Skeletal Tissues in Sharks

Abstract

Recent data on shark skeletal tissues have been reviewed. It is now reasonably certain that shark teeth and denticles are covered with a true ectodermal enamel, although the outer layer of these structures is structurally complex, consisting of both calcified ectodermal enamel and uncalcified areas of ectomesenchymal origin. The intradermal base of these structures most probably consists of acellular bone. The structural array of apatite crystallites in the teeth seems strongly correlated with the specific shape and function of individual teeth. The calcified cartilage of sharks differs significantly from that of other vertebrates, not only in its composition, but in the fact that the areas of calcification are composed of many vital and non-hypertrophic cells. Recent studies of the mineralization processes of other vertebrate tissues suggest a possible explanation for the classically described differential patterns of calcification of shark cartilages, but the specific details in elasmobranchs are as yet unknown, as indeed are many other aspects of their skeletal tissues whose future investigation would surely be useful in the elucidation of the general processes of vertebrate skeletal tissue mineralization.