Research interest in the fish heart has been focused classically on physiological activities and on gross morphological aspects (1–2). Thus, most surveys of the fish heart define the endocardium as a continuous lining formed by cells that may be squamous, cubic, or high. This simple definition summarizes the interest aroused by the endocardium in the field of fish research. However, increasing evidence suggests that the endocardium plays a crucial role in heart physiology. Moreover, the fish endocardium displays functions that are relevant for the entire organism. Most of these data are derived from studies of the teleost heart.

STRUCTURE OF THE ENDOCARDIUM

It is true that the endocardial cells lining the different chambers of the teleost heart may be squamous, cubic (with a more or less convex surface), or high (3–6). Less recognized is the fact that the histological appearance of endocardial cells may differ in the various heart chambers. In some species, endocardial cells bulge into the lumen of the bulbus arteriosus but are extremely flattened in the ventricle (Figure 8.1). In other species, the converse pattern is found. Histological differences in the endocardium also may occur in different areas of the same heart chamber (4). The extent to which these morphological differences reflect underlying functional heterogeneity remains unknown.

Under the transmission electron microscope, endocardial cells may show rough and smooth endoplasmic reticulum, Golgi apparatus, small mitochondria, microfilaments, and surface microvilli. The cells form a continuous endothelium, and are joined primarily through tight junctions and desmosome-like plaques (6). The interdigitations are complex and may result in cells with overlapping cytoplasm. The abluminal basement membrane is poorly organized.