Bone, dentine and cementum
The mesodermal tissues bone, dentine, and cementum are all similar in chemical composition and may be regarded as calcified collagens as their predominating organic component is the protein collagen. Cartilage, another mesodermal tissue that undergoes calcification, also contains collagen but differs from the other three in having a much higher content of chondroitin sulfate. The mineral constituent of all these tissues occurs as a separate phase of biological apatite crystals together with amorphous calcium phosphate. The extracellular substance of bone, dentine, and cementum is thoroughly and fairly uniformly impregnated with tiny crystallites of apatite between, on the surface of, and within the collagen fibrils, wherever there are small spaces between organic components. The apatite crystals are much smaller than those of enamel. The collagen fibrils are laid down in the ground substance by specialized types of fibroblast—osteoblasts, odontoblasts, and cementoblasts—at the developing edge of the tissue.
The mesodermal tissues bone, dentine and cementum are all similar in chemical composition and may be regarded as calcified collagens, since their predominating organic component is the protein collagen. Cartilage, another mesodermal tissue which undergoes calcification, also contains collagen but differs from the other three in having a much higher content of chondroitin sulphate. The mineral constituent of all these tissues occurs as a separate phase of biological apatite crystals together with amorphous calcium phosphate.
The extracellular substance of bone, dentine and cementum is thoroughly and fairly uniformly impregnated with tiny crystallites of apatite between, on the surface of and within the collagen fibrils, wherever there are small spaces between organic components. The apatite crystals are much smaller than those of enamel (see Chapter 28).
The collagen fibrils are laid down in the ground substance by specialized types of fibroblast – osteoblasts, odontoblasts and cementoblasts – at the developing edge of the tissue. At first the fibrils are uncalcified and the newly laid down osteoid tissue or predentine is distinguished from older calcified bone or dentine by its position as well as by the absence of crystals. Soon, amorphous calcium phosphate and, shortly afterwards, hydroxyapatite make their appearance. The initial tiny amorphous clusters rapidly grow to crystals of limited size, their growth slowing down as this size is approached. The crystals also multiply in numbers, with the result that the recently formed tissue cannot be readily distinguished from the neighbouring heavily mineralized area, laid down shortly beforehand. The osteoblasts, which laid down the osteoid, become embedded in the mineralized bone, and are then known as osteocytes, which are connected to one another by cytoplasmic processes. In dentine, odontoblasts move away from the amelodentinal junction and persist as a row of cells near the junction of the predentine and pulp, leaving behind long processes penetrating the thickness of the dentine.
The skeleton provides mechanical support for the body, a rigid framework for muscle attachment and a system of levers which enable different parts of the body, e.g. limbs and jaws, to be moved by muscular contraction.
Dentine, which constitutes the main body of a tooth, also gives mechanical support by acting as a tough but elastic cushion for the enamel which covers the crown. The cementum encasing the roots holds fast the uncalcified collagen fibres of the periodontal membrane or ligament, which anchor the teeth in the jaw. The biochemical functions of these tissues are also important, the skeleton functioning throughout life as a large reservoir for calcium and phosphate ions as well as the metabolically important ions lactate and citrate. In this way bone plays a crucial role in calcium and phosphate homoeostasis.
a. A fast exchange reaction occurs at the mineral surface, where there is a small, labile reserve of inorganic ions, including about 6 g of calcium, which is loosely adsorbed on the mineral surface and so can be augmented or diminished, without affecting the structure of the bone. Studies with radioisotopes have shown that complete exchange between plasma calcium and this reserve calcium takes place within about 1 min in young animals and 5 min in older animals.
b. A much slower irreversible reaction also occurs between tissue fluid and bone, and results in the gradual laying down of stable parts of the apatite lattice. Since the plasma and tissue fluids are normally supersaturated with respect to apatite (Chapter 31), this results in a steady drift of calcium and phosphate ions from solution in the blood to the solid calcium phosphates of bone. The degree of mineralization of a given piece of bone tissue will continue to increase indefinitely by this process, but as time goes on the rate of increase slows down as the crystals approach their maximum size and fill up all the available space within the tissue.
2. The second level involves two kinds of cellular participation: resorption of existing bone by osteoclasts and deposition of new bone by osteoblasts. Over short periods these two processes, acting at different sites, approximately balance each other. Acting in conjunction, they not only continuously remodel the surface contour of the bone but also play a major part in the control of calcium and phosphorus metabolism for the whole organism. Coordination at this second level is achieved through hormonal mediation of calcium and phosphate homoeostasis which ensures that the concentrations of these ions in the plasma and tissue fluid remain constant (Chapter 30).
Before a tissue can be analysed, it must be completely separated from neighbouring tissues which would otherwise contaminate it and invalidate the results. This is difficult with calcified tissues since they are hard and cannot be cut or broken readily without generation of heat which may cause decomposition; moreover, they are frequently in close juxtaposition to other tissues. Thus in addition to bone tissue the bones contain cartilage, periosteum (an uncalcified connective tissue), and fatty or haemopoietic marrow. Compact (cortical) bone is easily isolated in a relatively pure state from a long bone, such as a femur, of a large animal like the cow.
Dentine and cementum present greater difficulty since they are very firmly joined together and the dentine is also joined to the even harder enamel. After powdering the whole tooth, dentine and enamel can be separated by making use of the considerable difference between their densities (dentine 2·14; enamel 2·95). The main disadvantage of this is that both tissues may be contaminated by ‘junction particles’ of intermediate density. Cementum (density 2·03) and dentine do not differ sufficiently in density to permit a clear-cut separation.
Another method, which permits finer control, is to cut a longitudinal section through the tooth by means of a slicing machine with a diamond-impregnated disc and then systematically to dissect the section by hand. By using this technique it is possible to investigate how a given consituent varies in amount in differing areas of dentine.
Carefully ‘purified’ samples of mineralized tissue have been analysed in attempts to account for the whole of their weight as known substances. In this way, chemical ‘balance sheets’ can be drawn up for particular tissues. Tables 29.1 and 29.2 show the results of analyses of purified cow bone and human dentine, respectively. Information regarding the composition of cementum is limited but suggests that it closely resembles bone.