Early development and myogenesis of the posterior anuran lymph hearts.

The early development of anuran posterior lymph hearts studied by light- and electron-microscopy in frog larval stages 25-29 (Gosner 1960) can be subdivided into three phases. In phase I, mesenchymal myogenic cells are found, each possessing a single 9 + 0 cilium and numerous filopode-like processes aggregated near the vena caudalis lateralis, forming up to three metameric organ anlagen arranged like a cuff around the preexisting lymphatics (stages 26/27). In phase II, cell proliferation starts at stage 28 within the lymph heart wall as does the formation of primarily polynuclear myofibres by fusion of several myoblasts. At this stage immature myofibres show a vast sarcoplasm, a poorly developed SR and only few myofibrils with not yet distinguishable A- and I-bands. In phase III, the afferent and efferent valves are formed at the onset of pulsation in stage 29. Contractile myofibres contain large glycogen fields and a considerable amount of myofibrils which frequently branch and show distinct Z-lines, A-, I-, H- and M-bands; 1-3 cilia were found lying within a channel-like cell invagination. The peculiarities of organogenesis and myofibre development are discussed.