Heterotopic, not homotopic, fetal occipital allografts in adult hosts project to visual-related extracortical targets

Article type: Research Article

Authors: Gaillard, Frédéric

Affiliations: Institute of Physiology and Cellular Biology, UMR 6187 CNRS, Faculty of Sciences, 40 avenue du Recteur Pineau, F-86022 Poitiers, France

Note: [] orresponding author: F. Gaillard, Tel.: +33 549 453 853; Fax: +33 549 454 014; E-mail: [email protected]

Abstract: Purpose: Fetal occipital allografts implanted into the posterior cortex of adult mice project massively throughout the ipsilateral pallium of the host, but rarely outside this domain (Gaillard et al., 2004). The present study was undertaken to examine in detail whether this pattern is specific to graft location. Methods: Cortical fragments corresponding to presumptive occipital areas were harvested from E15 mice fetuses expressing ubiquitously the eGFP protein, and implanted in correct (homotopic) and incorrect (heterotopic) cortical loci in wild-type adults. Two months later, efferents were detected by immunohistochemistry and quantified on selected DAB-treated sections. Results: The present findings show (i) that robust projections are present in the ipsilateral host cortex regardless of the graft location; (ii) that 55% the grafts located in parietal and frontal cortices have obvious but sparse callosal and subcortical projections; and (iii) that grafts placed in occipital areas never contact ipsilateral subcortical targets, likely because graft-related axons are unable to cross obliquely the thalamocortical fascicles in the underlying white matter. Conclusions: These puzzling results question the use of transplantation strategies for repairing damaged networks in adults where rewiring involves complex white matter trajectories.

Keywords: Transplantation, embryonic visual cortex, eGFP-expressing cells, mature brain, callosal projections, subcortical projections

Journal: Restorative Neurology and Neuroscience, vol. 25, no. 2, pp. 161-175, 2007