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Some characteristics of muscle‐conditioned media with excitatory and inhibitory influences of neurite outgrowth from early neural tube explants

This study examined the growth of basal plate explants from 40‐hr chick embryos containing the trigeminal motor nucleus in the presence of target muscle‐conditioned medium (MCM) from day‐4, day‐10, and day‐18 embroys. We had previously found that MCM derived from the 4‐day target tissue enhanced neurite outgrowth from explants, while target MCM from 10‐ and 18‐day embryos inhibited it. For the present study, target MCM was treated with trypsin to assess the nature of the active fractions, or pre‐incubated in polyornithine‐coated dishes to determine the possible in polyornithine‐coated dishes to determine the possible presence and relative contributions of substrate‐binding vs. soluble components. Both trypsinization and pre‐incubation ablished the outgrowth‐enhancing potential of the 4‐day MCM, indicating a protein or polypeptide substeate‐binding active fraction, with no appreciable active souluble component. When the 10‐day target MCM were trypsinized, the inhibitory indluence was reversedm and neurite outgrowth was enhanced. Similarly, when the MCM from these embryos was preincinated over polyornithinem the inhibitory influence was lost and was replaced by a stimulatory one. These results suggest that the 10‐ and 18‐day MCM contain an active inhibitory fraction that is a protein or polypeptide, and which binds to a positively charged substate. When this inhibitory fraction is inactivated or depleted, an excitatory soluble fraction is revealed. Such a dual nature in MCM has not been seen previously and may be expressed only when the media are assayed with very early neural tissue.