Myogenesis in the Chicken: the Onset of Differentiation of Adult Myoblasts is Influenced by Tissue Factors.

Myoblasts from the pectoralis muscle of developing and adult chickens were cultured in various media and the number of differentiated myoblasts was monitored via indirect immunofluorescence using an antibody against sarcomeric myosin. Myoblasts from all age groups proliferated to a lesser extent in media lacking chicken embryo extract (CEE) when compared to myoblasts in standard medium containing 5% CEE. However, CEE influenced differentiation of myoblasts from 19-day-old embryos and adults but not that of myoblasts from 10-day-old embryos. Differentiation in cultures from 10-day embryos began at the same time and reached the same levels regardless of the presence or absence of the CEE in the medium. In contrast, differentiation of myoblasts from 19-day-old embryos and adult chickens was delayed by several days in cultures maintained with CEE compared to cultures in media lacking CEE. We do not know yet what is the active factor(s) in the CEE causing this delay of myogenic differentiation in cultures from older embryos and adults. Nevertheless, the present finding is in accordance with our previously published studies where we demonstrated that adult type chicken myoblasts, which differ from those dominant during mid-fetal development, emerge at late stages of embryogenesis and are the only one present during post-hatch and adult stages.