LHRH cells migrate on peripherin fibers in embryonic olfactory explant cultures: an in vitro model for neurophilic neuronal migration.

Luteinizing hormone releasing hormone (LHRH) neurons arise from progenitor cells in the olfactory placode. During prenatal development, these cells migrate via neurophilic interactions, in track-like arrangements along axons of the olfactory complex. The mechanisms by which these cells attain an adult-like distribution are unknown. In this study, we established an in vitro, embryonic mouse olfactory explant model to examine the factors directing LHRH cell migration. Cultures were generated from E11.5 embryos and maintained for up to 3 weeks. Typically 20-50% (160-400 LHRH cells) of the total LHRH neuronal population survived and maintained gene expression in these explants. Fibronectin and laminin staining delineated substratum producing cells which concentrically spread, from Days 1-7, from the periphery of the entire explanted tissue. In contrast, LHRH cells emigrated exclusively from inside olfactory pit areas to the surface of the culture after 3 days. The relationship between groups of LHRH cells emerging from bilateral olfactory pits was not random, but highly organized; in 93% of the cultures examined, the angular relationship between these groups was 180-270 degrees. After 5 or more days in vitro, LHRH cells were found in directional tracks similar to those observed in vivo forming a continuum from the olfactory pit out onto the substratum, where many bipolar LHRH cells were discretely located. The maximum distance away from the olfactory pit that LHRH cells were detected was 0.9 mm, which is compatible with the distance traversed by LHRH cells through the nasal region in vivo. The reproducible spatiotemporal appearance of LHRH cells was unrelated to the concentric spread of the fibronectin and laminin producing cells of the explant. Taken together, these data indicate that LHRH cells migrated directionally in these explants and that the molecular cues governing the initial migration of these neurons remained intact in this system in the absence of brain tissue. Double-label immunocytochemistry indicated that at least three populations of neuronal fibers existed in the explants: N-CAM-positive, peripherin-positive, and N-CAM/peripherin-positive. Although all three fibers groups showed highly organized spatiotemporal distribution patterns, only peripherin-positive fibers correlated with the location of LHRH cells. LHRH cells were always preceded by, and in close association with, peripherin-positive fibers. We hypothesize that signals arising from the peripherin-positive axons provide the appropriate guidance cues to LHRH cells as they emigrate from the olfactory pit.