The expression of neurotrophic factors in Schwann cells in denervated nerve stumps and their role in preferential reinnervation of appropriate pathways by regenerating femoral nerves. The time course of the gene expression measured with rtPCR in the cutaneous sensory nerve branch of the femoral nerve (A) and in ventral root motor nerves (B). The mRNA normalized to values in innervated nerve stumps increased to a peak at 15 days and receded thereafter. Two different retrograde dyes (fluorogold and rubyred) were applied for retrograde labelling and counting of the motoneurons that regenerated their axons into appropriate and inappropriate motor (muscle) and sensory (cutaneous) nerve branches (C), and 1.5 mm across the site of microsurgical repair of the transected femoral nerve (D). The regeneration of the motor axons is non-selective at 2 and 3 weeks for the appropriate and inappropriate motor and sensory femoral nerve branches (E). However, as also shown by the data in (F,G), motoneurons progressively regenerate their axons preferentially into the motor branch in the time frame in which Schwann cells in the sensory and motor nerve branches show selective expression of growth factors (A). The number of motoneurons that had regenerated their axons non-selectively into both branches did not change over time (F,G). Adapted from [33,98,99].