71
Two different types of cartilages have been described in hagfi shes. The fi rst one
is the “soft cartilage”. It is positive detected for Col2a1, contains large hypertrophic
chondrocytes and are surrounded by a thin extracellular matrix. The second one,
termed as “hard cartilage”, possesses no Col2a1, and contains smaller chondrocytes
that are surrounded by an abundance of extracellular matrix (see for review Zhang
and Cohn 2006 ; Rychel and Swalla 2007 ). Probably, the last common ancestor of
hagfi shes, lampreys, and gnathostomes also contains Col2a1 in the cartilages
because this protein is structurally similar in hagfi sh soft cartilage with that lamprey. Thus, “hagfi sh fi brillar collagens reveal that type II collagen-based cartilage
evolved in stem vertebrates” (Zhang and Cohn 2006 ).
In summary, the diversity of lamprey cartilages is shown by the following
properties:
“a) the ‘ hard cartilage ’ in the dorsal portion of the branchial basket skeleton
consists of disorganized polygonal chondrocytes and expresses an elastin-like
lamprin;
b) In the branchial and hypobranchial bars, discoidal chondrocytes expressing
fi brillar collagen and elastin generate so-called ‘ soft cartilage ’, and
c) ‘ mucocartilage ’ is the main skeletal tissue in the ventral pharynx and oral
region.
While biochemically similar to defi nitive cellular cartilage, mucocartilage is
histologically distinct, consisting of scattered mesenchymal cells embedded in
a mucopolysaccharide matrix” (Cattell et al. 2011 ).
There are no doubts that the fossil remnants of exceptionally preserved nonbiomineralized vertebrates play important role “in constraining the time of origin of
vertebrate cartilages (and thus the genome duplication), and in reconstructing cartilage development in the vertebrate common ancestor” (Sansom et al. 2011 ). The
fi nding of two Col2A1 orthologues in hagfi sh and lampreys suggest that genome
duplication have occurred on the vertebrate stem. For example, amphioxus
(Meulemans and Bronner–Fraser 2007 ) and tunicates have just one ancestral clade
A fi brillar collagen gene as recently reported by Sansom and co-workers (Sansom
et al. 2011 ).
The in-shore hagfi sh, Eptatretus burger possess small nodule-like structures
similar to vertebra-like cartilages. These formations are proposed to be “homologous to gnathostome vertebrae, implying that this animal underwent a secondary
reduction of vertebrae in most of the trunk” (Ota et al. 2011 ).
In mammals, three classic types of cartilage have been histologically identifi ed
based upon the relative contribution and distribution of fi bers within the extracellular matrix. Hyaline cartilage is the typical cartilage that is generally referred to
when one thinks of cartilage (Cole 2011 ). Elastic cartilage contains additional elastin fi bers, and fi brocartilage contains regions of organized fi brous tissue within the
extracellular matrix (Fig. 2.1 ).
In contrast to other animal groups, the cartilage of Mammalia is “a specialized
connective tissue in which the chondrocytes occupy only 5 % of the volume”
(Knudson and Knudson 2001 ). Chondrocytes are located into the matrix in such
2.1 From Non-mineralized to Mineralized Cartilage
Two different types of cartilages have been described in hagfi shes. The fi rst one
is the “soft cartilage”. It is positive detected for Col2a1, contains large hypertrophic
chondrocytes and are surrounded by a thin extracellular matrix. The second one,
termed as “hard cartilage”, possesses no Col2a1, and contains smaller chondrocytes
that are surrounded by an abundance of extracellular matrix (see for review Zhang
and Cohn 2006 ; Rychel and Swalla 2007 ). Probably, the last common ancestor of
hagfi shes, lampreys, and gnathostomes also contains Col2a1 in the cartilages
because this protein is structurally similar in hagfi sh soft cartilage with that lamprey. Thus, “hagfi sh fi brillar collagens reveal that type II collagen-based cartilage
evolved in stem vertebrates” (Zhang and Cohn 2006 ).
In summary, the diversity of lamprey cartilages is shown by the following
properties:
“a) the ‘ hard cartilage ’ in the dorsal portion of the branchial basket skeleton
consists of disorganized polygonal chondrocytes and expresses an elastin-like
lamprin;
b) In the branchial and hypobranchial bars, discoidal chondrocytes expressing
fi brillar collagen and elastin generate so-called ‘ soft cartilage ’, and
c) ‘ mucocartilage ’ is the main skeletal tissue in the ventral pharynx and oral
region.
While biochemically similar to defi nitive cellular cartilage, mucocartilage is
histologically distinct, consisting of scattered mesenchymal cells embedded in
a mucopolysaccharide matrix” (Cattell et al. 2011 ).
There are no doubts that the fossil remnants of exceptionally preserved nonbiomineralized vertebrates play important role “in constraining the time of origin of
vertebrate cartilages (and thus the genome duplication), and in reconstructing cartilage development in the vertebrate common ancestor” (Sansom et al. 2011 ). The
fi nding of two Col2A1 orthologues in hagfi sh and lampreys suggest that genome
duplication have occurred on the vertebrate stem. For example, amphioxus
(Meulemans and Bronner–Fraser 2007 ) and tunicates have just one ancestral clade
A fi brillar collagen gene as recently reported by Sansom and co-workers (Sansom
et al. 2011 ).
The in-shore hagfi sh, Eptatretus burger possess small nodule-like structures
similar to vertebra-like cartilages. These formations are proposed to be “homologous to gnathostome vertebrae, implying that this animal underwent a secondary
reduction of vertebrae in most of the trunk” (Ota et al. 2011 ).
In mammals, three classic types of cartilage have been histologically identifi ed
based upon the relative contribution and distribution of fi bers within the extracellular matrix. Hyaline cartilage is the typical cartilage that is generally referred to
when one thinks of cartilage (Cole 2011 ). Elastic cartilage contains additional elastin fi bers, and fi brocartilage contains regions of organized fi brous tissue within the
extracellular matrix (Fig. 2.1 ).
In contrast to other animal groups, the cartilage of Mammalia is “a specialized
connective tissue in which the chondrocytes occupy only 5 % of the volume”
(Knudson and Knudson 2001 ). Chondrocytes are located into the matrix in such
2.1 From Non-mineralized to Mineralized Cartilage