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The Role of Layer 4 in Thalamocortical Development

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Technical Report

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Uniformed Services University Of The Health Sciences Bethesda United States

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Thalamic innervation of cortex is a complex process in which specific regions of the thalamus must grow into precise cortical regions. Once within aspecific cortical area, thalamic afferents terminate largely upon a target population of cortical cells. Within the somatosensory cortex, thalamic afferentsterminate largely upon layer 4 cells. To further understand the importance of layer 4 in shaping the development of thalamic afferents and the specificity oftheir connectivity within somatosensory cortex, we disrupted the formation of layer 4 of ferret somatosensory cortex using the anti-mitotic drug, methylazoxymethanol acetate MAM on embryonic day 33 E33. As a control, MAM was administered to a second group of animals on E38 to interfere with layer 2 formation. Following MAM treatment, two sets of experiments were performed. In the first set of experiments, small crystals of the lipiphilic tracer, Oil wereplaced into the ventral basal VB region of thalamus of fixed brains obtained at P1, P7 and P14. The distribution of thalamic fibers within the somatosensory cortex was analyzed at each age. In the second set of experiments, organotypic cocultures were created composed of PO normal thalamus paired with normal, E33 or E38 MAM-treated cortex obtained from kits at PO or P7. Injections of anatomical tracers were made into the thalamic pieces of the cocultures and the resulting thalamic fibers and their terminations were analyzed with regard to their position of termination within the host cortex. Findings from the in vivo experiments demonstrate a pattern of development in normal cortex where thalamic afferents grow into developing cortical plate and begin the process of stopping and terminating by P1.

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