Developmental changes in spinal cord neurite-promoting activity from chick muscle extracts.

Muscle extracts from embryonic chicks aged 10-18 days of embryogenesis produced an increase in 6-day embryo spinal cord neurite initiation. A maximal effect was observed with 18-day extract. Extracts from older chicks produced no effect. These neurite promoting factors are produced at a time in which neuromuscular junctions are forming and becoming stabilized in ovo and may act to control terminal branching, selection, and maintenance of axonal endings.