Metamorphic development of the bronchial columella of the larval bullfrog (Rana catesbeiana)

Histological and immunohistochemical analyses of head and respiratory structures in bullfrog (Rana catesbeiana) tadpoles were undertaken to address the hypothesis that the bronchial columella (BC) is the primary sound conduction pathway in these larval anurans. In postembryonic tadpoles, the BC is composed of fibroblasts surrounded by a Type I collagen matrix, with Type II collagen located in basement membranes at the distal ends. It provides a highly flexible tendon-like attachment between the round window and the membranous sac of the primary bronchus of the ipsilateral lung. As the animals approach metamorphic climax stages, the fibroblasts decrease in number and the BC becomes almost exclusively collagenous. During metamorphic climax, the BC degenerates and is completely resorbed by the time the animal becomes a postmetamorphic froglet. At all larval stages examined, the BC is structurally and immunohistochemically different from both the opercularis muscle of tadpoles and the tympanic columella (stapes homolog) of postmetamorphic animals. These observations suggest that the BC may not be rigid enough to provide an effective coupling between the lungs and the round window. An alternative hypothesis for the function of the BC, based on its structure, is presented.