Hydroxyapatite represents a mineral of calcium hydroxyl phosphate. Overall, the mineral is not widely distributed, although there are individual abundant deposits. Vertebrate bones and teeth are also composed of a high percentage of hydroxyapatite.
What is hydroxyapatite?
Hydroxyapatite is composed of hydroxylated calcium phosphate. In the crystal, five calcium ions are associated with three phosphate ions and one hydroxyl ion. It is an ionic compound that crystallizes in a hexagonal crystal system. Thereby the hydroxyl group stabilizes the whole crystal. With fluorapatite and chlorapatite hydroxylapatite forms a gapless mixed series. Hydroxylapatite occurs in various minerals such as serpentinite, talc shale or pegmatite as an accompanying mineral. The mineral has been detected at about 250 locations so far. The appearance of the individual minerals depends on the composition and the mixing ratio with other accompanying minerals. Hydroxyapatite also occurs within living organisms. Especially the bones and teeth of vertebrates consist of a high percentage of this mineral. In addition to hydroxyapatite, they also contain organic material in the form of connective tissue and cells. Due to its almost pure mineral content, tooth enamel is the hardest material in the organism. Thus, its hydroxyapatite content is over 95 percent. Hydroxyapatite formation takes place during biomineralization. The material is very stable and extremely resistant to physical and chemical influences. Thus, bones and teeth represent an important archive of the living environment. Only acids, including fruit acids, slowly decompose hydroxyapatite.
Function, effect and tasks
In the human organism, hydroxyapatite is the most important supporting substance. It provides the skeletal system with the necessary strength. Together with special connective tissue material such as collagen, for example, the necessary tensile strength and stability is produced in bones. The composition of bones and teeth is different. The decisive factor here is the proportion of hydroxyapatite. Bones consist of about 65 percent of the mineral. The rest is made up of collagen and osteoblasts. The proportion of hydroxyapatite in teeth is much higher. Therefore, teeth are also much harder than bones. The determining factor for the composition is the function. Bones are part of the locomotor system. Their different loading by mechanical forces requires a certain flexibility. The teeth serve to grind food. This requires a much greater force and strength, which must also be reflected in a harder material. In this process, the teeth consist of the outer enamel, the dentin and the dental pulp. The enamel must be very strong and hard and is accordingly made up of over 95 percent hydroxyapatite. This makes it extremely resistant to external influences. Dentin, in turn, is a bone-like substance. It consists of 70 percent hydroxyapatite. The rest is mostly connective tissue. The dental pulp, or pulp, provides a conduit network of blood vessels and nerves to supply the tooth.
Formation, occurrence, properties and optimum values
The hydroxyapatite of bone and teeth is formed as part of biomineralization. Biomineralization is already an ancient process in the history of the earth. Ancient bacteria also formed limestones several billion years ago. The process is still similar today. Certain cells absorb the ions of the mineral in a dissolved state. Mineralization occurs by saturation of the solution with the appropriate ions. In the case of hydroxyapatite, these are calcium and phosphate ions. In the case of bones, the so-called osteoblasts are responsible for mineralization. During mineralization, they develop into osteocytes that are no longer able to divide and form a network within the solidified mineral. In a similar way, biomineralization also takes place in the teeth. Here, odontoblasts are responsible for mineralization.
Diseases and disorders
Hydroxyapatite is indeed very durable. But buildup and breakdown processes are constantly taking place within the bones. The bone shape must adapt to very different requirements. Thus, new bone substance is constantly being built up. However, there is also always a breakdown of bone substance. If the degradation process predominates, so-called osteoporosis develops. The processes are hormonally controlled.Thus, the parathyroid hormone is responsible for the balanced calcium level in the blood. In the case of calcium deficiency, it activates the mobilization of hydroxyapatite from the bones. The hormone calcitriol is responsible for calcium absorption from food in the intestine and mineralization in the bones. Both hormones are antagonists. If calcium absorption from food is disturbed because little calcitriol is produced due to a lack of vitamin D, bone resorption predominates over bone formation. Bone density decreases and bone fragility increases at the same time. However, these processes are very complicated and in many cases not yet fully understood. Hydroxyapatite can also be degraded in teeth. However, this is not a hormonal process. Physiologically, the tooth should last as long as possible in order to be able to grind the food. However, bacterial decomposition of food debris forms acids that can attack tooth enamel. The acid dissolves hydroxyapatite into calcium ions and phosphate ions, and the hydroxyl ion reacts with a hydrogen ion of the acid to form water. Calcium ions and phosphate ions then dissolve in water. Prolonged bacterial activity and constant acid formation eventually causes a hole to form in the tooth enamel. Without treatment, tooth decay leads to the destruction of the tooth. However, by using fluoride-containing toothpaste, hydroxyapatite can be transformed into the much more stable fluoroapatite. This makes it possible to stop the destruction process of teeth for a longer time.
