Development, Form, and Eruption

DEVELOPMENT AND FORM

During the sixth week of fetal life (7 or 8 months before birth), tiny tooth buds, sometimes called tooth germs, begin to grow within the alveolar process of the fetus. Tooth germs are small clumps of cells that have the ability to form the tooth tissues (dentin, enamel, cementum, and pulp). Both the primary and secondary teeth develop from these tooth germs, which later are located within cavities of the alveolar process called crypts (see Chapter 19 and Figs. 19-1 to 19-4)

At this time the dentin and enamel begin to form, followed later in development by the cementum. The type of dentin formed at this early stage is called primary dentin, and it occurs before root completion. Secondary dentin is continually formed within the tooth by the same odontoblasts that form regular dentin. This process continues throughout one’s entire lifetime. Secondary dentin differs from reparative dentin in that reparative dentin is laid down locally as protection for the pulp from irritation, caries, or trauma (see Chapter 20).

The earliest evidence of tooth formation occurs during the sixth week of embryonic life. At this time the dental lamina begins to form (see Chapter 19).

The primary teeth begin to calcify by about the fourth or fifth month of fetal life. The process of calcification is the hardening of the tooth tissues by the deposition of mineral salts within these tissues (Fig. 5-1). This process continues until about the third or fourth year after birth, when the deciduous roots become fully formed.

FIG. 5-1 Beginning of tooth calcification. The small hole in the bone in which a tooth bud germ forms is called a crypt; it later becomes a tooth socket, which houses the root of the tooth. (*Modified from Massler M, Schour I:* Atlas of the mouth in health and disease, *ed 2, Chicago, 1975, American Dental Association*.)

Soon after birth the permanent teeth begin to calcify and continue until about the twenty-fifth year, when the roots of the third molars become calcified. The last area of the tooth to become calcified is the apex of the root (see Chapter 21.)

Developmental Lobes

Each tooth begins to develop from four or more growth centers. These centers grow out from the tooth germ and are known as developmental lobes. The lobes grow and develop within their bony crypt until they fuse. This fusion of the lobes is called coalescence. The junction that forms the union of these lobes is marked by lines on the tooth called developmental grooves, which can be seen on the tooth after it has erupted.

The number of developmental lobes necessary for the formation of a tooth depends on the particular tooth and how many cusps it has. For instance, all the anterior teeth develop from four lobes—three labial and one lingual. The three labial lobes form the labial surface of each tooth. The mamelons, which are evident after the eruption of incisor teeth, are the incisal ridges of these three labial developmental lobes, and they also are separated by developmental grooves (see Fig. 2-22).

In Fig. 5-2 notice how the three labial lobes fuse to form the entire labial surface. The only evidence that three separate lobes existed is at the incisal ridge, where the mamelons are distinct and separate, and on the labial surface of the tooth, where developmental lines or grooves are evident. The three labial developmental lobes are called the mesiofacial, centrofacial, and distofacial lobes. The sole lingual lobe is appropriately called the lingual lobe and makes up the entire cingulum on the lingual surface of the tooth.