Osteoprogenitor cells and osteoblasts are developmental precursors of the osteocyte. Osteoclasts are phagocytotic cells derived from fusion of hemopoietic progenitor cells in bone marrow that give rise to the neutrophilic granulocyte and monocyte lineages. Each of these cells is described in more detail below.
Bones are the organs of the skeletal system; bone tissue is the structural component of bones.
Typically, a bone consists of bone tissue and other connective tissues, including hemopoietic tissue, fat tissue, blood vessels, and nerves. If the bone forms a freely movable or synovial joint, hyaline cartilage is present. The ability of the bone to perform its skeletal function is attributable to the bone tissue and, where present, the hyaline or articular cartilage.
Bone tissue is classified as either compact (dense) or spongy (cancellous).
If a bone is cut, two distinct structural arrangements of bone tissue can be recognized (Fig. 8.1 and Plate 12, page 246). A compact, dense layer forms the outside of the bone (compact bone); a spongelike meshwork consisting of trabe culae (thin, anastomosing spicules of bone tissue) forms the interior of the bone (spongy bone). The spaces within the meshwork are continuous and, in a living bone, are occupied by marrow and blood vessels.
Bones are classified according to shape; the location of spongy and compact bone varies with bone shape.
Spongy and compact bone tissues are located in specific parts of bones. It is useful, then, to outline briefly the kinds of bones and survey where the two kinds of bone tissue are located. On the basis of shape, bones can be classified into four groups:  Long bones are longer in one dimension than other bones and consist of a shaft and two ends (e.g., the tibia and the metacarpals). A schematic diagram of a long bone sectioned longitudinally through the shaft is shown in Figure 8.2.
 Short bones are nearly equal in length and diameter (e.g., the carpal bones of the hand).
 Flat bones are thin and platelike (e.g., the bones of the calvaria [skull cap] and the sternum). They consist of two layers of relatively thick compact bone with an intervening layer of spongy bone.
 Irregular bones have a shape that does not fit into any one of the three groups just described; the shape may be complex (e.g., a vertebra), or the bone may contain air spaces or sinuses (e.g., the ethmoid bone).
FIGURE 8.1 • Epiphysis of an adult long bone. This photo shows a longitudinally sectioned epiphysis of a long bone. The outer portion of the bone has a solid structure (arrows) and represents compact (dense) bone. The interior of the bone exhibits a spongy configuration and represents spongy (cancellous) bone. It consists of numerous interconnecting bony trabeculae separated by a labyrinth of interconnecting marrow spaces.
Long bones have a shaft, called the diaphysis, and two expanded ends, each called an epiphysis (see Fig. 8.2). The articular surface of the epiphysis is covered with hyaline cartilage. The flared portion of the bone between the diaphysis and the epiphysis is called the metaphysis. It extends from the diaphysis to the epiphyseal line. A large cavity filled with bone marrow, called the marrow or medullary cavity, forms the inner portion of the bone. In the shaft, almost the entire thickness of the bone tissue is compact; at most, only a small amount of spongy bone faces the marrow cavity. At the ends of the bone, the reverse is true. Here the spongy bone is extensive, and the compact bone consists of little more than a thin outer shell (see Fig. 8.1).