The teeth have the most important role and the hardest structure in human's jaws and many other spinescent animals. The main function of the teeth is cutting and grinding the food pieces. Further more, they help us in speaking properly.
Two kinds of teeth grow during every human's life cycle:
1-primary teeth.
2-permanent teeth .
The number of primary teeth is 20 which are distributed equally in the two jaws.
After a while, the 32 permanent teeth are replaced the primary ones gradually.
As the primary teeth are the base of the permanent teeth, having good oral hygiene is necessary.
Around the 8th week after conception, oval-shaped tooth buds start to form and harden in the embryo's mouth. Although the baby teeth aren't visible when a child is born, both the primary and permanent teeth are partially formed below the gums. The tooth crown develops first and after it is fully grown, the root begins to develop.
Then between about six months and 1 year old, the deciduous teeth begin to push through the gums. This process is called baby teeth eruption . A child gets his first complete set of primary teeth by the age of 3. The baby teeth chart shows the average age that each primary tooth is expected to erupt. But not all baby teeth follow these rules. Some infants may have their first primary tooth (usually one of the bottom middle teeth) erupted as early as 3 months.Others may reach their first birthday or more without getting their first deciduous tooth. If there is no sign of teething by this age you should ask for a pediatrician's advise. That kind of late development of primary teeth is more common in premature babies. In some rare cases (1 every 2.000) infants are born with one or more teeth (called natal teeth) or may have teeth emerge within the first month (called neonatal teeth). A dentist should evaluate the teeth's condition, as they could be loose, interfere with feeding or irritate the baby's tongue.
2- permanent (adult) teeth:
Adult teeth or permanent teeth replace the primary teeth. They aid in digestion, speech and general appearance. There are 32 permanent teeth, with the main difference from primary teeth is that there are 4 premolars and 6 molars in each arch. Permanent teeth begin erupting at 6yrs of age and end at 21yrs of age. Some people may not have some if not all their 3rd molars due to evolutionary changes leading to the lack of development of these teeth. 3rd molars may not erupt due to becoming embedded in jaw. The tooth consists of a crown and root, and the tooth is composed of different materials that aid in strengthening, preserving and maintaining its function.
Adult teeth begin to replace the front milk teeth from the age of about six. For the next six to eight years there is a gradual replacement of milk teeth by adult teeth. This stage is called mixed dentition, as both milk and adult teeth will be from the mouth at the same time. By the age of about 12-14 all adult teeth should have appeared using the exception of wisdom teeth (third molars). At this stage the mouth will contain twenty-eight teeth. These teeth have to last for life as we only get one set.
The permanent tooth begins to grow under the baby tooth.
- The root of the baby tooth begins to dissolve (reabsorb), and the baby tooth becomes loose.
- It takes three and a half years for the baby tooth to lose its roots, and for it to be replaced by the permanent tooth.
- The crown of the permanent tooth is the first part of the tooth to develop.
- It grows into the space left by the root of the baby tooth.
- The root of the permanent tooth then continues to form.
- When the crown erupts, it is covered by a cuticle or covering that protects the enamel. The cuticle is slowly worn away by chewing and tooth brushing.
During 7 years, 20 baby teeth are lost and are replaced by 28 permanent teeth.
The mixed dentition stage is the period when some baby and permanent teeth are visible in the mouth at the same time. 
In addition, 4 wisdom teeth will erupt by the age of 21 years, taking the total number of permanent teeth to 32.
The eruption of the 32 permanent teeth starts at age 6, and is completed between the ages of 20 and 23, after the wisdom teeth (3rd molars) have arrived.
The final four adult teeth should come through during late teens totaling 32 adult teeth. The descriptions below explain the functions of each of the teeth. To view a model showing all the milk and adult teeth erupting, take a look at the timeline showing the lifetime of teeth.
third molars often are not needed for chewing and are difficult to keep clean. Your dentist may recommend their removal to prevent potential complications when third molars are erupted partially or are impacted. The chart and photograph identify the names of the permanent teeth and provide the approximate ages at which you can expect the teeth to erupt. Heredity and other factors may influence the approximate ages at which children’s primary teeth shed and their permanent teeth emerge. Thorough brushing and flossing help remove food particles and plaque (a sticky film of bacteria) from the smooth surfaces of teeth. But toothbrush bristles cannot reach into the pits and fissures (depressions and grooves) of the chewing surfaces to remove food and plaque. Dental sealants protect these vulnerable areas by sealing out debris and plaque bacteria. A sealant is a plastic material that usually is applied to the
chewing surfaces of the back teeth—premolars and molars—where decay occurs most often. The plastic resin bonds into the pits and fissures of the chewing surfaces of back teeth. The sealant acts as a barrier, protecting enamel from plaque and acids.
chewing surfaces of the back teeth—premolars and molars—where decay occurs most often. The plastic resin bonds into the pits and fissures of the chewing surfaces of back teeth. The sealant acts as a barrier, protecting enamel from plaque and acids.
Eruption Sequence of Permanent Dentition:
- Central incisors= 6-8 yrs
- Lateral incisors= 6.5-8.5yrs
- Canine = 9-13 yrs
- 1st Premolars= 9.5-11.5yrs
- 2nd Premolars= 10-13yrs
- 1st Molars =5.5-7yrs
- 2nd Molars= 11-13yrs
- 3rd Molars= 17-21yrs
The 32 fit into the jaw space for 20 baby teeth
The bones of the face grow during the period of tooth development.
- The jawbone normally grows in size to accommodate the extra teeth.
- The jaws grow at a faster rate than the rest of the face, and eventually measure up to 1/3 of the size of the face. This is a dramatic increase from the proportion at birth.
- If the jaws do not grow sufficiently, crowding or impacted teeth will result.
Permanent incisors: These are the front teeth, and there are 8 of them altogether (four at the top and four at the bottom). They are usually the first permanent teeth to come through, and are good for cutting into food (for example biting into an apple). You can tell which are the incisors by their flat edge at the top, and because they each have only one root.
Permanent canines: These front teeth are also known as 'eye teeth', and are more pointed in shape than the incisors, but like the incisors they each have a single root. There are 4 altogether, and in animals such as dogs and cats the canines are much longer, and are used for tearing off chunks of food. In humans they are smaller in size, and are good for cutting into food similar to incisors.
Premolars: These are the new, adult back teeth which replace the first and second deciduous molars. There are eight altogether, four first premolars and four second ones. Deciduous teeth don't have premolars as back teeth, just molars.
Permanent molars: These are the larger back teeth, which are good for crushing food into smaller pieces before being swallowed. This is known as chewing. Molars usually have more than one root each, and have irregular, or 'bumpy' surfaces with grooves called 'fissures'. There are first, second and third permanent molars, which come through in that order.
First permanent molars: The four first permanent molars are similar in size to the second molars and come through at around age 6 to 8 years.
Second permanent molars: The four second permanent molars are similar in size to the first permanent molars and come through at around age 12 years.
Third permanent molars: These are the very last of the adult teeth to appear between age 18 and 21 years, and are also called 'wisdom teeth'. There are four, and they come through at the very back and sometimes cause problems if there isn't enough room for them.
and protecting the jaw and teeth roots from infections. Known informally as the gums, the gingiva are a very important part of the oral anatomy, and caring for them is critical to maintaining oral health. Problems with the gingiva can be identified during routine oral examinations, or noted by patients who experience changes in their mouths.
gingivitis. Classic symptoms of gingivitis can include changes in the color of the gingiva, along with swelling and bleeding. Patients may find that their gums are very tender after brushing their teeth, or that the gums bleed freely after oral care or eating. Gingivitis can lead to complications which include serious infections, and it is an issue which needs to be addressed.
it is really just a continuation of the connective tissue associated with the gingivo-dental fibers. In a healthy situation, there is never a direct attachment between the bone and the tooth itself. Such a direct attachment, when it occurs in pathological situations, is called ankylosis.
supported it will eventually be reabsorbed by the body. The part of the alveolar bone that lines the socket is a thin layer of dense cortical bone called the lamina dura. The bone that underlies the lamina dura is cancellous bone (sometimes called medulary bone). Cancellous bone looks spongy and contains blood producing "organ" called bone marrow. In fact, all three of the features discussed in this section, the lamina dura, the periodontal ligament and the cancellous bone can be seen on any intraoral dental x-ray. In the x-ray seen on the left, follow the edge of any of the three teeth present from the top of the crown down into the bone. The dark line that separates the tooth from the bone represents the space where the periodontal ligament lives. The thin bright strip of bone directly beside the periodontal ligament space is the lamina dura. Under the lamina dura is the less bright cancellous bone. If you look carefully you can see the trabeculii --the tiny spicules of bone crisscrossing the cancellous bone that make it look spongy. These trabeculii separate the cancellous bone into tiny compartments which contain the blood producing marrow. These marrow spaces are seen in the colored image of the PDL above as bright "blobs".
jaw and houses the nerves and blood vessels that carry nutrients to the tooth. A tough, yellowish bone-like tissue, cementum, covers the root and helps hold the tooth in its socket. The cementum contains the periodontal membrane and is in contact with the gum tissue that separates the tooth from the underlying bone. Fibers of the periodontal membrane are embedded in the cementum. The root end openings allow passage of the neural and circulatory elements out of the tooth. Each human tooth has from one to four root canals that carry veins, arteries, lymph vessels, and nerves to the living pulp of the tooth.
A "root canal" is actually only a part of the dental pulp. It has all the same characteristics and functions as the rest of the dental pulp, except that it is located inside the root portion of the tooth and is thus rather thin and spindly. When we tell a patient that they need a "root canal", we are not talking about the anatomical structure itself. We are talking about a procedure. We really mean that the nerve is sick and must be removed in its entirety from the tooth, the empty space where it used to live cleaned and sterilized and finally sealed with a form of rubber called gutta percha or one of the newer materials designed for this purpose. The technical name for this procedure is "endodontic treatment". The root canal(s) in any given tooth start out just like the rest of the nerve, as a solid piece of soft tissue. Blood vessels and nerves enter through a hole at the tip of each root. The tip of the root is called the "apex", and the hole that allows the nerve tissue, with its accompanying blood vessels to enter the tooth is called the apical foramen. Of course, blood must traverse through the root canals in order to infuse the nerve. As we age, the root canals too replace themselves with more and more dentin until they become less tube like and more like a network of blood vessels and nerves running down approximately the center of the root. The image to the right shows some of the complex anatomy that the dentist is presented with when he must perform a root canal procedure to relieve pain and infection. In fact, the nerve anatomy can become even more complex as we age. As the canal becomes thinner and thinner, we say that it has become sclerosed. One can see that it could be quite difficult to remove ALL the dead tissue in the root canals if its internal anatomy has become more and more sclerosed and difficult to negotiate as the tooth ages or becomes sick. While it is important for the endodontist to remove as much dead nerve tissue as possible from the pulp chamber and root canals, the final line of defense against endodontic failure is to make sure that any remaining dead nerve tissue inside the tooth is properly sealed off at the apical foramen and any other openings in the root by properly fitted and placed sealing materials.
project perpendicularly from the surface of the underlying dentin. When you are looking at a tooth in the mouth, you are seeing millions of these little enamel rods packed side by side, but you are seeing them end-on, as in the illustration on the right which is a reasonable representation of their cross section.They are packed together a bit like an Escher drawing.
tooth definition: 
four 
plucking, cutting, tearing, and holding. The biting portion of an incisor is wide and thin, making a chisel-shaped cutting edge. The upper incisors have a delicate tactile sense that enables them to be used for identifying objects in the mouth by nibbling. Next to the incisors on each side is a canine, or cuspid tooth. It frequently is pointed and rather peg like in shape and, like the incisors, has the function of cutting and tearing food.