The present study examines patterns of connectivity between the primary somatosensory cortex of the rat (SI) and surrounding cortical areas also implicated in the processing of somatosensory information. The impetus for the study was the recent reports of major differences in the organization of cortex lateral and caudal to the SI in two other rodent species; the mouse (Carvell and Simons, '86: Somatosens. Res. 3:213-237; '87: J. Comp. Neurol. 265:409-427) and the grey squirrel (Krubitzer et al., '86: J. Comp. Neurol 250: 403-430). Corticocortical connections between the somatosensory areas of the rat parietal cortex were examined by using the combined retrograde and anterograde transport of horseradish peroxidase as well as the retrograde transport of fluorescent tracers. Tracer injections were made into different locations within SI and dysgranular cortex as well as into more lateral regions of parietal cortex. The tangential patterns of distribution both of callosal connections and of cytochrome oxidase activity together provided points of reference in determining the relation between injection sites and the resultant patterns of label. The results indicate that two distinct somatosensory areas, SI and the dysgranular cortex, are interconnected with a further lateral somatosensory area referred to as the second somatosensory area (SII). These projections are organized in a topographic fashion, which we interpret as evidence for a single representation of the body surface in SII. The three somatosensory areas each exhibit unique laminar patterns of ipsilateral corticocortical projection neurons and terminations. In SI, projection neurons are found mainly in layers II, III, and Va, and terminations are largely restricted to the infragranular layers. In the dysgranular cortex, projection neurons and terminations are found in all layers except layer I in which only terminal label is detectable and layer Vb in which notably fewer neurons are labelled. In SII, projection neurons and terminations are found in all layers except layer I and are particularly dense in lower layer III and layer IV. Further, whereas the laminar and areal distributions of ipsilateral and contralateral corticocortical projections largely overlap in both SI and the dysgranular cortex, in SII they tend to be areally segregated. Neurons projecting bilaterally to both ipsilateral and contralateral somatosensory cortex were equally rare in all three somatosensory areas. These results are discussed in relation to the organization of SII in other rodent species, and it is concluded that in the rat, like the mouse, cortex lateral and caudal to SI contains a single representation of the body surface.
A characteristic feature of the rat somatosensory neocortex is a discrete topographic representation of the facial whiskers. Afferent fibers projecting to this vibrissae representation were "bulk-labeled" by injecting horseradish peroxidase into the white matter. Terminal arbors with the morphological characteristics of Lorente de No's (1949) "specific" thalamocortical afferents were then reconstructed through serial sections. These terminal arbors, characterized by the discrete organization of their dense plexus in layer IV, have a laminar distribution of boutons that parallels the laminar pattern of terminal degeneration resulting from lesions of the ventral posterior nucleus of the thalamus. The regional distribution of different-sized arbors corresponds to the distribution of vibrissae-related clusters of different sizes. Larger arbors were found in the posteromedial region corresponding to the mystacial vibrissae representation, while smaller arbors were found in the anterolateral region corresponding to the representation of the anterior sinus hairs. Terminal arbors were also reconstructed from sections stained simultaneously to demonstrate the pattern of vibrissae-related clusters. The greatest concentration of boutons on these axons occurred within a single vibrissae-related cluster. Furthermore, when 2 fibers terminated within a single cluster, their terminal arbors appeared to be largely coextensive. The morphology, size, and distribution of these terminal arbors support the hypothesis that the layer IV plexus of a single specific thalamocortical afferent tends to fill a vibrissae-related cluster. Thus, the organization of specific thalamocortical afferents may be responsible for clustered organization within the somatotopic map of the rodent neocortex.
The initial ingrowth of thalamocortical afferents into the presumptive somatosensory cortex was examined in the fetal rat. Thalamic fibers were labeled in fixed brains with the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI). On embryonic day 16, thalamocortical afferents arrive in the neocortex and course tangentially within the intermediate zone immediately underneath the cortical plate. By embryonic day 17, thalamocortical fibers have begun their radial growth into cortex and their arbors span the cell-sparse zone between layer VIb and the bottom of the cortical plate. By the day of birth (embryonic day 21), thalamocortical fibers form a dense plexus within layers VI and V below the dense cortical plate. Our observations indicate that in the rat thalamic afferents arrive in the cortex at a very early age and arborize within the forming cortical layers without an apparent "waiting" period.The mechanisms that control the development and differentiation of the neocortex remain the subject ofintense interest. Recent suggestions (1, 2) that thalamocortical projections play some role in these processes imply that thalamocortical projections make contact with the neocortex at a relatively early stage. However, the available evidence on this point is equivocal. On the one hand, Lund and Mustari (3) have reported that thalamocortical fibers reach the occipital cortex of the rat on embryonic day (E) 18 and then progressively invade the visual cortex on the following days. On the other hand, it has been reported that in the rat somatosensory cortex the thalamocortical fibers accumulate in the white matter below the cortical layers for several days and commence their innervation of the neocortical layers at about the time of birth (4, 5). This observation, as well as those of several other investigators (6,7), has led to the generally held belief that the development of thalamocortical projections is characterized by a "waiting" period. Further, it has been suggested that during this waiting period thalamocortical projections make contact with a transient population of cortical neurons, the subplate, and that these transient interactions play a particularly important role in the development and differentiation of the neocortex (8).We chose to investigate this question further because until recently the methods available to delineate thalamocortical projections were not particularly well-suited for use in the fetal brain. The introduction of carbocyanine dyes, such as 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), as neuronal tracers in previously fixed tissue (9) overcomes many of the shortcomings of previous methods. Our study focuses on the earliest development of thalamocortical projections to the presumptive somatosensory cortex of the rat (E16 to the day of birth). The later development of these afferents and their adult morphology has been studied (10-12). The available evidence suggests that the somatosensory cortex is one of the first po...
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