Observations on the ultrastructure and distribution of chromatophores in the skin of chelonians

The cytology and distribution of chromatophores responsible for skin pigmentation in chelonians is analyzed. Epidermal melanocytes are involved in the formation of dark spots or stripes in growing shelled and non-shelled skin. Melanocytes rest in the basal layer of the epidermis and transfer melanosomes into keratinocytes during epidermal growth. Dermal melanophores and other chromatophores instead remain in the dermis and form the grey background of the skin. When dermal melanophores condense they give origin to the dense spots or stripes in areas where no epidermal melanocytes are present. In the latter case the epidermis and the corneous layer are transparent and reveal the dermal distribution of melanophores and other chromatophores underneath. As a result of this basic process of distribution of pigment cells the dark areas visible in scales can have a double origin (epidermal and dermal) or a single origin (epidermal or dermal). Xanthophores, lipophores and a cell containing both pterinosomes and lipid droplets are sparse in the loose dermis while iridophores are rarely seen in the skin of chelonians analyzed in the present study. Xanthophores and lipophores contribute to form the pale, yellow or oranges hues present among the dark areas of the skin in turtles.