Teeth are highly mineralised structures retained in the alveoli - sockets situated in the alveolar process of the jaws (maxilla and mandible) - by means of periodontal ligament fibres (periodontium).
Like all mammals, humans are provided with two sets of teeth: the primary - or 'deciduous' - dentition (the so-called 'milk teeth') and the secondary - or 'permanent' - dentition (Hillson, 1986).
The primary dentition consists of 20 elements belonging to three morphological classes: incisors (8), canines (4), and molars (8). In modern humans, dental eruption starts at about the age of 6 months, and ends at about the age of 3 years.
The secondary dentition consists of 32 teeth belonging to four morphological classes: incisors (8), canines (4), premolars (8), and molars (12). Eruption starts at about the age of 6-7 years, ending at about 11-13 years (with the exception of the third permanent molar, which erupts at about 17-24 years).
For both the primary and secondary dentition, a considerable range of variation exists among living humans in the timing of eruption (for estimates in 'early Homo', see Smith, 1993).
From the age of 6 years, after resorption of the deciduous roots, the secondary dentition begins to replace the primary teeth. Twenty of the permanent teeth develop under their deciduous counterparts. The permanent molars, located behind the deciduous dentition, cannot erupt until jaws are large enough to accommodate them. Thus, the permanent dentition consists of 20 successional teeth (incisors, canines and premolars) and 12 accessional teeth (molars). From a biogenetic point of view, the accessional teeth belong to the primary dentition (having no predecessors). During the period between the eruption of the first permanent tooth (usually a lower M1) and the loss of the last deciduous tooth (usually an m2, making way for the permanent P4), the dentition is mixed.
With respect to their arrangement within the dental arch, teeth are classified as anterior (incisors), and posterior (premolars and molars). Due to their position and shape, the canines belong to both groups.
The external appearance of each tooth presents two main elements: the crown and the root (or roots), which are linked/separated by a constricted transitional portion, the neck.
Each tooth presents a buccal surface facing towards the cheek, a lingual one adjacent to the tongue, a mesial (anterior), and a distal (posterior) surface. The chewing/tearing surface of the crown coincides with the occlusal plane.
Shape and function are strongly correlated in the teeth. The anterior teeth are 'designed' for tearing, while the posterior multicuspal teeth are shaped for chewing. Incisors are single rooted teeth, with a cutting edge. Canines are single cusped teeth with a relatively long root and a cingulum ('a bulge or ridge found on the palatal or lingual aspects of incisor and canine teeth, near to their cervical margins', Harty and Ogston. 1992: 47). Premolars normally have two cusps and one or two roots of similar size, which are flattened mesiodistally. Molars commonly have 3-5 cusps and 2-3 roots (for morphological variation in modern human populations, see Taylor, 1978; Turner et al., 1991).
The crown is normally the only part of the tooth actually protruding into the mouth. It is coated by a layer of hard crystalline tissue - the enamel - while the root is coated with a layer of bone-like material - the cementum.
Underlying these surface layers there is a very tough resilient tissue, the dentine, protecting the pulp chamber. This chamber contains the nerves, blood vessels, and fibrous connective tissues that communicate with nerves and vessels in the jaws through a foramen located at the base of the root. The function of the pulp tissue is to feed the tooth for its entire life (Moss-Salentijn and Hendricks-Klyvert, 1985; Schroeder, 1991).
Enamel is a highly calcified tissue of ectodermal origin. It is composed of millions of closely packed calcified prisms , each extending from the inner to outer surface of the dentine. Deposition of the enamel matrix by the ameloblasts ('ectodermally derived cells primarily responsible for the formation of enamel', Harty and Ogston, 1992: 9) starts from the tip of the cusp towards the neck, and outwards from the dentino-enamel junction until the full thickness of enamel over dentine is complete. The enamel formation process is called amelogenesis.
Dentine is also a calcified tissue , but even though it is harder than bone tissue it has only about 1/5 the hardness of enamel. It is composed of a huge number of S-shaped tubules (the dental canaliculi) which are formed by odontoblasts. Its elastic properties (i.e., the ability to deform when forces are applied) support the much more brittle enamel, and make it more difficult to break because of this cushioning effect. Unlike amelogenesis, dentinogenesis continues throughout the lifetime of the tooth and proceeds towards the pulp cavity which decreases progressively in size.
Cementum is a non-homogeneous connective mineralised tissue and some forms of cellular cementum are closely related to bone. It anchors the collagen fibre bundles of the periodontal ligament to the root surface, and also carries out adaptive and reparative processes. Several types of cells derived from the ectomesenchyme are responsible for its development (cementoblasts, cementocytes, and fibroblasts). A portion of cementum is formed pre-eruptively during root development, while other portions form progressively during and after tooth eruption. As long as functioning periodontal ligament is present, the cementum continues deposition of different layers (annulations) in a roughly annual rhythm (see Stott et al., 1982; Charles et al., 1986; Condon et al., 1986; Geusa et al., 1996).
In human embryos, odontogenesis begins approximately 28 to 40 days after fertilisation (Schroeder, 1991) and goes on until about 18 to 25 years of age (Aiello and Dean, 1990). The deciduous tooth germs start development from the beginning of the 8th week and continue until the 17th week after fertilisation. During this period the tooth germs develop from initial cell clusters through three stages: the bud, cap, and bell stages. The bud and cap stages correspond to morphogenesis, while dental histodifferentiation takes place during the bell stage.
At the same time the epithelial structure - the enamel organ - is developing, together with the dental papilla and the dental sac. All these structures are components of the tooth germ.
The bud stage is reached by the germs of the anterior teeth at the end of the 7th week after fertilisation, by those of the first molars in the 8th week, and by those of the second molars in the 10th week. The tooth bud consists of a vestibularly-directed outgrowth and a thickening of the epithelial cell cluster.
The cap stage is reached between the 8th and 12th week after fertilisation. During this stage, the spatial relationship of the enamel organ changes with respect to the dental lamina. The cells of the enamel organ begin to simultaneously arrange themselves into structurally differentiated layers.
The anterior teeth are the first to reach the bell stage (12 to 16 weeks after fertilisation), while the deciduous molars reach the bell stage between the 15th and the 21st week. During this time the enamel organ assumes a shape appropriate to the various crown forms (morphological classes).
Lacking deciduous predecessors, the three molars of the secondary dentition arise posterior to the deciduous dentition, starting with M1 at about the 13th week after fertilisation. They originate from a distal extension of the deciduous dental lamina.
The successional teeth arise lingually to the deciduous teeth. Their germs grow and differentiate just like those of the deciduous teeth and the permanent molars, but their development extends over a broader span of time. This process begins during the 5th month after fertilisation (formation of buds for central incisors), and ends at the age of 2-3 years (beginning of formation of dental hard substance in P4).
