Bone is a specialized connective tissue characterized by a mineralized extracellular matrix. Calcium phosphate, in the form of hydroxyapatite crystals (Ca10(PO4)6OH2), is deposited along the collagen fibrils and in the proteoglycan ground substance. Bone serves as a storage site for calcium and phosphate, which can be released to the blood to maintain homeostatic levels. Osteocytes reside in lacunae in the bone ma-trix and extend fine cellular processes into canaliculi that connect the lacunae, thus forming a continuous network of cells within the mineral-ized tissue. Bones are organs of the skeletal system; bone tissue is the structural component of bones. Ground sections of bone are prepared from bone that has not been fixed but merely allowed to dry. Thin slices of the dried bone are then cut with a saw and further ground to a thinness that allows viewing in a light microscope. Slices may be treated with India ink to fill spaces that were formerly occupied by organic matter, e.g., cells, blood vessels, and unmineralized matrix. A simpler method is to mount the ground specimen on a slide with a viscous medium that traps air in some of the spaces, as in the specimen in this plate. Here, some of the osteonal canals and a perforating canal are filled with the mounting medium, making them translucent instead of black. Specimens prepared in this manner are of value chiefly to display the architecture of the compact bone.
Ground bone, long bone, human, ×80.
This figure reveals a cross-sectioned area of a long bone at low magnification and includes the outer or peripheral aspect of the bone, identified by the presence of circumferential lamellae (CL). (The exterior or periosteal surface of the bone is not included in the micrograph.) To their right are the osteons (O) or Haversian systems that appear as circular profiles. Between the osteons are interstitial lamellae (IL), the remnants of previously existing osteons.
Osteons are essentially cylindrical structures. In the shaft of a long bone, the long axes of the osteons are oriented parallel to the long axis of the bone. Thus, a cross section through the shaft of a long bone would reveal the osteons in cross section, as in this figure. At the center of each osteon is an osteonal (Haversian) canal (HC) that contains blood vessels, connective tissue, and cells lining the surface of the bone material. Because the organic material is not retained in ground sections, the Haversian canals and other spaces will appear black, as-they do here, if filled with India ink or air. Concentric layers of mineralized substance, the concentric lamellae,
PLATE 11 • BONE, GROUND SECTION
Ground bone-osteon, long bone, human, ×300.
This figure shows a higher-magnification micrograph of the labeled osteon from the upper figure. It includes some of the interstitial lamellae (IL) that are now seen at the bottom of the micrograph (the micrograph has been reoriented). Note the lacunae (L) and the fine thread-like profiles emanating from surround the Haversian canal and appear much the same as growth rings of a tree. The canal is also surrounded by concentric arrangements of lacunae. These appear as the small, dark, elongate structures.
During the period of bone growth and during adult life, there is constant internal remodeling of bone. This involves the destruction of osteons and formation of new ones. The breakdown of an osteon is usually not complete; however, part of the osteon may remain intact. Moreover, portions of adjacent osteons may also be partially destroyed. The space created by the breakdown process is reoccupied by a new osteon. The remnants of the previously existing osteons become the interstitial lamellae.