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.
