117
Trabecular dentine (Peyer 1968 ) is an example of hypermineralized tissue that
was found in the tooth plates of the Australian ghostshark Callorhinchus milii .
This specifi c biological material is overlain by a thin veneer of vitrodentine (Bargmann
1933 ) that is quickly worn away on exposed surfaces. Also tritoral hypermineralized
tissue, termed as “tubular dentine” (Moy Thomas 1939 ), and “pleromin” (Ørvig
1976 ), have been described in the large mandibular and palatine tooth plates of
some species. These occupy a central position and become exposed on the occlusal
surfaces by wear.
Some hypermineralized tissue are located not on the surface of the vomerine
tooth plates in chimeroids but deep within them in the form of a small region visible
only in sectioned material. Additionally, specifi c structures containing the hypermineralized tritoral tissue appears as slender rods or “perlstrings” in the interior of
the plates of extant chimaeroids like Chimaera (Didier et al. 1994 ).
Another example of hypermineralized tooth plates has been described in lungfi sh
(Dipnoi) (see for review Denison 1974 ; Kemp 2003 ). Anne Kemp gives us following
description:
“ The dentition develops from initially isolated cusps, arranged in a radiating
pattern, into tooth plates based on radiating ridges, sheathed in enamel and
containing several distinct forms of dentine and bone. Single cusps, made up of
enamel and mantle dentine, grow towards the underlying bone and fuse with
bone trabeculae to form a primary tooth plate, with four ridges in the upper jaw
and three in the lower. The ridges grow by the addition of new cusps, and the
tooth plate grows by the addition of new ridges, up to seven in all, but these processes are only partly responsible for growth of the tooth plate as a whole, ”
(Kemp 2002 ).
Here, we can speak about formation of hierarchical biocomposite structure
because with each “ new cusp, a layer of enamel and mantle dentine is added around
the external surface of the tooth plate, and layers of additional dentines grow within
the pulp cavity to increase the thickness of the tooth plate. These processes are
balanced by wear of hard tissue from the occlusal surface of the tooth plate, ”
(Kemp 2003 ).
The petrodentine , one of the principal constituents of the tooth plates of lungfi sh,
have been investigated using contact microradiography as well as by light and electron
microscopy as reported by Ishiyama and Teraki ( 1990 ). This biological material 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 have irregularly shaped columns.
Initially these crystals appeared extremely thin, with curled ones frequently noted.
The petrodentine of lungfi sh tooth plates is considered (Ishiyama and Teraki 1990 )
to be structurally distinct from the pleromin (cosmine) of holocephalian tooth plates
because of the difference between their constituent crystals.
3.2 Teeth
Trabecular dentine (Peyer 1968 ) is an example of hypermineralized tissue that
was found in the tooth plates of the Australian ghostshark Callorhinchus milii .
This specifi c biological material is overlain by a thin veneer of vitrodentine (Bargmann
1933 ) that is quickly worn away on exposed surfaces. Also tritoral hypermineralized
tissue, termed as “tubular dentine” (Moy Thomas 1939 ), and “pleromin” (Ørvig
1976 ), have been described in the large mandibular and palatine tooth plates of
some species. These occupy a central position and become exposed on the occlusal
surfaces by wear.
Some hypermineralized tissue are located not on the surface of the vomerine
tooth plates in chimeroids but deep within them in the form of a small region visible
only in sectioned material. Additionally, specifi c structures containing the hypermineralized tritoral tissue appears as slender rods or “perlstrings” in the interior of
the plates of extant chimaeroids like Chimaera (Didier et al. 1994 ).
Another example of hypermineralized tooth plates has been described in lungfi sh
(Dipnoi) (see for review Denison 1974 ; Kemp 2003 ). Anne Kemp gives us following
description:
“ The dentition develops from initially isolated cusps, arranged in a radiating
pattern, into tooth plates based on radiating ridges, sheathed in enamel and
containing several distinct forms of dentine and bone. Single cusps, made up of
enamel and mantle dentine, grow towards the underlying bone and fuse with
bone trabeculae to form a primary tooth plate, with four ridges in the upper jaw
and three in the lower. The ridges grow by the addition of new cusps, and the
tooth plate grows by the addition of new ridges, up to seven in all, but these processes are only partly responsible for growth of the tooth plate as a whole, ”
(Kemp 2002 ).
Here, we can speak about formation of hierarchical biocomposite structure
because with each “ new cusp, a layer of enamel and mantle dentine is added around
the external surface of the tooth plate, and layers of additional dentines grow within
the pulp cavity to increase the thickness of the tooth plate. These processes are
balanced by wear of hard tissue from the occlusal surface of the tooth plate, ”
(Kemp 2003 ).
The petrodentine , one of the principal constituents of the tooth plates of lungfi sh,
have been investigated using contact microradiography as well as by light and electron
microscopy as reported by Ishiyama and Teraki ( 1990 ). This biological material 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 have irregularly shaped columns.
Initially these crystals appeared extremely thin, with curled ones frequently noted.
The petrodentine of lungfi sh tooth plates is considered (Ishiyama and Teraki 1990 )
to be structurally distinct from the pleromin (cosmine) of holocephalian tooth plates
because of the difference between their constituent crystals.
3.2 Teeth