Vitamin A administration in lizards during tail regeneration determines epithelial mucogenesis and delays muscle and cartilage differentiation.

Regenerating epidermis and spinal cord is essential to maintain tail regeneration in lizards. The effects of vitamin A, an inhibitor of epithelial cornification, have been studied in lizards during tail regeneration. The injection of high doses of vitamin A induces regeneration of a thinner tail with gummy consistency and suppression of the formation of a normal cartilaginous axial skeleton. Microscopic analysis reveals that all epithelia increase the secretion of glycoprotein-mucus. During the analyzed period the epidermis does not form scales and keratinocytes limit or stop the production of bundles of intermediate filament keratins and packets of corneous beta-proteins (β-keratins). Differentiation of oberhautchen and β-layers is much reduced or inhibited while α-keratinization and the formation of a corneous layer are affected as well. The effects of vitamin A are dramatic also on mesoderm cells since the treatment stimulates an invasion of blood cells likely due to the disruption of the wall of blood vessels, mesenchymal cell death (pycnosis), and diffuse phagocytosis by immune cells. A delay of cartilage differentiation and cartilage degradation due to an increase of lysosomes in these cells or released by white blood cells explains the lack of stiffness of the regenerating tail after vitamin A treatment. Regenerating muscles are variably affected, ranging from a variable necrotic effect with partial degradation of internal organelles and myofilaments to a massive or complete loss of myofibrils that do not organize in sarcomeres. In general hypervitaminosis A appears to delay epithelial but also mesodermal cell differentiation and maintains the regenerating tail in an immature condition.