Contact mediated mechanisms of motor axon segmentation and the delineation of common axon pathways by inhibitory boundaries in the chick embryo.

Motor axon outgrowth to the hindlimb of the chick embryo is constrained by two barrier tissues that border axon pathways. Growth cones turn to avoid the posterior sclerotome and the pelvic girdle precursor. We asked if these functionally similar barriers share a common molecular profile. We found that both barrier tissues express peanut agglutinin (PNA) binding sites and chondroitin-6-sulfate immunoreactivity when growth cones are avoiding these tissues. Neither epitope was detected in axon pathways. PNA-negative paths also developed in the absence of axon outgrowth (following unilateral neural tube deletion). We conclude that these epitopes are markers for barriers to axon advance. Both barrier markers are also expressed in the perinotochordal mesenchyme (PNM). To determine if PNM is another barrier, we asked if motor axons would turn to avoid PNM after confrontation in vivo. Neural tube rotations directed motor axons toward the PNM. We found that axons turn to avoid the PNM. We also tested whether barrier function is linked to barrier-marker expression by deleting the notochord. Deletion resulted in a coordinate loss of PNA binding activity in the PNM and abnormal ramification of axons within this tissue. We conclude that the PNM is a barrier to axon advance and that barrier function is co-regulated with barrier marker expression. To elucidate mechanisms of motor axon segmentation, we used videomicroscopy to compare the behavior of motor growth cones interacting with anterior (A, path) or posterior (P, barrier) sclerotome cells in vitro. Upon 90$\sp\circ$ confrontation, growth cones turned to avoid P-cells by branching at 90$\sp\circ$. Growth cones retained motility but branched as contacting veils retracted. Upon tangential confrontation, veils failed to progress along filopodia that contact P-cells. In contrast, growth cones exhibited a selective affinity for A-cells; they preferred A-cells to the laminin substrate. Filopodial contact with A-cells stimulated protrusive activity and further contact enhanced consolidation of the nascent neurite. The behavior of motor growth cones in vitro suggests that both an inhibition of veil formation by P-cells and an enhancement of axon consolidation by A-cells contribute to the preferential advance of motor axons into anterior sclerotome in vivo.PhDNeuroscienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103288/1/9308414.pdfDescription of 9308414.pdf : Restricted to UM users only