Histomorphometric characterization of cardiac nodes in cattle, ponies, and cats.

The sinoatrial node (SAN) and atrioventricular node (AVN) are essential components of the cardiac conduction system and play a critical role in initiating and propagating the electrical impulse. To characterise these cardiac nodes across different species, we examined the hearts of ten cattle, ten cats, and five male ponies without evidence of cardiac disease using conventional histological and immunohistochemical techniques. Morphometric parameters of the nodes and their constituent cells were quantified at different magnifications. The SAN displayed an oval morphology in all species. Its diameter was 888.58 ± 6.36 μm in cats, 2143.17 ± 199.13 μm in ponies, and 2033.28 ± 939.44 μm in cattle. Mean nodal density was significantly higher in ponies than in cattle (p = 0.011). Pacemaker (P) cells of the AVN were significantly larger in ungulate species than in cats (p < 0.001). Heart bone (os cordis) was identified in all ten cattle hearts and in one cat heart, showing typical features of compact bone and bone marrow. Cartilaginous metaplasia was observed in all pony hearts and in 60% of cat hearts and was associated with reduced AVN size and cellularity in cats. These findings highlight important interspecies differences and contribute to the understanding of conduction system pathology in veterinary cardiology. The structural changes in the cardiac nodes in these species warrant more in-depth functional research, given the lack of functional cardiac assessment and the small sample size.