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Chapter 8: Bone

INTRODUCTION

As the main constituent of the adult skeleton, bone tissue (Figure 8–1) provides solid support for the body, protects vital organs such as those in the cranial and thoracic cavities, and encloses internal (medullary) cavities containing bone marrow where blood cells form. Bone (or osseous) tissue also serves as a reservoir of calcium, phosphate and other minerals, storing or releasing these ions in a controlled fashion to maintain constant concentrations in body fluids.

FIGURE 8–1

Components of bone.

A schematic overview of the basic features of bone, including the three key cell types: osteocytes, osteoblasts, and osteoclasts; their usual locations; and the typical lamellar organization of bone. Osteoblasts secrete the matrix that then hardens by calcification, trapping the differentiating cells now called osteocytes in individual lacunae. Osteocytes maintain the calcified matrix and receive nutrients from microvasculature in the central canals of the osteons via very small channels called canaliculi that interconnect the lacunae. Osteoclasts are monocyte-derived cells in bone required for bone remodeling.

The periosteum consists of dense connective tissue, with a primarily fibrous layer covering a more cellular layer. Bone is vascularized by small vessels that penetrate the matrix from the periosteum. Endosteum covers all trabeculae around the marrow cavities.

Diagrams depict the internal structure of bone.
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In addition, bones form a system of levers that multiply the forces generated during skeletal muscle contraction and transform them into bodily movements. This mineralized tissue therefore confers mechanical and metabolic functions to the skeleton.

Bone is a specialized connective tissue composed of calcified extracellular material, the bone matrix, and following three major cell types (Figure 8–2):

  • Osteocytes (Gr. osteon, bone + kytos, cell), which become localized in cavities (lacunae) between bone matrix layers (lamellae), with cytoplasmic processes in small canaliculi (L. canalis, canal) extending into the matrix (Figure 8–1b)

  • Osteoblasts (osteon + Gr. blastos, germ), growing cells which synthesize and secrete the organic components of the matrix

  • Osteoclasts (osteon + Gr. klastos, broken), giant, multinucleated cells involved in removing calcified bone matrix and remodeling bone tissue.

FIGURE 8–2

Bone tissue.

Newly formed bone tissue decalcified for sectioning and stained with trichrome in which the collagen-rich ECM appears blue. The tissue is a combination of pale-staining mesenchymal regions (M) containing capillaries, fibroblasts, and osteoprogenitor stem cells and variably darker blue regions of normally calcified matrix with varying amounts of collagen and the three major cell types found in all bone tissue.

Bone-forming osteoblasts (Ob) differentiate from osteoprogenitor cells in the periosteum and endosteum, and cover the surfaces of existing bone matrix. The osteoblasts secrete osteoid rich in collagen type I, but also containing proteoglycans and other molecules. As osteoid undergoes calcification and hardens, it entraps some osteoblasts which then ...

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