Int J Dev Biol
January 1, 2007;
Cell proliferation during the early compartmentalization of the Xenopus laevis inner ear.
The auditory and vestibular endorgans of the inner ear
which are essential for the senses of hearing and balance form early during development when the otocyst
undergoes a period of rapid growth and compartmentalization. Here we show the spatial and temporal patterns of proliferating cells in the Xenopus laevis inner ear
as this organ develops from an otic vesicle
at stage 31
until stage 47
, an age at which compartmentalization and the initial appearance of sensory structures are evident. Sites of new cell production were identified in specimens at stages 31, 37, 42, 45 and 47 using immunohistochemical methods to detect bromodeoxyuridine (BrdU) incorporation three hours after exposure to this thymidine analogue. Cells undergoing terminal mitosis at stages 37, 42 and 45 were detected by exposing specimens at these stages to BrdU and permitting development to proceed until stage 47
. Our results show that while newly replicating cells are uniformly distributed throughout the stage 31 otic vesicle
, they are spatially restricted in stages 37 through 45, with few dividing cells visible in the central patches of the emerging sensory epithelia. In contrast, no clear proliferative pattern was discerned at stage 47
. BrdU-positive cells that had undergone terminal mitosis at stage 37, 42 and 45 were detected in the central regions of nascent sensory epithelia at stage 47
. These findings are consistent with a developmental mechanism in which cells undergoing terminal mitosis during early X. laevis stages contribute to sensory epithelia and in which cell mixing and migration are features of inner ear
Int J Dev Biol
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Fig. 1. Cell proliferation in the stage 31 (A), 37 (B), 42 (C) and 45 (D) X. laevis inner ear, three hours after transient exposure to BrdU. Representative anterior to posterior sections (A1-A3; B1-B3; C1-C3; D1-D3) show BrdU-positive cells labelled with anti-BrdU antibody (red/brown; arrows) and counterstained with hematoxylin (blue/violet). (A) At stage 31, BrdU-positive cells are distributed throughout the epithelium of the otic vesicle (A1-A3). (B-D) Few BrdU-positive cells are detected in the emerging saccular epithelium (asterisks), while BrdU-positive cells can be seen flanking these regions in the stage 37 (B1-B3), 42 (C1-C3) and 45 (D1-D3) inner ear. BrdU-positive cells (arrows) can be seen restricted to other regions of the otic epithelium and within the sensory ganglion at stage 42 (C1-C3) and 45 (D1-D3). BrdU-positive cells (dashed circles) are visible in the ventricular zone of the brain in all stages (A4, B4, C4, D4). Control sections processed for immuno- histochemistry with secondary antibody but without primary (anti-BrdU) antibody and counterstained with hematoxylin (blue/violet) are shown in panels A5, C5, D5; panel B5 shows a control section that was not counterstained with hematoxylin. Scale bar, 100 μm.
Fig. 2 (Left). Cell proliferation in nascent sensory compartments of the stage 47 X. laevis inner ear, three hours after transient exposure to BrdU. Anterior to posterior sections representing the entire inner ear (A-D) show the presence of BrdU-positive cells (deep black; arrows) in the epithelia of all endorgan compartments. (A) BrdU- positive cells are more prevalent in the ventral regions of the utricle in anterior sections. (B) BrdU-positive cells are distributed throughout the epithelium of the utricle, horizontal canal, sacculus and anterior canal. (C) BrdU incorporation was detected in the saccular sensory epithelium and in the sensory ganglion. (D) BrdU-positive cells are visible in the lagena and posterior canal in posterior sections of the inner ear. (E) BrdU-positive cells were detected in the ventricular zone of the brain. (F) Control section processed for immunohistochemistry with secondary antibody but without primary (anti-BrdU) antibody. Hematoxylin counterstain (grey). Scale bar, 100 μm.
Fig. 3 (Right). Terminal mitosis in the inner ear of stage 47 X. laevis that were transiently exposed to BrdU at stages 37 (A), 42 (B) and 45 (C). Anterior to posterior sections representing the entire inner ear show the presence of BrdU-positive cells (black/dark-grey) with birthdates from earlier stages (S37, A1-A3; S42, B1-B3; S45, C1-C3) in stage 47 otic epithelia. Cells that had undergone terminal mitosis (arrows, dashed circles) are predominantly seen in the central regions of the emerging sensory epithelia of the utricle (A1, B1, C1) and of the sacculus (A2-A3, B2-B3, C2-C3) regardless of stage of BrdU exposure. Images of the saccular region within the dashed circles are shown at higher magnification underneath the corresponding panels. Control sections processed for immunohistochemistry with secondary antibody but without primary (anti-BrdU) antibody and counterstained with hematoxylin are shown in panels (A4, B4, C4) for the saccular epithelium. Hematoxylin counterstain (light grey). Scale bar, 50 μm.
Fig. 4 (Left). Comparison of new cell production (A) and terminal mitosis (B) in the saccular region of the X. laevis inner ear. (A) Cell proliferation observed after three hours of development following transient BrdU exposure at stages 37 (A1), 42 (A2) and 45 (A3). BrdU-positive cells (red/brown) flank the emerging saccular epithelium (dashed circles) at all stages. Few BrdU-positive cells are detected within the thickened region of the saccular epithelium. (B) Terminal mitosis observed at stage 47 following BrdU exposure at stage 37 (B1), 42 (B2) and 45 (B3). Images of the saccular region within the dashed circles are shown at higher magnification underneath the corresponding panels. In contrast to Fig. 4A, BrdU-positive cells (black/ dark-grey) with birthdates from earlier stages are prevalent in the thickened central region of the saccular epithelium (dashed circle). Hematoxylin counterstain (A, blue/violet; B, light grey). Scale bar, 50 μm.
Fig. 5 (Right). Schematic diagrams illustrating cell production in sections of the stage 31 (A), 37 (B), 42 (C), 45 (D) and 47 (E) X. laevis inner ear that contain saccular epithelium (dashed circles) and the sensory ganglion (sg). (A) At early stages, BrdU-positive cells (red) are distributed throughout the entire epithelium of the stage 31 otic vesicle. (B-D) At stages 37 (A), 42 (B) and 45 (C), BrdU-positive cells are detected throughout the otic epithelium but appear restricted to specific regions such as the outer boundaries of the emerging saccular sensory epithelium and flanking the protrusions of the emerging horizontal canals (D). (E) In the stage 47 inner ear, BrdU-positive cells are distributed throughout the otic epithelium without a distinct trend toward localization in a particular region. Hematoxylin counterstain (blue). Scale bar, 100 μm.