Bones in the heart skeleton of the otter (Lutra lutra)

In most mammalian species the cardiac skeleton is composed of coarse collag en fibres, fibrocartilage, and pieces of hyaline cartilage. Bone, the os co rdis, 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 x-ray analysis and light microscopy. Seri al sections were cut parallel to the atrioventricular plane and histochemic al staining methods were performed to identify connective tissue fibres, gl ycosaminoglycans, mineral deposits, and bone. Age and sex of the animals un der 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 laye r was missing and coarse connective tissue continuously transformed into fi brous 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 o f the otter, a small mammalian species. This finding indicates that differe ntiation of bone is not exclusively related to the size of the organ. Incre asing amounts of calcified cartilage and bone correlated with increasing ag e.