Reptiles as a Model System to Study Heart Development

Trabeculations of the porcine and human cardiac ventricles are different in number but similar in total volume

An intricate meshwork of trabeculations lines the luminal side of cardiac ventricles. Compaction, a developmental process, is thought to reduce trabeculations by adding them to the neighboring compact wall which is then enlarged. When pig, a plausible cardiac donor for xenotransplantation, is compared to human, the ventricular walls appear to have fewer trabeculations. We hypothesized the trabecular volume is proportionally smaller in pig than in human. Macroscopically, we observed in 16 pig hearts that the ventricular walls harbor few but large trabeculations. Close inspection revealed a high number of tiny trabeculations, a few hundred, within the recesses of the large trabeculations. While tiny, these were still larger than embryonic trabeculations and even when considering their number, the total tally of trabeculations in pig was much fewer than in human. Volumetrics based on high-resolution MRI of additional six pig hearts compared to six human hearts, revealed the left ventricles were not significantly differently trabeculated (21.5 versus 22.8%, respectively), and the porcine right ventricles were only slightly less trabeculated (42.1 vs 49.3%, respectively). We then analyzed volumetrically 10 pig embryonic hearts from gestational day 14-35. The trabecular and compact layer always grew, as did the intertrabecular recesses, in contrast to what compaction predicts. The proportions of the trabecular and compact layers changed substantially, nonetheless, due to differences in their growth rate rather than compaction. In conclusion, processes that affect the trabecular morphology do not necessarily affect the proportion of trabecular-to-compact myocardium and they are then distinct from compaction.

Heart morphology during the embryonic development of Podocnemis unifilis Trosquel 1948 (Testudines: Podocnemididae)

Cardiogenesis is similar in all vertebrates, but differences in the valvuloseptal morphogenesis among non-crocodilian reptiles, birds, and mammals are noted. The origin of mesenchymal structures such as valves that regulate the passage of blood and the formation of partial septa that prevent the complete mixing of oxygen-rich and low-oxygen blood present in adult chelonians are essential in the evolutionary understanding of complete septation, endothermy and malformations, even in mammals. In this context, this study analyzed the heart morphogenesis of Podocnemis unifilis (Testudines: Podocnemididae) from the 4th to the 60th day of incubation. We identified the tubular heart stage, folding of the cardiac tube and expansion of the atrial and ventricular compartments followed by atrial septation by the septum primum, ventricle septation by partial septa, outflow tract septation and the formation of bicuspid valves with cartilage differentiation at the base. The formation of the first atrial septum with the mesenchymal cap is noted during the development of the atrial septum, joining the atrioventricular cushion on the 17th day and completely dividing the atria. Small secondary perforations appeared in the mid-cranial part, observed up to the 45th day. Partial ventricle septation into the pulmonary, venous, and arterial subcompartments takes place by trabeculae carneae thickening and grouping on the 15th day. The outflow tract forms the aorticopulmonary and interaortic septa on the 16th day and the bicuspid valves, on the 20th day. Therefore, after the first 20 days, the heart exhibits a general anatomical conformation similar to that of adult turtles.

Anguimorpha as a model group for studying the comparative heart morphology among Lepidosauria: Evolutionary window on the ventricular septation

The group Anguimorpha represents one of the most unified squamate clades in terms of body plan, ecomorphology, ecophysiology and evolution. On the other hand, the anguimorphs vary between different habitats and ecological niches. Therefore, we focused on the group Anguimorpha to test a possible correlation between heart morphology and ecological niche with respect to phylogenetic position in Squamata with Sphenodon, Salvator, and Pogona as the outgroups. The chosen lepidosaurian species were investigated by microCT. Generally, all lepidosaurs had two well‐developed atria with complete interatrial septum and one ventricle divided by ventricular septa to three different areas. The ventricles of all lepidosaurians had a compact layer and abundant trabeculae. The compact layer and trabeculae were developed in accordance with particular ecological niche of the species, the trabeculae in nocturnal animals with low metabolism, such as Sphenodon, Heloderma or Lanthanotus were more massive. On the other hand athletic animals, such as varanids or Salvator, had ventricle compartmentalization divided by three incomplete septa. A difference between varanids and Salvator was found in compact layer thickness: thicker in monitor lizards and possibly linked to their mammalian‐like high blood pressure, and the level of ventricular septation. In summary: heart morphology varied among clades in connection with the ecological niche of particular species and it reflects the phylogenetic position in model clade Anguimorpha. In the absence of fossil evidence, this is the closest approach how to understand heart evolution and septation in clade with different cardiac compartmentalization levels.

Comparative and Functional Anatomy of the Ectothermic Sauropsid Heart

The heart development, form, and functional specializations of chelonians, squamates, crocodilians, and birds characterize how diverse structure and specializations arise from similar foundations. This review aims to summarize the morphologic diversity of sauropsid hearts and present it in an integrative functional and phylogenetic context. Besides the detailed morphologic descriptions, the integrative view of function, evolution, and development will aid understanding of the surprising diversity of sauropsid hearts. This integrated perspective is a foundation that strengthens appreciation that the sauropsid hearts are the outcome of biological evolution; disease often is linked to arising mismatch between adaptations and modern environments.

Anatomy of the heart of the leatherback turtle

Non‐crocodylian reptiles have hearts with a single ventricle, which is partially separated by a muscular ridge that provides some separation of blood flows. An exceptional situation exists in monitor lizards and pythons, where the ventricular left side generates a much higher systolic blood pressure than the right side, thus resembling mammals and birds. This functional division of the ventricle depends on a large muscular ridge and may relate to high metabolic demand. The large leatherback turtle (<1000 kg), with its extensive migrations and elevated body temperatures, may have similar adaptations. We report on the anatomy of the hearts of two leatherback turtles. One stranded in Ballum, Denmark in 2020, and was examined in detail, supplemented by observations and photos of an additional stranding specimen from Canada. The external morphology of the leatherback heart resembles that of other turtles, but it is large. We made morphometric measurements of the Ballum heart and created an interactive 3D model using high‐resolution MRI. The volume of the ventricle was 950 ml, from a turtle of 300 kg, which is proportionally almost twice as large as in other reptiles. The Ballum heart was compared to MRI scans of the hearts of a tortoise, a python, and a monitor lizard. Internally, the leatherback heart is typical of non‐crocodylian reptiles and did not contain the well‐developed septation found in pythons and monitor lizards. We conclude that if leatherback turtles have exceptional circulation needs, they are sustained with a relatively large but otherwise typical non‐crocodylian reptile heart.

Ventricular Septation and Outflow Tract Development in Crocodilians Result in Two Aortas with Bicuspid Semilunar Valves

Background: The outflow tract of crocodilians resembles that of birds and mammals as ventricular septation is complete. The arterial anatomy, however, presents with a pulmonary trunk originating from the right ventricular cavum, and two aortas originating from either the right or left ventricular cavity. Mixing of blood in crocodilians cannot occur at the ventricular level as in other reptiles but instead takes place at the aortic root level by a shunt, the foramen of Panizza, the opening of which is guarded by two facing semilunar leaflets of both bicuspid aortic valves. Methods: Developmental stages of Alligator mississipiensis, Crocodilus niloticus and Caiman latirostris were studied histologically. Results and Conclusions: The outflow tract septation complex can be divided into two components. The aorto-pulmonary septum divides the pulmonary trunk from both aortas, whereas the interaortic septum divides the systemic from the visceral aorta. Neural crest cells are most likely involved in the formation of both components. Remodeling of the endocardial cushions and both septa results in the formation of bicuspid valves in all three arterial trunks. The foramen of Panizza originates intracardially as a channel in the septal endocardial cushion.

Cardiac Structures in Marine Animals Provide Insight on Potential Directions for Interventions for Pediatric Congenital Heart Defects

Despite recent advances in pediatric diagnosis and surgical intervention, mortality and morbidity continue to be a prevalent issue in both Tetralogy of Fallot (ToF) and Hypoplastic Left Heart Syndrome (HLHS). Therefore, novel approaches to studying both of these conditions is warranted. Investigating cardiac anatomical features of different species in the animal kingdom similar to the defects and complications present in ToF and HLHS (as well as others) could serve as a new avenue for improving the management of congenital heart diseases (CHD). This review reveals that although structures found in HLHS and ToF are pathological, similar structures are found in diving mammals and reptiles that are adaptive. Pathologic aortic dilation in CHD resembles the aortic bulb present in diving mammals, but the latter is more elastic and distensible compared to the former. The unrepaired HLHS heart resembles the univentricular heart of non-crocodilian reptiles. Right ventricle hypertrophy is pathological in HLHS and ToF, but adaptive in crocodilians and diving mammals. Lastly, the increased pulmonary resistance due to pulmonary stenosis in ToF is comparable to increased pulmonary resistance in crocodilians due to the presence of an active valve proximal to the pulmonary valve. Some of these anatomical structures could potentially be adapted for palliative surgery in children with HLHS or ToF. Moreover, further investigating the underlying molecular signals responsible for the adaptive tissue responses seen in other species may also be useful for developing novel strategies for preventing some of the complications that occur after surgical repair in both of these CHDs.

High heart rate associated early repolarization causes J-waves in both zebra finch and mouse

High heart rates are a feature of small endothermic—or warm‐blooded—mammals and birds. In small mammals, the QT interval is short, and local ventricular recordings reveal early repolarization that coincides with the J‐wave on the ECG, a positive deflection following the QRS complex. Early repolarization contributes to short QT‐intervals thereby enabling brief cardiac cycles and high heart rates. We therefore hypothesized high hearts rates associate with early repolarization and J‐waves on the ECG of endothermic birds. We tested this hypothesis by comparing isolated hearts of zebra finches and mice and recorded pseudo‐ECGs and optical action potentials (zebra finch, n = 8; mouse, n = 8). In both species, heart rate exceeded 300 beats per min, and total ventricular activation was fast (QRS < 10 ms). Ventricular activation progressed from the left to the right ventricle in zebra finch, whereas it progressed from apex‐to‐base in mouse. In both species, the early repolarization front followed the activation front, causing a positive J‐wave in the pseudo‐ECG. Inhibition of early repolarization by 4‐aminopyridine reduced J‐wave amplitude in both species. Action potential duration was similar between ventricles in zebra finch, whereas in mouse the left ventricular action potential was longer. Accordingly, late repolarization had opposite directions in zebra finch (left‐right) and mouse (right‐left). This caused a similar direction for the zebra finch J‐wave and T‐wave, whereas in the mouse they were discordant. Our findings demonstrate that early repolarization and the associated J‐wave may have evolved by convergence in association with high heart rates.

An Appreciation of Anatomy in the Molecular World

Robert H. Anderson is one of the most important and accomplished cardiac anatomists of the last decades, having made major contributions to our understanding of the anatomy of normal hearts and the pathologies of acquired and congenital heart diseases. While cardiac anatomy as a research discipline has become largely subservient to molecular biology, anatomists like Professor Anderson demonstrate anatomy has much to offer. Here, we provide cases of early anatomical insights on the heart that were rediscovered, and expanded on, by molecular techniques: migration of neural crest cells to the heart was deduced from histological observations (1908) and independently shown again with experimental interventions; pharyngeal mesoderm is added to the embryonic heart (1973) in what is now defined as the molecularly distinguishable second heart field; chambers develop from the heart tube as regional pouches in what is now considered the ballooning model by the molecular identification of regional differentiation and proliferation. The anatomical discovery of the conduction system by Purkinje, His, Tawara, Keith, and Flack is a special case because the main findings were never neglected in later molecular studies. Professor Anderson has successfully demonstrated that sound knowledge of anatomy is indispensable for proper understanding of cardiac development.