The area in the rectangle in the orientation micrograph containing compact bone in the epiphysis is shown here at higher magnification. The lighter staining area is cartilage (C). It serves as the articular surface of the epiphysis. Note the presence of isogenous groups of chondrocytes (Ch), a charac teristic feature of a growing cartilage. Below this is bone tissue (BT). It can
Compact bone, long bone, human, H&E, ×135.
Bone from the diaphysis within the far right rectangle of the oritentation miocrograph is shown here at higher magnification. The outer surface of the bone is covered by dense connective tissue known as periosteum (P). The remaining tissue in be distinguished from the cartilage by the arrangement of its cells, the osteocytes (Oc). The osteocytes lie within the bone matrix, but are typically recognized only by their nuclei. Because bone matrix is laid down in layers (lamellae), bone characteristically shows linear or circular patterns that appear as striations. The irregular spaces seen within the bone tissue are vascular channels (VC) that contain, in addition to vessels, bone-forming tissue.
the micrograph is compact bone. The osteocytes (Oc) are again recognized by their nuclei within the bone matrix. Another feature worth noting in this growing bone is the presence of bone resorbing cells known as osteoclasts (Ocl). They are large multinucleated cells found at sites in bone where remodeling is taking place (see Plate 14).
Spongy bone, long bone, human, H&E, ×135.
The area in the rectangle in the orientation micrograph containing spongy bone in the epiphysis is shown here at higher magnification. Although the bone tissue at this site forms a three dimensional structure consisting of branching trabeculae, its structural organization and components are the same as that seen in compact bone. Note the osteocyte nuclei (N). As bone matures, the bone tissue becomes reorganized and forms osteons (O), which consist of a central vascular channel and surrounding layers (lamellae) of bone matrix. The two circular spaces are sites in which bone tissue has been removed and will be replaced by new tissue in the form of osteons. The spaces surrounding the spongy bone contain bone marrow consisting mainly of adipocytes. Other cells that have the capacity to form bone or hemopoetic tissue are also present.
KEY AS, articular surface BM, bone marrow BT, bone tissue C, cartilage CB, compact bone Ch, chondrocytes E, epiphysis GP, growth plate N, nuclei O, osteons Oc, osteocytes Ocl, osteoclasts P, periosteum SB, spongy bone T, trabeculae VC, vascular channels
Endochondral bone formation involves the continuing growth of a cartilage precursor, which serves as a fetal skeleton, and the simultane-ous removal of the cartilage and its replacement with bone tissue. In addition, as a bone grows, some of the bone tissue is removed while newer bone tissue is being laid down, a process called remodeling. Remodeling that alters the shape of the bone is called external remodel-ing; that which does not alter the shape of the bone, as in the formation of Haversian systems, is called internal remodeling. Two specialized cell types are identified with the process of bone growth and remodeling. The osteoblast is engaged in the formation of bone. Although the removal of bone is not as well described as its formation, it has been established that multinucleated cells, called osteo-clasts, are engaged in the removal of bone. Osteocytes, also, can alter and resorb bone in their immediate vicinity. The process is called os-teocytic osteolysis. It is important in calcium homeostasis, i.e., the maintenance of normal blood calcium concentrations.
Developing bone, short bone, monkey, H&E ×240.