RELATIONSHIP BETWEEN GROWTH AND THE PATTERN OF TOOTH INITIATION IN ALLIGATOR EMBRYOS

The temporal and spatial patterns in which teeth are initiated in the growing jaws of embryos are constant for a species but different for d ifferent species. The sources of the patterns have been explained in t wo ways. First, they are the outcome of reactions between molecules cr eated at stationary targets and those which diffuse through embryonic tissues (eg., Edmund, 1960). Second, Osborn (1978) supposed that the p atterns 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 alligat ors, 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 r eveal that new teeth, here called primary teeth,are added at a constan t rate at the back of the jaw. Interstitial growth of the cells betwee n primary teeth creates space for secondary teeth in secondary regions . The secondary regions increase in length exponentially with time. Th e sequence in which teeth are initiated in the growing secondary regio ns 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 seconda ry regions. The results suggest that the posterior progress zone in al ligator embryos grows about 125 mu m a day. Newly initiated tooth germ s are surrounded by an inhibitory zone about 250 mu m in diameter. The se zones contract from 20 to 30 mu m a day until they are about 170 mu m iri 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 whic h cells create molecules that initiate induction.