Enameloid is well conserved within the teeth, scales, and denticles of the fossils of extinct animals, and extant fish taxa, as well as in extant caudate amphibians. However, this tissue persists only during the larval stage in living caudates and is absent postmetamorphosis in amphibians. Little is known regarding the evolutionary fate of enameloid and whether it transitions into enamel or dentin. Here, the upper jaws of Xenopus laevis tadpoles at stages 55, 56, and 57 were examined to analyze: 1) the spatial patterning of tooth germs, 2) histological profiles of mineral deposition and collagen fiber distribution, and 3) the fine structure of enameloid during early odontogenesis using stereomicroscopy, light microscopy, and transmission electron microscopy. Stereomicroscopic observations revealed that tooth germs at the cap and bell stages form in an alternating sequence in stage 57 tadpoles. Von Kossa staining demonstrated mineral deposition within the dental matrix at the bell stage. Furthermore, the enameloid matrix, which is primarily composed of collagen fibers, was clearly distinguished from enamel via picrosirius red staining. In capstage tooth germs, collagen fibers within the amorphous matrix were sparsely distributed and attached to the inner enamel epithelium and dental papilla cells, respectively. Conversely, bell-stage enameloid was densely occupied with collagen fibers and encapsulated by enamel. Overall, these results suggest that X. laevis serves as an ideal experimental model for investigating tooth evolution, comparable to caudate amphibians.

Introduction
Materials and Methods
    1. Experimental animals and stereomicroscopicobservation
    2. Tissue preparation
    3. Hematoxyline-eosin, picrosirius red, and von Kossastaining
    4. Preparation for transmission electron microscopy
Results
    1. Stereomicroscopic and histological observations
    2. Ultrastructural findings
Discussion
Funding
Conflicts of Interest
References

• Young-Hoon Lee(Department of Oral Anatomy, School of Dentistry, Jeonbuk National University, Jeonju 54896, Republic of Korea, Institute of Oral Biosciences, Jeonbuk National University, Jeonju 54896, Republic of Korea)
• Byung Keon Park(Department of Oral Anatomy, School of Dentistry, Jeonbuk National University, Jeonju 54896, Republic of Korea, Institute of Oral Biosciences, Jeonbuk National University, Jeonju 54896, Republic of Korea) Corresponding author