonal to the brain surface and from sections from flattened brains cut parallel to the brain surface. Cortical fields varied in density of callosal connections, and the density of connections varied according to body part within sensory representations. Thus, Area 3b had few, Area 1 had more, and Area 2 had relatively dense callosal connections. Within each of these fields, connections were much less dense for the representations of the glabrous hand and foot and much more dense for the representations of the face and trunk. For the representation of the hand, retrogradely labeled cells were extremely sparse in Area 3b, moderately sparse in Area 1, and moderate in Area 2. There were less dense callosal connections in the hand representations of Areas 3b, 1, and 2 in macaque as compared to owl monkeys. Label in posterior parietal cortex was uneven with zones of extremely dense connections. A large region of very dense callosal connections was noted in motor cortex just medial to the probable location of the hand representation. In all regions, callosally projecting cells appeared to be more broadly distributed than callosal terminations. In no region was the discontinuous arrangement of callosal connections obviously organized into an extensive pattern of mediolateral or rostrocaudal bands or strips.

Animals↗
PubMed1987

Interhemispheric connections in the visual cortex of the squirrel monkey (Saimiri sciureus).

The callosal connections within the posterior parietal and occipital cortices were studied in the squirrel monkey with horseradish peroxidase tracing techniques. The data were evaluated with particular emphasis on the relationship of major callosal connections along the 17-18 border. The overall pattern of callosal connections in the squirrel monkey also was compared with callosal patterns in other New World simians. Our results show that the dense band of callosal connections along the 17-18 border in the squirrel monkey differs from the connections observed in other New World monkeys in that it is virtually confined to area 18 and avoids area 17. In addition to a continuous band of callosal connections in area 18 that parallels the 17-18 border, rostral extensions of the band are oriented perpendicular to the 17-18 border and present an obvious periodicity. The remaining parieto-occipital cortex contains a complex pattern of callosal connections that is strikingly similar to patterns reported for other New World monkeys. Thus, it is likely that the dorsolateral extrastriate visual cortex in the squirrel monkey is organized in a manner similar to that found within other New World monkeys.

Animals↗
PubMed1990

Maturation of the corpus callosum of the rat: II. Influence of thyroid hormones on the number and maturation of axons.

Quantitative electron microscopy has been used to study the number of callosal axons in the corpus callosum of normal and hypothyroid rats during postnatal development. At birth, the normal corpus callosum contains 4.4 x 10(6) axons. This number increases to 11.4 x 10(6) by 5 days of age (P5) and then, in contrast to cats and primates, remains constant until at least P60, the oldest age examined. The number of axons in the corpus callosum of hypothyroid animals is not significantly different from the values observed in normal rats at all ages studied, although the callosal axons of hypothyroid rats remain structurally immature. As extensive elimination of callosal axons has been shown to occur in normal rats past P5, we conclude that new callosal processes grow through the corpus callosum past this age that compensate numerically for the loss. Moreover, as the number of callosally projecting neurons seems to be higher in hypothyroid rats than in normal controls, it seems that the constant axon number derives from more parent neurons, and thus that there are more axon collaterals per callosal neuron in a normal animal than in a hypothyroid one. Taken together, these data indicate that although hypothyroidism does not alter the total number of callosally projecting axons, it interferes with the normal processes that define or sculpt the projection fields, thereby leading to a numerically normal projection with abnormal topography.

Animals↗
PubMed1989

A factor analysis of the rat's corpus callosum.

Previous work from our laboratory (Berrebi et al., Brain Research, 438 (1988) 216-224) demonstrated region-specific sexual dimorphisms in the size of the rat's corpus callosum, which are modifiable by extra stimulation in early life. These differences are assumed to reflect regional corticocortical fibers of passage which are altered differentially by gender and our experimental manipulations. In this paper, we report our findings when the original data are reanalyzed using a newly developed computer program. This program not only reproduced, with very high accuracy, the original means, but also permitted us to examine computer generated callosal width scores via a factor analysis procedure. Such a procedure yields useful information concerning the clustering of callosal fibers and thus contributes significantly to our hypothesis that discrete cortical regions are selectively sensitive to experimental variables. Factor analyses of the callosal variables and brain weight of 155 rats found 7 width factors, and an eighth factor which contained the variables of brain weight, callosal length, and callosal perimeter. Callosal area did not load significantly on any of these factors. The percentile locations of the width factors, starting at the anterior (genu) end were: widths 1-5, 6-17, 24-38, 46-57, 62-72, 79-95 and 96-99. Use of these factor scores in analyses of variance revealed that the male callosum is wider than the female's, with the differences most pronounced in the genu and the most posterior portion of the splenium. Both age and early handling experience influenced the callosal width factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed2004

Accumulation of 1,3-beta-D-glucans, in response to aluminum and cytosolic calcium in Triticum aestivum.

One of the most rapid responses to aluminum (Al) stress in plants is enhanced synthesis and deposition of 1,3-beta-D-glucans (callose) in root tips. Ironically, Al-induced synthesis and deposition of callose occurs in vivo, despite evidence from in vitro systems that suggests that Al is a powerful inhibitor of 1,3-beta-D-glucan synthase. We set out to test the hypothesis that an Al-induced increase in the activity of free calcium in the cytoplasm ([Ca(2+)](cyt)) is the trigger for enhanced synthesis of callose in in vivo systems, an effect that would not be observed in in vitro systems. Root tips of an Al-sensitive cultivar of Triticum aestivum were treated with Al (0-100 microM) or the Ca ionophore A23187 (0-3 micro M) for 3-24 h, and the effects on [Ca(2+)](cyt) and synthesis of callose were measured using confocal laser scanning microscopy. Treatment with Al induced a rapid increase in both [Ca(2+)](cyt) (4.7-fold) and synthesis of callose (30-fold). Treatment with the Ca ionophore, A23187, also elicited an increase in [Ca(2+)](cyt) (6.6-fold). Despite a greater increase in [Ca(2+)](cyt) in the presence of A23187, this increase was accompanied by a smaller increase in callose deposition (11-fold) than was observed in the presence of Al. These data suggest that an increase in [Ca(2+)](cyt) is not the only factor modulating increases in callose synthesis and deposition in the presence of Al.

Aluminum↗
PubMed2000

Aluminum-induced 1-->3-beta-D-glucan inhibits cell-to-cell trafficking of molecules through plasmodesmata. A new mechanism of aluminum toxicity in plants.

Symplastic intercellular transport in plants is achieved by plasmodesmata (PD). These cytoplasmic channels are well known to interconnect plant cells to facilitate intercellular movement of water, nutrients, and signaling molecules including hormones. However, it is not known whether Al may affect this cell-to-cell transport process, which is a critical feature for roots as organs of nutrient/water uptake. We have microinjected the dye lucifer yellow carbohydrazide into peripheral root cells of an Al-sensitive wheat (Triticum aestivum cv Scout 66) either before or after Al treatment and followed the cell-to-cell dye-coupling through PD. Here we show that the Al-induced root growth inhibition is closely associated with the Al-induced blockage of cell-to-cell dye coupling. Immunofluorescence combined with immuno-electron microscopic techniques using monoclonal antibodies against 1-->3-beta-D-glucan (callose) revealed circumstantial evidence that Al-induced callose deposition at PD may responsible for this blockage of symplastic transport. Use of 2-deoxy-D-glucose, a callose synthesis inhibitor, allowed us to demonstrate that a reduction in callose particles correlated well with the improved dye-coupling and reduced root growth inhibition. While assessing the tissue specificity of this Al effect, comparable responses were obtained from the dye-coupling pattern in tobacco (Nicotiana tabacum) mesophyll cells. Analyses of the Al-induced expression of PD-associated proteins, such as calreticulin and unconventional myosin VIII, showed enhanced fluorescence and co-localizations with callose deposits. These results suggest that Al-signal mediated localized alterations to calcium homeostasis may drive callose formation and PD closure. Our data demonstrate that extracellular Al-induced callose deposition at PD could effectively block symplastic transport and communication in higher plants.

Aluminum↗
PubMed1992

[Interhemispheric transfer in multiple sclerosis. Morphofunctional correlations].

Signs of cerebral disconnection, especially left ear suppression to dichotically presented verbal stimuli, have been reported in multiple sclerosis patients and found to be correlated to morphological atrophy of the corpus callosum on magnetic resonance imaging. To reinvestigate this issue, 26 patients satisfying criteria for definite multiple sclerosis were proposed 3 tasks aimed at evaluating interhemispheric function: a dichotic listening task, a motor finger-tapping task and a sensory transfer task. Performance at these tasks suggested impaired callosal function in MS patients, compared to normal controls. Callosal morphology was assessed on midsagittal MRI sections using a digitalised method of partition of the callosal area into 6 subregions and automatized surface measurements. Results of correlations between task performance and callosal areas showed a significant correlation between total callosal atrophy and severity of interhemispheric impairment on each functional task. Moreover, impaired motor transfer was specifically related to atrophy of the anterior callosal regions. These results suggest that MS patients may constitute a suitable population to studying interhemispheric transfer of information through the callosal commissure and that this approach may be useful in the clinical management of MS patients.

Adolescent↗
PubMed2003

Corpus callosum deficiency in transgenic mice expressing a truncated ephrin-A receptor.

The A-class of the erythropoietin-producing hepatocellular carcinoma cell-derived (EphA) tyrosine kinase receptors and their ligands, the A-ephrins, play critical roles in the specification of topographic axon projection maps during development. In this study, the role of the EphA subfamily in callosal projections was investigated using transgenic mice expressing a kinase deletion mutant of EphA5. In approximately half of these transgenic mice, cerebral cortical neurons in various cortical regions (primary and secondary somatosensory cortices and frontal as well as visual areas) failed to project to the contralateral cortex. When commissural axons were examined with DiI labeling, few callosal fibers were found to traverse the midline in both the adult and neonatal transgenic mice. This defect in callosal development correlates with the expression of the transgene, because neurons in the superficial layers of the motor cortex, where transgene expression is low, show normal contralateral projection through the corpus callosum. In addition, multiple EphA receptors are expressed in callosal neurons and ephrin-A5 stimulates neurite outgrowth of callosal neurons in vitro. The midline glia structures important for callosal axon midline crossing appear normal in the transgenic mice, suggesting that the defects are unrelated to defective guidance structures at the midline. These observations suggest critical functions for EphA receptor in establishing callosal connections during brain development.

Agenesis of Corpus Callosum↗
PubMed1988

Glutamate-positive corticocortical neurons in the somatic sensory areas I and II of cats.

Combined retrograde transport-immunocytochemical experiments were carried out on cats to study the morphology, laminar distribution, and percentages of corticocortical projecting neurons of somatosensory area I (SI) and II (SII) showing immunoreactivity to an antiserum raised against the amino acid glutamate (Glu). A previously characterized anti-Glu serum (Conti et al., 1987a, b; Hepler et al., 1987) was used in conjunction with HRP. This tracer was injected either in SI to label retrogradely neurons in ipsilateral SII (SII-SI association neurons) and contralateral SI (SI-SI callosal neurons) or in SII to label retrogradely neurons in ipsilateral SI (SI-SII association neurons) and contralateral SII (SII-SII callosal neurons). In sections from SI and SII processed for simultaneous visualization of Glu and HRP (Bowker et al., 1982), and containing the cells from which every one of the 4 corticocortical projections arise, 3 types of labeled neurons were observed: (1) single-labeled neurons showing the homogeneous brown immunoreaction product of Glu (Glu-positive neurons); (2) single-labeled neurons containing the granular black reaction product of retrogradely transported HRP (Glu-negative, association or callosal neurons); and (3) double-labeled neurons in which both the black HRP granules and the brown immunostaining were present (Glu-positive, association or callosal neurons). Double-labeled neurons were all pyramidal in shape and were distributed intermingled with Glu-negative corticocortical neurons in all layers of SI and SII known to give rise to association and callosal projections. Counts from 25-micron-thick sections showed that of 432 association and callosal neurons sampled from SI and SII, 214 (49.5%) were Glu-negative and 218 (50.5%) Glu-positive. In counts carried out on 5-micron-thick sections, the percentage of Glu-positive corticocortical neurons raised to about 70%. The 2 populations of single- and double-labeled corticocortical neurons showed no difference in their perikaryal cross-sectional areas. The present results show that a large fraction of association and callosal neurons of SI and SII are immunoreactive for Glu, and, therefore, these neurons probably use this excitatory amino acid, or a closely related compound, as neurotransmitter.

Animals↗
PubMed1994

Dynamic behaviors of growth cones extending in the corpus callosum of living cortical brain slices observed with video microscopy.

During development, axons of the mammalian corpus callosum must navigate across the midline to establish connections with corresponding targets in the contralateral cerebral cortex. To gain insight into how growth cones of callosal axons respond to putative guidance cues along this CNS pathway, we have used time-lapse video microscopy to observe dynamic behaviors of individual callosal growth cones extending in living brain slices from neonatal hamster sensorimotor cortex. Crystals of the lipophilic dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) were inserted into the cortex in vivo to label small populations of callosal axons and their growth cones. Subsequently, 400 microns brain slices that included the injection site, the corpus callosum, and the target cortex were placed in culture and viewed under low-light-level conditions with a silicon-intensified target (SIT) camera. Time-lapse video observations revealed striking differences in growth cone behaviors in different regions of the callosal pathway. In the tract, which is defined as the region of the callosal pathway from the injection site to the corresponding target cortex, growth cones advanced rapidly, displaying continual lamellipodial shape changes and filopodial exploration. Forward advance was sometimes interrupted by brief pauses or retraction. Growth cones in the target cortex had almost uniform compact shapes that were consistently smaller than those in the tract. In cortex, axons adhered to straight radial trajectories and their growth cones extended at only half the speed of those in the tract. Growth cones in subtarget regions of the callosum beneath cortical targets displayed complex behaviors characterized by long pauses, extension of transitory branches, and repeated cycles of collapse, withdrawal, and resurgence. Video observations suggested that extension of axons into cortical targets could occur by interstitial branching from callosal axons rather than by turning behaviors of the primary growth cones. These results suggest the existence of guidance cues distinct for each of these callosal regions that elicit characteristic growth cone behaviors.

Animals↗