Secondary cartilage in the auditory bulla of the hamster.

The auditory bulla, the bony capsule enclosing the middle ear, has been extensively studied in dry skulls and fossils, but detailed histological descriptions of the connective tissues involved are lacking. In rodents the membraneously ossified tympanic bone forms the major part of the enlarged auditory bulla. This study demonstrated fairly extensive secondary cartilage formation on the developing tympanic bone of the hamster. The first sign of the secondary cartilage was visible with Alcian blue staining on the 5th postnatal day. The amount of cartilage diminished after 15 days of age, but some cartilage was still present on the tympanic bone at the end of the observation period (25 days of age). While the secondary cartilage was present, a great amount of translative growth occurred in the bony coverage of the tympanic cavity, not only to accommodate the enchondrally growing cochlea within the cavity, but also to create space for the enlarged auditory bulla as an adaptation to low-frequency sensitivity in hearing. The tympanic bone is a derivative of the angular bone in the reptilian lower jaw. The mammalian mandible, a derivative of another membraneous bone of the reptilian lower jaw, the dentary, possesses secondary cartilages in the angular and condylar processes. Do the reptilian dentary and angular utilize secondary cartilage in the process of their transformation to the mammalian phylogenetic level?