221
Richter and Smith 1995 ) Experiments carried out on regenerating scales of
Lepisosteidae (Sire 1994 ) as well as of Polypteridae (Sire et al. 1987 ) confi rmed
ectodermal origin of ganoine in both living primitive actinopterygians even on ultrastructural level. Similarity with tooth enamel was also confi rmed with respect to
mineralization features (Ørvig 1967 ; Sire 1995 ) as well as “the basal epidermal
cells show a differentiation pattern similar to that described for the ameloblasts during mammalian tooth morphogenesis,” (Zylberberg et al. 1997 ; see also Sire et al.
1987 ; Sire 1994 ). These ameloblasts synthesize the ganoine matrix that is a noncollagenous. Resorption and replacement of ganoin by dentine has been reported in
a fossil polypterid (Daget et al. 2001 ). “Unlike enamel, ganoine is multilayered and,
as evidenced by modern taxa, always localized deep to an epithelium” (Vickaryous
and Sire 2009 ).
The formation of ganoine is excellently described by Sire ( 1994 ) in Lepisosteus
oculatus (Holostei) in following way:
“Thus, nonregenerated scales of this fi sh 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 fi rst 2 months of
regeneration, the bony plate forms. It differs from the bony plate of nonregenerated scales only by its large, woven-fi bered 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 only by a
short epithelial bridge 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,” (Sire 1994 ).
Fig. 4.2 Photograph of thin ground section, showing cosmine morphology of Low Devonian
lungfi sh Uranolophus wyomingensis (FMNH 5089) overlying denticles ( dt ). pc pore-canals,
e enameloid, b bone. Scale =100 µm (Reprinted from Meinke ( 1986 ) with permission of John
Wiley and Sons. Copyright © 1986 Wiley-Liss, Inc)
4.2 Dentine and Dentine-Based Composite
Richter and Smith 1995 ) Experiments carried out on regenerating scales of
Lepisosteidae (Sire 1994 ) as well as of Polypteridae (Sire et al. 1987 ) confi rmed
ectodermal origin of ganoine in both living primitive actinopterygians even on ultrastructural level. Similarity with tooth enamel was also confi rmed with respect to
mineralization features (Ørvig 1967 ; Sire 1995 ) as well as “the basal epidermal
cells show a differentiation pattern similar to that described for the ameloblasts during mammalian tooth morphogenesis,” (Zylberberg et al. 1997 ; see also Sire et al.
1987 ; Sire 1994 ). These ameloblasts synthesize the ganoine matrix that is a noncollagenous. Resorption and replacement of ganoin by dentine has been reported in
a fossil polypterid (Daget et al. 2001 ). “Unlike enamel, ganoine is multilayered and,
as evidenced by modern taxa, always localized deep to an epithelium” (Vickaryous
and Sire 2009 ).
The formation of ganoine is excellently described by Sire ( 1994 ) in Lepisosteus
oculatus (Holostei) in following way:
“Thus, nonregenerated scales of this fi sh 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 fi rst 2 months of
regeneration, the bony plate forms. It differs from the bony plate of nonregenerated scales only by its large, woven-fi bered 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 only by a
short epithelial bridge 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,” (Sire 1994 ).
Fig. 4.2 Photograph of thin ground section, showing cosmine morphology of Low Devonian
lungfi sh Uranolophus wyomingensis (FMNH 5089) overlying denticles ( dt ). pc pore-canals,
e enameloid, b bone. Scale =100 µm (Reprinted from Meinke ( 1986 ) with permission of John
Wiley and Sons. Copyright © 1986 Wiley-Liss, Inc)
4.2 Dentine and Dentine-Based Composite