Sites of cell proliferation during scute morphogenesis in turtle and alligator are different from those of lepidosaurian scales

Cell proliferation in forming shield scutes has been studied by immunofluorescence in embryos of turtle, alligator and snake after injection of 5-bromo-deoxy-uridine. Hinge regions of scutes in alligator and turtle carapace derive from an initial waving and invagination of the epidermis that contains 5-bromo-deoxy-uridine-labelled cells. This suggests that down growth of the epidermis into the dermis is driven by local proliferation in addition to dermal anchorage and stabilization of hinge regions. Few keratinocytes migrate into suprabasal layers 1 day after injection of 5-bromo-deoxy-uridine and keratinocytes reach the precorneous layer in about 5 days. Proliferating keratinocytes are randomly distributed in the outer scale surface of symmetric scutes but are more numerous in the outer scale surface of asymmetric or overlapped scutes indicating epidermal expansion. Higher localization of proliferating cells along hinge regions of embryonic turtle and alligator scutes is maintained in adult scutes where most growth occurs. In snake, skin proliferation becomes prevalent on the elongating outer side of the asymmetric scale. Comparison between proliferation sites in turtle–alligator–chick scales with lepidosaurian scales indicates that placodes are present only in turtle–alligator–chick scales. Conversely, scale primordia detected only using gene markers are found in most crocodilian and lepidosaurians embryonic skin.