Species-specific features of the heart morphology of Rana temporaria

During amphibian metamorphosis, the single atrium becomes divided into the right and left atria, resulting in the formation of a three-chambered heart consisting of one ventricle and two atria. The venous sinus adjoins the right atrium and receives blood returning from the body organs, whereas oxygenated arterial blood from the respiratory organs enters the left atrium. Both atria communicate with the ventricle through a common opening, causing partial mixing of blood. However, this mixing remains incomplete due to muscular trabeculae projecting from the ventricular walls, which form a system of interconnected chambers. The arterial conus with a spiral valve, represented by a longitudinal fold directing blood flow during cardiac contraction, arises from the right portion of the ventricle , where venous blood predominates. Topographically, the heart is located on the ventral side of the anterior body cavity beneath the esophagus. The lateral surfaces of the right and left atria contact the medial surfaces of the bases of the right and left lungs, while the apex of the heart is directed caudally and to the right. The macro- and histoarchitecture of the common frog heart exhibit distinctive characteristics at different levels of organization. In particular, the morphometric parameters and structure of myocardial cardiomyocytes are associated with the absence of coronary vessels, indicating that the heart lacks its own blood supply system. The heart of an adult frog is ellipsoid in shape, dorsoventrally flattened, and belongs to the elongated-expanded type, as confirmed by the development index (158.2 ± 3.9%). The absolute heart weight is 0.14 ± 0.0 1 g, while the relative heart weight is 0.21 ± 0.02%. The atria are completely separated by a septum and communicate with the ventr i cle through a common opening. The arterial conus extends from the ventricle and is positioned ventrally, whereas the venous sinus adjoins the dorsal part of the right atrium . The ventricular myocardium has a spongy structure, containing a central cavity from which additional hollow chambers extend. The muscle fibers are arranged irregularly and, together with the cavities, form a reticular structure. Contractile cardiomyocytes are mononucleated, and their longitudinal and transverse striations are less pronounced than those observed in warm-blooded animals. Cytometric analysis revealed cardiomyocytes of small (6.75 ± 0.28 μm), medium (9.34 ± 0.46 μm), and large (11.86 ± 0.52 μm) widths in the ve n tricular myocardium. The corresponding nuclear volumes were 32.58 ± 2.08, 48.75 ± 2.20, and 64.82 ± 2.82 μm³, r e spectively. Histometric analysis demonstrated that the average area occupied by muscle fibers within the ventricular myocardium was 66.8 ± 2.0%, whereas the connective tissue stroma accounted for 34.2 ± 1.9%.