MORPHOLOGY OF VISUAL CALLOSAL NEURONS WITH DIFFERENT LOCATIONS, CONTRALATERAL TARGETS OR PATTERNS OF DEVELOPMENT

In kittens, callosally projecting neurons were labeled by retrograde t ransport of FITC- (fluorescein isothiocyanate)- and TRITC- (tetramethy lrhodamine isothiocyanate)-conjugated latex microspheres injected in t wo different visual areas (17, 17/18, 19, or postero-medial lateral su prasylvian; PMLS) at postnatal day 3. At postnatal day 57 more than 12 00 labeled neurons in visual cortical areas were intracellularly injec ted with 3% lucifer yellow (LY) in perfusion-fixed slices of the contr alateral hemisphere. The distribution of labeled neurons was charted, and LY-filled neurons were classified on the basis of their area and l ayer of location, and dendritic pattern. The dendritic arbors of 120 n eurons were computer reconstructed. For the basal dendrites of supragr anular pyramidal neurons a statistical analysis of number of nodes, in ternodal and terminal segment lengths, and total dendritic length was run relative to the area of location and axonal projection. Connection s were stronger between homotopic than between heterotopic areas. Over all tangential and laminar distributions depended on the area injected . Qualitative morphological differences were found among callosally pr ojecting neurons, related to the area of location, not to that of proj ection. In all projections from areas 17 and 18, pyramidal and spinous stellate neurons were found in supragranular layers. In contrast, spi nous stellate neurons lacked in projections from area 19, 21 a, PMLS a nd postero-lateral lateral suprasylvian (PLLS). In all areas, the infr agranular neurons showed heterogeneous typology, but in PMLS no fusifo rm cells were found. Quantitative analysis of basal dendrites did not reveal significant differences in total dendritic length, terminal, or intermediate segment length among neurons in area 17 or 18, and this was related to whether they projected to contralateral areas 17-18 or PM LS. All injections produced exuberant labeling in area 17. No diffe rences could be found between neurons in area 17 (with transient axons through the corpus callosum) and neurons near the 17/18 border (which maintain projections to the corpus callosum). In conclusion, morpholo gy of callosally projecting neurons seems to relate more to intrinsic specificities in the cellular composition of each area than to the are a of contralateral axonal projection or the fate of callosal axons.