Teeth and jaws of Xenopus laevis: the time scales of tooth development and replacement in Xenoous laevis (Daudin), with observations on the growth of the jaws

Abstract

The prime concern of the Thesis was to establish absolute time scales for tooth development and tooth replacement, but the relationship between the dentition and growth of the jaws was also considered. The work was undertaken in three parts as follows:

(1) A longitudinal study was made of the first teeth to form and erupt (first generation teeth) in larval and newly metamorphosed animals. This was done by rearing 172 larvae in such a way that their rates of body growth (as measured by external criteria) were similar, and so the course of dental development could be followed histologically in a cross-sectional study. In this way the events in tooth development (amelogenesis, dentinogenesis, eruption, ankylosis and resorption) were observed and a time scale applied. On average each tooth took 26 days to develop and erupt, and then remained in its functional position for about 7 days. Individual tooth replacement was assessed to occur about every 16 days.

In a follow up longitudinal study 96 animals were used to assess the time taken for resorption of the first generation teeth. It was found that resorption could occur at any time after a tooth completed ankylosis, but that the process occurred in two phases - a phase of slow resorption lasting up to 8 days (called erosion), followed by a 48 hour phase of rapid resorption (called absorption). The histological differences between the phases are described.

(2) A longitudinal study of the dentitions of three large adult females was made by anaesthetising them at regular intervals and taking wax impressions of their mouths. Charts were constructed, and the data analysed to obtain (a) the functional life span of the teeth, (b) the period of time tooth loci were unoccupied before eruption of the successional teeth, and (c) the replacement cycle time (defined as the period between eruption of a tooth and the eruption of its successor). Variation was found between animals, and the median functional life span ranged between about 24 and 29 days, and the median replacement cycle time between 38 and 42 days. These time scales were compared with the functional life span of the first generation teeth, and the pattern of tooth replacement discussed.

(3) In order to assess the relationship of the dentition to the jaws, metamorphosed animals of various sizes and ages were used to study proportional changes in the upper and lower jaws with increase in body size, and to study variations in the number and size of the teeth. Significant, but small, differences in jaw proportions were found between animals, and these are discussed in relation to local factors. An assessment was made of the importance of the dentition and jaws for feeding.

Since the first generation teeth began to form during the larval stages of the animal, and in view of the dramatic change in feeding patterns at metamorphosis, the larval lower jaw was examined from a functional viewpoint, and the rate at which it changed shape during metamorphosis assessed.

Part three of the work is presented first in the Thesis to enable the reader to appreciate the jaws and dentition as a unit.