Histology of Polypterus senegalus fin rays revisited

The present comparative histological study of the pectoral, caudal and anal fins of the polypterid Polypterus senegalus reveals the presence of a layer of dentine identified between the superficial ganoine patches and the bony part of the lepidotrichia in the three fins. Its extent varies depending on the fins. Similarly, the ganoine layer present at the surface of the proximal lepidotrichia shows fin-dependent differences in extent and distribution. The dentine layer is crossed by a system of thin worm-like vascular canaliculi that reach the ganoine layer and even penetrate within it as in the scales. In the lepidotrichia, the dentine lays directly on bone, which differs from the scales where dentine lies on isopedine, a plywood-like structure. Another difference between scales and lepidotrichia is the presence of actinotrichia that are unmineralised, fusiform rods of elastoidine located at the tip of the fins. Ontogenesis with differentiation of actinotrichia has no equivalent in scale formation. Although structural features are shared by lepidotrichia and scales in P. senegalus, observations on the scales and lepidotrichia support the hypothesis of Schaeffer (1977) that “scales and lepidotrichia are somewhat differently shaped manifestations of the same morphogenetic system”.