Ganoine formation in the scales of primitive actinopterygian fishes, lepisosteids and polypterids.

The scales of primitive living actinopterygian fishes, lepisosteids and polypterids, have retained ganoine, a hypermineralized layer which covered the scales of the osteichthyan ancestors. To know finally its tissue origin in the actinopterygian lineage, ganoine formation was described in Lepisosteus oculatus, with scales devoid of dentin, and was compared to ganoine formation in two polypterids, Calamoichthys calabaricus and Polypterus senegalus, with scales possessing a dentin layer. The events taking place before, during and after ganoine deposition were studied in experimentally regenerated scales using light and transmission electron microscopy. In spite of differences in tissue composition and in organization of the epidermal cells on the scale surface, ganoine formation is similar in both types of scales. Preganoine is deposited by epidermal cells and constitutes a thick layer which mineralizes progressively to become ganoine, a true enamel. The cellular processes involved in ganoine formation were compared to those described for enamel in mammalian teeth.