2. The Human Dentition

Teeth are highly mineralised structures formed within the alveolar process of the upper and lower jaws (the maxilla and mandible), and are held firmly in place by means of periodontal ligament fibres (periodontium).

The human dentition follows the standard mammalian pattern, consisting of two sets of teeth. The primary - or 'deciduous' - dentition (the so-called 'milk teeth') consists of 20 elements belonging to three morphological classes: incisors (8), canines (4), and molars (8). In modern humans, eruption of the deciduous dentition starts at about the age of 6 months, and ends at about the age of 3 years.

The secondary - or 'permanent' - dentition consists of 32 teeth belonging to four morphological classes: incisors (8), canines (4), premolars (8), and molars (12). Eruption starts at about 6-7 years of age, and ends at about 11-13 years of age, with the exception of the third permanent molar, which erupts between the age of 17-24 years (Hillson, 1986).

For both sets of dentition, there is a considerable range of variation among living humans with respect to the timing of eruption (for estimates in 'early Homo', see Smith, 1993).

Starting around 6 years of age, the secondary dentition begins to replace the primary teeth. Twenty of the permanent teeth develop under their deciduous counterparts, and after the deciduous roots have been resorbed, the permanent teeth begin to erupt. The permanent molars, which develop behind the deciduous dentition, cannot erupt until the jaws have grown 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 because they have no predecessors. Usually, the first permanent tooth to erupt is a lower, or mandibular, M1. During the period between its eruption and the loss of the last primary tooth (usually an m2, making way for the permanent P4), the dentition is mixed.

Teeth are also classified according to their position in the dental arch. Incisors are classified as anterior, whereas premolars and molars are classified as posterior. The canines belong to both groups due to their position and shape.

The gross morphological structure of a tooth displays two main elements: the crown and the root (or roots), which are joined/separated by a constricted transitional portion, the neck. The crown is usually the only part of the tooth that is actually visible in the mouth. It is coated by a layer of hard, largely inorganic, tissue - the enamel - while the root is coated with a layer of bone-like material - the cementum.

Each tooth crown has five surfaces. The buccal surface faces the cheek and the lingual one is adjacent to the tongue. The mesial surface is towards the front of the mouth (anterior) and the distal surface is towards the back (posterior). The occlusal surface of the crown is the plane on which the upper and lower dentition come into contact, and where chewing and tearing occurs.

Form and function are strongly correlated in the teeth. The structure of the anterior teeth is 'designed' for tearing, while the multiple cusps of the posterior teeth are well adapted for chewing. Incisors have a single root, and a sharp occlusal surface for cutting. Canines are single cusped teeth with a relatively long root. Both incisors and canines can possess a cingulum, which is a ridge of enamel found on the lingual aspect of the crown near the cervical margin (Harty and Ogston, 1992). Premolars normally have two cusps and one or two roots of similar size, which are constricted mesiodistally. Molars usually have 3-5 cusps and 2-3 roots (in modern human populations there is a certain degree of morphological variability, see Taylor, 1978 and Turner et al., 1991 for further discussion).



Underlying the surface layers of enamel on the crown and cementum on the root there is a very tough resilient tissue, the dentine, the function of which is to protect the pulp chamber. This chamber contains the nerves, blood vessels, and fibrous connective tissues that are connected 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 provide the tooth with nourishment throughout the life of the tooth (Moss-Salentijn and Hendricks-Klyvert, 1985; Schroeder, 1991).

Enamel is a highly calcified tissue of ectodermal origin, made up of millions of tightly packed calcified prisms, each extending from the inner to outer surface of the dentine. Deposition of the enamel matrix occurs through the activity of ameloblasts ("ectodermally derived cells primarily responsible for the formation of enamel", Harty and Ogston, 1992: 9). The enamel formation process, or amelogenesis, starts from the tip of the cusp towards the neck, and moves outwards from the dentino-enamel junction until the full thickness of enamel over dentine is complete.

Dentine is also a calcified tissue and is formed through the process of dentinogenesis. Although dentine is much harder than bone tissue, it has only about one-fifth the hardness of enamel, and thus is a much more elastic tissue. It is composed of many S-shaped tubules (the dental canaliculi) which are formed by odontoblasts. Thus, when forces are applied to the tooth (e.g., chewing) the dentine provides considerable support for the much more brittle enamel, and makes it more resistant to breakage because of this cushioning effect. Unlike amelogenesis, dentinogenesis is a continuous process and moves towards the pulp cavity, which decreases in size over time.

Cementum is a mineralised connective tissue that anchors the collagen fibre bundles of the periodontal ligament to the surface of the root, 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), and some forms of cellular cementum are structurally very similar to bone. Part of the cementum is formed during root development prior to the eruption of the tooth, while other portions form progressively during and after eruption. As long as functioning periodontal ligament is present, the cementum continues to deposit layers (annulations) in a roughly annual rhythm (see Stott et al., 1982; Charles et al., 1986; Condon et al., 1986; Geusa et al., 1996b).

In human embryos, odontogenesis (the formation of the dental tissues) begins approximately 28 to 40 days after conception (Schroeder, 1991) and continues until about 18 to 25 years of age (Aiello and Dean, 1990).