First step of selective motoneuron axonal growth: selective outgrowth at discrete sites in the spinal cord.

Selective axonal growth at a series of choice points along pathways is essential for the establishment of precise motoneuron projections. To reveal some of the molecules responsible for this selective growth of motoneuron axons, this study investigates the phenomenon of why motoneurons extend axons outside the spinal cord, whereas interneurons do not. Axonal growth in the chick embryonic spinal cord at early stages of development was examined immunohistochemically. MAb SC1 staining of serial sections selectively revealed the entire distribution of motoneuron axons in the embryo. In the cervical segments, some motoneurons, called dorsal motoneurons, extended axons outside the cord via the dorsal root entry zone. The axons of both dorsal and ventral motoneurons were distributed only in the anterior half of the sclerotome; inside the cord motoneuron cell bodies and axons were distributed evenly along the anterior-posterior axis. Motoneurons and interneurons extended axons during the same period, and although the growth cones of both were intermixed in the same locations, only motoneuron axons grew out from the cord. The outgrowth of all st 19 motoneurons from the cord through either the dorsal or ventral exit zone strongly suggests that extension of their axons outside the cord is a selective rather than random process.