CUTANEOUS AND MUSCLE AFFERENTS - INTERACTIONS WITH POTENTIAL TARGETS IN-VITRO

Previous studies suggest that cutaneous and muscle afferents use diffe rent environmental cues in growing to their peripheral targets, As a f irst approach to learning whether these earlier observations reflect d ifferences in the behavior of individual growth cones, trigeminal cuta neous and muscle afferents of embryonic day 10 chicks were cocultured with explants of epidermis or dermis or with myotubes, and interaction s of their growth cones with these potential targets were followed wit h time lapse video microscopy. Cutaneous and muscle afferents differed in their response to all three targets, In birds, few cutaneous affer ents innervate epidermis. Accordingly, most cutaneous neurites retract ed within minutes of touching an epidermal cell. In contrast, most mus cle afferents stopped growing but remained in contact with epidermis f or as long as they were observed (>1 hr), Further, most cutaneous affe rents grew readily across explants of dermis, their normal target, at rates comparable to their growth on the substrate, In contrast, most m uscle afferents advanced only poorly on dermis, Finally, most cutaneou s afferents grew readily across myotubes, the normal targets of muscle afferents, In contrast, few muscle afferents grew across myotubes; mo st either retracted or changed course and grew along the myotube, Over all, muscle afferents stayed in contact with myotubes longer than cuta neous afferents. These cell-type-specific responses reflect in large p art the patterns of cutaneous and muscle afferent growth in vivo. Furt her studies are required to determine whether these observed differenc es between the behavior of regenerating cutaneous and muscle afferent growth cones could potentially play a role in the selection of targets or pathways during embryonic development.