Bone and Its Relation to Extracellular Calcium and Phosphate

Bone and Its Relation to Extracellular Calcium and Phosphate

Bone is composed of a tough organic matrix that is greatly strengthened by deposits of calcium salts. Average compact bone contains by weight about 30 per cent matrix and 70 per cent salts. Newly formedbone may have a considerably higher percentage ofmatrix in relation to salts.

Organic Matrix of Bone. The organic matrix of bone is 90to 95 per cent collagen fibers, and the remainder is a homogeneous gelatinous medium called ground sub-stance. The collagen fibers extend primarily along thelines of tensional force and give bone its powerful tensile strength.

The ground substance is composed of extracellular fluid plus proteoglycans, especially chondroitin sulfate and hyaluronic acid. The precise function of each of these is not known, although they do help to control the deposition of calcium salts.

Bone Salts. The crystalline salts deposited in theorganic matrix of bone are composed principally of calcium and phosphate. The formula for the majorcrystalline salt, known as hydroxyapatite, is the following:

Ca₁₀ (PO₄)₆(OH)₂

Each crystal—about 400 angstroms long, 10 to 30 angstroms thick, and 100 angstroms wide—is shaped like a long, flat plate. The relative ratio of calcium to phosphorus can vary markedly under different nutri-tional conditions, the Ca/P ratio on a weight basis varying between 1.3 and 2.0.

Magnesium, sodium, potassium, and carbonate ionsare also present among the bone salts, although x-ray diffraction studies fail to show definite crystals formed by them. Therefore, they are believed to be conjugated to the hydroxyapatite crystals rather than organized into distinct crystals of their own. This ability of many types of ions to conjugate to bone crystals extends to many ions normally foreign to bone, such as strontium, uranium, plutonium, the other transuranic elements, lead, gold, other heavy metals, and at least 9 of 14 of the major radioactive products released by explosion of the hydrogen bomb. Deposition of radioactive sub-stances in the bone can cause prolonged irradiation of the bone tissues, and if a sufficient amount is deposited, an osteogenic sarcoma (bone cancer) even-tually develops in most cases.

Tensile and Compressional Strength of Bone. Each collagenfiber of compact bone is composed of repeating peri-odic segments every 640 angstroms along its length; hydroxyapatite crystals lie adjacent to each segment of the fiber, bound tightly to it.This intimate bonding pre-vents “shear” in the bone; that is, it prevents the crys-tals and collagen fibers from slipping out of place, which is essential in providing strength to the bone. In addition, the segments of adjacent collagen fibers overlap one another, also causing hydroxyapatite crys-tals to be overlapped like bricks keyed to one another in a brick wall.

The collagen fibers of bone, like those of tendons, have great tensile strength, whereas the calcium salts have great compressional strength. These combined properties plus the degree of bondage between the collagen fibers and the crystals provide a bony struc-ture that has both extreme tensile strength and extreme compressional strength.