114
cavity and odontodes, these denticles consist of a pulp cavity surrounded by
dentine, itself covered by enameloid. The denticles are held onto the bony surface
by means of ligaments and attachment bone. These features make them identical to
teeth and odontodes. Moreover, the denticles project beyond the skin, as do teeth,
and are certainly replaced as indicated by the observation of several buds in serial
sections, ” (Sire 2001 ).
From the structural and morphological point of view, the denticles in A. elymus
are similar to those of teeth inside the oral cavity. According to Sire ( 2001 ) these
denticles are not evolutionarily related to odontodes of early vertebrates. In spite of
homology between odontodes and teeth, the origin of the denticles is not to be found
in odontodes, but in teeth. “ The denticles are simply teeth that form outside the
mouth, probably derived from a sub-population of odontogenically pre-specifi ed
neural crest cells. These ‘accidental’ extra-oral teeth have arisen independently
in these lineages and were selectively advantageous in a hydrodynamic context, ”
(Sire 2001 ). Thus, extra-oral teeth are denticle-like structures that are not organized
into functional systems and cover only selected parts of the dermal skeleton.
Except for the extra-oral teeth described above, fi sh can possess tooth-sets
outside their jaws. Such kind of extra-mandibular dentition has been very recently
described by Finarelli and Coates ( 2012 ). They studied the earliest known holocephalans, Chondrenchelys problematica . As described by these authors, the
species “possess a set of individual teeth in conjunction with toothplates external to
the mandibular arch and distinct from the mandibular toothplate dentition. The
outer surfaces of these extra-mandibular teeth have a pin-cushion appearance,
refl ecting vertically oriented, closely spaced tubules emerging onto the outermost
dentine surface. Chondrenchelys, like other holocephalans, shows no trace of a
pharyngeal dentition; yet there is evidence for a dentition external to the mandibular
arch, possibly supported on labial cartilages. This extant animal now provides
evidence for the odontogenic potential of extra-mandibular neural crest cell
populations, ” (Finarelli and Coates 2012 ).
3.2.6 Vertebrate Oral Teeth
Many osteichthyan fi sh possess teeth, which are instead regularly patterned and
replaced. In these species, a dental lamina morphologically typical for tetrapods, is
absent. However, according to the classical view, the development of a sub- epithelial,
permanent dental lamina is the crucial step for the initiation of a pattern order for
teeth at the mouth margin. Thus, “ classically the oral dentition of teeth regulated
into a successional iterative order was thought to have evolved from the superfi cial
skin denticles migrating into the mouth at the stage when jaws evolved, ” (Fraser and
Smith 2011 ).
It was suggested generally that, an ectodermal invagination is the base element
for development of the oral cavity of vertebrates. Correspondingly, “ oral teeth are
proposed to arise exclusively from ectoderm, contributing to tooth enamel
3 Biocomposites and Mineralized Tissues
cavity and odontodes, these denticles consist of a pulp cavity surrounded by
dentine, itself covered by enameloid. The denticles are held onto the bony surface
by means of ligaments and attachment bone. These features make them identical to
teeth and odontodes. Moreover, the denticles project beyond the skin, as do teeth,
and are certainly replaced as indicated by the observation of several buds in serial
sections, ” (Sire 2001 ).
From the structural and morphological point of view, the denticles in A. elymus
are similar to those of teeth inside the oral cavity. According to Sire ( 2001 ) these
denticles are not evolutionarily related to odontodes of early vertebrates. In spite of
homology between odontodes and teeth, the origin of the denticles is not to be found
in odontodes, but in teeth. “ The denticles are simply teeth that form outside the
mouth, probably derived from a sub-population of odontogenically pre-specifi ed
neural crest cells. These ‘accidental’ extra-oral teeth have arisen independently
in these lineages and were selectively advantageous in a hydrodynamic context, ”
(Sire 2001 ). Thus, extra-oral teeth are denticle-like structures that are not organized
into functional systems and cover only selected parts of the dermal skeleton.
Except for the extra-oral teeth described above, fi sh can possess tooth-sets
outside their jaws. Such kind of extra-mandibular dentition has been very recently
described by Finarelli and Coates ( 2012 ). They studied the earliest known holocephalans, Chondrenchelys problematica . As described by these authors, the
species “possess a set of individual teeth in conjunction with toothplates external to
the mandibular arch and distinct from the mandibular toothplate dentition. The
outer surfaces of these extra-mandibular teeth have a pin-cushion appearance,
refl ecting vertically oriented, closely spaced tubules emerging onto the outermost
dentine surface. Chondrenchelys, like other holocephalans, shows no trace of a
pharyngeal dentition; yet there is evidence for a dentition external to the mandibular
arch, possibly supported on labial cartilages. This extant animal now provides
evidence for the odontogenic potential of extra-mandibular neural crest cell
populations, ” (Finarelli and Coates 2012 ).
3.2.6 Vertebrate Oral Teeth
Many osteichthyan fi sh possess teeth, which are instead regularly patterned and
replaced. In these species, a dental lamina morphologically typical for tetrapods, is
absent. However, according to the classical view, the development of a sub- epithelial,
permanent dental lamina is the crucial step for the initiation of a pattern order for
teeth at the mouth margin. Thus, “ classically the oral dentition of teeth regulated
into a successional iterative order was thought to have evolved from the superfi cial
skin denticles migrating into the mouth at the stage when jaws evolved, ” (Fraser and
Smith 2011 ).
It was suggested generally that, an ectodermal invagination is the base element
for development of the oral cavity of vertebrates. Correspondingly, “ oral teeth are
proposed to arise exclusively from ectoderm, contributing to tooth enamel
3 Biocomposites and Mineralized Tissues