Temporal and spatial distribution of growth-associated molecules and astroglial cells in the rat corticospinal tract...

To understand better the role of growth-promoting and -inhibiting molecules in the development of the corticospinal tract (CST), temporospatial expression of embryonic neural cell adhesion molecule (E-NCAM), growth-associated protein-43 (GAP-43), and chondroitin sulfate proteoglycan (CSPG) was studied in developing rats. Transverse sections of the seventh cervical (C7), seventh thoracic (T7), and fourth lumbar (L4) segments were examined at postnatal days (P) 2, 6, 10, 14, and 28. The highest E-NCAM immunoreactivity appeared at the C7 level on P2 and shifted caudally to the T7 on P6 and L4 on P10, which correlated closely with the time course of CST development. The peak expression of GAP-43 emerged at C7 on P2 and shifted to the T7 and L4 levels at a relatively lagging pace compared with that of E-NCAM. Conversely, a transient reduction in CSPG immunoreactivity was found within the CST at the C7 level on P2, T7 level on P6, and L4 level on P10, corresponding well with the arrival of CST-leading axons at these levels. Interestingly, higher levels of CSPG were found to surround the growing CST, suggesting a repulsive environment that channels the growth of CST. Moreover, a transition from immature to mature astrocytes in a rostrocaudal direction during CST development was evidenced by anti-vimentin and anti-glial fibrillary acidic protein (GFAP) immunostaining, suggesting a guidance role of immature astroglia in axonal outgrowth. Our study thus demonstrated dynamic changes of multiple growth-related molecules and astroglial environment that contribute to postnatal development of the CST.