75
In some fi sh the tesserae represent multi-layered, quite thick structure (see for
review Dean and Summers 2006 ). For example, very complex tesserae have been
demonstrated by the myliobatid stingrays (Summers 2000 ). Evolutionary expediency concerning development of these structures especially in representatives of
this cartilaginous fi sh can be determined by their feeding behaviour. Usually, they
are “durophagous”, means crushing hard shelled molluscs between cylindrical tooth
plates. This manner of feeding necessarily imposes large loads on their own skulls.
Correspondingly, some species possess heavily mineralised jaws containing up to
six layers of tesserae.
Other interesting structures are mineralised trabeculae of cartilaginous fi sh.
These are hollow and structurally similar to that in bone. Their function, however,
is to take loads from the tooth plates to the thickened parts of the jaws and skull.
The overall diameter of these thinwalled trabeculae is about 800 μm, and the
lumen being on the order of 500 μm (Currey 2010 ). According to Currey ( 2010 ),
“this is a better arrangement than that in the solid bony trabeculae usual in tetrapods, because it produces a greater stiffness per unit mass than do solid struts.
Producing hollow trabeculae does not seem to be part of the adaptive repertoire of
bone,” (Currey 2010 ).
Distribution and structural organization of tesserae are also crucial in faster
swimming sharks which need increased scleral skeletal support for their eyes. Thus,
in the larger, more active shark’s species tessellated cartilage provides such kind of
support stronger in comparison to regular hyaline cartilage (Pilgrim and Franz–
Odendaal 2009 ).
Although the biomineralization may share some principal similarities with
mammalian endochondral calcifi cation concerning the cell organization “(e.g.
alignment of fl attened chondrocytes at the tissue periphery) and perhaps matrix
reorganization (e.g. the expression of alkaline phosphatase and reduced sulfation
in zones of mineralization),” (Dean et al. 2009a ) it is believed the similarities to
end there (Dean et al. 2009a ).
Recently, Adam Summers and co-workers (Dean et al. 2010 ) investigated the
ultrastructure of tessellated skeleton without damaging the delicate relationships between constituent tissues or to the tesserae themselves using synchrotron radiation tomography (SRT) as well as cryo-electron microscopy. In this
way they observed previously unknown internal structures, namely passages
connecting the lacunar spaces – the intratesseral canaliculi – within tesserae
(Dean et al. 2010 ). These formations link consecutive lacunar spaces into long
lacunar strings. It was shown that these strings radiate outward from the center
of tesserae.
The skeletal tissues of elasmobranch fi shes defi nitively illustrate the challenges of
studying the principles of hierarchical organization of biological materials (Dean et al.
2009b ). From this point of view, “each tessera is a geometric block (hundreds of
microns deep and wide in adults), comprised of hydroxyapatite crystals on a collagen
scaffold. The skeleton is therefore a fi bro-mineral composite (perichondriumtesseraeintertesseral fi bers) wrapping a fi ber-reinforced gel (uncalcifi ed cartilage)
2.1 From Non-mineralized to Mineralized Cartilage
In some fi sh the tesserae represent multi-layered, quite thick structure (see for
review Dean and Summers 2006 ). For example, very complex tesserae have been
demonstrated by the myliobatid stingrays (Summers 2000 ). Evolutionary expediency concerning development of these structures especially in representatives of
this cartilaginous fi sh can be determined by their feeding behaviour. Usually, they
are “durophagous”, means crushing hard shelled molluscs between cylindrical tooth
plates. This manner of feeding necessarily imposes large loads on their own skulls.
Correspondingly, some species possess heavily mineralised jaws containing up to
six layers of tesserae.
Other interesting structures are mineralised trabeculae of cartilaginous fi sh.
These are hollow and structurally similar to that in bone. Their function, however,
is to take loads from the tooth plates to the thickened parts of the jaws and skull.
The overall diameter of these thinwalled trabeculae is about 800 μm, and the
lumen being on the order of 500 μm (Currey 2010 ). According to Currey ( 2010 ),
“this is a better arrangement than that in the solid bony trabeculae usual in tetrapods, because it produces a greater stiffness per unit mass than do solid struts.
Producing hollow trabeculae does not seem to be part of the adaptive repertoire of
bone,” (Currey 2010 ).
Distribution and structural organization of tesserae are also crucial in faster
swimming sharks which need increased scleral skeletal support for their eyes. Thus,
in the larger, more active shark’s species tessellated cartilage provides such kind of
support stronger in comparison to regular hyaline cartilage (Pilgrim and Franz–
Odendaal 2009 ).
Although the biomineralization may share some principal similarities with
mammalian endochondral calcifi cation concerning the cell organization “(e.g.
alignment of fl attened chondrocytes at the tissue periphery) and perhaps matrix
reorganization (e.g. the expression of alkaline phosphatase and reduced sulfation
in zones of mineralization),” (Dean et al. 2009a ) it is believed the similarities to
end there (Dean et al. 2009a ).
Recently, Adam Summers and co-workers (Dean et al. 2010 ) investigated the
ultrastructure of tessellated skeleton without damaging the delicate relationships between constituent tissues or to the tesserae themselves using synchrotron radiation tomography (SRT) as well as cryo-electron microscopy. In this
way they observed previously unknown internal structures, namely passages
connecting the lacunar spaces – the intratesseral canaliculi – within tesserae
(Dean et al. 2010 ). These formations link consecutive lacunar spaces into long
lacunar strings. It was shown that these strings radiate outward from the center
of tesserae.
The skeletal tissues of elasmobranch fi shes defi nitively illustrate the challenges of
studying the principles of hierarchical organization of biological materials (Dean et al.
2009b ). From this point of view, “each tessera is a geometric block (hundreds of
microns deep and wide in adults), comprised of hydroxyapatite crystals on a collagen
scaffold. The skeleton is therefore a fi bro-mineral composite (perichondriumtesseraeintertesseral fi bers) wrapping a fi ber-reinforced gel (uncalcifi ed cartilage)
2.1 From Non-mineralized to Mineralized Cartilage