The outer surface of the tooth root is covered with mineralized connective tissue, which is known as the cementum (substantia ossea dentis) and extends from the margin of the dental crown to the apex of the root (Fig 3-13). The cementum is very thin around the neck of the tooth (approximately 0.015 mm) and slightly thicker at the apex of the root (approximately 0.4 mm). Cementum is similar to bone tissue in structure and composition.
Dentin is the largest mass in the tooth and is formed and later supplied by the odontoblasts. In the bell stage of tooth germs, the odontoblasts differentiate from the cells of the dental papilla. These cells come from the embryonic connective tissue and the neural crest. The differentiation into odontoblasts can be seen from the increase in cell volume and the cell organelles contained therein. The initially star-shaped (stellate) cells become narrow and columnar, and a thick cytoplasmic process develops, which protrudes from the outer end of the cell. The odontoblasts thus formed lose their ability to divide and become highly specialized secretory cells.
The enamel and cementum layer together surround the dentin core, which makes up the principal mass of the tooth. Dentin (dentinum, substantia eburnea) forms a layer up to 3 mm thick. This layer encloses a hollow cavity in the tooth, the pulp cavity (cavum dentis; cavitas dentis; ca-vum pulpae), and the
root canal (cavum radicis dentis). The dental pulp (pulpa dentis) lies inside this cavity.
Dental enamel is a complex product of differentiated cell activities of ameloblasts (Fig 3-7). It arises during three processes taking place in the cells simultaneously, namely the formation of enamel matrix, mineralization of the matrix, and maturation of the crystalline structure. Enamel is the mature crystalline framework of prisms (Fig 3-8) and the hardest substance in the human body.
The crown of the tooth is covered with a very hard layer of enamel. The enamel is thinnest at the neck of the tooth and increases in thickness up to the incisal or occlusal surface of the tooth. On the occlusal surface, the enamel layer is up to 2 mm thick.
The individual tooth is composed of five different tissue layers, which can be seen without magnification in a cross section of a tooth (Figs 3-1 and 3-2). These tissues can be divided into soft and hard substances:
Cartilage tissue, like bone tissue, is classified as intercellular substance-rich connective and supporting tissue. Supporting tissues give the body its form because of their strength; they surround important organs and create stable protective spaces. The intercellular substance in supporting tissue is formed not by fibers but by the ground substance. This ground substance comprises 75% water, 5% protein, 15% collagenous substance, and 5% mucopolysaccharides as a strengthening material (put simply, these are macromolecules of amino sugars and one sulfuric acid residue). The cartilage-forming cells are called chondroblasts.
Bony substance comprises two differentiated structures: the outer compact cortical layer and the inner spongy material (lamellar or cancellous bone).
The basic substance of bone tissue is two-thirds inorganic mineral salts and one-third organic substances (living cells). The collagenous connective tissue fibers are responsible for the high bending strength of bone, while the mineral salts create the enormous compressive strength (15 kPa/mm2). The mineral salts are mainly calcium phosphate in the form of hydroxyapatite crystals. Apatites are crystallized molecules that were once easily confused with other crystals, hence the name (apatan = they deceive).
This tissue group includes many different cell complexes, which, apart from their histologic origin, still have a few similarities of form that relate to the main mechanical function of that tissue. For instance, it is noticeable in connective tissue cells that the intercellular substance (ie, connecting substance between the cells) is highly developed.