Heart Pigmentation in the Gray Bichir,Polypterus senegalus(Actinopterygii: Polypteriformes)

Pigmentation of the aortic and pulmonary valves in C57BL/6J x Balb/cByJ hybrid mice of different coat colours

Neural crest-derived melanocytes have been recorded in several parts of the mammalian heart but not in the pulmonary valve. We report here the presence of melanin-containing cells in the leaflets (cusps) of both the aortic and pulmonary valves. A total of 158 C57BL/6J x Balb/cByJ hybrid mice exhibiting four coat colours, namely black, white, agouti and non-agouti brown, were examined. We sought for any relationship between the presence of melanocytes in the valves and the coat colour of the animals. The pigmentation levels of the leaflets were accomplished using a scale of five pigment intensities. White mice lacked pigment in the heart. In 10.5% of the remaining animals, there were melanocytes in the pulmonary valve leaflets. Thus, this is the first study to report the presence of such cells in the pulmonary valve of mammals. Melanocytes occurred in the leaflets of the aortic valves of 87.2% of mice. The incidence of melanocytes and the pigmentation level of the leaflets did not statistically differ according to the coat colours of the animals. This disagrees with previous observations, indicating that the amount of melanocytes in the heart reflects that of the skin. The incidence and distribution of melanocytes in aortic and pulmonary valves are consistent with the notion that the formation of the arterial valves is mediated by specific subpopulations of neural crest cells. We hypothesize that melanocytes, even not producing melanin, may be more frequent in the heart than previously thought, exerting presumably an immunological function.

Development and evolution of the metazoan heart

The mechanisms of the evolution and development of the heart in metazoans are highlighted, starting with the evolutionary origin of the contractile cell, supposedly the precursor of cardiomyocytes. The last eukaryotic common ancestor is likely a combination of several cellular organisms containing their specific metabolic pathways and genetic signaling networks. During evolution, these tool kits diversified. Shared parts of these conserved tool kits act in the development and functioning of pumping hearts and open or closed circulations in such diverse species as arthropods, mollusks, and chordates. The genetic tool kits became more complex by gene duplications, addition of epigenetic modifications, influence of environmental factors, incorporation of viral genomes, cardiac changes necessitated by air‐breathing, and many others. We evaluate mechanisms involved in mollusks in the formation of three separate hearts and in arthropods in the formation of a tubular heart. A tubular heart is also present in embryonic stages of chordates, providing the septated four‐chambered heart, in birds and mammals passing through stages with first and second heart fields. The four‐chambered heart permits the formation of high‐pressure systemic and low‐pressure pulmonary circulation in birds and mammals, allowing for high metabolic rates and maintenance of body temperature. Crocodiles also have a (nearly) separated circulation, but their resting temperature conforms with the environment. We argue that endothermic ancestors lost the capacity to elevate their body temperature during evolution, resulting in ectothermic modern crocodilians. Finally, a clinically relevant paragraph reviews the occurrence of congenital cardiac malformations in humans as derailments of signaling pathways during embryonic development.

The cardiac regulatory toolkit contains many factors including epigenetic, genetic, viral, hemodynamic, and environmental factors, but also transcriptional activators, repressors, duplicated genes, redundancies and dose‐dependancies.

Numerous toolkits regulate mechanisms including cell‐cell interactions, EMT, mitosis patterns, cell migration and differentiation and left/right sidedness involved in the development of endocardial cushions, looping, septum complexes, pharyngeal arch arteries, chamber and valve formation and conduction system.

Evolutionary development of the yolk sac circulation likely preceded the advent of endothermy in amniotes.

Parallel evolutionary traits regulate the development of contractile pumps in various taxa often in conjunction with the gut, lungs and excretory organs.

Heart structure in the Amazonian teleost Arapaima gigas (Osteoglossiformes, Arapaimidae)

The fish heart ventricle has varied morphology and may have a specific morpho-functional design in species adapted to extreme environmental conditions. In general, the Amazonian ichthyofauna undergoes constant variations in water temperature, pH and oxygen saturation, which makes these species useful for investigations of cardiac morphology. Arapaima gigas, a member of the ancient teleost group Osteoglossomorpha, is one of the largest freshwater fish in the world. This species has a specific heart metabolism that uses fat as the main fuel when O2 supplies are abundant but also can change to glycogen fermentation when O2 content is limiting. However, no information is available regarding its heart morphology. Here, we describe the heart of A. gigas, with emphasis on the ventricular anatomy and myoarchitecture. Specimens of A. gigas weighing between 0.3 and 4040 g were grouped into three developmental stages. The hearts were collected and the anatomy analyzed with a stereomicroscope, ultrastructure with a scanning electron microscope, and histology using toluidine blue, Masson's trichrome and Sirius red stains. The ventricle undergoes morphological changes throughout its development, from the initial saccular shape with a fully trabeculated myocardium and coronary vessel restricted to the subepicardium (Type I) (group 1) to a pyramidal shape with mixed myocardium and coronary vessels that penetrate only to the level of the compact layer (Type II) (groups 2 and 3). The trabeculated myocardium has a distinct net-like organization in all the specimens, differing from that described for other teleosts. This arrangement delimits lacunae with a similar shape and distribution, which seems to allow a more uniform blood distribution through this myocardial layer.

The bulbus arteriosus of the holocephalan heart: gross anatomy, histomorphology, pigmentation, and evolutionary significance

This study was designed to determine whether the outflow tract of the holocephalan heart is composed of a myocardial conus arteriosus and a non-myocardial bulbus arteriosus, as is the case in elasmobranchs. This is a key issue to verify the hypothesis that these two anatomical components existed from the onset of the jawed vertebrate radiation. The Holocephali are the sister group of the elasmobranchs, sharing with them a common, still unknown Palaeozoic ancestor. The sample examined herein consisted of hearts from individuals of four species, two of them belonging to the Chimaeridae and the other two to the Rhinochimaeridae. In all specimens, the cardiac outflow tract consisted of a conus arteriosus, with myocardium in its walls and two rows of valves at its luminal side, and an intrapericardial bulbus arteriosus shorter than the conus and devoid of valves. The bulbus, mainly composed of elastin and smooth musculature, was covered by the epicardium and crossed longitudinally by coronary artery trunks. These findings give added support to the viewpoint that the outflow tract of the primitive heart of the gnathostomes was not composed of a single component, but two, the conus and the bulbus. All rabbitfish (Chimaera monstrosa) examined had pigment cells over the surface of the heart. The degree of pigmentation, which varied widely between individuals, was particularly intense in the cardiac outflow tract. Pigment cells also occurred in the bulbus arteriosus of one of the two hearts of the straightnose rabbitfish (Rhinochimaera atlantica) included in the study. The cells containing pigment, presumably derived from the neural crest, were located in the subepicardium.