The fine structure of the heart and connective tissue sheath surrounding the stomach of the brachiopod Rhynchonella psittacea has been studied. The stomach wall is lined externally with peritoneal epithelium. Between the bases of the peritoneal epithelial cells and those of the stomach epithelial cells is an extracellular amorphous matrix. The exterior part of the matrix is occupied by smooth muscle cells and the interior part by fibroblasts. The heart wall shows continuity with the peritoneal epithelium covering the stomach wall and consists of three layers: an outer layer of smooth myoepithelial and epithelial cells, an intermediate thick layer of extracellular matrix, and an inner discontinuous layer of fibroblasts. In myoepithelial cells, nucleated heads protruding freely into the coelom and contractile parts embedded in the extracellular matrix can easily be distinguished. These cells contain no sarcoplasmic reticulum or any elements of a T system. The epithelial cells are non-muscular mononucleated cells scattered among the myoepithelial cells and closely associated with these basally. They possess a well-developed rough endoplasmic reticulum. In rare cases, a small amount of myofibrils occurs basally in the epithelial cells. Morphologically the epithelial cells in the myocardium are very similar to the peritoneal epithelial cells covering the stomach wall. Both epithelial and myoepithelial cells are ciliated. No nerve elements have been found in the brachiopod heart. The structure of the brachiopod heart is compared with that of other invertebrates; similarity of cellular composition of the brachiopod heart and stomach cover is considered evidence of origin of the heart cells from the cells of the connective tissue sheath of the stomach. The myogenic role of the peritoneal cells and epithelial cells of the myocardium is suggested. J. Morphol. 234:69-77, 1997. © 1997 Wiley-Liss, Inc.
Copyright © 1997 Wiley-Liss, Inc.