Blood vessels reach the Haversian canals from the marrow through other tunnels called perforating (Volkmann’s) canals (VC). In some instances, as here, Volkmann’s canals travel from one Haversian canal to another. Volkmann’s canals can be distinguished from Haversian canals in that they pass through lamellae, whereas Haversian canals are surrounded by concentric rings of lamellae.
the lacunae. These thread-like profiles represent the canaliculi, spaces within the bone matrix that contained cytoplasmic processes of the osteocyte. The canaliculi of each lacuna communicate with canaliculi of neighboring lacunae to form a three-dimensional channel system throughout the bone.
Ground bone, long bone, human, ×400.
In a still higher magnification, the circumferential lamellae are found around the shaft of the long bone at the outer as well as the inner surface of the bone. The osteoblasts that contribute to the formation of circumferential lamellae at these sites come from the periosteum and endosteum, respectively, whereas the osteons are constructed from osteoblasts in the canal of the developing Haversian system. This figure reveals not only the canaliculi but also the lamellae of the bone. The latter are just barely defined by the faint lines (arrows) that extend across the micrograph. Collagenous fibers in neighboring lamellae are oriented in different directions. This change in orientation accounts for the faint line or interface between adjacent lamellae.
KEY CL, circumferential lamellae HC, Haversian canal IL, interstitial lamellae L, lacuna VC, Volkmann’s canal arrow, lamellar boundary
PLATE 11 • BONE, GROUND SECTION VCVCILILHCHCCLCLOO CL HC IL ILILIL VC LL LL LL LL HCHCHC HCHCHC
Bone represents one of the specialized connective tissues. It is characterized by a mineralized extracellular matrix. It is the mineralization of the matrix that sets bone tissue apart from the other connective tissues and results in an extremely hard tissue that is capable of providing support and protection to the body. The mineral is calcium phosphate in the form of hydroxyapatite crystals. In addition to its supporting role, bone also provides a storage site for calcium and phosphate. Both can be mobilized from the bone matrix and taken up by the blood as needed to maintain normal levels. Bone matrix contains type I collagen and, in small amounts, a number of other types of collagen, i.e., types V, III, XI, and XIII. Other matrix proteins that constitute the ground substance of bone such as proteoglycan macromolecules, multiadhesive glycoproteins, growth factors, and cytokines are also present. Bone is typically studied in histological prepara-tions by removing the calcium content of the bone (decalcified bone), thus allowing it to be sectioned like other soft tissues. ORIENTATION MICROGRAPH: The orientation micrograph shows the upper end of a decal-cified humerus from an infant. The interior of the head of the bone, the epiphysis (E), consists of spongy cancellous bone made up of an anastomosing network of trabeculae (T) in the form of spicules of bone tissue. The outer portion consists of a dense layer of bone tissue known as compact bone (CB). Its thickness varies in different parts of the bone. The shaft of this bone, the diaphysis (D), is also made up of compact bone (CB) and in its interior, spongy bone (SB). Also within the shaft of the bone is bone marrow (BM), which at this stage of life is in the form of hemopoetic tissue. Lastly, cartilage is also a component of the bone where it is present as an articular surface (AS) and as a growth plate (GP). The latter is described in a later plate. ASECBDCBCBSBBMCBTGPAS E CB D CB CB SB BM CB T GP
Compact bone, long bone, human, H&E, ×178.