From ontogeny to phylogeny : the olfactory system of Ascaphus truei

Tetrapods began the water-to-land transition during the Devonian period, approximately 400 million years ago. To succeed in this new environment the sensory systems of the early terrestrial explorers underwent specific adaptations. A similar challenge faces amphibians as they develop from aquatic larvae to terrestrial adults. In this study, the morphological and ultrastructural changes associated with metamorphosis in the primitive frog Ascaphus truei were examined to shed light on how the olfactory system may have evolved to function in a terrestrial versus aquatic environment. Light microscopy, transmission electron microscopy and scanning electron microscopy were used. The main olfactory chamber of larval A. truei contains both microvillar and ciliated supporting cells and both microvillar and ciliated receptor cells. In the adult stage the principal chamber contains a "larval-type" of epithelium anteriorly, but posteriorly the principal chamber is lined with an "adult-type" of epithelium containing solely ciliated receptor cells and microvillar supporting cells. The presence of two distinct types of olfactory epithelium in the principal chamber is unique among previously described anurans. The presence of both microvillar and ciliated receptor cells may serve to detect both water-borne and air-borne odorants and may represent the primitive condition for amphibians.