A description of the times of appearance and regression of marginal lingual papillae in human fetuses and newborns

Evolution of the structure and function of the vertebrate tongue

Studies of the comparative morphology of the tongues of living vertebrates have revealed how variations in the morphology and function of the organ might be related to evolutional events. The tongue, which plays a very important role in food intake by vertebrates, exhibits significant morphological variations that appear to represent adaptation to the current environmental conditions of each respective habitat. This review examines the fundamental importance of morphology in the evolution of the vertebrate tongue, focusing on the origin of the tongue and on the relationship between morphology and environmental conditions. Tongues of various extant vertebrates, including those of amphibians, reptiles, birds and mammals, were analysed in terms of gross anatomy and microanatomy by light microscopy and by scanning and transmission electron microscopy. Comparisons of tongue morphology revealed a relationship between changes in the appearance of the tongue and changes in habitat, from a freshwater environment to a terrestrial environment, as well as a relationship between the extent of keratinization of the lingual epithelium and the transition from a moist or wet environment to a dry environment. The lingual epithelium of amphibians is devoid of keratinization while that of reptilians is keratinized to different extents. Reptiles live in a variety of habitats, from seawater to regions of high temperature and very high or very low humidity. Keratinization of the lingual epithelium is considered to have been acquired concomitantly with the evolution of amniotes. The variations in the extent of keratinization of the lingual epithelium, which is observed between various amniotes, appear to be secondary, reflecting the environmental conditions of different species.

[Intraepithelial nerve endings in the lingual marginal papillae of newborn piglets]

The structure and occurrence of nerve fibres and nerve terminals in the lingual marginal papillae of newborn piglets are investigated by means of the osmium-zinc-iodate method and by transmission-electron microscopy. Subepithelial nerve fibres primarily accompany the blood vessels. They cross the epithelium with an almost vertical course to terminate in the stratum superficiale or the stratum corneum. Due to rows of varicosities, the axons look like a 'string of pearls'. As a result of the superficial localization and the typical ultrastructure, they seem to have a sensory function. Therefore, the papillae marginales are considered to function as mechanoreceptors.

[Morphology of the marginal lingual papillae in newborn piglets]

The morphology of the marginal lingual papillae in eight newborn pigs aged 1-5 days was examined at the light-microscopical level, as well as, for the first time, using electron transmission and scanning methods. The papillae marginales are arranged tuft-like or rosetted with chief and accessory papillae, the individual papillae varying considerably in shape. Leaf-, tongue- or fingerlike and conical shapes predominate. The stratified squamous epithelium is either keratinized, resembling the epidermal differentiation pathway, or non-keratinized, according to the mucosal pathway. The characteristic morphology of keratinocyte differentiation is shown with special reference to the establishment of the permeability barrier. The connective tissue papillae are intensively surface-enlarged by means of multiple fingerlike protrusions.