Histological observation of the palate in Alligator Mississippiensis

The palatine rugae are specialized structures located on the fused hard palate; however, the number, arrangement and direction of the palatine rugae differ among species. Since crocodilians were evolutionally the first animals to show fusion of the palate, the authors focused on the structure of the crocodilian palate. The surface structure of the palate in Alligator mississippiensis at postnatal 60 days was examined by stereomicroscopy and scanning electron microscopy. In addition, histological observation was made by hematoxylin-eosin and Azan staining. Moreover, the urea silver nitrate method was used for nerve staining. On the palate of Alligator mississippiensis, many papilla-like, small conic-shaped projections of about 450-800 m in width and about 500 m in height were observed, although there were no palatine rugae. These projections were distributed over the whole palate, and there was no difference in density by region (anterior, middle, and posterior portion of the hard palate). These projections consisted of keratinized stratified squamous epithelium and connective tissue. In particular, many collagen fibers were observed in connective tissue. The bundles of nerve fibers ran from the connective tissue toward the oral epithelium. Collagen fibers were sparse in the area with the fascicle of nerve fibers under the epithelium of these projections. It is generally known that many nerve fibers and nerve endings are located in the connective tissue of the palatine rugae. Since there are many nerve fibers in the connective tissue of these projections, it is suggested that these projections may act as sensitive organs such as the palatine rugae.