Pharyngeal dentition of medaka: tooth replacement and stem cell niche

Background: Medaka has nearly 1,000 functional teeth in pharyngeal regions, which are continuously replaced with new ones throughout life. Objectives: To determine precise replacement cycles of pharyngeal teeth and identify dental stem cells enabling continual tooth renewal. Methods: We examined cell proliferation in pharyngeal odontogenic regions of medaka by BrdU labeling combined with fluorochrome labeling of growing teeth. The adult medaka was IP injected with BrdU (3mg/ml) and killed at various time intervals up to 8W, and processed for paraffin embedding. After BrdU injection, some were allowed to swim in calcein-water for 5 to 15 min and processed as above, but the pharyngeal bones/teeth were dissected and examined by fluorescent microscopy as whole mount samples. Results: By 2 hrs after BrdU injection, nuclei of the epithelial cells were labeled with BrdU ranging from basal to the mid zone of odontogenic epithelia and spread out throughout the epithelial layer by 24-48 hrs. Most of labeled cells in pharyngeal epithelia disappeared by 2W except in the narrow regions posterior to the rows of developing tooth germs. In calcein labeling study, labeled tooth germs located posterior to the rows of unlabeled functional teeth replaced the functional teeth in 2W and remained functional for the following 2 to 3W until they were finally replaced with the next generation of teeth. Conclusions: These data indicate existence of possible stem cell niche in the dental epithelia of eternally replacing pharyngeal teeth of medaka and their replacement cycles.