Muscle trophic factor: II. Ontogenic development of activity of a muscle trophic factor in chicken serum

The activity of chicken skeletal muscle cell trophic factor was greater in chicken serum than in other tissues examined. The variation of this activity in chicken serum was studied pre- and post hatching. The activity of the factor in the embryo was high from day 11 to day 13 of incubation and then decreased, reaching a minimum on day 16. It subsequently increased again, reaching a peak around the day of hatching. After hatching, the activity in serum began to decrease on day 4, reaching a minimum at day 18. In the hen, activity did not very significantly beyond day 18. In the rooster, however, activity increased gradually from about day 50, reached a new maximum by day 110, and was maintained at this level thereafter. The ratio of the average activity of serum from roosters older than 110 days to that of hens of the same age was about 4:3. The time course of activity of the trophic factor closely parallels that of the growth and differentiation of the muscle in vivo.