Short bones possess a shell of compact bone and have spongy bone and a marrow space on the inside. Short bones usually form movable joints with their neighbors; like long
FIGURE 8.2 • Structure of a typical long bone. The diaphysis (shaft) of a long bone contains a large marrow cavity surrounded by a thick-walled tube of compact bone. A small amount of spongy bone may line the inner surface of the compact bone. The proximal and distal ends, or epiphyses, of the long bone consist chiefly of spongy bone with a thin outer shell of compact bone. The expanded or flared part of the diaphysis nearest the epiphysis is referred to as the metaphysis. Except for the articular surfaces that are covered by hyaline (articular) cartilage, indicated in blue, the outer surface of the bone is covered by a fibrous layer of connective tissue called the periosteum, indicated in pink.
bones, their articular surfaces are covered with hyaline cartilage. Elsewhere, periosteum, a fibrous connective tissue capsule covers the outer surface of the bone.
Outer Surface of Bones
Bones are covered by periosteum, a sheath of dense fibrous connective tissue containing osteoprogenitor cells.
Bones are covered by a periosteum except in areas where they articulate with another bone. In the latter case, the articulating surface is covered by cartilage. The periosteum that covers an actively growing bone consists of an outer fibrous layer that resembles other dense connective tissues and an inner, more cellular layer that contains the osteoprogenitor cells. If active bone formation is not in progress on the bone surface, the fibrous layer is the main component of the periosteum, and the inner layer is not well defined. The relatively few cells that are present, the periosteal cells, are, however, capable of undergoing division and becoming osteoblasts under appropriate stimulus.
In general, the collagen fibers of the periosteum are arranged parallel to the surface of the bone in the form of a capsule. The character of the periosteum is different where ligaments and tendons attach to the bone. Collagen fibers from these structures extend directly, but at an angle, into the bone tissue, where they are continuous with the collagen fibers of the extracellular matrix of the bone tissue. These fibers are called Sharpey’s fbers.
Bones that articulate with neighboring bones possess movable (synovial) joints.
Where a bone articulates with a neighboring bone, as in synovial joints, the contact areas of the two bones are referred to as articular surfaces. The articular surfaces are covered by hyaline cartilage, also called articular cartilage because of its location and function; articular cartilage is exposed to the joint cavity. This cartilage is not covered with perichondrium. The details of articular cartilage are discussed in Chapter 7 (page 203 and in Folder 8.1 (Clinical Correlation: Joint Diseases).
Bone cavities are lined by endosteum, a layer of connective tissue cells that contains osteoprogenitor cells.
The lining tissue of both the compact bone facing the marrow cavity and the trabeculae of spongy bone within the cavity is referred to as endosteum. The endosteum is often only one cell layer thick and consists of osteoprogenitor cells that can differentiate into bone matrix–secreting cells, the osteoblasts, and bone-lining cells. Osteoprogenitor cells and bone-lining cells are difficult to distinguish at the microscopic level. They are both flattened in shape with elongated nuclei and indistinguishable cytoplasmic features. Because of their location within the bone cavities they are frequently called endosteal cells.
The marrow cavity and the spaces in spongy bone contain bone marrow.