Abstract
We showed the development of rat's corticospinal (CS) innervation consists of at least two phase. The first stage peaks at postnatal day 7 (P7), when CS terminals are distributed in the whole spinal gray. However, the CS terminals in the ventral side are eliminated until P10. After this elimination, "the second wave" of CS innervation arrives: CS terminals again increase in the ventral and also dorsal side. This second wave quickly occurs from P12 and completes until P18. In this study we investigated which level of CS projection produces the second wave. A small amount of biotin dextran was injected at one site in the sensorimotor cortex and animals were fixed after desired survival time. Serial sagittal or horizontal sections were obtained from the cortex to cervical cord and transverse sections from C5 to C8. Axonal arborization was reconstructed by camera lucida drawing on serial longitudinal sections through the pathway of CS tract as well as transverse section at C5 to C8. In the longitudinal sections, the number of branches derived from the dorsal funiculus at P18 did not increase compared with P10. The number of axons in the medullary pyramid showed no difference between P7 and P14. Therefore it is unlikely that CS neurons not involved in the first wave newly participate in the second wave. On the other hand, the number of branching in the gray matter at P18 was much larger than that at P10. These suggest that the branches give rise to the second wave are generated within the spinal gray matter of the same segment rather than in the more proximal axons. [J Physiol Sci. 2007;57 Suppl:S151]
