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Abstract
The development of the ventricular myocardial trabeculae occurs in three steps: emergence, trabeculation and remodeling. The whole process has been described in vertebrates with two different myocardial structural types, spongy (zebrafish) and compact (chicken and mouse). In this context, two alternative mechanisms of myocardial trabeculae emergence have been identified: (1) in chicken and mouse, the endocardial cells invade the two-layered myocardium; (2) in zebrafish, cardiomyocytes from the monolayered myocardium invaginate towards the endocardium. Currently, the process has not been studied in detail in vertebrates having a mixed type of ventricular myocardium, with an inner trabecular and an outer compact layer, which is presumptively the most primitive morphology in gnathostomes. We studied the formation of the mixed ventricular myocardium in the lesser spotted dogfish (Scyliorhinus canicula, Elasmobranchii), using light, scanning and transmission electron microscopy. Our results show that early formation of the mixed ventricular myocardium, specifically the emergence and the trabeculation steps, is driven by an endocardial invasion of the myocardium. The mechanism of trabeculation of the mixed ventricular myocardium in chondrichthyans is the one that best reproduces how this developmental process has been established from the beginning of the gnathostome radiation. The process has been apparently preserved throughout the entire group of sarcopterygians, including birds and mammals. In contrast, teleosts, at least those possessing a mostly spongy ventricular myocardium, seem to have introduced notable changes in their myocardial trabeculae development.
Highlights
The heart of Scyliorhinus canicula, as in other chondrichthyans, is anatomically composed of sinus venosus, atrium, ventricle and an outflow tract formed by a conus arteriosus and a bulbus arteriosus
Our report seems to be the first to provide an extensive description of the development of myocardial trabeculae in a ventricle with mixed myocardium, i.e., with a thick compacta, in S. canicula, a representative of chondrichthyans
Our results show that the formation of trabeculae in this type of myocardium takes place in three steps: emergence, trabeculation and remodeling
Summary
The heart of Scyliorhinus canicula, as in other chondrichthyans, is anatomically composed of sinus venosus, atrium, ventricle and an outflow tract formed by a conus arteriosus and a bulbus arteriosus. All these segments, contained within the pericardial cavity, have myocardium in their walls, except for the bulbus arteriosus, which consists mainly of elastin and smooth muscle cells[1,2]. Knowledge about the formation of the mixed type of myocardium is notably scarce, except for some brief accounts included in a general study of the heart development in the lesser spotted dogfish, Scyliorhinus canicula[19]. The species is oviparous, what facilitates obtaining embryos and determining their embryonic stage in the laboratory according to the criteria described by Ballard et al.[22]
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