Teeth

I

Introduction

Teeth, hard, calcified structures, attached to the upper and lower jaws of vertebrates and of a few lower animals, and used primarily for mastication. In some animals teeth serve other functions, such as gnawing, digging, and fighting. In the course of evolution various forms of teeth have developed, from the simple, scalelike rows of cone-shaped teeth found in sharks to the more complex structures common in mammals.

II

Human Teeth

Human teeth also serve major functions other than chewing. The teeth are directly involved in the process of speech, acting as a brace against which the tongue presses in the formation of certain sounds. The teeth also affect the dimensions and the expression of the face, the appearance of which may be unpleasantly modified by the loss of neighbouring teeth or by any irregularity in tooth growth or colouring.

A

Tooth Structure

Human teeth consist of an external portion, called the crown, and a root that is embedded within the jaw. The outer layer of the crown is composed of calcified tissue known as enamel—the hardest substance in the body. Inside the enamel is the dentine, a bonelike substance extending from the inner surface of the enamel into the jaw to form the root. Covering the dentine of the root is a thin layer of a hard tissue called cementum. The roots are held in place by elastic fibres that constitute the periodontal membrane, which extends from the cementum to a thickened layer of bone, known as the lamina dura, within the jaw.

The dentine of the crown encloses the pulpal chamber, which penetrates into the root as the root canal. Passing through the root-canal opening at the end of the root are blood vessels, nerves, and connective tissue, which fill the root canal and pulpal chamber.

B

Embryological Development

The development of the tooth bud in the human embryo begins in the second month after conception. The tooth bud is composed of outer, or ectodermal, and inner, or mesodermal, tissues. The ectoderm calcifies into enamel rods that develop to cover the crown. Following the depositing of enamel, the mesoderm differentiates into the dentinal portion of the crown and the pulpal chamber. As the embryo develops, continuing calcification results in the formation of the root and a wide root canal, through which blood vessels, nerves, and connective-tissue elements extend into the pulpal chamber. As the tooth crown erupts and the root lengthens, the pupal chamber and canal become more constricted because of the continuous production of dentine by special cells within the pulp. The crown is pushed through the gum by an eruptive force as the tooth continues to develop.