Postnatal development of the rat dorsal funiculus.

The present study shows that there are approximately 21,000 axons in the neonatal rat dorsal funiculus, as compared to 26,000 in 2-week-old animals. We attribute this increase primarily to arriving corticospinal fibers. In adult animals, however, there are approximately 15,000 axons. This is a decline of 58% from the 2 week level, and the decrease is proportionately similar in the corticospinal area and the dorsal funiculus proper. Thus, axon numbers decline in later postnatal development, and since the decline seems to be well past the time of the histogenetic death of the cells that give rise to these axons, we propose that the loss is not caused by the death of neurons. This is further evidence that a reduction in axon numbers not accompanied by cell death is a widespread phenomenon in mammalian postnatal neural development. We infer that the mechanism of axon loss is a reduction in axon branching and that its function is to sharpen synaptic connections.