Postnatal loss of axons in normal rat sciatic nerve.

Myelinated and unmyelinated axons were counted in sciatic nerves of newborn, 5-day-old, 14-day-old, and adult rats. Myelinated axons increase from essentially none at birth to approximately 8,000 in adulthood, but total axon numbers decrease steadily from 33,954 at birth to 22,872 in adulthood. Thus there is a significant postnatal loss of axons from rat sciatic nerve. This loss is, in our opinion, not associated with the death of the cells that give rise to these axons. This is thus an example of a regressive event that probably is of importance in normal neural development, namely the postnatal elimination of axons unaccompanied by death of the neurons that give rise to axons. These findings presumably imply a considerable amount of proximal peripheral axon branching, and the postnatal elimination of axons in the sciatic nerve presumably results from a reduction of this branching. Thus postnatal elimination of processes on, for example, somatic muscle cells may be at least partially the result of long axon elimination rather than local withdrawal of presynaptic processes, as is usually thought to be the case. In addition, an increased number of axons resulting from early postnatal manipulations may indicate cessation of axon loss rather than formation of new axons.