Histological and morphometric study of the components of the sinus and atrioventricular nodes in horses and dogs

The cardiac nodes are the source of the electrical impulse that is transmitted to the heart, the aim of this work is study the histological and morphometric characteristics of the different components of the sinus and atrioventricular nodes in horses and dogs that help to know the physiopathology of these nodes. A group of ten horse hearts and five dog hearts were used. The region of the sinus and atrioventricular nodes was sectioned serially, and the block of tissue removed for study. The samples were assessed using a morphometric analysis with the Image-Pro Plus 7.1 software and the acquisition of the images using a Leica DMD108 optic microscope. The shape of the horse's sinus node is oblong and its P cells are large. The shape of the dog's sinus is rounded or oblong. The P cells are large and pale. The area of P cells in horses was 976 (SD 223.7) μm² and in dogs the area for P cells was 106 (SD 30.4) μm², which indicates that the value for P cells in horses are significantly higher than in dogs (p = .001). The horse atrioventricular node presented an oblong shape and in dogs, presents a spindle shape. The lower cell density in any of the cardiac nodes, especially in P cells of sinus node, can decrease electrical conduction within the nodes and in the internodal tracts, which would reflect the presence of cardiac arrhythmias derived from poor conduction, even in morphologically normal hearts.