Histology and growth of the cetacean petro-tympanic bone complex

Abstract

The histological organization and the main growth trends of the periotic and tympanic bones are described in known-aged common dolphins. The maximum sizes of these bones are already acquired in new-born specimens, and their full mineralization (84.62% and 83.3%, respectively) is reached either at birth (tympanic) or within the first 6 months. Mass, compactness and density of the periotic and tympanic quickly rise during the first year of life, and remain stable afterwards. Mean values of compactness and density observed in adults are 94% and 2.65 g/cm³ for the periotic, and 98% and 2.66 g/cm³ for the tympanic. The periotic and tympanic are composed of a fibro-lamellar tissue, initially deposited as a loose spongiosa with hypermineralized trabeculae. Before the end of the first year, this spongiosa is made compact by the perivascular formation of primary osteons. The vascular network of bone, originally very extensive, is obliterated rapidly by the osteons. The collagen matrix is composed of very thin collagen fibrils (diameter: 10 to 30 nm) arranged in a dramatically reduced network compared to normal bone. Hence, the room available for the mineral phase is considerably increased. Whatever the age of the animals, there is no inner (Haversian) remodelling in the periotic and tympanic. Since the compactness and density of these bones augment during the nursing period, it seems likely that the full hearing capacities of the dolphins are not reached before weaning.