EMBRYONIC WING AND LEG MOTONEURONS HAVE INTRINSICALLY DIFFERENT SURVIVAL PROPERTIES

Although spinal motoneurons in the chick embryo are born in a rostro-c audal gradient, the timing of their naturally occurring cell death var ies in the opposite sense: brachial motoneurons (MNs) die later than l umbar MNs. We used in vitro methods to determine whether this differen ce results from factors intrinsic or extrinsic to the MNs. Embryonic M Ns were purified from E5 lumbar and brachial spinal cord by a method t hat enriches for MNs with heightened trophic requirements; they were c ultured in serum-free medium. In the absence of muscle extract. death of both populations was rapid; similar results are obtained in ovo aft er limb ablation. In the presence of muscle extracts, however, lumbar MNs always died more rapidly than brachial MNs, as in the normal embry o. We tested the ability of wing and leg bud extracts to provide troph ic factors for MNs in culture. MNs from a given level responded equall y well to wing or leg extracts. However, dose-response studies showed that near-maximal survival of brachial MNs was obtained with lower con centrations of limb extract than those required for optimal survival o f lumbar MNs. These results suggest that the delayed apoptosis in the brachial MN population is not a result of differing peripheral trophic support, but is intrinsically programmed, at least partially through a higher sensitivity to trophic factors.