Review: the value of the American alligator (Alligator mississippiensis) as a model for research in craniofacial development.

American alligators exhibit a number of morphological features which are more characteristic of mammals than reptiles. This unique combination makes an extremely useful animal for experimental investigations of craniofacial development. Thus, it is possible to make a longitudinal study of the development of the alligator's secondary palate by utilizing shell-less culture techniques. Incubation of eggs at 28 degrees C, as opposed to 3 degrees C, slows development sufficiently to enable both the recovery of critical intermediate stages, and also great precision in the timing of teratogen administration. Injection of either 0.01 mg of 5-fluoro-2-desoxyuridine or 0.05 mg of hydrocortisone into the yolks of Days 12, 14, and 17 embryos produces cleft lip, cleft lip and palate, and cleft palate, respectively. Application of the teratogen at critical developmental stages on Day 12 reliably induces unilateral cleft lip on either the right or left sides, while injection on Day 10 yields embryos with normal lips and palates but virtually no lower jaw. The spontaneous malformation rate is very low, but highest in eggs laid by young and old animals. Changes in eggshell banding provide a noninvasive method for monitoring embryonic development. Detailed comparisons of palatal development in birds and alligators permit an accurate phylogenetic study of physiological clefting in the aves. The alligator dentition is readily amenable to experimental studies.