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13 2 The Biology of Bone Adam K. Huttenlocker, Holly N. Woodward, and Brian K. Hall This chapter examines the topic that, with elegant simplicity, Hancox (1972) called “Biology of Bone.” Whereas cartilage may be found in vertebrates and in many invertebrates ,boneisaunique,typicallyvascularizedskeletaltissuefoundonlyinvertebrate animals (Hall, 2005). In this section, we discuss the complex hierarchical structure of bone and highlight research into its structural evolution and development. The topics included in this chapter represent a condensed version of those discussed by Francillon-Vieillot et al. (1990), de Ricqlès et al. (1991), and parts of Hall (2005), and they strongly emphasize fossil and extant terrestrial vertebrates—in particular, mammals and birds, the model organisms in which many skeletogenic processes and patterns have been most thoroughly described. Later, we examine different types of bone tissue found among different groups of vertebrates, both extant and extinct. For more detailed reviews of comparative osteohistology, see these and other references at the end of this chapter (including Huysseune 2000; Hall, 2003, 2005). THE HIERARCHICAL STRUCTURE OF BONE Bone is a complex organ system comprised of a composite of tissues; as such, it can be studied at many different levels of biological organization (e.g., tissue and organ levels). Its intricate composition and anisotropic structure mean that bone behaves differently on different scales, a finding that must be considered in any study concerned with bone deposition, structure, or mechanics. Francillon-Vieillot et al. (1990) presented four levels (or “orders”) of the integration of bone, based on the earlier work of Petersen (1930). A variation of this scheme was also presented by Rho et al. (1998) and illustrated here in Figure 2.1. Bone on the 14    Biology of Bone first (macrostructural) order (~1 m–1 mm) can be imagined as an integrated, functional organ. The overall shape and functional attributes of the macrostructure of an individual bone result from the sum of its ontogenetic, phylogenetic (historical), mechanical, and environmental influences and may incorporate considerable histovariability at the microscopic level. On the second (meso- or micro structural) order (1 mm–100 μm), bone as a tissue consists of a network of cells within an organic and mineral extracellular matrix (or ECM) and is perforated by neurovascular bundles. The organization of the ECM and vascularization can be highly variable , even within a single section of bone. The third (sub-micro structural) order (100 μm–1 μm) concerns the structure of bone at the cellular level, including how osteogenic cells are organized and interact. Finally, bone on the fourth (nanostructural ) order (1 μm–10 nm) refers to the chemical and structural organization of its organic and mineral components (e.g., Rho et al. 1998) (Fig. 2.1). The Histological Level Here we are concerned with bone at the first and second order; that is, what is the structure of bone at the tissue level, and how is its organization influenced by its gross structure and mechanical environment? Bone is an aerobic, vascularized tissue that requires variable levels of oxygen consumption, depending largely on its growth rate. Appropriately classified as a connective tissue, bone has many structural functions; it supports the body, protects major organs such as the brain and spinal cord, and is a site of attachment for ligaments, tendons, and muscles. In addition, bone has significant physiological functions that include the housing of bone marrow (a blood-forming organ) Figure 2.1. The hierarchical organization of bone (modified from Rho et al. 1998: Fig. 1). 8] Project MUSE (2024-04-26 06:21 GMT) Biology of Bone    15 and production of red blood cells, the storage of calcium and phosphate, and the metabolic regulation of mineral homeostasis. Compacta Versus Spongiosa Bone architecture can be divided into two general types based on the degree of porosity: compacta and spongiosa (Fig. 2.2). If the mineral volume of bone tissue is greater than 50% of its porous space, the bone is identified as compacta (or compact; Fig. 2.2A), which is usually confined (but not restricted to) the outer cortex of any bone. The inverse is true for spongiosa (or cancellous bone; Fig. 2.2B), which is extremely porous and often confined to the endosteal diploë. Spongiosa can further be divided into fine cancellous (the least porous), coarse cancellous, and trabecular (the most porous). The cortex is usually composed of compacta, and the endosteal region of spongiosa, but this is not always the case. Compacta may become more porous during growth and secondarily converted to cancellous spongiosa with the...

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