The developing alligator tongue undergoes a soft form of cornification associated with intermediate filament keratins

Little is known on the keratinisation and cornification in the alligator tongue. This process has been studied during late and pre-hatching embryonic stages, using histochemical, immunolabelling and electron microscopy. The study revealed that PAS-positive, Alcian blue and Blue Nile Sulphate reactive glycoprotein/glycolipids are produced and accumulated in the corneous layer of the lingual epithelium before hatching. Also, Intermediate Filament Keratins (IFKs) but not Corneous Beta Proteins (CBPs) are synthesised in the tongue epithelium. Alligator IFKs have variable composition and cysteine content, 0.1%-1.1%, and few contain 2.5%-4.9% cysteine, and participate in the formation of a hard-corneous layer. These keratins also contain numerous hydrophobic amino acids, in particular leucine and valine, suggesting that they are partially hydrophobic. The predicted cross-reactivity with the IFK-antibodies here employed suggests that these antibodies can recognise some alligator IFKs. Light and transmission electron microscopy show that the stratified corneous layer is formed by narrow alpha-corneocytes storing peripheral corneous material and a central electron-pale and likely glycolipid core. Numerous membrane coating granules 0.1-0.2 μm in diameter and reactive to silver-methenamine reaction for glycoproteins are accumulated and merge with keratin bundles along the plasma membrane or are extruded among pre-corneous keratinocytes. Corneocytes of 0.1-0.2 μm in thickness pile up on embryonic tongue before hatching, forming the stratum corneum, but they do not accumulate detectable CBPs, the prevalent proteins instead present in alligator scales and claws. Whether CBPs are later produced in juveniles and adult alligator tongue to mechanically strengthen the lingual epithelium is unknown but it is hypothesised that the presence of cysteine-rich IFKs may contribute to the hardness of the tongue corneous layer.