Morphological Differences in the Musculature of the Pulmonary Venous Wall between Three Species of Caviomorph, the Nutria, Guinea Pig and Chinchilla

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The musculature of the pulmonary vein of three species of caviomorph, the nutria Myocastor coypus, the guinea pig Cavia porcellus, and the chinchilla Chinchilla laniger, was examined by light and transmission electron microscopy. In the nutria and the guinea pig, cardiac musculature was localized only in the extrapulmonary region, whereas in the chinchilla it was distributed not only in the large-sized but also in the medium-sized intrapulmonary venous walls. These results suggest that the pattern of the distribution of cardiac musculature may not necessarily reflect the phylogenic relationships between species, since such a major morphological difference was found within a single phylogenic group. The hypothesis that body size may determine the distribution pattern is supported by the data which show that the cardiac musculature is distributed in intrapulmonary veins in small rodents or insectivores, but not in those of mammals larger than the guinea pig. Furthermore, when the ultrastructure of cardiac myocytes in the pulmonary venous wall of chinchillas was examined, morphological similarities were found between the cardiac myocytes in the pulmonary vein and the atrial myocytes of the heart. This suggests that active contraction of the musculature of the pulmonary vein may also occur.