228
As reviewed by Sharif et al. ( 2011 ), the collagen matrix of elasmoid scales is
mineralised on the external (episquamal) side with hydroxyapatite crystals. The
episquamal side of the scale possesses concentric ridges (circuli) and grooves (radii)
radiating from the central focus to the edges of the scale. Each radius is covered by
a dermal space with blood vessels and cells, which are embedded within a loose
matrix. Scleroblasts synthesize and shape the scale matrix during ontogeny and
regeneration. The external layer is synthesised fi rst, followed by the elasmodine
layer. The collagens of the elasmodine layer mineralize slowly from the external
layer (Sharif et al. 2011 ).
As reported above, ganoid scales observed in basal taxa like actinopterygian
clade, are thickly juxtaposed rhomboid structures. According to Meunier ( 2011 ):
“these scales have evolved into imbricated thin and fl exible elasmoid scales in various,
more recent taxa. This evolutionary process has contributed to a lightening of the dermal
skeleton, and has improved the effi ciency of swimming. Similar specializations can be
pointed out in the sarcopterygian clade, e.g. the thick cosmoid scales of extinct osteolepids
and the elasmoid scales of extant dipnoids and coelacanths. In Neoceratodus and Latimeria,
elasmoid scales present an extreme specialization. The basal plate is composed of an
unmineralized network of elasmodine (isopedine), and the collagenous fi bres are organized
into a double twisted plywood,” (Meunier 2011 ; see also Giraud et al. 1978 ).
The observations demonstrated that highly specialized structure isopedine is
evolved from a bony tissue. For example, it was reported that “the fi brous basal
plate of a Latimeria scale is homologous to the osseous basal plate of a cosmoid
scale,” (Meunier 1980 ).
The limiting layer develops in close proximity to the basalmost epidermal cells
similarly to hyaloine and ganoine. It has been demonstrated that epidermal cells
play a role in the formation of the limiting layer (Sire 1988 ). The main organic
structural component of this well-mineralized layer is represented by collagenous
tissue (exclusive of Sharpey’s fi bers). The limiting layer is localized superfi cially on
the posterior fi eld of teleost elasmoid scales (Sire et al. 2009 ). Its deposition is periodic because the limiting layer is superfi cially separated from basal most epidermal
cells by a narrow mesenchymal space similar to hyaloine. The odontogenic pathway
is the key way for initiation of skeletogenesis using elasmoid scales: “a scale papilla
is formed in the dermis, immediately adjacent to the basal layer cells of the epidermis. Each presumptive elasmoid scale begins as a discrete accumulation of
woven- fi bered matrix, then is underpinned by multiple lamellae of collagen fi brils
organized into a plywood-like arrangement” (Sire et al. 2009 ; see also Sire and
Huysseune 2003 ; Sire and Akimenko 2004 ).
Leptoid Scales These scales have been observed on higher order bony fi sh and
appear in two forms, cycloid and ctenoid structures. Both scales add concentric
layers during their growth. Localization and orientation of leptoid scales seems to
be ideal to reduce drag as well as to allow a smoother fl ow of water over the fi sh
body.
Cycloid Scales These very common type of scales with a smooth outer edge can
be observed on fi sh with soft fi n rays, such as carp and salmon.
4 Fish Scales as Mineral-Based Composites
As reviewed by Sharif et al. ( 2011 ), the collagen matrix of elasmoid scales is
mineralised on the external (episquamal) side with hydroxyapatite crystals. The
episquamal side of the scale possesses concentric ridges (circuli) and grooves (radii)
radiating from the central focus to the edges of the scale. Each radius is covered by
a dermal space with blood vessels and cells, which are embedded within a loose
matrix. Scleroblasts synthesize and shape the scale matrix during ontogeny and
regeneration. The external layer is synthesised fi rst, followed by the elasmodine
layer. The collagens of the elasmodine layer mineralize slowly from the external
layer (Sharif et al. 2011 ).
As reported above, ganoid scales observed in basal taxa like actinopterygian
clade, are thickly juxtaposed rhomboid structures. According to Meunier ( 2011 ):
“these scales have evolved into imbricated thin and fl exible elasmoid scales in various,
more recent taxa. This evolutionary process has contributed to a lightening of the dermal
skeleton, and has improved the effi ciency of swimming. Similar specializations can be
pointed out in the sarcopterygian clade, e.g. the thick cosmoid scales of extinct osteolepids
and the elasmoid scales of extant dipnoids and coelacanths. In Neoceratodus and Latimeria,
elasmoid scales present an extreme specialization. The basal plate is composed of an
unmineralized network of elasmodine (isopedine), and the collagenous fi bres are organized
into a double twisted plywood,” (Meunier 2011 ; see also Giraud et al. 1978 ).
The observations demonstrated that highly specialized structure isopedine is
evolved from a bony tissue. For example, it was reported that “the fi brous basal
plate of a Latimeria scale is homologous to the osseous basal plate of a cosmoid
scale,” (Meunier 1980 ).
The limiting layer develops in close proximity to the basalmost epidermal cells
similarly to hyaloine and ganoine. It has been demonstrated that epidermal cells
play a role in the formation of the limiting layer (Sire 1988 ). The main organic
structural component of this well-mineralized layer is represented by collagenous
tissue (exclusive of Sharpey’s fi bers). The limiting layer is localized superfi cially on
the posterior fi eld of teleost elasmoid scales (Sire et al. 2009 ). Its deposition is periodic because the limiting layer is superfi cially separated from basal most epidermal
cells by a narrow mesenchymal space similar to hyaloine. The odontogenic pathway
is the key way for initiation of skeletogenesis using elasmoid scales: “a scale papilla
is formed in the dermis, immediately adjacent to the basal layer cells of the epidermis. Each presumptive elasmoid scale begins as a discrete accumulation of
woven- fi bered matrix, then is underpinned by multiple lamellae of collagen fi brils
organized into a plywood-like arrangement” (Sire et al. 2009 ; see also Sire and
Huysseune 2003 ; Sire and Akimenko 2004 ).
Leptoid Scales These scales have been observed on higher order bony fi sh and
appear in two forms, cycloid and ctenoid structures. Both scales add concentric
layers during their growth. Localization and orientation of leptoid scales seems to
be ideal to reduce drag as well as to allow a smoother fl ow of water over the fi sh
body.
Cycloid Scales These very common type of scales with a smooth outer edge can
be observed on fi sh with soft fi n rays, such as carp and salmon.
4 Fish Scales as Mineral-Based Composites