The postnatal development of the tract of Lissauer in the rat.

This study determined the numbers of axons in the tract of Lissauer of the S2 segment of rat spinal cord during postnatal development. One day after birth there were an average of 39 myelinated and 9,520 unmyelinated axons in this pathway. These numbers changed steadily to the 722 myelinated and 2,759 unmyelinated axons that characterize the adult. Thus there was a 20-fold increase in myelinated axons but a threefold decrease in total axons in this pathway from birth to adulthood. The axonal changes occurred so long after the changes in the number of cells that give rise to these axons that the axonal changes were almost certainly not due to the death of their parent neurons. We do not yet understand the mechanisms that control axonal numbers in this pathway, but in our opinion these mechanisms are as important as those that control neuronal numbers in considerations of the development of the nervous system.