Scale morphogenesis during embryonic development in the lizard Anolis lineatopus

The development of scales was analysed in embryos of the Jamaican Iguanid lizard Anolis lineatopus after the injection of cell proliferation markers [3H]thymidine and 5-bromodeoxyuridine. Embryos were fixed at successive postinjection periods, from 2 h up to 13 d and sections of developing skin were studied by autoradiography and immunocytochemistry. The epidermis during the initial stages of morphogenesis (flat epidermis and symmetric scale stages) expands chiefly by the tangential proliferation of the basal layer. The superficial periderm took part in its own laminar expansion. The rates of proliferation in the epidermis and dermis were similar during the flat epidermal stage, but dermal proliferation decreased under the wave-like epidermis of symmetric scales. No specific localisation of proliferating cells was visible either in the epidermis and or the dermis at these 2 early stages. Scale asymmetry is brought about by cell multiplication and hypertrophy of the cells of the future outer surface of the scale that behaves as an epithelial placode capable of producing beta-keratinised cells. The growth of the outer surface determines the asymmetry of the scale. Conversely, the future inner side and hinge region showed a lower degree of cell proliferation and cells remained cuboidal or became flat. Also the proliferation of the dermis under the scaling epidermis was significantly diminished. During the asymmetric scale stage, labelled cells were more common in the hypertrophied basel epithelium from 2 h until 4 d postinjection, during which time a few cells moved into the upper keratinizing layers. At 6-13 d postinjection, labelled cells were commonly seen in the upper beta-keratinising layers.