Histological, histochemical, and morphometric analysis of epidermal Leydig cells and histochemical characterization of epidermal apical cells in juvenile and adult axolotls (Ambystoma mexicanum)

Ambystoma mexicanum, also known as the axolotl, is a paedomorphic urodele. Metamorphosis can be induced experimentally, and the most significant changes occur in the skin. These include thinning of the epidermis, increased keratinization of the stratified squamous epithelium, and loss of Leydig cells (LCs). Similar epidermal changes are observed in other metamorphic urodeles. Epidermal cells are responsible for the secretory function of the skin in juvenile amphibians, whereas dermal glands perform this function in adults after metamorphosis. In the axolotl, this occurrence is still partially understood. The only recognized epidermal secretory cells in juvenile A. mexicanum are the LCs, whose specific secretion products have not yet been characterized from the histochemical standpoint. Additionally, the persistence of LCs in adulthood, when mucous and serous (granular-protein secretion) glands are abundant, remains a matter of debate. The present study aims to describe the morphological and histochemical changes in the epidermis of 10 cutaneous regions from juvenile (4 months old) and adult (24 and 48 months old) non-metamorphic A. mexicanum, with a particular focus on the amount and histochemical characteristics of LCs. Results indicate that the juvenile epidermis is a stratified cuboidal epithelium formed by three strata: basal, spinosum (containing the LCs), and apical. The most superficial layer contains cuboidal cells that lack the characteristics of a true stratum corneum. In adults, the stratum apical is also formed by squamous cells, suggesting a transition to a cornified and squamous layer as age increases. Histochemical methods demonstrated that LCs are most likely serous and not mucous cells. On the other hand, cuboidal cells of the juvenile apical stratum would be responsible for producing mucous secretion components. Morphometric analysis revealed a significant decrease in both LCs and the epidermal thickness in the 24-month-old adult axolotl compared to the juvenile. While LC count and epidermal thickness in the 48-month-old adult showed a slight increase compared to the 24-month-old adult, these differences were not statistically significant and far lower than those observed in the juvenile axolotl, which exhibited the highest number of LCs and a thicker epidermis. These natural axolotl epidermal changes indicate a gradual transition toward a morphology resembling metamorphic skin as age advances. The decreased number of LCs and the transition from cuboid cells to squamous cells in the stratum apical suggest that both cell types may naturally disappear entirely at some point during development.