Английский язык для медиков: конспект лекций

Embedded in the matrix are the bone cells, or osteocytes. Bone matrix consists of both organic and inorganic materials. The organic portion is made up chiefly of collagen fibres. The inorganic portion of matrix constitutes about two thirds of a bone's total weight. The chief inorganic substance is calcium phosphate, which is responsible for the bone's hardness. If the calcium were removed from a bone, the bone would loose its rigidity and become flexible. If the organic portion were burned out, or calcined, the bone would retain its shape but crumble under the slightest pressure.

In the formation of intramembraneous bone, certain cells of the embryonic connective tissue congregate in the area where the bone is to form. Small blood vessels soon invade the area, and the cells, which have clustered in strands, undergo certain changes to become osteoblasts. The cells then begin secreting collagen fibers and an intercellular substance. This substance, together with the collagen fibers and the connective tissue fibers already present, is called osteoid. Osteoid is very soft and flexible, but as mineral salts are deposited it becomes hard matrix. The formation of endochondral bone is preceded by the formation of a cartilaginous structure similar in shape to the resulting bone. In a long bone, ossification begins in the area that becomes the center of the shaft. In this area, cartilage cells become osteo-blasts and start forming bone tissue in the same manner as intramem-braneous bone is formed. This process spreads toward either end of the bone, but while this is happening the cartilage cells at each end also become osteoblasts and start forming bone. The only areas where cartilage is not soon replaced by bone tissue are the regions where the shaft joins the two epiphyses. These areas, called epiphyseal plares, are responsible for the bone's continuing growth in length, As the bone grows, each epiphyseal plate forms new cartilage, which is then replaced by bone. When the epiphyseal plates stop forming cartilage, the bone stops growing. The bone's growth in diameter is due to the addition of layers of bone around the outside of the shaft. As they are formed, layers of bone on the inside of the shaft are removed. Thus, when a long bone grows in diameter, the compact bone of the shaft does not get thicker, but the marrow cavity gets larger. In all bones, the matrix is arranged in layers called lamellae.