The cardiac internuncial cell in Carassius auratus longsdorffii and other 19 teleost species: a fine-structural study.

Thin-section studies of the sino-auricular border region in the hearts of 20 species of teleost fish revealed the cardiac internuncial cell (CIC), hitherto known only in Misgurnus, to occur also in 8 species (Acheilognathus lanceolatus, Carassius auratus longsdorffii, Cyprinus carpio, Girella punctata, Kareius bicoloratus, Rhodeus ocellatus ocellatus, Rhyncopelates oxyrhynchus, Tilapia nilotica), but to be absent in the other 12 species. There was an indication that the CIC was close in nature to myocardial cells because of the findings of Z band-like structures, especially myosin-like thick filaments (ca. 15 nm thick) in the cytoplasm. However, the myosin-like thick filaments were not associated with the actin-like thin filaments (ca. 5 nm thick) or Z band-like structures. Interestingly, the thick filaments showed considerable variation in their occurrence among individuals of a species (Carassius auratus longsdorffii); they were scarce in all of the CICs (Type I) observed in 12 out of 15 individuals studied, and very numerous in all of the CICs (Type II) in the remaining 3 individuals. All of the CICs in Kareius bicoloratus, Girella punctata, Rhodeus ocellatus ocellatus and Rhyncopelates oxyrhynchus were likely to be Type I and all of the CICs in Acheilognathus lanceolatus, Cyprinus carpio and Tilapia nilotica, Type II.