Embryonic wing and leg motoneurons have intrinsically different survival properties

Although spinal motoneurons in the chick embryo are born in a rostro-caudal gradient, the timing of their naturally occurring cell death varies in the opposite sense: brachial motoneurons (MNs) die later than lumbar MNs. We used in vitro methods to determine whether this difference results from factors intrinsic or extrinsic to the MNs. Embryonic MNs were purified from E5 lumbar and brachial spinal cord by a method that enriches for MNs with heightened trophic requirements; they were cultured in serum-free medium. In the absence of muscle extract, death of both populations was rapid; similar results are obtained in ovo after limb ablation. In the presence of muscle extracts, however, lumbar MNs always died more rapidly than brachial MNs, as in the normal embryo. We tested the ability of wing and leg bud extracts to provide trophic factors for MNs in culture. MNs from a given level responded equally well to wing or leg extracts. However, dose-response studies showed that near-maximal survival of brachial MNs was obtained with lower concentrations of limb extract than those required for optimal survival of 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.