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The formation of the eight independent endorgan compartments (sacculus, utricle, horizontal canal, anterior canal, posterior canal, lagena, amphibian papilla, and basilar papilla) of the Xenopus laevis inner ear is illustrated as the otic vesicle develops into a complex labyrinthine structure. The morphology of transverse sections and whole-mounts of the inner ear was assessed in seven developmental stages (28, 31, 37, 42, 45, 47, 50) using brightfield and laser scanning confocal microscopy. The presence of mechanosensory hair cells in the sensory epithelia was determined by identification of stereociliary bundles in cryosectioned tissue and whole-mounts of the inner ear labeled with the fluorescent F-actin probe Alexa-488 phalloidin. Between stages 28 and 45, the otic vesicle grows in size, stereociliary bundles appear and increase in number, and the pars inferior and pars superior become visible. The initial formation of vestibular compartments with their nascent stereociliary bundles is seen by larval stage 47, and all eight vestibular and auditory compartments with their characteristic sensory fields are present by larval stage 50. Thus, in Xenopus, inner ear compartments are established between stages 45 and 50, a 2-week period during which the ear quadruples in length in the anteroposterior dimension. The anatomical images presented here demonstrate the morphological changes that occur as the otic vesicle forms the auditory and vestibular endorgans of the inner ear. These images provide a resource for investigations of gene expression patterns in Xenopus during inner ear compartmentalization and morphogenesis.
Figure 3. The F-actin cytoskeleton (green) of stereociliary bundles is detected with Alexa 488âphalloidin fluorescence in the Xenopus laevis otic vesicle. AâD: Arrows point to stereociliary bundles in confocal images of cryosections from stages 31 (A), 37 (B), 42 (C), and 45 (D). Propidium iodide counterstain (red) labels RNA (cytosol) and DNA (nuclei). Scale bar = 50 μm.Download figure to PowerPoint
Figure 4. Alexa 488–phalloidin fluorescence marks the F-actin cytoskeleton (green) of stereociliary bundles in confocal sections from S47 (A) and S50 (B) inner ear whole-mounts. Arrows point to stereociliary bundles. Propidium iodide counterstain (red) labels RNA (cytosol) and DNA (nuclei). A: S47 endorgan compartments contain regions where stereociliary bundles are identifiable. A1: Sacculus, utricle. A2: Crista of the anterior canal. A3: Lagena, cristae of posterior and horizontal canals. B: Representative confocal sections show that stereociliary bundles are visible in the epithelia of all S50 vestibular and auditory compartments. B1: Utricle, sacculus. B2: Cristae of horizontal and anterior canals. B3: Crista of horizontal canal (transverse view). B4: Lagena, sensory ganglion. B5: Basilar papilla, amphibian papilla, crista of posterior canal, sensory ganglion adjacent to amphibian papilla. Scale bar = 50 μm.Download figure to PowerPoint
actb (actin, beta) gene expression in Xenopus laevis embryos, NF stage 31, as assayed by phalloidin fluorescent staining. transverse section, left is up, dorsal is right.
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