96
osteoclasts are the main players in formation of the skeleton by endochondral ossifi -
cation (Karsenty and Wagner 2002 ). At the same time, “cartilage morphogenesis is
a key rate-limiting step in bone development” (Reddi 2000a , b ). Furthermore,
endochondral bone development is a very complex process that includes a multistep, sequential, developmental cascade with activities of micro RNAs (Nakamura
et al. 2011 ), bone morphogenetic proteins (Retting et al. 2009 ), endocrine/paracrine
factors (e.g., PTHrP, 1,25(OH)(2)D(3), IGF-1, FGFs, and prolactin), as well as
requiring the presence of collagens and dietary calcium, silizium and vitamin D (see
for review Wongdee et al. 2012 ). The origin of the calcium phosphate phases and
biochemical, structural, and physico-chemical mechanisms in their formation of
endochondral bone (Glimcher 2006 ) as well as the special role of polyphosphates
(Omelon et al. 2009 ) are still intriguing research topics in understanding the principles of endochondral bone biomineralization.
The endochondral ossifi cation can also be accepted as the remodeling of cartilage
templates. As briefl y described by Scotti et al. ( 2010 ):
“this process relies on the specialized morphoregulatory functions of hypertrophic
chondrocytes. Hypertrophic chondrocytes derive from the condensation of mesenchymal precursors and produce a type X collagen-rich avascular cartilaginous matrix.
At the periphery of this cartilage tissue, the so-called ‘borderline’ hypertrophic
chondrocytes instruct surrounding mesenchymal cells to differentiate into osteoblasts, which results in the formation of a ‘bony collar.’ In parallel, chondrocytes in
the central regions direct mineralization of the hypertrophic cartilage by initiating
remodeling via the production of specifi c matrix metalloproteinases (MMP), and
attract blood vessels by releasing vascular-endothelial growth factor (VEGF).
The in-growing blood vessels deliver osteoblastic, osteoclastic, and hematopoietic
precursors, which mediate resorption of the cartilaginous template and formation of
vascularized bone containing the so-called stromal sinusoids, which provide the
microenvironment for hematopoiesis”.
However, what about the location where endochondral ossifi cation usually
occurs? This place is to fi nd in the expanding growth plate, a “dynamic region of the
young skeleton located beneath the soft articular cartilage that caps the ends of the
growing long bones, and above the mineralized bone itself. These bones grow along
their vertical axis through the progressive expansion of the growth plate. Within the
active growth plates, the bone elongates as new cartilage forms on its ends” (Omelon
et al. 2009 ). According to Brighton ( 1994 ), “older cartilage beneath that newformed cartilage mineralizes with apatite; it is then resorbed by bone-resorbing cells
(osteoclasts) that remove both calcifi ed cartilage and mineralized bone. Finally,
osteoblasts build new bone to replace the resorbed calcifi ed cartilage. This is one
process that increases the size of the skeleton,” (Omelon et al. 2009 ). Endochondral
bones are usually three dimensional.
Variations in the densities and composition of the bone of marine vertebrates
may be related to adaptations for buoyancy or locomotion, or to habitat or
phylogeny (see for review Tont et al. 1977 ). The diversity of bones within different
clades of marine vertebrates is huge, however, and thus here I wish to concentrate
special attention on whale bones as examples of the biggest animal bone known.
3 Biocomposites and Mineralized Tissues
osteoclasts are the main players in formation of the skeleton by endochondral ossifi -
cation (Karsenty and Wagner 2002 ). At the same time, “cartilage morphogenesis is
a key rate-limiting step in bone development” (Reddi 2000a , b ). Furthermore,
endochondral bone development is a very complex process that includes a multistep, sequential, developmental cascade with activities of micro RNAs (Nakamura
et al. 2011 ), bone morphogenetic proteins (Retting et al. 2009 ), endocrine/paracrine
factors (e.g., PTHrP, 1,25(OH)(2)D(3), IGF-1, FGFs, and prolactin), as well as
requiring the presence of collagens and dietary calcium, silizium and vitamin D (see
for review Wongdee et al. 2012 ). The origin of the calcium phosphate phases and
biochemical, structural, and physico-chemical mechanisms in their formation of
endochondral bone (Glimcher 2006 ) as well as the special role of polyphosphates
(Omelon et al. 2009 ) are still intriguing research topics in understanding the principles of endochondral bone biomineralization.
The endochondral ossifi cation can also be accepted as the remodeling of cartilage
templates. As briefl y described by Scotti et al. ( 2010 ):
“this process relies on the specialized morphoregulatory functions of hypertrophic
chondrocytes. Hypertrophic chondrocytes derive from the condensation of mesenchymal precursors and produce a type X collagen-rich avascular cartilaginous matrix.
At the periphery of this cartilage tissue, the so-called ‘borderline’ hypertrophic
chondrocytes instruct surrounding mesenchymal cells to differentiate into osteoblasts, which results in the formation of a ‘bony collar.’ In parallel, chondrocytes in
the central regions direct mineralization of the hypertrophic cartilage by initiating
remodeling via the production of specifi c matrix metalloproteinases (MMP), and
attract blood vessels by releasing vascular-endothelial growth factor (VEGF).
The in-growing blood vessels deliver osteoblastic, osteoclastic, and hematopoietic
precursors, which mediate resorption of the cartilaginous template and formation of
vascularized bone containing the so-called stromal sinusoids, which provide the
microenvironment for hematopoiesis”.
However, what about the location where endochondral ossifi cation usually
occurs? This place is to fi nd in the expanding growth plate, a “dynamic region of the
young skeleton located beneath the soft articular cartilage that caps the ends of the
growing long bones, and above the mineralized bone itself. These bones grow along
their vertical axis through the progressive expansion of the growth plate. Within the
active growth plates, the bone elongates as new cartilage forms on its ends” (Omelon
et al. 2009 ). According to Brighton ( 1994 ), “older cartilage beneath that newformed cartilage mineralizes with apatite; it is then resorbed by bone-resorbing cells
(osteoclasts) that remove both calcifi ed cartilage and mineralized bone. Finally,
osteoblasts build new bone to replace the resorbed calcifi ed cartilage. This is one
process that increases the size of the skeleton,” (Omelon et al. 2009 ). Endochondral
bones are usually three dimensional.
Variations in the densities and composition of the bone of marine vertebrates
may be related to adaptations for buoyancy or locomotion, or to habitat or
phylogeny (see for review Tont et al. 1977 ). The diversity of bones within different
clades of marine vertebrates is huge, however, and thus here I wish to concentrate
special attention on whale bones as examples of the biggest animal bone known.
3 Biocomposites and Mineralized Tissues