Cardiac fine structure in selected arthropods and molluscs

The ultrastructure of the single-chambered hearts of selected arthropods is compared with that of the multi-chambered hearts of three molluscs. I used the following four systems to make the comparison: (1) contractile apparatus, (2) sarcoplasmic reticulum and surface invaginations, (3) cell to cell junctions, and (4) nerves. The contractile apparatus is composed of thin and thick filaments. While the thin filaments have the same diameter, the diameter of the thick filaments differs from one heart to another. Evidence is presented to indicate that this is due to varying amounts of paramyosin in the thick filaments. The arthropod cardiac cells have an extensive system of sarcoplasmic reticulum, the terminal vesicles of which are coupled to the plasmalemma and to the invaginations of the plasmalemma, the T-system. The molluscan cardiac cells lack a typical T-system, which is presumably due to their small cell size (about 10 μm). They possess, however, an elaborate system of sarcoplasmic reticulum which extends from just under the plasmalemma to the middle of the cell. In addition to elaborate sarcoplasmic reticulum, the heart of the whelk (Busycon canaliculatum) possess many small invaginations of the plasmalemma, called sarcolemmic tubules. These invaginations of the cell surface are not found in the hearts of the few bivalves examined. All arthropod and molluscan hearts have intercalated discs which can be seen in the light microscope. Two types of junctions can be distinguished in the electron microscope. The mechanical junction is at the level of the terminal sarcomere where the thin filaments are embedded in the cell wall and dense granular material appears to cause the two adjacent cells to adhere to each other. The electrical junction is found along the lateral borders of cells of both the molluscan and arthropod hearts. Finally, while nerves appear to be absent in the myogenic moth heart, they are abundant in the myogenic cockroach heart and in the neurogenic lobster heart. Furthermore, two types of nerves appear very prominently in the myogenic molluscan hearts.