Ultrastructural studies on the heart of bony-fish larvae.

The ultrastructure of the heart in 2 and 6 d old larvae of Melanogrammus aeglefinus and 1, 7, 14, and 21 d old larvae of Poecilia reticulata, is described. Additional studies were done on prenatal specimens of P. reticulata. In the atrium of M. aeglefinus the endocardium is separated from the muscle wall by a wide, electron lucent space, which probably contains cardiac jelly (Davis 1924). The myocardium is an 1 cell-thick layer in the atrium, whereas it consists of 2 to 4 cells in the ventricle. Cardiac trabeculae seem to be absent. The contractile material appears more developed at the endocardial side of the muscle wall than at the epicardial side. Thus, in the latter area the myofibrils are thin, few in number, and embedded in large amounts of free ribosomes. Often they radiate from a spot of electron dense material or an intercalated disc. Generally, intercalated discs, short nexuses and specific heart granules (Jamieson and Palade 1964) occur in ventricular as well as in atrial myocardial tissue. In P. reticulata cardiac jelly seems absent, whereas cardiac trabeculae occur regularly, particularly in the ventricle. The myofibrillar apparatus displays a nearly adult structure. However, in the ventricular wall, there occur spots and bands of intracellular, electron dense material at that sarcolemma facing the subepicardial space. Normally, myofibrils extend from this material into the cellular cytoplasm. Subsarcolemmal electron dense material was only scarcely seen in the atrial wall.(ABSTRACT TRUNCATED AT 250 WORDS)