SENSORY AXONS ARE GUIDED BY LOCAL CUES IN THE DEVELOPING DORSAL SPINAL-CORD

During development, different classes of sensory neurons establish dis tinctive central projections within the spinal cord. Muscle spindle af ferents (Ia fibers) grow ventrally through the dorsal horn to the vent ral cord, whereas cutaneous sensory collaterals remain confined to the dorsal horn, We have studied the nature of the cues used by Ia fibers in establishing their characteristic projections within the dorsal ho rn, An organotypic culture preparation of embryonic chicken spinal cor d and sensory ganglia was used to test the influence of ventral spinal cord and local cues within the dorsal spinal cord on the growing Ia a fferents, When the ventral half of the spinal cord was replaced with a n inverted duplicate dorsal half, Ia fibers entering through the dorsa l columns still grew ventrally within the host dorsal horn, After the fibers entered the duplicate dorsal half, they continued growing in th e same direction, With respect to the duplicate dorsal tissue, this wa s in an opposite, ventral-to-dorsal, direction, In both cases, however , Ia collaterals remained confined to the medial dorsal laminae, Restr iction to these laminae was maintained even when the fibers had to cha nge their direction of growth to stay within them, These results show that cues from the ventral cord are not required for the development o f correct Ia projections within the dorsal horn, Local, rather than lo ng-range directional, cues appear to determine the pattern of these pr ojections, When the ventral half of the spinal cord was left intact bu t sensory axons were forced to enter the dorsal gray matter growing ro strally or caudally, their collateral axons grew in random directions, further showing the absence of directional cues even when the ventral cord was present, Taken together, these observations suggest that Ia fibers are guided by local positional cues that keep them confined to the medial gray matter within the dorsal horn, but their direction of growth is determined primarily by their orientation and position as th ey enter the dorsal gray matter.