Histological and ultrastructural aspects of the nasal complex in the harbour porpoise, Phocoena phocoena.

During the evolution of odontocetes, the nasal complex was modified into a complicated system of passages and diverticulae. It is generally accepted that these are essential structures for nasal sound production. However, the mechanism of sound generation and the functional significance of the epicranial nasal complex are not fully understood. We have studied the epicranial structures of harbor porpoises (Phocoena phocoena) using light and electron microscopy with special consideration of the nasal diverticulae, the phonic lips and dorsal bursae, the proposed center of nasal sound generation. The lining of the epicranial respiratory tract with associated diverticulae is consistently composed of a stratified squamous epithelium with incomplete keratinization and irregular pigmentation. It consists of a stratum basale and a stratum spinosum that transforms apically into a stratum externum. The epithelium of the phonic lips comprises 70-80 layers of extremely flattened cells, i.e., four times more layers than in the remaining epicranial air spaces. This alignment and the increased number of desmosomes surrounding each cell indicate a conspicuous rigid quality of the epithelium. The area surrounding the phonic lips and adjacent fat bodies exhibits a high density of mechanoreceptors, possibly perceiving pressure differentials and vibrations. Mechanoreceptors with few layers and with perineural capsules directly subepithelial of the phonic lips can be distinguished from larger, multi-layered mechanoreceptors without perineural capsules in the periphery of the dorsal bursae. A blade-like elastin body at the caudal wall of the epicranial respiratory tract may act as antagonist of the musculature that moves the blowhole ligament. Bursal cartilages exist in the developmental stages from fetus through juvenile and could not be verified in adults. These histological results support the hypothesis of nasal sound generation for the harbor porpoise and display specific adaptations of the echolocating system in this species.