title: Precardiac mesoderm differentiation in vitro. Actin-isotype synthetic transitions, myofibrillogenesis, initiation of heartbeat, and the possible involvement of collagen.
abstract: The differentiation of precardiac mesoderm into beating heart tissue was examined during explant culture. Explanted tissue forms tubular heart-like vesicles and initiates rhythmic contractility within 18-24 h in vitro, a developmental time-course approximating that observed during in vivo development. Electron-microscopic observations reveal that beating heart cells are rich in cytoplasmic myofibrils in varying degrees of order, with some regions containing highly organized myofibrillar arrays. The analysis of actin-isotype biosynthesis, using metabolic labeling with [35S]-methionine and isoelectric-focusing resolution of the synthesized radioactive polypeptides, demonstrates that the initiation of cardiac alpha-actin synthesis and the pattern of transition in the synthesis of alpha-, beta-, and gamma-actin isotypes is equivalent to the initiation time and pattern observed in vivo. A possible collagen involvement in the differentiation process was investigated by assessing the effects of collagen-synthesis inhibitors on the development of the explant cultures. Two different agents, alpha, alpha'-dipyridyl and L-azetidine-2-carboxylic acid, exhibited a dose-dependent ability to inhibit the formation of beating heart tissue. When examined by electron microscopy, the nonbeating tissue exhibited a drastic depression of myofibrillogenesis, but otherwise appeared healthy. Further examination of the effect of L-azetidine-2-carboxylic acid demonstrated that the inhibition of myofibril formation and heartbeat was correlated with a 60% inhibition of native collagen synthesis; however, the time-course and pattern of actin-isotype biosynthesis was completely unaffected. The data suggest a possible involvement in heart differentiation that is necessary for either the synthesis of non-actin cardiac contractile proteins or the assembly of cardiac contractile proteins into myofibrils.
mesh_terms: D000199:Actins / Q000032:analysis*; D000818:Animals; D002454:Cell Differentiation; D002642:Chick Embryo; D003094:Collagen / Q000378:metabolism*; D006321:Heart / Q000196:embryology* / Q000502:physiology; D008648:Mesoderm / Q000166:cytology / Q000502:physiology* / Q000648:ultrastructure; D008854:Microscopy, Electron; D009206:Myocardium / Q000648:ultrastructure; D009210:Myofibrils / Q000648:ultrastructure*; D009924:Organ Culture Techniques
chemical_list: D000199:Actins; D003094:Collagen