Light and TEM study of nonregenerated and experimentally regenerated scales of Lepisosteus oculatus (Holostei) with particular attention to ganoine formation.

BACKGROUND: The structure of nonregenerated and experimentally regenerated scales of the holostean fish Lepisosteus oculatus and the events taking place before and during ganoine deposition on the scale surface were studied. The aim of this study was to answer the question of the origin of the ganoine in lepisosteids, the scales of which are devoid of dentine, and to compare them to ganoine formation in polypterid scales and to enamel formation in teeth.
METHODS: Two adult specimens were used and the scale structure was studied using light and transmission electron microscopy. Regeneration was used as an alternative to the lack of developmental stages and to induce ganoine deposition on the scale surface.
RESULTS: Nonregenerated scales are composed of a thick, avascular bony plate capped by ganoine that is covered either by the epidermis or by dermal elements. The ganoine surface is separated from the covering soft tissues by an unmineralized layer, the ganoine membrane. During the first 2 months of regeneration, the bony plate forms. It differs from the bony plate of nonregenerated scales only by its large, woven-fibered central region and by the presence of numerous vascular canals. Shortly before ganoine deposition, the osteoblasts cease their activity and an epithelial sheet comes to contact them and spreads on the bony surface. This epithelial sheet is connected to the epidermis by a short epithelial bridge only and is composed of two layers: the inner ganoine epithelium (IGE), in contact with the bone surface and composed of juxtaposed columnar cells that synthesize the ganoine matrix, preganoine; the outer ganoine epithelium (OGE), composed of elongated cells, the surface of which is separated from the overlying dermal space by a basal lamina. Isolated patches of preganoine are deposited by the IGE cells in the upper part of the osteoid matrix of the scale. The interpenetrated preganoine and osteoid matrices constitute an anchorage zone between ganoine and bone. Preganoine patches fuse and a continuous layer of preganoine is progressively synthesized by the IGE cells. Preganoine progressively mineralizes to become ganoine.
CONCLUSIONS: The processes of ganoine formation are similar to those known for the ganoine in the polypterid scales and to those described for enamel deposition in teeth. Ganoine is enamel.