engleski

Immunohistochemical Characterization of Midline Cellular Structures Associated with Growing Corpus Callosum in Human Fetus

The telencephalon midline is an axonal choice point for a number of fiber systems, were axons respond to environmental guidance cues and/or fasciculate with other axons and project either ipsilaterally or contralaterally via the corpus callosum (CC). Several cell populations composing different midline structures: the masa commissuralis of early development, the "glial-neuronal" sling, the glial wedge, the subcallosal zone, the indusium griseum, have been implicated in callosal axon guidance. We analyzed thirteen fetal human post-mortem brains, age ranged from 15 weeks of gestation (w.g.) to newborn, processed with immunoperoxidase staining to enquire expression of axonal guidance molecules, neuronal markers, neurite-growth-related markers and glial markers in the cells and the extracellular matrix (ECM) of supra-, sub- and intracallosal moiety of the midline during development. We found transient SEMA3A expression in the median and the paramedian parts of anterior and posterior callosal extent at 20^th w.g. Intracallosally, cellular immunoreactivity was restricted to the "septa", which divide major axonal bundles, and to the ventral callosal surface, while callosal axonal bundles showed principally weak extracellular reactivity in the ventral part. At the same time, we found strong cellular and extracellular SEMA3A immunoreactivity in the upper subcallosal zone. Neogenin and ephrinA4 specific immunoreactivity showed very similar pattern, but expression predominantly in extracellular matrix with less positive soma and weaker overall staining compared to SEMA3A. TuJ1 and MAP1b specific immunoreactivity of commissural fibers through entire anterior-posterior extent of CC, lacking reactivity of cell bodies, was steadily present during analyzed developmental period. In addition, NeuN, MAP2 and SMI312 specific immunoreactivity of neurones prominent on the border between CC and adjoining subcallosal zone with coincidence of specific GFAP immunoreactivity, showed contemporary presence of neuronal and glial growing substrate. This work elucidate requirement of coexistence of number of different guidance mechanisms in the interhemispheric cerebral wall to achieve complexity of morphogenetic events during the growth of axons and formation of the CC.

semaphorin3A; development; telencephalon; contralateral projections

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