Distribution of catecholamine-containing cells in the atrial region in rats and ground squirrels

The number and distribution of small intensely fluorescent (SIF) cells in the atrial region were determined, using fluorescence histochemical and histological techniques, in two species of the Rodentia order. In rats, SIF cells bore a fairly consistent anatomical relationship to the intracardiac ganglia and the variation in their distribution in individual animals appeared to reflect in part variations in extent and conformation of the intracardiac ganglia. In ground squirrels of similar age, fewer SIF cells were associated with intracardiac ganglia but they were in grape-like clusters between the aorta and the pulmonary artery. The clusters, varying from sparse to extensive, were present in all six ground squirrels examined. These observations indicate considerable intraorder differences in both qualitative and quantitative distribution of SIF cells in the atrial region, and prompt speculation that morphological differences reflect different functional roles.