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PubMed1986

Synaptic connections of callosal projection neurons in the vibrissal region of mouse primary motor cortex: an electron microscopic/horseradish peroxidase study.

Reciprocal axonal projections between homotypic areas of the vibrissal region of mouse primary motor cortex (MsI) (Porter and White: Neurosci. Lett. 47:37-40, '84) suggested the existence of reciprocal synaptic connections between callosal projection neurons and callosal afferents. In the present study, the retrograde transport of horseradish peroxidase (HRP) was combined with lesion-induced degeneration to identify synapses between callosal afferents and callosal neurons in the corresponding region of the contralateral cortex. The procedure was as follows: MsI was injected with HRP and aspirated on the following day. After 4 days, the animals were perfused and motor cortex was processed for HRP according to a variation of the Adams (Brain Res. 176:33-47,'77) technique, and postfixed in OsO4. The methods used consistently filled fine dendritic branches and spines with dense reaction product, thus allowing examination of synaptic contacts with these processes. All callosal projection neurons were identified as pyramidal neurons, having somata in cortical layers II/III and V. Labeled cells from each of the two levels were prepared for electron microscopy, and that part of each cell's apical dendrite that traversed the superficial cortical layers, where most callosal axons terminate, was cut in an unbroken series of thin sections. Micrographs were taken of all labeled profiles in each thin section, and tracings of the profiles were assembled to reconstruct the apical dendrites. Data on the distribution, type, and amount of callosal and other synapses with the shaft and spines of the apical dendrites were obtained by examining the reconstructions. In addition, profiles of basal dendrites of layer II/III cells were examined in thin sections to ascertain the numbers of callosal and other synapses formed with their shafts and spines. The proportion of synapses that each dendrite formed with callosal axon terminals was compared to the concentration of callosal afferents in the neuropil. Dendrites of both layer II/III and layer V pyramidal cells synapsed with callosal axon terminals. The apical and basal dendrites of layer II/III neurons formed a similar proportion of their synapses with callosal afferents, and this was similar to the concentration of callosal synapses in the surrounding neuropil. Segments of apical dendrites belonging to layer V and layer II/III neurons course through neuropil containing nearly the same concentration of degenerating callosal terminals, but the layer V cells form fewer callosal synapses.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Topography of interhemispheric connections in neocortex of mice with congenital deficiencies of the callosal commissure.

Normally, axons within the corpus callosum are ordered according to the cortical regions from which they originate, and callosal cells and terminations form elaborate cortical patterns related to the underlying topographic representations of the sensory periphery. About 30% of mice of the BALB/c strain show congenital deficiencies of the callosal commissure which range from total absence of the corpus callosum to a moderate reduction in the size of this commissure. In the light of current theories about the origin of these callosal deficiencies, it seems likely that fibers crossing the midplane in mutant mice have to circumvent local disturbances along their migration path. Since these disturbances in fiber trajectory may, in turn, alter the overall pattern of callosal projections, we set out to investigate whether the distribution of callosal connections in mice with marked deficiencies of the corpus callosum is as ordered as in normal mice. In groups of normal and mutant mice, we used multiple injections of horseradish peroxidase to reveal the overall distribution of callosal connections and restricted injections of horseradish peroxidase conjugated with wheat germ agglutinin to reveal finer aspects of the organization of the callosal pathway in these animals. Our results show that the number of labeled cells is reduced in mice with a small corpus callosum and that no labeled cells are present in the neocortex of acallosal mice. Furthermore, the topographic distribution of fibers within the corpus callosum of mutant mice can be significantly less ordered than in normal mice. However, even in mice with extreme deficiencies of the corpus callosum, callosal fibers originate from and terminate in all major areas of the cortex, and, within these areas, callosal cells and terminations are distributed according to the normal plan. The laminar distribution of callosal cells also appears normal in these mice. These findings indicate that gross developmental anomalies of the corpus callosum do not prevent normal specification of the callosal pattern during development. Within the context of current theories about the origin of congenital callosal deficiencies, our findings suggest that callosal fibers are able to establish appropriate contralateral connections in spite of alterations of their migration route. They also suggest that fiber topography within the corpus callosum does not play an important role in guiding migrating axons to their correct contralateral targets. Finally, our failure to find labeled fibers within the anterior commissure indicates that this commissure does not serve as an alternative route for deviated callosal axons.

Agenesis of Corpus Callosum↗
PubMed2001

Dynamics of callose deposition and beta-1,3-glucanase expression during reproductive events in sexual and apomictic Hieracium.

Callose accumulates in the walls of cells undergoing megasporogenesis during embryo sac formation in angiosperm ovules. Deficiencies in callose deposition have been observed in apomictic plants and causal linkages between altered callose deposition and apomictic initiation proposed. In apomictic Hieracium, embryo sacs initiate by sexual and apomictic processes within an ovule, but sexual development terminates in successful apomicts. Callose deposition and the events that lead to sexual termination were examined in different Hieracium apomicts that form initials pre- and post-meiosis. In apomictic plants, callose was not detected in initial cell walls and deficiencies in callose deposition were not observed in cells undergoing megasporogenesis. Multiple initial formation pre-meiosis resulted in physical distortion of cells undergoing megasporogenesis, persistence of callose and termination of the sexual pathway. In apomictic plants, callose persistence did not correlate with altered spatial or temporal expression of a beta-1,3-glucanase gene (HpGluc) encoding a putative callose-degrading enzyme. Expression analysis indicated HpGluc might function during ovule growth and embryo sac expansion in addition to callose dissolution in sexual and apomictic plants. Initial formation pre-meiosis might therefore limit the access of HpGluc protein to callose substrate while the expansion of aposporous embryo sacs is promoted. Callose deposition and dissolution during megasporogenesis were unaffected when initials formed post-meiosis, indicating other events cause sexual termination. Apomixis in Hieracium is not caused by changes in callose distribution but by events that lead to initial cell formation. The timing of initial formation can in turn influence callose dissolution.

Cell Wall↗
PubMed1991

Organization of callosal connections in the visual cortex of the rabbit following neonatal enucleation, dark rearing, and strobe rearing.

The organization of visual callosal projections was studied in (1) normal adult rabbits; (2) adult rabbits which had undergone monocular enucleation (ME) or binocular enucleation (BE) at birth; and (3) adult rabbits which had been deprived of normal visual experience during development by dark rearing (DR) or strobe rearing (SR). Previously published observations (Murphy and Grigonis, Behav Brain Res 30:151, 1988) on callosal organization in adult rabbits in which retinal ganglion cell activity was eliminated during development by intraocular tetrodotoxin (TTX) injections, are also summarized for comparison with these data. The tangential extent of the callosal cell zone was significantly larger than normal in DR, TTX, and ME rabbits, was unchanged in BE rabbits, and was significantly reduced in SR rabbits. An analysis of the laminar distribution of the callosal cells revealed a significant increase in the percentage of callosal cells in lamina IV in ME, DR, and TTX animals. Measurements of density of callosal cells showed a significant increase in the density of the callosal projection in ME and SR rabbits and a decrease in density in BE rabbits compared with normal. The data suggest that the mechanisms involved in the development of the tangential and laminar organization of the callosal cell zone are different. In addition, the data suggest that the mechanisms involved in the maintenance of callosal projections are different from the mechanisms involved in the elimination of callosal projections during development. The effects of these developmental manipulations on callosal organization in other mammals are reviewed and compared with the effects in rabbits. The data suggest that species differences in the degree of maturity of the visual system at birth and in the extent of callosal development at the time of eye opening, may underlie species differences in the effects of these manipulations on the organization of visual callosal projections during development.

Animals↗
PubMed1982

The distribution of the cells of origin of callosal projections in cat visual cortex.

The distribution of neurons projecting through the corpus callosum (callosal neurons) was examined in retinotopically defined areas of cat visual cortex. As many callosal neurons as possible were labeled in a single animal by surgically dividing the posterior two-thirds of the corpus callosum and exposing the cut ends of callosal axons to horseradish peroxidase. The distribution of callosal neurons within a visual field representation was related to standard electrophysiological maps as well as to recording sites marked by electrolytic lesions. Callosal neurons were found in every retinotopically defined cortical area. The portion of the visual field representation that contained callosal neurons increased progressively from the area 17/18 border to area 19, to areas 20 and 21, and to the lateral suprasylvian visual areas. In area 17, the portion of the visual field representation containing callosal neurons extended from the vertical meridian out to a maximum of 10 degrees azimuth. In the posteromedial lateral suprasylvian visual area, callosal neurons were present in a region extending from the vertical meridian representation out to a representation of 60 degrees azimuth. Most callosal neurons were medium to large pyramids at the border of layers III and IV. A few layer IV stellates were among the callosal neurons of areas 17 and 18. In area 19 and even more so in the lateral suprasylvian visual areas, callosal neurons included pyramidal and fusiform-shaped cells in layers V and VI. The laminar distributions of callosal neurons in areas 20 and 21 were similar to those of area 19 and the lateral suprasylvian visual areas. The widespread distribution of callosal neurons in areas 20 and 21 and in the lateral suprasylvian visual areas suggests that the regions of peripheral visual field representation in cat cortex, as well as the representations of the vertical meridian, have access to the opposite cerebral hemisphere. This finding is significant in light of demonstrations of the importance of some of these cortical areas in the interhemispheric transfer of visual learning.

Animals↗
PubMed1998

Callosal terminals in the rat prefrontal cortex: synaptic targets and association with GABA-immunoreactive structures.

The callosal projections of the cerebral cortex play an important role in the functional integration of the two hemispheres, and the anatomy of these connections has been extensively studied in primary sensory and motor regions. In the present investigation, we examined the synaptic targets of callosal terminals in a limbic association area, the prefrontal cortex (PFC) in the rat. In addition, we examined the relationship of callosal afferents to GABA local circuit neurons within the PFC. Callosal terminals were labeled by either anterograde transport of Phaseolus vulgaris leucoagglutinin from superficial or deep layers or by anterograde degeneration following electrolytic lesion of the contralateral PFC. Callosal terminals in either the superficial or deep layers labeled by either method formed primarily asymmetric axo-spinous synapses (approximately 95%), while the remainder formed axo-dendritic synapses. Some of the dendrites postsynaptic to callosal terminals exhibited a morphology characteristic of local circuit neurons. This observation was confirmed in tissue immunolabeled for GABA, in which degenerating callosal terminals sometimes formed asymmetric synapses on GABA-labeled dendrites. In addition, GABA-labeled terminals and callosal afferents were sometimes observed to converge onto common postsynaptic dendritic shafts or spines within the PFC. These results indicate that callosal terminals in limbic association cortex, consistent with sensory and motor cortices, primarily target the spines of pyramidal neurons. In addition, the results suggest that callosal afferents to the PFC interact with GABA local circuit neurons at multiple levels. Specifically, a proportion of callosal terminals appear to provide excitatory drive to GABA cells, while GABA terminals may modulate the excitation from callosal inputs to the distal dendrites and spines of PFC pyramidal neurons.

Afferent Pathways↗
PubMed1979

The anatomical substrate of callosal messages from SI and SII in the cat.

Horseradish peroxidase (HRP) was injected into the first (SI) or second (SII) somatosensory areas of 21 adult cats. The radial and tangential (normal and parallel to the pial surface, respectively) distribution and morphology of the callosal neurons were studied. HRP injections were combined with single unit recording in the contralateral cortex in order to determine which part of the somatosensory periphery is represented within the regions containing callosal neurons, the callosal (efferent) zones, in SI and SII. The callosal zone of SI extends over the trunk and part of the forepaw representation. In the forepaw and hindlimb representations callosal neurons projecting only to the contralateral SII are found, while in the trunk representation callosal neurons projecting to contralateral SI or SII are found. The callosal zone in SII extends widely throughout the forepaw representation in this area and projects to the contralateral SII but not to SI. In both SI and SII the callosal neurons are mainly located in layer III. A few of them are also found in layer VI. They are very rare in other layers. Callosal neurons in layer III are mostly pyramidal but exceptionally stellate; in layer VI they are pyramidal, triangular, and occasionally stellate. These data indicate that transformations of the cortical somatosensory maps are achieved in the message sent through the corpus callosum. These transformations are i) determined by the extent and location of the callosal zones and perhaps by the distribution of callosal neurons within them, ii) different in different areas, iii) different in a same area, according to the cortical targets to which they are conveyed. The existence of callosal connections originated from areas of distal forepaw representation supplies a possible anatomical substrate for those types of intermanual transfer of tactile learning which depend upon the integrity of the corpus callosum.

Animals↗
PubMed2001

Callosal connections correlate preferentially with ipsilateral cortical domains in cat areas 17 and 18, and with contralateral domains in the 17/18 transition zone.

Previous studies have shown that the distribution of callosal connections in the 17/18 callosal zone of the cat is patchy at a small scale, but the mechanisms that determine this periodic pattern remain unclear. The present study investigated this issue by correlating the distribution of retrogradely labeled callosal cells with the underlying patterns of ocular dominance columns (ODCs) revealed transneuronally after intraocular injections of wheat germ agglutinin-horseradish peroxidase. The density of labeled callosal cells was found to vary significantly between adjacent territories dominated by different eyes, indicating that the distribution of callosal cells is significantly biased toward domains that are eye specific. Moreover, callosal connections relate to the pattern of ODCs in a rather unique way: callosal cells correlate preferentially with contralateral ODCs within the 17/18 transition zone (TZ), and with ipsilateral ODCs in regions of areas 17 and 18 located outside the TZ. Similar results were obtained in cats raised with strabismus, indicating that the overlap between right and left ODCs present in normal cats does not influence the correlation between callosal neurons and ODCs. The results are consistent with the hypothesis that callosal linkages are stabilized during development by interhemispheric correlated activity driven by bilateral projections from temporal retina. It is proposed that developmental constraints imposed by both this retinally driven mechanism and the pattern of ODCs are likely to determine not only the association of callosal clusters with specific sets of ODCs, but also important aspects of the functional characteristics of the callosal pathway in cat striate cortex.

Animals↗
PubMed1982

Modification of visual callosal projections in rats.

The effects of a variety of developmental manipulations on the distribution of the callosal pathway to visual cortex were examined by using the Fink Heimer technique in adult rats. First, the callosal projections in albino and pigmented rats were compared and found to be similar. The callosal pathway was limited in area 17 to a region adjoining its lateral border with area 18a. Second, dark-reared rats were found to have normal callosal projections. Third, rats bilaterally enucleated at birth and expanded callosal inputs within area 17. Fourth, monocular enucleation at birth produced an expanded callosal pathway to area 17 contralateral to the enucleation and normal callosal projections to the opposite hemisphere. The expanded callosal inputs after enucleation showed a patchy distribution and usually avoided the most medial part of area 17. Fifth, a reduction in the callosal projections to the area 17/18a border was found after neonatal unilateral optic tract lesions. Sixth, expanded callosal inputs to area 17 were found following unilateral thalamic lesions at birth. The abnormal projection occupied mainly layers IV and III. The results of the different experiments indicate that the detailed distribution of the visual callosal projection within area 17 depends heavily on the organization of the retinogeniculocortical pathways to each hemisphere.

Animals↗
PubMed1986

Ontogenetic change in the distribution of callosal projection neurons in the postcentral gyrus of the fetal rhesus monkey.

In the postcentral gyrus of the mature rhesus monkey the distribution of callosal projection neurons is discontinuous. The density of callosal projection neurons, which are mainly located in the supragranular layers, varies both within and across cytoarchitectonic areas (Killackey et al., '83). In the present study, we investigated the ontogeny of corpus callosum projections of the postcentral gyrus in five fetal rhesus monkeys, ranging in age from embryonic day (E) 108 to E 133. Multiple large injections of horseradish peroxidase that involved the underlying white matter were made into the postcentral gyrus of one hemisphere and the distribution of labeled neurons in the ipsilateral thalamus and the other hemisphere was determined. The pattern of thalamic label indicated that the tracer was effectively transported from all portions of the postcentral gyrus. We found that the areal distribution pattern of labeled callosal projection neurons varied at the different fetal ages. At early fetal ages (E 108, E 111, and E 119) callosal projection neurons were continuously distributed throughout the postcentral gyrus. As in the adult animal, the vast majority of labeled callosal projection neurons were found in the supragranular layers, although a few labeled cells were located in the infragranular layers. From the earliest age, there was regional variation in the width of the band of labeled supragranular callosal projection neurons. The difference between the precentral and postcentral gyrus was most obvious, but there was also a difference between anterior and posterior portions of the postcentral gyrus. The first indication of some discontinuity in the distribution of callosal projection neurons was noted at E 126. By E 133, approximately 1 month before birth, the distribution of callosal projection neurons appeared remarkably mature. On E 119 aggregations of anterograde label could be detected in restricted portions of the posterior postcentral gyrus beneath the cortical layers. By E 133 anterograde label was found within the cortical layers (most densely in layer IV) in these regions of the postcentral gyrus. Thus, the emergence of the discrete pattern of callosal projection neurons appears to be temporally correlated with the ingrowth of callosal afferents. On the basis of these observations, as well as those of others (discussed in the text), we propose that the ontogenetic changes in the distribution of callosal projection neurons reflect the unique strategy employed by cortical projection neurons in establishing their patterns of connectivity. It is hypothesized that this strategy may involve multiple processes.

Animals↗
PubMed1982

Synaptic proliferation in the auditory cortex of the young adult rat following callosal lesions.

The long-term effects of partial deafferentation in the neocortex of adult rats were studied in four-month old rats in which the corpus callosum had been completely sectioned when they were one-month old. Quantitative light microscopy was used to identify morphological changes in the auditory cortex resulting from the loss of established callosal connections. Particular attention was directed at those cortical layers known to receive the heaviest callosal projection (layers II and III) and at neurons known to be postsynaptic to callosal afferents (layer V pyramidal neurons). The comparative analysis of both semithin plastic sections and Golgi-impregnated material from long-term, callosally-lesioned rats and age-matched control animals reveals no differences in the overall cortical thickness, the thickness of cortical layers, the numbers of neurons or the density of spines along apical dendrites of layer V pyramidal neurons. However, as a result of the callosal lesion, large diameter apical dendrites are significantly thinner in the callosally deafferented cortex and there is a small increase in the number of neuroglial cells in the deeper cortical layers. To determine whether another system of afferents to the auditory cortex spreads into the deafferented callosal domain, geniculate lesions were made in long-term, callosally-lesioned animals and age-matched controls. The terminal projection patterns of thalamic afferents were compared using the Fink-Heimer technique and quantitative electron microscopy. Normally in the auditory cortex there is only a small region of overlap between the terminal projection fields of callosal afferents and thalamic afferents, the latter projecting chiefly to layer IV and low layer III. However, three months after callosal lesions, thalamic axons had proliferated superficially into part of the callosal domain. Furthermore, in the normal auditory cortex after geniculate lesions, there were three rostrocaudally oriented bands of relatively dense thalamocortical terminal degeneration separated by regions of less dense degeneration. In the doubly lesioned animals these bands of degeneration were less distinct due to a proliferation of thalamic axons into the regions characterized by sparse projections.

Aging↗
PubMed1995

Cellular aspects of callosal connections and their development.

Detailed visualization, three-dimensional reconstruction, and quantification of individual callosal axons interconnecting the visual areas 17 and 18 of the cat was undertaken in order to clarify the structural basis for interhemispheric interaction. These studies have generated the notion of macro- vs micro-organization of callosal connections. The first refers to the global distribution of callosal connections in the hemisphere as well as to the pattern of area-to-area connections. The latter refers to the fine radial and tangential distributions of individual callosal axons. A discrete disjunctive, 'columnar' pattern of termination of callosal axons, previously unknown for the visual areas, was found. The consequence of caliber and distribution of callosal axons and their branches on the dynamic properties of interhemispheric interactions were analyzed by computer simulations. These studies suggested that callosal axons could synchronize activity within and between the hemispheres in ways relevant for the 'binding' of perceptual features. These new concepts prompted a reexamination of the normal development of callosal connections. The central issue is whether intrinsic developmental programs, or else cellular interactions open to environmental information specify the morphological substrate of interhemispheric interactions. The answer to this question is still incomplete. In development, transient, widespread arbors of callosal axons, which could provide the basis for plastic changes of callosal connections were found in the white matter and the deep cortical layers. On the other hand, growth into the cortex and synaptogenesis of callosal axons appear to be highly, topographically specific albeit not necessarily independent of visual experience.

Animals↗
PubMed1991

Organization, Development and Enucleation-induced Alterations in the Visual Callosal Projection of the Hamster: Single Axon Tracing with Phaseolus vulgaris leucoagglutinin and Di-I.

The distribution of callosal axons interconnecting lateral area 17 and medial area 18 of the rodent's occipital cortex is dramatically altered by neonatal enucleation, but it is not known how this manipulation affects the morphology of individual callosal axons or whether the enucleation-induced changes in this pathway reflect maintenance of a transient developmental state by these fibres. In the present study, these questions were addressed by tracing the individual callosal axons in normal adult and neonatally enucleated adult hamsters with Phaseolus vulgaris leucoagglutinin (PHAL) and by anterograde labelling of developing callosal axons with the carbocyanine dye, Di-I. In normal adults, injections of PHAL into the region of the 17 - 18a border produced dense labelling in all layers in the region of the contralateral 17 - 18a border. Larger injections resulted in callosal labelling that extended across the lateral one-half of area 17, primarily in layers I and V. Thirty-four callosal axons from normal adult hamsters were reconstructed through all the cortical laminae. Most of these had very simple terminal arbors. They gave off short collaterals in the infragranular layers and branched more extensively in the uppermost part of layer II - III and in lamina I. Small injections of PHAL into the occipital cortex of neonatally enucleated adult hamsters resulted in labelled axons throughout most of areas 17 and 18a in the contralateral hemisphere. The terminal arbors of most individual callosal axons in eyeless hamsters were not appreciably different from those in sighted animals. However, 26.8% of 28 fibres reconstructed through all cortical laminae in the neonatally enucleated hamsters had much more widespread branches than any of the axons recovered from normal hamsters. As a result, the average total length of the callosal axons from the blinded hamsters was significantly greater than that for such fibres from the sighted animals. Anterograde labelling with Di-I demonstrated axons in the anterior commissure and anterior part of the corpus callosum on P-0. Labelled fibres extended into the white matter underlying the occipital cortex on P-1 and entered the cortical plate on P-2. Some of these axons reached into the marginal layer. Many developing callosal axons had short branches in the white matter, but generally extended only a single collateral into the cortical grey matter. Callosal axons in perinatal animals branched very little within the cortex and, in this respect, resembled fibres labelled with PHAL in adult hamsters. These results support the conclusion that the expanded tangential distribution of the occipital callosal projection in neonatally enucleated adult hamsters results, at least in part, from individual axons with abnormally widespread terminal arbors which are not present in large numbers at any time during normal development.

Journal Article↗
PubMed1978

Sources and terminations of callosal axons related to binaural and frequency maps in primary auditory cortex of the cat.

The distributions of sources and terminals of callosal fibers in the high-frequency representation of AI were related to binaural and frequency maps in combined anatomical and electrophysiological experiments. Sources of callosal axons were retrogradely labeled with HRP. Distributions of axon terminals were determined by autoradiographic labeling with [3H]-proline and anterograde degeneration following callosal section. Regions in which cells exhibit summation or ipsilateral dominance and suppression contain higher concentrations of sources and terminals of callosal fibers than do regions in which cells exhibit monaural contralateral responses or contralateral dominance and suppression. Callosal axon terminals aggregate into columns. In sections cut parallel to the cortical surface callosal columns take on complex forms that exhibit certain consistent features. Two prominent elongated columns separated by a narrow zone of sparse callosal innervation run in a rostrodorsal to caudoventral direction through AI crossing several octaves of the frequency representation. Ventral to these columns, along the AI-AII border, less densely labeled callosal columns are in evidence. Low frequency representations of AI are interconnected but details of their innervation patterns were not worked out. Outside of AI there are regions that contain complex configurations of callosal columns. Several morphologically distinct types of neurons, located in laminae III through VI, were retrogradely labeled following injections of HRP into the opposite AI. About 94% of callosal neurons are pyramidal cells of layers III and IV.

Animals↗
PubMed1984

Callosal connections of the posterior neocortex in normal-eyed, congenitally anophthalmic, and neonatally enucleated mice.

Following multiple injections of horseradish peroxidase into the posterior neocortex of one hemisphere, we examined the distribution of retrogradely labeled cells and anterogradely labeled terminations in tangential and coronal sections through contralateral areas 17 and 18 in three groups of adult mice: normal-eyed (ZRDCT-n and C57Bl/6J strains), congenitally anophthalmic (ZRDCT-an strain), neonatally enucleated (ZRDCT-n strain). In agreement with previous studies, we observed that the pattern of callosal connections in areas 17 and 18 of normal-eyed mice contains the following features: (1) a dense band of callosal cells and terminations separating the interiors of areas 17 and 18, which have relatively few callosal connections, (2) a ring-like configuration anterolateral to area 17, (3) a region of dense labeling lateral to area 18, (4) a narrow band of labeling bridging the posterior portion of area 18, and (5) a region of labeling anteromedial to area 17. We find that all these features of the normal callosal pattern are recognizable in congenitally anophthalmic mice. Their presence in mice that never had eyes supports the hypothesis that central visual pathways can develop many aspects of their connectivity in the absence of input from the periphery. However, we also find that the details of certain features of the callosal pattern in congenitally eyeless mice often differ from those of the same features in normal-eyed mice, and that the between-animal variability in the appearance of these features is higher in eyeless mice. These latter findings indicate that the eyes are needed during normal development to fine-tune the pattern of callosal connections. Our results also reveal that the callosal pattern in neonatally enucleated mice does not differ significantly from that in congenitally anophthalmic mice, indicating that the period in which the eyes guide callosal development extends into postnatal life. While the present data do not delineate the time course of this period, the finding of similarly abnormal callosal patterns in congenitally anophthalmic and neonatally enucleated mice suggests that the eyes exert little if any influence prenatally. Finally, examination of coronal sections indicates that the laminar distribution of callosal connections develops normally in both groups of eyeless mice.

Animals↗
PubMed1986

Callosal projections from area SII to SI in monkeys: anatomical organization and comparison with association projections.

The present research was aimed at ascertaining in the macaque monkey the reciprocity of the heterotopical callosal connections between SI and SII, with particular regard to the connectivity of the hand representation, and at comparing the topographical and laminar pattern of these callosal connections with those of association connections entertained by these areas. Horseradish peroxidase (HRP) was unilaterally injected into area SI in five monkeys. The sites of HRP delivery included the trunk and the hand zones preliminarily identified by recording multi-unit responses to peripheral stimulation by means of microelectrodes. Anterograde and retrograde labelling was studied in SII of both sides. The results showed the complete reciprocity of the heterotopical callosal connections between SI and SII. In the latter area both callosal axon terminals and neurones were found, which were labelled from either the trunk or the hand zone of contralateral SI. Labelling of callosal axon terminals occurred mainly in layer IV and in the lowermost part of layer III. Labelled callosal neurones were mainly in the lower half of layer III, whereas few occurred in infragranular layers. Topographically, the distribution of callosal terminals and cell bodies duplicated the distribution of association terminals and cell bodies labelled in SII on the side ipsilateral to HRP injection. The laminar pattern of termination of association fibres from SI was similar to that of callosal fibres. However, the distribution of association-projecting neurones in SII showed a striking difference from that of callosal-projecting neurones. Unlike the latter neurones, which were mainly located in supragranular layers, association cell bodies overwhelmingly dwelt in layers V and VI and were less numerous in layers II and III. This laminar pattern of association SII-SI cells corresponds to the "feed-backward" model and fits the laminar pattern of their axon terminations (Friedman: Brain Res. 273: 147-151, '83). The association and callosal inputs and outputs of area SII are discussed in relation to the function of the forward and backward type of reciprocal connections entertained with SI in the ipsilateral hemisphere and to the function of SII in the interhemispheric exchange of somatosensory information.

Animals↗
PubMed1989

Callosal connections of the somatic sensory areas II and IV in the cat.

The homotopic and heterotopic callosal connections in the forelimb representations of the second (SII) and fourth (SIV) somatic sensory areas of cats were investigated by means of the axonal transport of horseradish peroxidase (HRP) in conjunction with microelectrode recording. The tracer was injected in the electrophysiologically identified hand and/or digit zone of SII (six cats) or SIV (four cats). The homotopic area in the contralateral hemisphere was explored with microelectrodes in five animals (three injected in SII and two in SIV) to map neuronal receptive fields. The aim was to correlate in the same experimental case the topography of labelled callosal neurons with the physiological map of the forelimb. Labelled cells and recording sites were plotted on planar maps reconstructed with the aid of a computer from serial coronal sections from the anterior ectosylvian gyrus. After SII injections, labelled callosal neurons were observed throughout the forelimb representation in the contralateral area, but in the tangential plane their distribution was uneven. Each somatotopic zone composing the forelimb map, that is, the arm, hand, and digit zones, contained several subzones in which callosal neurons were either dense or rare. Microelectrode explorations showed that receptive fields mapped from callosal and relatively acallosal subzones representing the same body part were similar in extent and location. After SIV injections, labelled callosal neurons were observed throughout the forelimb and proximal body representation of the contralateral area. Although slight regional variations in the density of labelled cells were apparent, no subzones bare of callosal labelling were observed in SIV. In both SII and SIV, callosal neurons were concentrated mainly in layer III, but a significant number was also evident in the infragranular layers. After HRP injections in the digit zone of SII or SIV, labelled cell bodies were also observed in heterotopic areas of the contralateral hemisphere. Most of these neurons were clustered in the medial bank of the coronal sulcus and in two other heterotopic cortical regions lying, respectively, in the anterior suprasylvian sulcus and in the lateral branch of the ansate sulcus. Some callosal cells interconnecting SII and SIV were also labelled. The results show that the distal forelimb zones in SII and SIV are callosally connected with the respective homotopic zones and with several somatosensory fields located heterotopically in the contralateral hemisphere.

Afferent Pathways↗
PubMed1990

Maturation of the corpus callosum of the rat: I. Influence of thyroid hormones on the topography of callosal projections.

The normal adult rat corpus callosum contains numerous axonal profiles that are immunoreactive for the high molecular weight subunit of the neurofilament triplet (NF-H). NF-H immunoreactivity develops gradually during the first 2 postnatal weeks. The expression of NF-H immunoreactivity is almost completely suppressed in rats rendered hypothyroid by neonatal treatment with propylthiouracil. To ensure that the cytoskeletal deficit was due to a shortage of thyroid hormones rather than to unspecific, toxic effects of propylthiouracil, hypothyroid animals kept on the propylthiouracil diet were given restorative thyroxine injections daily. Such animals express NF-H at normal levels. This suggests that the callosal axons may be arrested at an immature stage of development. The immaturity of the hypothyroid corpus callosum can also be revealed by a comparison of the myelin content in the corpus callosum between normal rats, hypothyroid rats, and hypothyroid rats under thyroxine therapy. Hypothyroid rats are severely deficient in myelin, and again this deficit can be corrected by postnatal thyroxine treatment. During normal callosal development, there is a progressive spatial restriction of the transcallosal projection that creates in the adult patches of callosally projecting cortex interposed by acallosal regions. Given the structural immaturity of the hypothyroid callosal axons, it was interesting to investigate the state of development of their topography. For this purpose, multiple injections of wheat germ agglutinin-horseradish peroxidase were made into the occipital and parietal cortices of adult hypothyroid animals. In normal rats, the majority of visual callosally projecting cells are located in three groups--in area 18b, at the boundary of area 17 and 18a, and in the lateral portion of area 18a. Within these areas projecting cells are concentrated in layers II-III, Va, and Vc-VIa. The callosal axon terminals are concentrated in these same regions, with a laminar distribution as far as the somata plus layer I. In the midportion of areas 17 and 18a, fewer callosal cells are found, and they occupy mainly layers Vc-VIa, as in the case for terminals in these same areas. In the parietal cortex, callosal cells and terminals are disposed in vertical arrays alternating with almost empty zones. Most are concentrated in layers III and V. The topography of the callosal axon terminal fields is unaffected by hypothyroidism. However, there is a dramatic redistribution of the callosally projecting cell somata.(ABSTRACT TRUNCATED AT 400 WORDS)

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