Fine structure of the heart in the Japanese Medaka, Oryzias latipes

Heart morphology in the small freshwater teleost, Japanese Medaka, (oryzias latipes) was studied by light and electron microscopy. Particular attention was directed to the three heart layers, epicardium, myocardium, and endocardium in the atrial and ventricular portions of the organ. The trabeculae originate and insert into the myocardial wall forming many complex anastomoses and interdigitations. The entire ventricle is trabeculated giving the chamber a "spongy" appearance; the atrium is less extensively trabeculated. The epicardial layer forms an outer covering over the organ and is composed of simple squamous epithelial cells. The myocardial cells of the trabeculae have small diameters but extend for considerable distances. The myofibrils usually are located peripherally, while the nucleus, mitochondria, and other cellular organelles are located centrally. The myofibrils exhibit a typical vertebrate structure with distinct A-bands, I-bands, and Z-lines; H-zones are discernible but appear less prominent than in higher vertebrates. Intercalated disks have fasciae adherentes and maculae adherentes (desmosomes) as component parts. Although a distinct T-system is absent, prominent subsarcolemmal cisternae are located in the I-band areas and there is a moderately well-developed sarcoplasmic reticulum. The endocardium is composed of a continuous layer of cells that appear to be metabolically very active. Filling the cytoplasm of these cells are large numbers of rough endoplasmic reticulum cisternae and associated mitochondria, vesicles containing amorphous material, and large round bodies with consistencies varying from dense to amorphous to filamentous.