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Bone is a type of C.T. in which the intercellular matrix is highly specialized for rigidity and strength. It is a highly dynamic tissue; the structure of mature bone seen in the microscope is the last of numerous cycles in which bony tissue is formed, then resorbed, followed by formation of new bony tissue, etc. Bone is usually formed in layers or lamellae which contain collagen fibers in a nearly parallel array, and a small proportion of proteoglycans and other substances. This organic matrix is soon mineralized by formation of minute apatite-like crystals oriented along the collagen fibers. The collagen fibers of adjacent lamellae are laid down obliquely or perpendicularly. This plywood-like arrangement increases the overall strength.
A specialized layer of dense C.T. called the periosteum covers the outer surface of the bone. The inner surface enclosing the marrow cavity is lined with a very thin, delicate C.T., the endosteum. The cells of the endosteum and the inner layer of the periosteum resemble fibroblasts, but can differentiate into osteoblasts under appropriate stimulation. The principal bone cells are the osteocyte, the osteoblast, and the osteoclast. The osteocyte, or bone cell proper, is found in lacunae lying in or between the lamellae. Very fine channels (canaliculi) containing osteocyte cell processes connect lacunae to each other and to tissue fluid spaces, providing for nutrition and metabolic exchange. The osteocyte is derived from the osteoblast as it becomes surrounded by matrix. The osteoblast is found on the surfaces of bone and is involved with bone deposition. Active sites of bone formation are characterized by large basophilic osteoblasts. The osteoclast is usually found in depressions (Howship's lacunae) at the surfaces of bony tissue and is associated with bone resorption; it is a large multi-nucleated cell derived from blood monocytes.
Bony tissue is classified as either spongy (trabecular, cancellous) or compact bone, depending on the relative proportions of mineralized and soft tissue; most bones contain both spongy and compact regions. Compact bone and the larger trabeculae of spongy bone contain lamellae. In compact bone, however, the predominant structural unit is the osteon, or Haversian system. In the osteon, lamellae are arranged as concentric cylinders surrounding a central Haversian canal. This canal contains nerves, connective tissue and blood vessels. The long axis of an osteon is approximately parallel to the major axis of stress, usually the long axis of the bone. The formation of an osteon can be visualized as a cycle of bone resorption proceeding radially outward from a blood vessel, followed by deposition of new lamellae starting at the outer edge and ending at what becomes a new Haversian canal.
Those of you with odd boxes should begin with slide #14, described in Section (b) below.
(a) Decalcified bone (slide #25)
These lamellae may be discontinuous, and are usually present on one side of the bone only. Look for roughly circular arrays of lamellae which are cross-sections through osteons (Haversian systems) and are found in the middle part of the bone tissue, with the Haversian canal surrounded by concentric lamellae. There may be larger spaces within the compact bone filled with a loose C.T. and often lined with basophilic osteoblasts. These are usually areas where bone was previously resorbed and new osteons were in the process of being formed (remodeling). The inner circumferential lamellae form the inner layer of bone adjacent to the marrow cavity. They do not necessarily form continuous layers, and may not be present in all specimens. Volkmann's canals penetrating from the periosteal or endosteal surfaces may be present; they are easily confused with Haversian canals until you recognize that they are oriented roughly at right-angles to the osteon lamellae. Volkmann's canals are tubular passages in the bone through which blood vessels in the periosteum or the marrow cavity send branches to the Haversian canals. On some slides, Volkman's canals may be seen connecting adjacent Haversian canals. The endosteum, which is best seen under high power, is a thin layer of C.T. consisting of a single layer of osteoblasts on the inner margin of the bone.
(b) Ground bone (slide #14, odd boxes)
The basic process in bone formation consists of the deposition of a calcifiable organic matrix by osteoblasts, and its subsequent mineralization. In early development, osteoblasts differentiate from mesenchyme cells and from fibroblast-like cells with mesenchymal potentialities. In later development, they differentiate from fibroblast-like osteogenic cells only. Osteoblasts remain relatively stationary while laying down the matrix and become completely embedded in matrix, at which time, they become known as osteocytes.
Bone formation is classified into 2 main types, according to the milieu in which it occurs:
(1) Intramembranous: The bone is formed in and replaces a pre-existing membrane of embryonic connective tissue. A periosteum develops on the surfaces of the newly formed bone, and osteoblasts, which differentiate in it, continue the process of osteogenesis. Bones of the skull and jaw are examples.
(2) Endochondral bone formation: A cartilage model is formed initially; the cartilage is a temporary structure and is eventually destroyed except for the articular surface. Short and long bones are formed this way.
The long bones are formed by a combination of endochondral and intramembranous bone formation. The initial bone collar at the primary center is made by intramembranous ossification. halfway between the ends of the cartilage model, the osteogenic capacity of cells of the perichondrium is activated leading to development of the bone collar. Subsequently, blood vessels penetrate into the cartilage model leading to endochondral ossification. Growth in width of the diaphysis of large bones occurs in a manner similar to that of the "membrane" bones (e.g. calvaria). Most of the true endochondral bone serves merely as a temporary framework which is ultimately resorbed. Mature bone of intramembranous or endochondral origin has essentially the same histological structure, i.e., the lamellar structure just studied.
A non-lamellar form of bone (woven bone) is normally found in rapidly growing areas of embryonic or developing bone, and in healing fractures. Most woven bone is replaced by lamellar bone during remodeling, but woven bone persists at certain sites in the adult (e.g. in tooth sockets, bony sutures, and tendonous and ligamentous insertions). The process of bone remodeling occurs continually in the embryo, fetus and the adult in order to maintain the proper shape of the bone during growth, and for adaptation to normal variations in physiological conditions and to changes in stress throughout life. Remodeling involves the removal of osseous tissue (resorption) in some locations and adding tissue (accretion) in others. In bone resorption, normally both the organic matrix and the mineral crystals are removed simultaneously.
Intramembranous Bone Formation
Endochondral Bone Formation
A later stage of endochondral bone formation is shown in a section of femur, slide #15. Starting with the reversed ocular, locate: (1) articular cartilage of the epiphysis, a smooth surfaced region (2) a secondary center of ossification and marrow cavity in the epiphysis between the articular surface and a belt of basophilic cartilage, (3) the epiphyseal growth cartilage, located between the primary and secondary marrow cavities, and (4) the primary marrow cavity in the diaphysis containing eosinophilic bony trabeculae.
Examine the developing joints in slide #19. Observe that the joint capsule extends from one bone to the other, but does not cover the articular cartilage at the apposing bone surfaces. At the inner surface of the joint capsule, a synovial membrane extends into the joint cavity where the contour of the articular cartilage is rounded. Examine the articular cartilage on slide #15 and notice the transition from cartilage to bone of the secondary center of ossification. Observe the smooth surface of the articular cartilage which can be seen on one surface of the bone. Study its structure and note that its surface is smooth and lacks a perichondrium.