Tongue development in several developmental stages of the metamorphosing newt Hynobius leechi (Hynobiidae) and the paedomorphic Ambystoma mexicanum (Ambystomatidae) before and after artificially induced metamorphosis was studied by light microscopy (LM) and scanning electron microscopy (SEM). In H. leechi the anlage of the glandular field (lingual glands) of the secondary tongue appears under the free tip of the primary tongue and is clearly seen in late larvae (developmental stage approx. 65). The epithelium of the primary tongue is stratified and composed of epithelial cells, AB-positive goblet cells, some superficial ciliated cells, very few Leydig cells and typical taste buds. Later more or less radially arranged tubular glands (lingual glands) develop in the anterior portion of the prospective secondary tongue, which open in furrows lined in their upper region by the surface epithelium (“neck portion”). Fully developed glands are variously long, moderately branched and contain columnar secretory cells that are preferably AB-positive in their upper region, but AB- and PAS-negative in their terminal portions. Posteriorly the tubular glands become shorter in favour of the neck portion and then are abruptly replaced by a heavily ciliated area containing indentations of the epithelium interspersed with numerous goblet cells (crypts). This zone is considered as a modified remnant of the former primary tongue. Formation of the secondary tongue, often described as “fusion” of the glandular field with the primary tongue, is considered as a process levelling the free, probably regressive end of the primary tongue and the posterior part of the growing glandular field. The development of the secondary tongue of metamorphosing A. mexicanum follows the same pattern. However, the putative anlage of lingual glands in semiadult paedomorphic specimens may be considered as a further character indicating partial metamorphosis in this species. In the transformed axolotl we demonstrate the secondary tongue with lingual glands, epithelial folds with noticeable numbers of AB-PAS-positive goblet cells at the lower surface of the free tip of the secondary tongue, and, contrary to H. leechi, tubular glands immediately behind the dentary.

Urodela, tongue development, lingual glands, metamorphosis, primary and secondary tongue