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 compartmentalization.