Histological effects of vitamin A on limb regeneration in the larval axolotl, Ambystoma mexicanum

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Abstract
Vitamin A causes profound changes in the development of pattern during amphibian limb regeneration. Vitamin A effects include the induction of duplications of skeletal structures in the anteroposterior, proximodistal, and dorsoventral axes. The purpose of this investigation was to study the underlying histological changes that are induced in the regenerating limb of the larval axolotl, Ambystoma mexicanum, by treatment with vitamin A. Axolotl larvae (7–10 cm in length) had forelimbs amputated through the radius and ulna and were then immersed in 75 IU/mL retinol palmitate for 14 days. Limbs were removed and fixed at intervals over the period of regeneration, both during and beyond the period of retinoid treatment. They were then examined in the light microscope, scanning electron microscope, or transmission electron microscope. Compared with the controls, the retinol palmitate treated regenerating limbs exhibited the development of an eccentric epidermal cap which was always displaced towards the posterior edge of the limb. Beneath this epidermal cap, the density of the cells of the blastema was greater than that of the cells towards the anterior edge of the developing blastema where the cells were much less densely arranged. Epidermal changes induced by retinol palmitate included the development of a very uneven and creviced surface, with a great deal of variation in cell size, and the development of ciliated cells in the surface layer of the epidermis. The significance of these observations for pattern modification by vitamin A are discussed.