Journal of Dental Research, Vol 77, 1730-1738, Copyright © 1998 by International & American Associations for Dental Research Online Journals
Relationship between growth and the pattern of tooth initiation in alligator embryos
J. W. Osborn
Department of Oral Health Sciences, Faculty of Medicine and Oral Health Sciences, University of Alberta, Edmonton, Canada.
The temporal and spatial patterns in which teeth are initiated in the
growing jaws of embryos are constant for a species but different for
different species. The sources of the patterns have been explained in two
ways. First, they are the outcome of reactions between molecules created at
stationary targets and those which diffuse through embryonic tissues (e.g.,
Edmund, 1960). Second, Osborn (1978) supposed that the patterns mirror the
way a (mixed) population of parent cells, the tooth clone, grows.
Westergaard and Ferguson (1986, 1987, 1990) concluded, from their
observations of the sequence of tooth initiation in alligators, that the
complicated sequences in which 20 teeth are initiated in each tooth
quadrant could not be explained by jaw growth. The present study attempts
to refute this criticism by means of measurements made from the raw data
published by Westergaard and Ferguson. These data reveal that new teeth,
here called primary teeth, are added at a constant rate at the back of the
jaw. Interstitial growth of the cells between primary teeth creates space
for secondary teeth in secondary regions. The secondary regions increase in
length exponentially with time. The sequence in which teeth are initiated
in the growing secondary regions was found to be the same in every part of
the upper and lower jaws. It was accurately reproduced by a computer
program based on a linear contraction rate of inhibitory zones and
exponential growth of secondary regions. The results suggest that the
posterior progress zone in alligator embryos grows about 125 microm a day.
Newly initiated tooth germs are surrounded by an inhibitory zone about 250
microm in diameter. These zones contract from 20 to 30 microm a day until
they are about 170 microm in diameter. The sequences in which tooth
positions are initiated in embryos may be more the result of the pattern in
which cells escape from molecules that inhibit induction rather than the
pattern in which cells create molecules that initiate induction.
