Fine Structure of the Myocardium of the Blue Crab Callinectes sapidus Rathbun

The myocardium of the blue crabCallinestes sapidus has the following ultrastructural features. Myocardial cells are transversely striated with A, I, and Z-bands, H-zones, and M-lines. Thick myofilaments are arranged in a hexagonal pattern and each thick myofilament is surrounded by six thin myofilaments. The sacrotubular system consist of the sacroplasmic reticulum, transverse tubules, and longitudinal tubules. Intercalated discs having gaps 40–600 A wide connect adjacent muscle cells end-to-end. Interstitial cells are often observed beneath the external laminae of the myocardial cells. Nerve terminals and glial elements are also embedded in the myocardial cells. The nerve terminals contain both dense and clear vesicles. The dense vesicles are round (800–1000 A in diameter), whereas clear vesicles are either round (360–600 A in diameter) or flattened (350×700 A). *** DIRECT SUPPORT *** A00BY066 00004