In this months exploration we look at some teeth with the help of some magnifiers.

Everyday we use our teeth to help us break up our food and grind it up to make it easier for us to digest. As children we work our way through one set of teeth until our mouth is large enough to take on the size of the final set of teeth that will hopefully last us the rest of our life. What is it that makes these teeth so specially suited for this purpose. The shape of the crown and the root of a tooth vary among different teeth and also among different species of animals. Here are a couple of different teeth from the same child. Notice how different they look.

All teeth have the same general structure and consist of three layers. An outer layer of enamel, which is the hardest tissue in the body, covers part or all of the crown of the tooth. On the left is a picture of the surface of the enamle of a tooth, magnified 1000 times. The middle layer of the tooth is composed of dentine, which is less hard than enamel and similar in composition to bone. The dentine forms the main bulk, or core, of each tooth and extends almost the entire length of the tooth. Dentine is nourished by the pulp, which is the innermost portion of the tooth. The pulp consists of cells, tiny blood vessels, and a nerve and it occupies a cavity in the centre of the tooth. The pulp canal extends almost the whole length of the tooth and communicates with the body's general nutritional system through holes at the end of the roots. On the right is a picture of a tooth that has been broken in half. Can you see the differences between the smooth outer enamel layer, the middle dentine layer and the bright region of the pulp?

Below are higher magnification scanning electron microscope images of the enamel layer from a broken tooth on the left, and from the pulp area. The pulp area has mostly dried up, so a lot of cracks are visible, just like you would see in dried mud.

Like most other mammals, humans have two successive sets of teeth during life. The first set of teeth are called primary teeth (or milk teeth) and the second set are called permanent ones. Primary teeth are different from permanent teeth because they are smaller, have more pointed cusps, are whiter and more prone to wear, and have small, delicate roots. The primary teeth begin to appear about six months after birth, and are usually all present by age 2 1/2; shedding when you begin to loose your primary teeth, begins about age 5 or 6 and is finished by age 13.

There are 20 primary teeth - four incisors, two canines, and four molars in each jaw. The primary molars are replaced in adults by the premolars, or bicuspid teeth. The 12 adult molars of the permanent teeth erupt (emerge from the gums) behind the primary teeth and do not replace any of these, giving a total of 32 teeth. These are four incisors, two canines, four premolars, and six molars in each jaw.

• Incisor teeth are the teeth at the front of the mouth. They are adapted for plucking, cutting, tearing, and holding. The biting portion of an incisor is wide and thin, making a chisel-shaped cutting edge.
  
• Next to the incisors on each side is a canine, or cuspid tooth. It frequently is pointed and rather peglike in shape and, like the incisors, has the function of cutting and tearing food.
  
• Premolars and molars have a series of elevations, or cusps, that are used for breaking up particles of food. Behind each canine are two premolars, which can both cut and grind food. Each premolar has two cusps (hence the name bicuspid). The molars, by contrast, are used exclusively for crushing and grinding. They are the teeth farthest back in the mouth. Each molar typically has four or five cusps.

The three final images below show different views of the enamel layer at high magnification. Where the tooth is broken we can see the smooth surface of the tooth on the right side of the first image, and the column-like structure of the enamel below the surface on the left side. Most of the toughness of teeth comes from the wat in which the enamel and dentine layer meet, and this is shown in the ssecond image, with the enamel on top of the dentine. The third image shows the column-like structure of the enamel at higher magnification.

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Last Update: 2/13/98