73
Developmentally, cartilage can be followed histologically through three general
phases (Hall and Miyake 2000 ). Prior to cell differentiation, a cellular condensation
forms within the mesenchyme. This condensation is known as either the cartilage
anlagen (Cameron et al. 2009 ), protocartilage (Cole and Hall 2009 ), or cartilage
condensation (Hall and Miyake 2000 ). These cells then begin to secrete the cartilage specifi c matrix. As development progresses the chondrocytes continue to proliferate and generate extracellular matrix, which then calcifi es as the cells enter the
hypertrophic phase of differentiation. This calcifi ed cartilage matrix is then replaced
by bone – in a process known as endochondral bone formation.
From genetics point of view, in mammals the Runt genes are the key players of
skeletogenesis because even the stem species of chordates harboured a single Runt
gene. It is accepted that the Runt locus duplications occurred during early vertebrate
evolution. Probably, “Runt was part of a core gene network for cartilage formation.
This network was already active in the gill bars of the common ancestor of cephalochordates and vertebrates, and diversifi ed after Runt duplications occurred during
vertebrate evolution ” (Hecht et al. 2008 ).
Mineralized Cartilage Some kind of non-mineralized and non-collagen-based
cartilaginous endoskeleton was the earliest skeleton in the vertebrate lineage. In
such groups as lancelets, hagfi sh and lampreys, this skeleton was associated mostly
with the pharynx. The collagen-based cartilage arose only after the evolution of collagen type II from earlier simple forms of the collagen’s family. “In contrast to
animals with completely non-collagenous skeletons, some of the primitive chondrichthyans (such as sharks) were able to form skeletal parts though the process of
endochondral ossifi cation” (Obradovic-Wagner and Aspenberg 2011 ). Principally,
the calcifi ed cartilage as a tissue type in the endoskeleton of sharks is suggested to
be a primitive vertebrate characteristic (Coates et al. 1998 ). Matrix calcifi cation in
cartilaginous fi sh is well known (see for review Egerbacher et al. 2006 ); and is
found as cortical mineralisation next to the cartilage surface, deep in the vertebra
body, and in the neural arch of vertebrae. It will be described in more detail, with
particular emphasis placed on concomitant changes in the organic matrix components which precede or accompany mineralisation but do not result in chondrolysis and ossifi cation. In this way, calcifi cation of cartilage in chondrichthyes provides
a model to study direct metaplasia of chondrocytes, morphologically indicated by
the remodelling of cartilage matrix and mineral deposition.
According to Ørvig ( 1951 ) there are “three principal kinds of calcifi ed cartilage
in elasmobranchs:
(a) globular calcifi cation, considered to be an early stage of mineralization both
ontogenetically and phylogenetically;
(b) prismatic or granular calcifi cation, the type which forms the tesserae;
(c) areolar calcifi cation, which occurs in the vertebral centra of Euselachii,” (Kemp
and Westrin 1979 ).
Endoskeletal tesserae (Applegate 1967 ) or Kalkplättchen (Roth 1911 ) of elasmobranchs and holocephalans are formations of calcifi ed tissue which are responsible
2.1 From Non-mineralized to Mineralized Cartilage
Developmentally, cartilage can be followed histologically through three general
phases (Hall and Miyake 2000 ). Prior to cell differentiation, a cellular condensation
forms within the mesenchyme. This condensation is known as either the cartilage
anlagen (Cameron et al. 2009 ), protocartilage (Cole and Hall 2009 ), or cartilage
condensation (Hall and Miyake 2000 ). These cells then begin to secrete the cartilage specifi c matrix. As development progresses the chondrocytes continue to proliferate and generate extracellular matrix, which then calcifi es as the cells enter the
hypertrophic phase of differentiation. This calcifi ed cartilage matrix is then replaced
by bone – in a process known as endochondral bone formation.
From genetics point of view, in mammals the Runt genes are the key players of
skeletogenesis because even the stem species of chordates harboured a single Runt
gene. It is accepted that the Runt locus duplications occurred during early vertebrate
evolution. Probably, “Runt was part of a core gene network for cartilage formation.
This network was already active in the gill bars of the common ancestor of cephalochordates and vertebrates, and diversifi ed after Runt duplications occurred during
vertebrate evolution ” (Hecht et al. 2008 ).
Mineralized Cartilage Some kind of non-mineralized and non-collagen-based
cartilaginous endoskeleton was the earliest skeleton in the vertebrate lineage. In
such groups as lancelets, hagfi sh and lampreys, this skeleton was associated mostly
with the pharynx. The collagen-based cartilage arose only after the evolution of collagen type II from earlier simple forms of the collagen’s family. “In contrast to
animals with completely non-collagenous skeletons, some of the primitive chondrichthyans (such as sharks) were able to form skeletal parts though the process of
endochondral ossifi cation” (Obradovic-Wagner and Aspenberg 2011 ). Principally,
the calcifi ed cartilage as a tissue type in the endoskeleton of sharks is suggested to
be a primitive vertebrate characteristic (Coates et al. 1998 ). Matrix calcifi cation in
cartilaginous fi sh is well known (see for review Egerbacher et al. 2006 ); and is
found as cortical mineralisation next to the cartilage surface, deep in the vertebra
body, and in the neural arch of vertebrae. It will be described in more detail, with
particular emphasis placed on concomitant changes in the organic matrix components which precede or accompany mineralisation but do not result in chondrolysis and ossifi cation. In this way, calcifi cation of cartilage in chondrichthyes provides
a model to study direct metaplasia of chondrocytes, morphologically indicated by
the remodelling of cartilage matrix and mineral deposition.
According to Ørvig ( 1951 ) there are “three principal kinds of calcifi ed cartilage
in elasmobranchs:
(a) globular calcifi cation, considered to be an early stage of mineralization both
ontogenetically and phylogenetically;
(b) prismatic or granular calcifi cation, the type which forms the tesserae;
(c) areolar calcifi cation, which occurs in the vertebral centra of Euselachii,” (Kemp
and Westrin 1979 ).
Endoskeletal tesserae (Applegate 1967 ) or Kalkplättchen (Roth 1911 ) of elasmobranchs and holocephalans are formations of calcifi ed tissue which are responsible
2.1 From Non-mineralized to Mineralized Cartilage