Conus arteriosus of the teleost heart: Dismissed, but not missed

Abstract

The heart outflow tract (OFT) of primitive fish is formed by two portions: a proximal conus arteriosus and a distal bulbus arteriosus. The OFT of modern teleosts is considered to be formed by a single component, the bulbus, the conus having been lost through evolution. This article challenges the concept of the disappearance of the conus arteriosus in the teleost heart. A total of 28 teleost species belonging to 19 families and 10 orders were analyzed. The hearts were divided into two large groups: those having entirely trabeculated ventricles, and those possessing a compacta. In the hearts having entirely trabeculated ventricles, the conus arteriosus appears as a distinct segment interposed between the ventricle and the bulbus arteriosus, being formed by compact vascularized myocardium. However, the conus of several species lacks vessels. In these cases, the conus presents large intercellular spaces bounded by collagen. In the hearts possessing a ventricular compacta, the conus either appears as a muscular ring of variable length connecting the ventricle and the bulbus or forms a crown or ring of myocardium apposed to the ventricular base. In all the teleosts studied, the conus can be recognized as an anatomic entity different from the ventricle. Furthermore, the conus appears as a distinct heart segment in the developing fish. Therefore, the conus arteriosus has not been lost in evolution and constitutes a fundamental part of the teleost OFT. In all the species studied, the conus supports the OFT valves, which should properly be named conus valves. Anat Rec Part A, 288A:900 -908, 2006.

The Teleost Heart: A Morphological Approach

“…In fact, the overall picture that emerges from the different studies is that the OFT of all primitive fi sh abides to the same general rule. Like primitive fi sh (Parsons 1930 ;Icardo 2006 ;Grimes and Kirby 2009 ) , ancient teleosts show a conus and a bulbus. In all genera studied (Albula, Pterothrissus, Megalops, Elops, Tarpon), the heart shows a muscular conus arteriosus (Parsons 1930 ;Santer 1985 ;Satchell 1991 ) .…”

“…The loss of the conus was attributed to intussusception into the ventricle (Smith 1918 ) , or it was considered a direct consequence of heart evolution (Santer 1985 ) . However, evidence raised in the last decade indicates that the conus is not lost at all in the heart of modern teleosts (Schib et al 2002 ;Icardo et al 2003 ;Grimes et al 2006 ;Icardo 2006 ) . Examination of up to 28 species belonging to different families and orders (Icardo 2006 ) has revealed that the conus arteriosus is a distinct anatomical segment interposed between the ventricle and the bulbus arteriosus.…”

“…However, evidence raised in the last decade indicates that the conus is not lost at all in the heart of modern teleosts (Schib et al 2002 ;Icardo et al 2003 ;Grimes et al 2006 ;Icardo 2006 ) . Examination of up to 28 species belonging to different families and orders (Icardo 2006 ) has revealed that the conus arteriosus is a distinct anatomical segment interposed between the ventricle and the bulbus arteriosus. The Macrouridae (Greer Walker et al 1985 ) could also be added to that list (discussed in Icardo et al 2003 ) .…”

“…With very few exceptions, the conus contains vessels (Fig. 2.3a ) even when the neighboring myocardium is not vascularized (Icardo 2006 ) . As occurs with other fi sh hearts having conus, the teleost conus arteriosus supports the out fl ow valves, which should more properly be named conus valves (Schib et al 2002 ) .…”

“…Recent studies indicate that, in addition to those four chambers, all teleosts contain a conus arteriosus (Schib et al 2002 ;Grimes et al 2006 ;Icardo 2006 ) and a distinct atrioventricular segment (Icardo and Colvee 2011 ) . Accordingly, the morphology of the heart is reviewed to include these two segments.…”