This figure shows the area circumscribed by the rectangle in figure above at higher magnification. The chondrocytes are contained within lacunae (arrows), and their cytoplasm stains deeply. The surrounding cartilage matrix material is scant and blends into the dense connective tissue. Cartilage matrix material can be detected best by observing the larger group of chondrocytes at the left of this figure and then observing this same area in figure above. Note the light homogeneous area around the cell nest in the lower-power view. This is the region of cartilage matrix. At the greater magnification of this figure, it is possible to see that some of the collagen fibers are incorporated in the matrix, where they appear as wispy bundles.
OVERVIEW OF BONE / 218 BONES AND BONE TISSUE / 219 GENERAL STRUCTURE OF BONES / 220 Outer Surface of Bones / 220 Bone Cavities / 221 Mature Bone / 221 Immature Bone / 223 CELLS OF BONE TISSUE / 223 Osteoprogenitor Cells / 225 Osteoblasts / 225 Osteocytes / 227 Bone-Lining Cells / 227 Osteoclasts / 227 BONE FORMATION / 232 Intramembranous Ossification / 234 Endochondral Ossification / 235 Growth of Endochondral Bone / 237 Development of the Osteonal (Haversian) System / 240 BIOLOGIC MINERALIZATION AND MATRIX VESICLES / 241 PHYSIOLOGIC ASPECTS OF BONE / 242 Folder 8.1 Clinical Correlation: Joint Diseases / 221 Folder 8.2 Clinical Correlation: Osteoporosis / 233 Folder 8.3 Clinical Correlation: Nutritional Factors in Bone Formation / 234 Folder 8.4 Functional Considerations: Hormonal Regulation of Bone Growth / 242
Bone is a connective tissue characterized by a mineralized extracellular matrix.
Bone is a specialized form of connective tissue that, like other connective tissues, consists of cells and extracellular matrix. The feature that distinguishes bone from other connective tissues is the mineralization of its matrix, which produces an extremely hard tissue capable of providing support and protection. The mineral is calcium phosphate in the form of hydroxyapatite crystals [Ca10(PO4)6(OH)2].
By virtue of its mineral content, bone also serves as a storage site for calcium and phosphate. Both calcium and phosphate can be mobilized from the bone matrix and taken up by the blood as needed to maintain appropriate levels throughout the body. Thus, in addition to support and protection, bone plays an important secondary role in the homeostatic regulation of blood calcium levels.
Bone matrix contains mainly type I collagen along with other matrix (noncollagenous) proteins.
The major structural component of bone matrix is type I collagen and, to a lesser extent, type V collagen. Trace amounts of other types such as type III, XI, and XIII collagens have also been found in the matrix. All collagen molecules constitute about 90% of the total weight of the bone matrix proteins.