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Types of Skeletal Tissues
Chapter
Types of Skeletal Tissues
‘In the pioneering stages of Natural Science we recognize the work of collecting, describing and classifying the typical units as a fundamental necessity. The study of their morphology and their history belongs to a more advanced period.’1
Skeletal tissues are ancient, their origins reaching back perhaps three-quarters of a billion years. The number of skeletal tissues or organs is far more limited. In 2000, Thomas and colleagues evaluated 182 characters of skeletal design as possible design options in morphospace, a 3D representation of the distribution of all known morphologies. Of these 182 characters, 146 were already in use in animals of the Burgess Shale fauna 530 mya. Indeed, within 15 million years of the appearance of the crown groups of the major phyla, 80 per cent of the design elements were already in use.2 Four classes of mineralized tissues are found in vertebrates. The four are bone, cartilage, dentine and enamel. This may seem an unlikely list. Normally, we think of cartilage and bone as skeletal tissues, enamel and dentine as dental tissues. But enamel and dentine arose evolutionarily as skeletal tissues in the exoskeleton (dermal, dermoskeleton) of early vertebrates (Fig. 1.1, Table 1.1). Bone is also a primitive tissue of that exoskeleton. Cartilage, on the other hand, provided the basis for the second vertebrate skeletal system, the endoskeleton (Table 1.1), and has an even more diverse distribution and, potentially, a longer evolutionary history than bone.3 This is because cartilage and cartilage-like tissues form endoskeletal elements in many invertebrates. Although most invertebrates have non-cartilaginous endoskeletons the diversity of taxa with cartilage is astonishing. Where there is an exoskeleton or cuticle in invertebrates – and there almost always is – it is composed of chitin (sometimes with glycoproteins, sometimes mineralized) or calcium carbonate,
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but not calcium phosphate, which is the major componen of bone.4 Neither bone nor mineralized cartilage is eve found in invertebrates, although some invertebrate cart lages have surprisingly ‘bone-like’ features (Chapter 4). Mineralization is not the exclusive property of the fou vertebrate mineralized tissues. Mineralization is ubiqu tous within metazoans as well as being a property o many single-celled organisms. In Table 1.2, I summariz the range and diversity of mineralized biological tissues i various groups and the major organic component(s) asso ciated with mineralization in each. Mineralization ca place enormous demands on an organism, for exampl when deer regrow their annual set of antlers (Chapter 8 or hens lay eggs (Chapter 25). A
enamel dentine
bone of attachmen basal bone
perichondrial bone
B
endochondral bone cartilage
calcified cartilage
Figure 1.1 Diagrammatic representations of the tissues that comprise (A) the dermal (exo) and (B) the endoskeletons of vertebrates. The major difference is that the dermal skeleton is based on dentine and associated bone, while the endoskeleton is based on cartilage. (A) Enamel caps the dentine in the dermal skeleton. Individual exoskeletal units (odontodes), which produce dentine and bone of attachment, fuse to adjacent basal bone. (B) The cartilage of the endoskeleton may (i) remain unmineralized, (ii) mineralize and remain as a permanent mineralized (calcified) cartilage, (iii) be surrounded by perichondrial bone, or (iv) be invaded and replaced by endochondral bone.
Types of Skeletal Tissues
‘In the pioneering stages of Natural Science we recognize the work of collecting, describing and classifying the typical units as a fundamental necessity. The study of their morphology and their history belongs to a more advanced period.’1
Skeletal tissues are ancient, their origins reaching back perhaps three-quarters of a billion years. The number of skeletal tissues or organs is far more limited. In 2000, Thomas and colleagues evaluated 182 characters of skeletal design as possible design options in morphospace, a 3D representation of the distribution of all known morphologies. Of these 182 characters, 146 were already in use in animals of the Burgess Shale fauna 530 mya. Indeed, within 15 million years of the appearance of the crown groups of the major phyla, 80 per cent of the design elements were already in use.2 Four classes of mineralized tissues are found in vertebrates. The four are bone, cartilage, dentine and enamel. This may seem an unlikely list. Normally, we think of cartilage and bone as skeletal tissues, enamel and dentine as dental tissues. But enamel and dentine arose evolutionarily as skeletal tissues in the exoskeleton (dermal, dermoskeleton) of early vertebrates (Fig. 1.1, Table 1.1). Bone is also a primitive tissue of that exoskeleton. Cartilage, on the other hand, provided the basis for the second vertebrate skeletal system, the endoskeleton (Table 1.1), and has an even more diverse distribution and, potentially, a longer evolutionary history than bone.3 This is because cartilage and cartilage-like tissues form endoskeletal elements in many invertebrates. Although most invertebrates have non-cartilaginous endoskeletons the diversity of taxa with cartilage is astonishing. Where there is an exoskeleton or cuticle in invertebrates – and there almost always is – it is composed of chitin (sometimes with glycoproteins, sometimes mineralized) or calcium carbonate,
1
but not calcium phosphate, which is the major componen of bone.4 Neither bone nor mineralized cartilage is eve found in invertebrates, although some invertebrate cart lages have surprisingly ‘bone-like’ features (Chapter 4). Mineralization is not the exclusive property of the fou vertebrate mineralized tissues. Mineralization is ubiqu tous within metazoans as well as being a property o many single-celled organisms. In Table 1.2, I summariz the range and diversity of mineralized biological tissues i various groups and the major organic component(s) asso ciated with mineralization in each. Mineralization ca place enormous demands on an organism, for exampl when deer regrow their annual set of antlers (Chapter 8 or hens lay eggs (Chapter 25). A
enamel dentine
bone of attachmen basal bone
perichondrial bone
B
endochondral bone cartilage
calcified cartilage
Figure 1.1 Diagrammatic representations of the tissues that comprise (A) the dermal (exo) and (B) the endoskeletons of vertebrates. The major difference is that the dermal skeleton is based on dentine and associated bone, while the endoskeleton is based on cartilage. (A) Enamel caps the dentine in the dermal skeleton. Individual exoskeletal units (odontodes), which produce dentine and bone of attachment, fuse to adjacent basal bone. (B) The cartilage of the endoskeleton may (i) remain unmineralized, (ii) mineralize and remain as a permanent mineralized (calcified) cartilage, (iii) be surrounded by perichondrial bone, or (iv) be invaded and replaced by endochondral bone.