Structure

There are several different calcified tissues in the human body and several different ways of categorizing them. All calcified tissues have one thing in common: in addition to the principal protein component, collagen,1 and small amounts of other organic phases, they all have an inorganic component hydroxyapatite (abbreviated OHAp) or Cai0(PO4)6(OH)2.2 In the case of long bones such as the tibia or femur, an understanding of the organization of these two principal components is the beginning phase of the characterization of bone structure according to scale, (i.e., the level of the observation technique). It has been convenient to treat the structure of compart cortical bone (e.g., the dense bone tissue found in the shafts of long bones) on four levels of organization (Fig. 1) (Katz, 1980a; Park, 1984).

The collagen triple helical structure (tropocollagen) and OHAp crystallography, comprise the initial or molecular level. The structure of OHAp is shown in Fig. 2 (Young, 1975). It

'There is one exception to this, enamel, which is found in the outer sheath of teeth; enamel has a small amount of another protein, enamelin.

2In actuality, this stoichiometric formula is not achieved in biological apatites. There are carbonate and other ions present as well in bones and teeth.

forms a hexagonal unit cell with space group symmetry P 6,/m and lattice constants a = 9.880 A and c = 6.418 A, containing two molecular units, Cas (P04)30H, per unit cell. How this mineral phase is produced by cells and whether it is the first calcium phosphate laid down are subjects of considerable research at present. Because of its small crystallite size in bone (approximately 2 x 20 x 40 nm), the X-ray diffraction pattern of bone exhibits considerable line broadening, compounding the difficulty of identifying additional phases. As we will see later, the fact that a Ca-bearing inorganic compound is one of the components of calcified tissues has led to the development of a whole class of ceramic and glass-ceramic materials that are osteophilic within the body (i.e., they present surfaces that bone chemically attaches to).

To appreciate the way in which collagen contributes to the hierarchical structural levels of bone, it is necessary to discuss the structure of collagen. However, this would require an introduction to amino acids, how they link to form polypeptides, and how these lead to proteins, which the constraints of space will not allow. A simplified picture of how the alpha chain is formed, how three chains coil together to form the tropocollagen molecule, and how molecules form a protofibril is shown in Fig, 3. For further reading, see Glimcher and Krane (1968).

As yet, we do not know fully how the two components, collagen and OHAp, are arranged or what forms hold them together at this molecular level. Whatever the arrangement, when it is interfered with, as is apparently the case in certain bone pathologies in which the collagen structure is altered during formation, the result is a bone that is formed with seriously compromised physical properties.

The second, or ultrastructural, level may be loosely defined as the structural level observed with transmission electron microscopy (TEM) or high-magnification scanning electron microscopy (SEM) (Fig. 1), Here too, we have not yet achieved a full understanding of the collagen-OHAp organization. It appears that the OHAp can be found both inter- and intra-fibrillarly within the collagen. As we shall see later, at this level, it appears that we can model the elastic properties of this essentially two-component system by resorting to some sort of linear superposition of the elastic modules of each component, weighted by the percent volume concentration of each.

These fibrillar composites form larger structures, fibers, and fiber bundles, which then pack into lamellar-type units that can be observed with both SEM and optical microscopy. This is the third, or microstructural level of organization. Figure 4 shows two such types of lamellar organizations (5). Figure 4a illustrates the circular (or nearly circular) lamellar units forming the secondary osteons (Haversian systems) found in mature human bone. Figure 4b shows the straight lamellar units forming the plexiform (lamellar) bone found generally in young quadruped animals the size of cats and larger. This is the structural level that is being described when the term "bone tissue" is used or when histology is generally being discussed. At this level, composite analysis can also be introduced to model the elastic properties of the tissue, thus providing an understanding of the macroscopic properties of bone (i.e., those associated with the behavior of the whole bone, or fourth level of structure). Unfortunately, this modeling is much more

Articular cartilage

Spongy Done Compact bane

Periosteum

Nutrient artery

Intramedullary cavity

Line of epiphyseal fusion

Articular cartilage

Spongy Done Compact bane

Periosteum

Nutrient artery

Intramedullary cavity

Line of epiphyseal fusion

Concentric lamella

Micro- to Ultra-

Concentric lamella

Apatite mineral crystals (200-400A long)

Micro- to Ultra-

Apatite mineral crystals (200-400A long)

Ultra- to Molecular

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