Structure of the conus arteriosus of the sturgeon (Acipenser naccarii) heart. I: the conus valves and the subendocardium.

Sturgeons are bony fish that retain structural traits typical of the more primitive Chondrostei. From an evolutionary viewpoint, sturgeons are considered relic fish. However, they show remarkable ecological plasticity and are well adapted to contemporary environmental conditions. Although development of the cardiovascular system is critical for all organs and systems, and is affected by evolutionary changes, the structure of the sturgeon heart has been mostly overlooked. This is also true for the conus arteriosus, which, as in Chondrostei, is endowed with several rows of valves and a layer of contractile myocardium. This work reports on the structure of the valves, the endocardium, and the subendocardium of the conus arteriosus of the sturgeon (Acipenser naccarii) heart. It is part of a broader study that aims to cover the entire structure of the sturgeon heart. The conus arteriosus of 15 A. naccarii hearts, ranging in age from juveniles to sexually-differentiated adults, has been studied by conventional light, transmission (TEM), and scanning electron microscopy (SEM). In addition, maceration of the soft tissues with NaOH, and actin localization by fluorescent phalloidin has been used. The conus is a tubular chamber that arises from the right ventricular side and presents two constrictions at the conus-ventricle and conus-aorta junctions. The conus is endowed with three rows of valves: one distal and two proximal. The segment of the conus located between the distal and the two proximal rows is devoid of valvular structures. The distal row has four leaflets, while the two proximal rows show the greatest variation in leaflet number, size, and shape. All leaflets have collagenous chordae tendineae arising from the free border and from the parietal side of the leaflets. The endocardium is a flat endothelium which shows a thick, irregular basement membrane. The leaflet body is formed by a loose connective tissue which blends with the subendocardium. The subendocardium is a connective tissue consisting of myofibroblasts, collagen, and elastin. It is divided into two distinct areas: one proximal, which shows little elastin and poorly organized collagen; and one distal, which is rich in elastin, with cells and extracellular fibers organized into layers that are oriented in alternative circumferential and longitudinal directions. The present report is the first systematic analysis of the structure of the sturgeon conus. Descriptions of the conus valves should recognize the existence of three valve rows only. The variability in valve morphology, and the loose structure of the leaflet tissue make it unlikely that the valves play an effective role in preventing blood backflow. In this regard, the ventricle-conus constriction may act as a sphincter. The subendocardium is an elastic coat capable of actively sustaining the tissue deformation that accompanies the heart contractile cycle. Further comparative studies are needed to provide deeper insight into the structural changes that accompany phyletic diversification.

The formation, septation and fate of the truncus arteriosus in man.

The results of our studies enable us to draw the following conclusions. The truncus appears in the human embryo, between Stages XII and XIII, as a portion of the aortic sac which invaginates into the interior of the pericardial cavity. Therefore it is an arterial portion which is added to the heart. It lengthens progressively. The sigmoid valves form in the angle between the bulbus cordis and the truncus. Septation of the truncus begins when the sixth arterial arches appear in embryos of 6 to 8 mm. The process is very rapid; commencing in embryos of 6 to 7 mm, it is complete in embryos of 10 to 11 mm, that is to say, during only five days. The septation mechanism is extrinsic. The peribranchial mesenchyma which accompanies the aortic sac in its invagination advances principally on the right inferior part and insinuates itself between the fourth and sixth arterial arches, separating the truncus pulmonalis from a portion of the ascending aorta. An aorticopulmonary communication exists for a certain period prior to fusion of the two blocks of mesenchyma; there is a mesenchymal island. On the contrary, in the bulbus cordis septation is effected by the bulbar ridges. Septation of the truncus, which does not exist in the primitive cardiac tube, occurs prior to that of the bulbus cordis. While septation of the truncus has been already completed in embryos of 10 mm, septation of the bulbus cordis is completed only in embryos of 14 to 16 mm. Therefore the bulbus and the truncus are two portions, different in respect of both structure and septation. There is no continuity between the bulbar ridges and the septation of the truncus. They are separated by the sigmoid valves. This makes it possible to observe independent malformations in the bulbus and in the truncus. In the truncus the mesenchyma passes between the two vessels. They do not have a common septum, and it is for this reason that the surgeon can separate them in the mature heart.

Experimental study of the development of the truncus and the conus in the chick embryo.

The development of the truncus and the conus was studied in the chick embryo by in vivo labelling techniques. The earliest labels were placed at the stage of fusion of the myocardial troughs (stage 9-) and they were traced until the mature heart stage (stage 35). Microdissections and light microscopic studies were also carried out. The results are discussed in relation to the human heart. Our experiments permit the following conclusions: (1) At stage 9- fusion of the myocardial troughs takes place at the level of the primordium of the trabeculated portion of the right ventricle, when neither the conus nor the truncus are present. (2) At stage 12 (loop stage) there appears the caudal portion of the conus, which constitutes the cephalic end of the cardiac tube. (3) The truncus appears between stages 13 and 22. (4) At stage 22 angular junction between the conus and the truncus is the area where the semilunar valve cusps of the great arteries will develop and that, at this same stage, the junction between the conus and the trabeculated portion of the right ventricle seen from the right surface corresponds to the inferior edge of the crista supraventricularis. (5) It was confirmed that the pulmonary semilunar valve cusps originate from the walls of the truncus. (6) The development of the conus and truncus are similar in chick and man. (7) Histologically, in the chick, the wall of the truncus and the conus contain cardiac muscle as late as stage 28, but from then on the walls of the truncus are transformed into connective tissue and plain muscle.