SURVIVAL OF NEWLY POSTMITOTIC MOTONEURONS IS TRANSIENTLY INDEPENDENT OF EXOGENOUS TROPHIC SUPPORT

We compared the survival requirements of early- and late-born motoneur ons from E5 chicken spinal cord, Density gradient centrifugation follo wed by immunopanning using SC1 antibody allowed us to purify two size classes of motoneuron, Large motoneurons retained by 6.8% metrizamide were shown by BrdU labeling in ovo to be born on average 1.5 d earlier than the small motoneurons recovered from the metrizamide pellet, Lar ge motoneurons were both biochemically and functionally more mature: t hey expressed higher levels of choline acetyltransferase and low-affin ity neurotrophin receptor, and had an acute requirement for trophic su pport from muscle-derived factors, After 24 hr in culture in basal med ium, all early-born motoneurons died, whereas 60% of late-born motoneu rons survived, Small motoneurons can develop into large motoneurons in ovo, suggesting that they represent a general transitional stage in m otoneuron development, Our results suggest that a defined period elaps es between birth of a motoneuron and its acquisition of trophic depend ence, possibly corresponding to the time required for target innervati on, This property may have important consequences for the timing and r egulation of developmental motoneuron death.