The fine structure and formation of hypermineralized petrodentine in the tooth plate of extant lungfish (Lepidosiren paradoxa and Protopterus sp.)

Histological features of the tooth plate of lungfish were investigated by light and electron microscopy and contact microradiography with primary reference to the fine structure and formation of petrodentine, one of the principal constituents of the tooth plates. Petrodentine is a highly mineralized tissue noticeably analogous to that of enameloid, and is deposited intermittently in a proximal direction by the sole participation of mesenchymal petroblasts. The petroblasts appear very likely to have prominent biphasic functions of secreting the petrodentine matrix and of eliminating the matrix by resorption. The petrodentine thus may be capable of attaining hypermineralization with the participation mesenchymal cells alone. Mineral crystals constituting the petrodentine are large, hexagonal or similar but irregularly shaped columns. Initially these crystals appeared extremely thin, with curled ones frequently noted. The petrodentine of lungfish tooth plates is considered to be structurally distinct from the pleromin (Kosmin) of holocephalian tooth plates because of the difference between their constituent crystals.