Ultrastructure of the olfactory organ in the clawed frog,Xenopus laevis, during larval development and metamorphosis

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 metamorphosis 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 metamorphosis.

Ancestral amphibian v2r s are expressed in the main olfactory epithelium

“…Moreover, MOE-specific expression was found in a large percentage (50%) of the v2r genes analyzed here. This pattern of expression is very different from the ectopic or broad expression of sporadic mammalian or and v1r genes (20,21) and diametrically opposite to the very rare cells occasionally seen for more modern v2r genes of subgroup A2 and A3 both in larval MOE [this study and results by Hagino-Yamagish et al (2)] and adult middle cavity (2), a subdivision of the MOE arising during metamorphosis (22). In fact, the restriction of the more ancestral v2r genes to the MOE is even stricter than that of the more modern v2r genes to the VNO, because we did not find a single exception in over 400 counted cells.…”

Expression Patterns of Glycoconjugates in the Three Distinctive Olfactory Pathways of the Clawed Frog, Xenopus laevis.

“…Morphological characteristics between the olfactory epithelium and the middle chamber epithelium are also different. The olfactory epithelium contains ciliated receptor neurons, whereas the middle chamber epithelium contains both ciliated and microvillous receptor neurons [16,27]. Although the functional significance of the vomeronasal epithelium in amphibians remains unclear, these previous studies strongly suggest the presence of three types of distinct olfactory subsystems in the Xenopus primary olfactory system.…”

Coordinated shift of olfactory amino acid responses and V2R expression to an amphibian water nose during metamorphosis

“…Anurans reconstruct the main olfactory epithelium (MOE) of their aquatic tadpoles into a so-called air nose (principal cavity) during metamorphosis [2]. Secondarily aquatic pipid frogs such as Xenopus laevis have an additional olfactory epithelium, the so-called water nose (middle cavity), that develops newly during metamorphosis [3][4][5], and is responsible for detecting water-borne odors (in terrestrial anurans the middle cavity is non-sensory [2]). …”