Ultrastructure of the parietal pericardium in teleosts.

The ultrastructure of the parietal pericardium in Melanogrammus aeglefinus, Paracheirodon innesi, and Poecilia reticulata is described in larvae of variable age. This structure is composed of a mesothelial layer facing the pericardial cavity and an outer layer of collagen tissue. The latter is continuous with the surrounding tissue, i.e. mainly skeletal muscles. Generally, the parietal pericardium is attached to loose connective tissue dorsally, whereas its ventral and lateral parts are connected to muscles and tendons. Posteriorly, it is fused with the peritoneum. In prenatal larvae of P. reticulata the parietal pericardium is in contact with yolk material. The parietal mesothelial cells are attenuated and contain numerous ribosomes, a well developed Golgi apparatus, and a number of membrane-bound inclusion bodies (0.1-0.4 micron) of moderate electron density. In addition, numerous pinocytotic vesicles (80-100 nm) occur in differentiated mesothelial cells, whereas they are scarce in undifferentiated cells. Furthermore, bristle-coated vesicles are very rare in differentiated mesothelial cells, whereas they are seen regularly in undifferentiated cells. The collagen fibre bundles of the parietal pericardium are often orientated perpendicularly to each other, and in postnatal larvae of P. reticulata a one cell thick layer of melanophores occurs within the fibrous tissue. These cells contain numerous membrane-bound, highly electron-dense inclusion bodies (0.3-0.6 micron). They are occasionally seen in Paracheirodon innesi also, whereas they are absent in 2 and 6 days old larvae of M. aeglefinus and prenatal larvae of P. reticulata. Macrophages, capillaries, and nerve bundles occur in the pericardial and subpericardial connective tissue. The present results are discussed and compared with those previously reported for mammalian parietal pericardium.