TWO SYNAPTIC COMPARTMENTS AND TWO FIBER DIVISIONS OF CINGULUM BUNDLE DIFFERENTIALLY CONNECT TRANSMODAL FRONTO-PARIETAL AREAS VIA CINGULATE BELTS DURING EARLY MID-GESTATION IN HUMANS

Previously, we have described (Kostović et al., FENS Forum 2004) differential distribution of synapses along ventro-dorsal axis (mesocortex-neocortex) in the early midfetal cingular cortex. Herewith, we analyzed rostro-caudal extent of synaptic and fibrillar compartments within cingulate cytoarchitectonic belts on postmortem human material 11-17 PCW. We have found that both synaptic compartments subplate (SP), and marginal zone (MZ) of the cingular cortical belts, form a continuous tangential nexus (synaptically engaged neurons, axonal plexuses, ECM-glia) along rostro- caudal axis. However, they show differential distribution in the ratio in deep vs superficial synaptic compartments along ventro-dorsal gradient. Deep-superficial synaptic nexus predominates in the dorsal isocortical portion of cingulate, while superficial MZ synaptic nexus is main connectivity route in ventral mesocortical cingulate. Cingulum bundle, the main associative fiber system, is composed of (1) a well-developed ventro-medial division, which is strongly AchE reactive and may be visualized on in vitro DTI images, and (2) dorso-lateral division, which is poorly developed. In conclusion, both synaptic compartments in cingulate cortical belts show differential ventro-dorsal gradients (mesocortex-isocortex) but together form pronounced tangential nexus that connects transmodal medial cortical regions along rostro-caudal (fronto-parietal) axis. Cingulum bundle consists of two major components: early developing division, which contains subcortical afferents (from thalamus and basal forebrain) together with efferent fibres of cingulate projection neurons, and secondly late maturing division, which contains long cortico-cortical fibres. Altogethe,r our results indicate early midfetal development of the main, medial connectivity backbone in the human brain.