The fine structure of the heart of the teleost, Pleuronectes platessa L.

Abstract

Tissue from the four chambers of the heart of the plaice (Pleuronectes platessa, L.) has been examined in the electron microscope in order to describe the morphology of the heart at a fine structural level. The sinus venosus is a thin walled chamber between 60–90 μ thick consisting of a connective tissue matrix in which are situated the plexus of the parasympathetic cardiac ganglion and localised bundles of myocardial cells. The myocardial cells do not form a continuous layer but are associated in particular with the region of the cardiac ganglion and are innervated by it. The sino-auricular junction has hitherto been described as a pacemaker region but the myocardial cells in this region are identical in morphology to myocardial cells in other parts of the heart. There is a large complex of nerves, derived from the cardiac plexus, that runs around the junction before branching to innervate the auricle. The myocardial tissues consist of an outer layer of myocardium forming the wall of the heart and a profusion of trabeculae. The endocardium invaginates into the endocardium to divide up the cells into populations of approximately 25 cells in profile. There is no well-defined coronary blood supply although capillaries are occasionally seen. The myocardial cells themselves are small in diameter (3.5–5.5 μ) and show some primitive features which are: a short sarcomere (1.4–2.0 μ), the absence of any sarcoplasmic reticulum, and very scarce fasciae occludentes. In the atrium in particular, there are many groups of 1500 A membrane-bound, dense-cored vesicles in the myocardial cells. Ventricular cells contain more myofilaments and mitochondria than do atrial cells and have many vesicles of 0.1–0.3μ diameter whose function and contents are unknown. Connective tissue is very evident in the plaice heart, being an integral part of the sinus venosus and the auriculo-ventricular junction and being the sole constituent of the auriculoventricular valve and the bulbus arteriosus.