Ultrastructure of muscle fibers and cells synthesizing DNA in lymph hearts of developing frogs and chick embryos.

The ultrastructure of striated muscle fibers and 3H-thymidine (3HTdr)-labeled cells adjacent to them in the lymph hearts of larvae of Rana temporaria, yearling frogs, and 9- to 13-day-old chick embryos was studied by use of electron-microscopic autoradiography. A comparatively high level of differentiation of lymph-heart muscle fibers was observed not only in yearling frogs but also in larvae. Myosatellites occurred in all stages of development. No mitoses were found in muscle fibers. In 9- to 13-day-old chick embryos the myofibers of lymph hearts were somewhat less differentiated than those of the larvae and yearling frogs. Differentiating sarcomeres were often seen in the sarcoplasm of myofibers of chick embryos. The analysis of the ultrastructure of 3HTdr-incorporating cells shows that 2-4 h after the single 3HTdr administration only mononucleated cells devoid of myofilaments are commonly labeled both in tadpoles and chick embryos. When fixation is postponed by 48-70 h, myonuclei frequently become labeled. Thus, the data obtained support the evidence that proliferation and differentiation processes in the developing muscle tissue of the lymph heart of both species studied are mutually exclusive, similar to the situation in differentiating vertebrate skeletal muscle.