A light and scanning electron microscope study of microstructure, growth and development of the dermal skeleton of Polypterus (Pisces: Actinopterygii)

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Abstract
The microstructure, growth and development of the dermal skeleton of Polyptenus (Pisces, Actinopterygii) shows many similarities to that of other living fishes and, by inference, to a variety of fossil groups. Scales, dermal bones and fin spines contain vascular and lamellar bone, dentine and hypermineralized ganoine arranged in distinctive, repeating configurations. Teeth and the jawbone exhibit both types of bone and dentine, an enameloid tooth cap and a collar tissue that bears a close resemblance to ganoine, though in Polypterus the collar is a radially‐fibered dentine and not enamel or enameloid. Regenerated fin spines and larval material of fin spines and scales show that bone tissue develops first in the mesoderm. Development of dentine and ganoine is specifically associated with the epithelio‐mesenchymal interface, contrary to earlier descriptions of the process. It is unclear whether ganoine is enamel or enameloid but an epithelial origin is favoured. The dental tissues form separately from bone and then fuse with it, a process like that which occurs in oral teeth. Growth of fin spines occurs by addition of bone on all surfaces. Units of dentine and ganoine or ganoine alone are added to the dorsal and lateral surfaces by continued epithelio‐mesenchymal interaction; thus, growth processes are an extension of the original ontogenetic pathway. The presence of enameloid, which is a product of combined epithelial and dermal secretions, and ganoine, possibly an epithelial product, in the same organism, is further evidence that changes in timing of gene activity play a large role in ontogeny of, and may have played an equally important part in evolutionary change in, the dermal skeleton of fishes.

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