Epithelial crypts: A complex and enigmatic olfactory organ in African and South American lungfish (Lepidosireniformes, Dipnoi).

African lungfish (Protopterus) seem unique among osteognathostomes in possessing a potential vomeronasal organ homolog in form of accessory epithelial crypts within their nasal cavity. Many details regarding structural and functional properties of these crypts are still unexplored. In this study, we reinvestigate the issue and also present the first data on epithelial crypts in the South American lungfish Lepidosiren paradoxa. The nasal cavities of L. paradoxa and Protopterus annectens were studied using histology, scanning electron microscopy, and alcian blue and PAS staining. In both species, the epithelial crypts consist of a pseudostratified sensory epithelium and a monolayer of elongated glandular cells, in accordance with previously published data on Protopterus. In addition, we found a new second and anatomically distinct type of mucous cell within the duct leading into the crypt. These glandular duct cells are PAS positive, whereas the elongated glandular cells are stainable with alcian blue, suggesting distinct functions of their respective secretions. Furthermore, the two lungfish species show differently structured crypt sensory epithelia and external crypt morphology, with conspicuous bilaterally symmetrical stripes of ciliated cells in L. paradoxa. Taken together, our data suggest that stimulus transport into the crypts involves both ciliary movement and odorant binding mucus.

Ontogenetic Development of the Derived Olfactory System of the Mantellid Frog Mantidactylus betsileanus.

The nasal cavity of Mantidactylus betsileanus, a frog of the Madagascar-Comoroan endemic family Mantellidae, is characterized by a unique internal architecture. Unlike the state commonly observed in anurans, the two discernible olfactory subsystems of M. betsileanus (the main olfactory organ and the vomeronasal organ) are anatomically separated from each other, suggesting an enhanced functional differentiation. Here we evaluate the ontogenetic formation of this extraordinary anatomical state based on a histological study of a developmental series of M. betsileanus. The olfactory system of premetamorphic tadpoles, and most of its changes during metamorphosis, resembles that of other anurans. At the end of metamorphosis however, a growing obstruction of the passage between main olfactory organ and vomeronasal organ takes place, leading to the deviant morphological state previously described for adults. The late appearance of this atypical anatomical feature in the course of ontogeny agrees with the phylogenetic hypothesis of the observed obstruction representing a derived state for these frogs. From a functional point of view, the apparent autonomy of the vomeronasal organ is possibly linked to the presence of clade-specific femoral glands that are known to produce pheromones and that likewise are fully expressed in adults only. Anat Rec, 299:943-950, 2016. © 2016 Wiley Periodicals, Inc.

The nasolacrimal duct of anuran amphibians: suggestions on its functional role in vomeronasal perception.

Tear secretions discharged by the Harderian gland are suggested to function as a solvent for molecules sensed by the vomeronasal organ (VNO) in anurans. It has been assumed that chemical stimuli are absorbed at the surface of the eye to be carried - together with the lacrimal fluid - into the nasal cavity via the nasolacrimal duct. In the study presented herein, we examined the intranasal anatomy of 10 different anuran species to analyse the opening region of the nasolacrimal duct and its functional relationship with the VNO and the external naris. In addition, vital staining of the nasal cavities was conducted. Our results indicate that stimuli reaching the VNO are more likely to be ingested through the nostril than via the eye. In many cases the intranasal orifice of the nasolacrimal duct shows a close proximity to the external naris and simultaneously we observed a noticeable distance to the VNO. We suggest that the secretions of the Harderian gland are carried to the external naris by the nasolacrimal duct, where they bind chemical stimuli that are subsequently actively transported into the VNO. In some of the investigated species the opening region of the tear duct was situated in a more caudal part of the nasal cavity and closer to the VNO. In these cases a conspicuous system of channels can be found, which is suspected to carry the intruding medium of smell from the nostril to the nasolacrimal aperture.