In most mammalian species the cardiac skeleton is composed of coarse collagen fibres, fibrocartilage, and pieces of hyaline cartilage. Bone, the os cordis, is a regular constituent of the ruminant heart. The cardiac skeleton of the otter (Lutra lutra) has not previously been described. The skeleton in 30 otter hearts was studied by -ray analysis and light microscopy. Serial sections were cut parallel to the atrioventricular plane and histochemical staining methods were performed to identify connective tissue fibres, glycosaminoglycans, mineral deposits, and bone. Age and sex of the animals under investigation were considered. The otter heart skeleton was composed of coarse collagen fibres with intercalated pieces of fibrous and\or hyaline cartilage, calcified cartilage, and lamellar bone with red or white marrow. Pieces of hyaline cartilage were not clearly defined : a perichondrial layer was missing and coarse connective tissue continuously transformed into fibrous and hyaline cartilage. In both sexes the amount of cartilage and bone were found to increase with age.Our results establish the presence of bony material in the heart skeleton of the otter, a small mammalian species. This finding indicates that differentiation of bone is not exclusively related to the size of the organ. Increasing amounts of calcified cartilage and bone correlated with increasing age.Key words : Cardiac skeleton ; os cordis ; cartilage ; extracellular matrix. The musculature of the atrial and ventricular walls is inserted into the cardiac skeleton, which comprises the fibrous rings (annuli fibrosi) and the fibrous trigone (trigona fibrosa). The fibrous rings are mainly composed of interwoven bundles of collagen fibres with a few elastic fibres surrounding the atrioventricular openings and the openings of aorta and pulmonary artery. The fibrous trigones consist of areas of dense connective tissue between the aorta and the atrioventricular openings and are connected by the aortic-mitral curtain. The right trigone, together with the membranous septum, constitutes the central fibrous body (Gray, 1995). In certain mammalian species, the trigones contain fibrocartilage, hyaline cartilage, and even bony material. The latter forms a right and left cardiac bone (os cordis) related to the trigone between atrioventricular ostia and the aortic Correspondence to Dr M. Egerbacher,

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