Development of the dentition in Alligator mississippiensis. Early embryonic development in the lower jaw

Abstract

Development of the dentition in a close series of accurately staged and aged embryos of Alligator mississippiensis was studied by macroscopy, light microscopy, scanning electron microscopy (SEM) and detailed reconstructions. Dental development in the lower jaw up to stage 18; day 26 is described in this paper. In the lower jaw, the first formed tooth germ appears at stage 14 in tooth family 3; the second at stage 15 in family 6; the third, fourth, fifth, sixth and seventh at stage 17 in families 12,2,4,9 and 16, respectively. Neither ‘Zahnreihen’ nor ‘perfect alternation’ theories of tooth initiation explain the data. Rather the present data suggest that initiation of teeth is related to jaw growth, the distance between existing teeth and the size and developmental maturity of the latter. Each tooth is postulated to have a ‘zone of inhibition’ around it. The first formed embryonic teeth are discernible by macroscopy and SEM as surface elevations (projecting tooth germs). The dental tissues of these are poorly differentiated and lack enamel. Projecting tooth germs sink into the jaw mesenchyme, their dental epithelia degenerate and they persist as non‐functional dentine rudiments for varying periods before becoming completely resorbed or shed. Formation of the dental lamina occurs by an overgrowth of the operculum (on which a distinct line of cobblestoned cells is visible by SEM) and closure of the dental furrow, as well as by the formation of a dental prolamina. The scaled fate maps and descriptive appearances of the developing alligator dentition highlight its value for future experimental studies of pattern formation and positional information.

Evolution and developmental diversity of tooth regeneration

“…A similar mirror image development of palatal teeth is also found in Sphenodon [45]. In most toothed reptiles a non-functional set of teeth is observed developing early on in the embryo, these teeth forming superficially on the oral surface rather than being associated with a dental lamina [13,48,52,54]. These non-functional tooth germs have been proposed to be remnants of a larval dentition, that became redundant with the extension of the embryonic period [52].…”

Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development

“…The primordial null-generation teeth are resorbed, incorporated into the jaws, or shed in ovo (3,66,67). For example, American alligators (Alligator mississippiensis) go through two to four tooth-replacement cycles before hatching (95)(96)(97). The timing for the establishment of amniote hatchling functional dentitions is well established.…”

Reversing opinions on Dollo’s Law

“…The amniote tooth developmental program, as exemplified by modern mammals and crocodiles [43,52], begins when the oral epithelium forms a dental lamina (see Figure 2). This invaginates (or evaginates in some reptile teeth) into the underlying mesenchyme and becomes dental epithelium, while the mesenchyme condenses to form tooth buds.…”