Embryonic motoneurones grafted into the spinal cord of an adult rat can innervate a muscle

We have previously shown that motoneurone-like cells from embryonic grafts survive and migrate into the host neuropil of adult rat spinal cord, depleted of some of its own motoneurones. We moreover demonstrated that a muscle, when connected at the site of the graft to the spinal cord of the host by its own nerve, was reinnervated by motoneurones that could be identified by retrograde labelling with HRP [11]. However, it was not clear whether these retrogradely labelled motoneurones were of graft origin. In this study we combined the use of an embryonic marker with retrograde labelling to demonstrate that grafted neurones of embryonic origin can indeed innervate a soleus muscle implant. Embryonic donor cells were labelled with bromodeoxyuridine (BrDU) by its incorporation into replicating DNA during neurogenesis. The nuclei of grafted cells were then identified in host cords by immunocytochemistry, visualising the BrDU positive nuclei with the fiuorophore Texas Red, while the fluorescent dyes Fast Blue and Diamidino Yellow were used for retrograde labelling. Examination of frozen spinal cord sections by fluorescence microscopy, at wavelengths appropriate to each fiuorophore, showed that about 12% of the neurones innervating the muscle implant also contained detectable amounts of BrDU and therefore were of graft origin.