94
alternative pathway has been reported (Meunier and Huysseune 1992 ), in galeaspids
and placoderms, where it was shown that bone can develop through spheritic
mineralization in the absence of a collagenous matrix.
Dermal Bone Nowadays, the most diverse collections of dermal elements are
found in reptiles like crocodylians. However, in early representatives of many
stem gnathostomes (structural-grade ostracoderms) especially the dermal skeleton
was once the predominant skeletal system. During the evolution, it has undergone extensive reduction and modifi cation visible in most modern forms today (Sire
and Huysseune 2003 ). As reviewed by Vickaryous and Hall ( 2008 ), “for tetrapods,
the most obvious remnants include the craniofacial skeleton (dermatocranium
or desmocranium), dental tissues, and one or more elements of the pectoral
apparatus. The dermal skeleton also includes bones developing within the eye
(scleral ossicles), eyelid (palpebral), integument (osteoderms), and across the
abdomen (gastralia)”.
Additional type of bones, which arise directly through differentiation of mesenchyme, initially compacted in sheets or membranes, are known as intramembranous
bones. The process of intramembranous bone formation on example of craniofacial
skeleton is reviewed by Helms and Schneider ( 2003 ).Abzhanov et al. ( 2007 ) proposed
following classifi cation of the intramembranous bones:
(α) “the sesamoid bones, which form in tendons as a result of mechanical stress
(such as the patella in the tendon of the quadriceps femoris);
(β) the periosteal bones, which form from connective tissue and add to the thickness
of long bones;
(χ) the dermal bones, which form within the dermis of the skin”.
Dermal bones tend to be fl at and plate-like. Dermal bones of the skull and the
pectoral girdle develop within the integument, generally in the lower layer of the
dermis (Castanet et al. 2003 ). These ossifi cations are frequently penetrated by
numerous canals that carried blood vessels and nerves (including the lateral line
system of fi shes and non-amniote basal tetrapods) to the external bone surface and
into the directly overlying integument (see for review Witzmann 2009 ). As reviewed
by Janis et al. ( 2012 ), heavy dermal cover of some of early tetrapods possessed various
types of dermal mineralized elements like pits and ridges. Interestingly, blood
vessels in these animals run through the superfi cial integument to the epidermis
because dermal bone does not block interaction between it and the dermis. This
kind of vascularization within dermal sculpture seems to be crucial from physiological point of view. Thus, according to Janis and co-workers ( 2012 ), various
hypotheses for the presence of sculptured dermal bones in early tetrapods have
been proposed:
– “cutaneous respiration;
– protection from desiccation;
– strengthening adaptation;
– mechanical protection of the soft-tissue dermis including vessels, nerves, and
thermoregulation”.
3 Biocomposites and Mineralized Tissues
alternative pathway has been reported (Meunier and Huysseune 1992 ), in galeaspids
and placoderms, where it was shown that bone can develop through spheritic
mineralization in the absence of a collagenous matrix.
Dermal Bone Nowadays, the most diverse collections of dermal elements are
found in reptiles like crocodylians. However, in early representatives of many
stem gnathostomes (structural-grade ostracoderms) especially the dermal skeleton
was once the predominant skeletal system. During the evolution, it has undergone extensive reduction and modifi cation visible in most modern forms today (Sire
and Huysseune 2003 ). As reviewed by Vickaryous and Hall ( 2008 ), “for tetrapods,
the most obvious remnants include the craniofacial skeleton (dermatocranium
or desmocranium), dental tissues, and one or more elements of the pectoral
apparatus. The dermal skeleton also includes bones developing within the eye
(scleral ossicles), eyelid (palpebral), integument (osteoderms), and across the
abdomen (gastralia)”.
Additional type of bones, which arise directly through differentiation of mesenchyme, initially compacted in sheets or membranes, are known as intramembranous
bones. The process of intramembranous bone formation on example of craniofacial
skeleton is reviewed by Helms and Schneider ( 2003 ).Abzhanov et al. ( 2007 ) proposed
following classifi cation of the intramembranous bones:
(α) “the sesamoid bones, which form in tendons as a result of mechanical stress
(such as the patella in the tendon of the quadriceps femoris);
(β) the periosteal bones, which form from connective tissue and add to the thickness
of long bones;
(χ) the dermal bones, which form within the dermis of the skin”.
Dermal bones tend to be fl at and plate-like. Dermal bones of the skull and the
pectoral girdle develop within the integument, generally in the lower layer of the
dermis (Castanet et al. 2003 ). These ossifi cations are frequently penetrated by
numerous canals that carried blood vessels and nerves (including the lateral line
system of fi shes and non-amniote basal tetrapods) to the external bone surface and
into the directly overlying integument (see for review Witzmann 2009 ). As reviewed
by Janis et al. ( 2012 ), heavy dermal cover of some of early tetrapods possessed various
types of dermal mineralized elements like pits and ridges. Interestingly, blood
vessels in these animals run through the superfi cial integument to the epidermis
because dermal bone does not block interaction between it and the dermis. This
kind of vascularization within dermal sculpture seems to be crucial from physiological point of view. Thus, according to Janis and co-workers ( 2012 ), various
hypotheses for the presence of sculptured dermal bones in early tetrapods have
been proposed:
– “cutaneous respiration;
– protection from desiccation;
– strengthening adaptation;
– mechanical protection of the soft-tissue dermis including vessels, nerves, and
thermoregulation”.
3 Biocomposites and Mineralized Tissues