Gail D. Burd

Ultrastructure of the Olfactory Organ in the Clawed Frog, Xenopus laevis, During Larval Development and Metamorphosis

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.

Metamorphic remodeling of the primary olfactory projection in Xenopus: Developmental independence of projections from olfactory neuron subclasses

In adult Xenopus, the nasal cavity is divided into separate middle (MC) and principal (PC) cavities; the former is used to smell water-borne odorants, the latter air-borne odorants. Recent work has shown that olfactory neurons of each cavity express a distinct subclass of odorant receptors. Moreover, MC and PC axons project to distinct regions of the olfactory bulb. To examine the developmental basis for this specificity in the olfactory projection, we extirpated the developing MC from early metamorphic (stage 54-57) tadpoles and raised the animals through metamorphosis. In most lesioned animals, the MC partly regenerated. Compared with the unlesioned side, reduction of the region of the glomerular layer of the olfactory bulb receiving MC afferents ranged from 70% to 95%. PC afferents did not occupy regions of the olfactory bulb deprived of MC afferents. These results support a model in which intrinsic cues in the olfactory bulb control the projection pattern attained by ingrowing olfactory axons.

Neurogenesis in the olfactory bulb of the frogXenopus Laevis shows unique patterns during embryonic development and metamorphosis

We determined the time of origin of neurons in the olfactory bulb of the South African clawed frog,Xenopus laevis. Tritiated thymidine injections were administered to frog embryos and tadpoles from gastrulation (stage 11/12) through metamorphosis (stage 65), paraffin sections were processed for autoradiography, and the distribution of heavily and lightly labeled cells was examined. In the ventral olfactory bulb, we observed that the mitral cells were born as early as stage 11/12 and continued to be generated through the end of metamorphosis. Interneurons (periglomerular and granule cells) were not born in the ventral bulb until stage 41, and birth of these cells also continued through metamorphosis. Labeled cells were observed in the accessory olfactory bulb, beginning at stage 41. In contrast, the cells of the dorsal olfactory bulb were not born until the onset of metamorphosis (stage 54); at this stage in the dorsal bulb, the genesis of mitral cells, interneurons, and glial cells completely overlapped. The results indicate that olfactory axon innervation is not necessary to induce early stages of neurogenesis in the ventral olfactory bulb. On the other hand, the results on the dorsal olfactory bulb are consistent with the hypothesis that innervation from new or transformed sensory neurons in the principal cavity induces neurogenesis in the dorsal bulb.

Neuronal turnover in the Xenopus laevis olfactory epithelium during metamorphosis

Metamorphic changes in the amphibian olfactory system present many interesting questions concerning the competing possibilities of neuronal respecification versus replacement. For example, are olfactory neurons retained during this transition with their presumed sensitivity to waterborne versus airborne stimuli respecified, or are olfactory neurons completely replaced? We address this question using the African clawed frog (Xenopus laevis) as a model. The water‐sensing nose (principal cavity; PC) of larval X. laevis is respecified into an air‐sensing cavity in adults, with changes in odorant receptor gene expression, ultrastructure, and site of innervation of the receptor neurons. The vomeronasal organ (VNO) does not appear to change function, structure, or innervation during metamorphosis. We labeled PC and VNO olfactory receptor neurons with injections of retrogradely transported fluorescent microspheres into the main and accessory olfactory bulbs. Injections were performed in larvae, and animals were allowed to survive through metamorphosis. After metamorphosis, few labeled cells were observed in the PC, whereas the VNO and the olfactory bulbs remained heavily labeled. Animals that were killed before metamorphosis always had extensive label in the PC epithelium regardless of how long the beads were present. This suggests that changes in the PC olfactory epithelium that are seen during metamorphosis are due primarily to turnover of the neurons in this epithelium rather than to respecification of existing neurons. These results also are discussed in terms of natural turnover time of olfactory receptor neurons. J. Comp. Neurol. 433:124–130, 2001.

Development of the Olfactory System in the African Clawed Frog, Xenopus Laevis

The olfactory system of adult mammals has been studied extensively (see Halasz, 1990). The basic structure of the mammalian olfactory epithelium and olfactory bulb is outlined in this section to serve as background for the development and adult structure of the olfactory system in the African clawed frog, Xenopus laevis. The peripheral sensory tissue of the vertebrate olfactory system is the olfactory epithelium (see Farbman, 1992). The olfactory epithelium lines the dorsal roof, septum, and lateral turbinates of the caudal region of the mammalian nasal cavity. Another area of chemosensory tissue located at the floor of the nasal cavity of many mammals, snakes, and amphibians is the vomeronasal organ (see Farbman, 1992; Eisthen, 1997). In both the olfactory epithelium and vomeronasal organ, there are three basic cell types: olfactory receptor neurons, supporting cells, and basal cells. The receptor neurons are primary sensory neurons that are responsible for transducing odorant stimuli. They are bipolar neurons with an axon that projects to the olfactory bulb. The supporting cells can be secretory or ciliated cells that span the sensory epithelium from the apical border to the basal lamina. The basal cells are stem cells that have the ability to divide and differentiate into receptor cells during development and throughout the life of the organism.