Substantial Amounts of Calcium and Phosphate Enter and Leave Bone Each

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Although bone may be considered as being a relatively inert material, it is active metabolically. Considerable amounts of calcium and phosphate both enter and exit bone each day, and these processes are hormonally controlled.

Composition of Bone. Mature bone can be simply described as inorganic mineral deposited on an organic framework. The mineral portion of bone is composed largely of calcium phosphate in the form of hydroxyapatite crystals, which have the general chemical formula Ca10 (PO4)6(OH)2. The mineral portion of bone typically comprises about 25% of its volume, but because of its high density, the mineral fraction is responsible for approximately half the weight of bone. Bone contains considerable amounts of the body's content of carbonate, magnesium, and sodium in addition to calcium and phosphate (see Table 36.2).

The organic matrix of bone on which the bone mineral is deposited is called osteoid. Type I collagen is the primary constituent of osteoid, comprising 95% or more. Collagen in bone is similar to that of skin and tendons, but bone collagen exhibits some biochemical differences that impart increased mechanical strength. The remaining noncollagen portion (5%) of organic matter is referred to as ground substance. Ground substance consists of a mixture of various proteoglycans, high-molecular-weight compounds consisting of different types of polysaccharides linked to a polypeptide backbone. Typically, they are 95% or more carbohydrate.

Electron microscopic study of bone reveals needle-like hydroxyapatite crystals lying alongside collagen fibers. This orderly association of hydroxyapatite crystals with the collagen fibers is responsible for the strength and hardness characteristic of bone. A loss of either bone mineral or organic matrix greatly affects the mechanical properties of bone. Complete demineralization of bone leaves a flexible collagen framework, and the complete removal of organic matrix leaves a bone with its original shape, but extremely brittle.

Cell Types Involved in Bone Formation and Bone Resorption. The three principal cell types involved in bone formation and bone resorption are osteoblasts, osteocytes, and osteoclasts (Fig. 36.3).

Osteoblasts are located on the bone surface and are responsible for osteoid synthesis. Like many cells that actively synthesize proteins for export, osteoblasts have an abundant rough ER and Golgi apparatus. Cells actively engaged in osteoid synthesis are cuboidal, while those less active are more flattened. Numerous cytoplasmic processes connect adjacent osteoblasts on the bone surface and connect osteoblasts with osteocytes deeper in the bone. Os-teoid produced by osteoblasts is secreted into the space adjacent to the bone. Eventually, new osteoid becomes mineralized, and in the process, osteoblasts become surrounded by mineralized bone.

Bone Growth Osteoid Mineralized Bone

As osteoblasts are progressively incorporated into mineralized bone, they lose much of their bone-forming ability and become quiescent. At this point they are called osteocytes. Many of the cytoplasmic connections in the osteoblast stage are maintained into the osteocyte stage. These connections become visible channels or canaliculi that provide direct contact for osteocytes deep in bone with other osteocytes and with the bone surface. It is generally believed that these canaliculi provide a mechanism for the transfer of nutrients, hormones, and waste products between the bone surface and its interior.

Osteoclasts are cells responsible for bone resorption. They are large, multinucleated cells located on bone surfaces. Osteoclasts promote bone resorption by secreting acid and proteolytic enzymes into the space adjacent to the bone surface. Surfaces of osteoclasts facing bone are ruffled to increase their surface area and promote bone resorption. Bone resorption is a two-step process. First, osteoclasts create a local acidic environment that increases the solubility of surface bone mineral. Second, proteolytic enzymes secreted by osteoclasts degrade the organic matrix of bone.

Bone Formation and Bone Remodeling. Early in fetal development, the skeleton consists of little more than a cartilaginous model of what will later form the bony skeleton. The process of replacing this cartilaginous model with mature, mineralized bone begins in the center of the cartilage and progresses toward the two ends of what will later form the bone. As mineralization progresses, the bone increases in thickness and in length.

The epiphyseal plate is a region of growing bone of particular interest because it is here that the elongation and growth of bones occurs after birth. Histologically, the epi-physeal plate shows considerable differences between its leading and trailing edges. The leading edge consists primarily of chondrocytes, which are actively engaged in the synthesis of cartilage of the epiphyseal plate. These cells gradually become engulfed in their own cartilage and are replaced by new cells on the cartilage surface, allowing the process to continue. The cartilage gradually becomes calcified, and the embedded chondrocytes die. The calcified cartilage begins to erode, and osteoblasts migrate into the area. Osteoblasts secrete osteoid, which eventually becomes mineralized, and new mature bone is formed. In the epiphyseal plate, therefore, the continuing processes of cartilage synthesis, calcification, erosion, and osteoblast invasion result in a zone of active bone formation that moves away from the middle or center of the bone toward its end.

Chondrocytes of epiphyseal plates are controlled by hormones. Insulin-like growth factor I (IGF-I), primarily produced by the liver in response to growth hormone, serves as a primary stimulator of chondrocyte activity and, ultimately, of bone growth. Insulin and thyroid hormones provide an additional stimulus for chondrocyte activity.

Beginning a few years after puberty, the epiphyseal plates in long bones (as in the legs and arms) gradually become less responsive to hormonal stimuli and, eventually, are totally unresponsive. This phenomenon is referred to as closure of the epiphyses. In most individuals, epiphyseal closure is complete by about age 20; adult height is reached at this point, since further linear growth is impossible. Not all bones undergo closure. For example, those in the fingers, feet, skull, and jaw remain responsive, which accounts for the skeletal changes seen in acromegaly, the condition of growth hormone overproduction (see Chapter 32).

The flux of calcium and phosphate into and out of bone each day reflects a turnover of bone mineral and changes in bone structure generally referred to as remodeling. Bone remodeling occurs along most of the outer surface of the bone, making it either thinner or thicker, as required. In long bones, remodeling can also occur along the inner surface of the bone shaft, next to the marrow cavity. Remodeling is an adaptive process that allows bone to be reshaped to meet changing mechanical demands placed on the skeleton. It also allows the body to store or mobilize calcium rapidly.

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  • niina
    How calcium and phosphorus enter in bones?
    8 years ago

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