Initiation and time course of mitosis of non-neuronal cells after spinal motoneuron axotomy.

The mitotic response of non-neuronal cells following motor axon transection was measured after in vitro incorporation of [3H]thymidine in frog spinal cord. This predominantly ipsilateral response occurs more rapidly and is of greater magnitude when motor axons are unilaterally transected at the ventral root than after sciatic nerve transection. No increase in incorporation occurred when regenerating fibers were transected a second time before reinnervation, but an increase was observed when the second operation was performed after the formation of functional neuromuscular connections had taken place. Autoradiographic studies after dorsal or ventral root transection showed that the distribution of labeled cells approximated the anatomical extent of the injured cellular elements within the spinal cord. These data are discussed in relation to the characteristics of the dividing cells and the nature of the events eliciting mitosis.