Abstract:Juveniles of the Mekong giant catfish, Pangasianodon gigas (Teleostei), have 3 sorts of toothupper and lower jaw teeth, palatal teeth, and pharyngeal teeth-but adults are toothless. To investigate the histogenesis and disappearance of the teeth, we made serial sections of the mouth and teeth of juvenile fish at 10 developmental stages (from ca. 8.5 to ca. 30 cm in total length) and examined them under scanning electron microscope and light microscope. Observations of teeth and surrounding tissues in the serial sections revealed the process of tooth resorption by active odontoclast-like cells. Numbers of jaw and palatal teeth decreased with age. When the fish reached ca. 14 cm in total length, the numbers of functional upper jaw teeth and successional tooth germs decreased rapidly, and the developmental rate of successional tooth germs slowed. When the fish reached ca. 24 cm, no teeth existed in the upper jaw. It is clear that tooth disappearance results from the shedding of functional teeth and the lack of replacement tooth germs. (J. Oral Sci. 45, 213-221, 2003)

The histological structure of upper-and lower-jaw teeth of Sicyopterus japonicus, a fish known to exhibit rock-climbing behavior, were examined by light microscopy and scanning electron microscopy. The upper jaw teeth and the lower jaw teeth were entirely different in form. The characteristics of the upper jaw teeth were that the enameloid constituting the tip of the tooth had a shovel-like shape and that there were many eruptive teeth behind each of which lay a large number of successional teeth ready for continuous replacement. The shape of the enameloid was suited to the work of scraping algae from the surface of stones. The lower jaw teeth had a hinged structure. It was suggested that the forms of teeth in Sicyopterus japonicus were the result of the fish's adaptation to its feeding habit, rather than its rock-climbing nature.

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