Microscopic anatomy of the heart and aortic crest of the sea hare Aplysia depilans (Gastropoda: Euopisthobranchia)

The heart of the sea hare Aplysia depilans , located within the pericardial cavity, consists of an auricle and a ventricle connected to an aortic crest. The auricular epicardium is composed of thin, flattened epithelial cells containing vesicles that may be involved in transport by transcytosis between the heart lumen and the pericardial cavity. The extracellular matrix beneath this epithelium is rich in collagen fibrils. The muscular bundles that traverse the auricular lumen, constituting the auricular myocardium, are anchored to the connective tissue layer of the epicardium, and are formed by numerous cross-striated muscle fibres. Due to the absence of an endocardial epithelium, the very thin connective tissue layer surrounding the muscle bundles are in direct contact with the haemolymph. The ventricular epicardium comprises an almost cubic epithelium, including some myoepithelial cells, overlying a thin connective tissue layer. The ventricular myocardium consists of numerous muscular bundles traversing the ventricular cavity, formed by cross-striated muscle fibres coated by an extremely thin layer of connective tissue. An endocardium is also absent from the ventricle. The aortic crest located within the pericardial cavity is lined by a columnar epithelium whose cells contain numerous cisternae of endoplasmic reticulum, indicating active protein synthesis. Beneath the epithelium, the aortic crest contains extensive portions of connective tissue interspersed with some smooth muscle fibres. Some connective tissue cells of the aortic crest possess cisternae of rough endoplasmic reticulum filled with fine particles, as well as vesicles containing particles with dimensions compatible with those of haemocyanin and other haemolymph proteins.