Evidence for a conserved myogenic program
An invariant, temporal sequence in the accumulation of muscle specific proteins is described, based on studies of nascent, postmitotic, mononucleated myoblasts from three chick skeletal muscle systems; (1) cultured breast muscle, (2) in vivo myotome and (3) in vivo limb. Desmin was the first muscle specific protein found, followed by titin and shortly thereafter, coordinately, by a cohort of myofibrillar proteins. The myofibrillar proteins were first detected in the form of small, irregularly shaped aggregates. Although titin appeared just prior to the other myofibrillar proteins, in this aggregate form, it was not detected as organized filaments earlier than the others. It is therefore unlikely that titin can act as a scaffold in sarcomere formation. If titin acted in this capacity, it would be expected to be exist as a filament earlier than the other myofibrillar proteins, in order that they could assemble upon it. The accumulation and assembly of these proteins was also found to be independent of fusion. In all three of the systems examined, unfused myoblasts were found which contained sarcomeres, identical to those found in adult muscle, already assembled into myofibril. Mononucleated cells were able to synthesize and assemble myofibrillar proteins without the prerequisite of fusion. Although newly born in vivo myoblasts in both myotome and limb displayed the muscle specific protein sequence already described, the source of the precursors of these cells has been a question of considerable interest. Are these precursor cells already postmitotic myoblasts or are they still replicating cells? The myotome proper was found to contain cells that were desmin +/myofibrillar protein +. However, a region identical in each somite--the dorsalmost portion of the somite--displayed cells that were desmin +/myofibrillar protein $-$. These desmin +/myofibrillar protein--cells suggest a local pool of replicating myoblasts. Limb stained for desmin, indicated the cells that invest the limb are not as yet differentiated myoblasts. At no stage at which somitic cells have been found to invest the limb were cells that were desmin + found to extend from the ventral aspect of the somite to the limb.
Anatomy & physiology|Biology
DiLullo, Camille, "Evidence for a conserved myogenic program" (1991). Dissertations available from ProQuest. AAI9200331.