Muscle cell differentiation in the ascidian heart

The simple tubular heart of tunicates consists of a single layer of striated muscle cells which display distinct electrical properties at the luminal and extraluminal surfaces. We have investigated heart morphogenesis and cytodifferentiation in the ascidian, Botryllus schlosseri. Myocardium is formed by invagination from the wall of the heart primordium. Cell polarity is clearly apparent in the undifferentiated cells of the heart primordium and is maintained throughout the whole course of cardiac muscle differentiation. Myocardium cells are initially cubic in appearance, then undergo a progressive flattening with the formation of characteristic protrusions at the luminal surface. The first sign of muscle cell differentiation is the formation of close associations between sarcoplasmic reticulum cisterns and the plasma membrane at the luminal and junctional surface. Myofibrillogenesis also occurs near the luminal surface, whereas the cell portion facing the pericardial cavity maintains an undifferentiated structure. The findings support the hypothesis that membrane changes precede and influence myofibril formation in developing muscle cells.