Ultrastructure of the Olfactory Organ in the Clawed Frog, Xenopus laevis , During Larval Development and Metamorphosis ANNE HANSEN, 1 JOHN O. REISS, 2 CYNTHIA L. GENTRY, 2 AND GAIL D. BURD 2 * 1 Zoological Institute and Zoological Museum, University of Hamburg, D-20146 Hamburg, Germany 2 Department of Molecular and Cellular Biology, University of Arizona, Life Sciences South 444, Tucson, Arizona 85721 ABSTRACT Development of the olfactory epithelia of the African clawed frog, Xenopus laevis, was studied by scanning and transmission electron microscopy. Stages examined ranged from hatching through the end of metamorphosis. The larval olfactory organ consists of two chambers, the principal cavity and the vomeronasal organ (VNO). A third sensory chamber, the middle cavity, arises during metamorphosis. In larvae, the principal cavity is exposed to water-borne odorants, but after metamorphosis it is exposed to airborne odorants. The middle cavity and the VNO are always exposed to waterborne odorants. Electron microscopy reveals that in larvae, principal cavity receptor cells are of two types, ciliated and microvillar. Principal cavity supporting cells are also of two types, ciliated and secretory (with small, electron-lucent granules). After metamorphosis, the principal cavity contains only ciliated receptor cells and secretory supporting cells, and the cilia on the receptor cells are longer than in larvae. Supporting cell secretory granules are now large and electron-dense. In contrast, the middle cavity epithelium contains the same cell types seen in the larval principal cavity. The VNO has microvillar receptor cells and ciliated supporting cells throughout life. The cellular process by which the principal cavity epithelium changes during metamorpho- sis is not entirely clear. Morphological evidence from this study suggests that both microvillar and ciliated receptor cells die, to be replaced by newly generated cells. In addition, ciliated supporting cells also appear to die, whereas there is evidence that secretory supporting cells transdifferentiate into the adult type. In summary, significant developmental additions and neural plasticity are involved in remodeling the olfactory epithelium in Xenopus at metamor- phosis. J. Comp. Neurol. 398:273–288, 1998.  1998 Wiley-Liss, Inc. Indexing terms: amphibia; chemosensory; ontogeny; scanning electron microscopy; transmission electron microscopy As their name implies, amphibians typically lead a dual life, with an aquatic larva and a terrestrial adult. In this way, they are intermediate between fishes and amniotes. The transition from water to land clearly places many demands on sensory systems, including the olfactory sys- tem. Classical studies at the light microscopic level have shown that the olfactory organ remodels at metamorpho- sis in all living amphibian groups: salamanders (Seydel, 1895; Schuch, 1934), caecilians (Sarasin and Sarasin, 1890; Badenhorst, 1978), and frogs (Born, 1876; Hinsberg, 1901; Fo ¨ske, 1934; Rowedder, 1937; Khalil, 1978). This remodeling is far more extensive in frogs than in the other groups. However, little is known about the ultrastructural changes that accompany the changes in gross morphology. Grant sponsor: NIDCD NRSA; Grant number: DC-00189; Grant sponsor: University of Arizona Foundation; Grant sponsor: Office of the Vice President for Research at the University of Arizona; Grant sponsor: Howard Hughes Medical Institute; Grant number: 71195-521303; Grant sponsor: NIDCD; Grant number: DC-00446; Grant sponsor: NINDS; Grant number: NS-37147. Anne Hansen and John O. Reiss contributed equally as first author of this manuscript. John O. Reiss’s present address: Department of Biology, Humboldt State University, Arcata, CA 95521. Cynthia L. Gentry’s present address: Program in Neuroscience, Univer- sity of Arizona, Tucson, AZ 85721. *Correspondence to: Gail D. Burd, Department of Molecular and Cellular Biology, University of Arizona, Life Sciences South 444, Tucson, AZ 85721. E-mail: gburd@u.arizona.edu Received 13 January 1998; Revised 20 April 1998; Accepted 27 April 1998 THE JOURNAL OF COMPARATIVE NEUROLOGY 398:273–288 ( 1998)  1998 WILEY-LISS, INC.