Essential Facts About Teeth for Biology Students

Our teeth are the strongest part of our body, they are made of proteins such as collagen and minerals such as calcium. In addition to helping us chew through even through tough foods, they also help us speak clearly. We see 32 teeth in most of the adults, whereas children have 20 teeth. Our teeth help us chew our food while eating, they assist us while talking and make us look better when we flash a smile for selfies. But there is a lot to learn about this essential tool called the teeth. Although we are born with 20 baby teeth they aren’t visible. Until they start to come through the gums when you are about 6 to 12 months old. By the time we are 2 to 3 years of age, all the teeth will usually appear. This process is called teething and the new set of teeth are called primary teeth, baby teeth, or milk teeth. As we get older around 5 to 6 years of age these primary teeth begin to fall one after the other. This happens because it is pushed out of the way by the permanent tooth that is behind it, this gradually replaces the primary teeth. By the time we are about 12 or 13 most of us will have lost the baby teeth and have a full set of permanent teeth. Between the age of 17 and 25, most of us will have 4 teeth called the wisdom teeth which grow at the back of the mouth to complete the adult set of 32 shining white teeth. These 32 sets are divided into 4 different types which are 8 incisors, 4 canines also called cuspids 8 premolars also called bicuspids 12 molars including 4 wisdom teeth.

Types of Teeth

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1. Incisors

 All of the 8 incisors teeth are located in the front part of the mouth. We have 4 of them in the upper jaw and 4 of them in the lower jaw. These help us bite the food as they tend to have sharp edges. Whenever we sink our teeth into something such as an apple we use incisor teeth. The very first teeth we grow are incisors appearing when we are around 6 months old. Incisors have 3 major functions The 1st is to incise or cut into the material and the 2nd is esthetics and the 3rd is for phonetics or pronunciation. The maxillary incisors are wider than the mandibular incisors. The incisors have much smaller and weaker roots. 

2. Canines

Canines are 4 teeth that sit next to the incisors. We have 2 canines at the top of the mouth and 2 canines at the bottom. Canines have a sharp pointy surface that is used for tearing the food. Canines are also called cuspids. These are the only cusp teeth on the mouth. When the maxillary and the mandibular incisors touch each other as they usually do, this means the mandibular canine is a half tooth closer to the midline than the maxillary canine. This starts to allow for some interdigitation of these cusps. When we come into the lateral excursion you will note that the maxillary canine is a half cusp posterior to the mandibular canine, which has its cusps come up between maxillary lateral and maxillary canine. This is to know that they have the same basic overbite and overjet as we see in our incisors. That is the mandibular canine is inside or lingual to the maxillary arch and crudes to the lingual surface of the maxillary surface and the canine. The canine is often called the cornerstone of the arch. Canine seems to be the pivot point for both the posterior teeth and the anterior teeth. The canine is a very strong tooth, it is often referred to as the longest and the strongest root of any single tooth in the mouth.

3. Premolars

Premolars are 8 in number and sit next to the canines. There are 4 premolars on the top and 4 on the bottom. Premolars are bigger than the canines and the incisors. They have a flat surface with ridges for cutting and grinding the food into smaller pieces which makes it easier to swallow. Baby molar teeth are replaced by the premolars. Infants and young children do not have premolars because these teeth do not start to come in until around age 10. Premolars are said to be intermediate between the moles and the canines. The 1st premolars will be the one closer to the canines and the 2nd premolars will be the once close to the molar. Ther 1st premolars are more similar to the canines, as they are sharp and better for ripping, whereas the 2nd premolars are flat and are better for grinding. They erupt around 10 or 12 years and like all other permanent teeth, their root finishes developing 2 ½ years after the eruption. The maxillary 1st premolar is going to have 2 roots, whereas the other premolars are going to have 1 root. It is different then mandibular molar such that in maxillary 1st premolar the routs bifurcate in the middle or apical third. As they have 2 cusps they are going to have 2 pulp horns, 2 roots, 2 root canals once for each root. It is going to have a longer mesial cusp slope than the distal cusp slope. It is the only tooth that will consistently have a mesial crown concavity. 

4. Molars

Molars are 12 in number and are our biggest and strongest teeth. We have 6 on the top and 6 at the bottom. The large surface area of the molar helps them grind up the food. When you eat your tongue pushes the food to the back of your mouth. Then the molar breaks up the food into pieces small enough for you to swallow. The molars include 4 wisdom teeth, which are the last set of teeth to come in. Wisdom teeth are also called 3rd molars, they usually come between the age of 17 and 25. Not everyone has enough room in their mouth for this last group of teeth. Sometimes the wisdom teeth are impacted, meaning they are stuck under the gums. This implies they have enough space to grow. When you do not have room for the wisdom teeth you will likely like to have them removed, as they tend to crowd and damage other teeth causing severe pain. 

Development of Teeth

Development and growth is a complex process of interactions between the primitive oral epithelium and the underlying ectomesenchyme cells. The epithelium is derived from the ectoderm, the 1st pharyngeal arch, where the cells of the ectomesenchyme are neural crust in origin. Since the neural crest cells are primarily derived from the ectoderm and eventually give rise to the mesenchymal tissues they are called ectomesenchyme. Tooth development through a continuous process can be divided into 3 stages called the bud, cap, and the bell stage. These stages are so named because of the shape of the enamel organ assumed in each stage. 

Formation of Dental Lamina

During the early development stage of the teem in the mouth as they are still in the developing stage, it is called the primitive oral cavity or stomodeum. This is lined by 25 squamous epithelium, which is called the oral ectoderm. The ectoderm contacts the endoderm with the help of a membrane called the Bucco pharyngeal membrane. At 27 days of pregnancy, the Bucco pharyngeal membrane will rupture and now the oral ectoderm is in contact with the endoderm. Considering our ectoderm, beneath the ectoderm we have connective tissue. Most of these connective tissues are ectomesenchyme kind in origin. They instruct the ectoderm to start the tooth development. When the tooth develops, this development takes place in the anterior portion of the maxilla, ie future maxilla, and the future mandible as they are still not developed. 2 to 3 after the rupture of the Bucco pharyngeal membrane, certain areas of the basal cells. Certain areas of the basal cell proliferate more rapidly compared to the other areas and they form something called a primary epithelial band. Eventually, this band will divide into 2 parts, once will be lingual and the other will be the buckle. The epithelial band will divide into the inner lingual process and the outer buckle process. The inner lingual process is called the dental lamina and the outer buckle process is called the vestibular lamina. This vestibular lamina forms the vestibule of the mouth. When we place our tongue between the cheeks and your teeth we find a space there, that is the vestibule. The dental lamina will form the ectodermal portion of deciduous teeth and the permanent successes will arise from the lingual extension and permanent molars will arise from its distal extension. The total activity of dental lamina will extend for a period of 5 years. We can find the dental lamina still active in the third molar region after it has disintegrated elsewhere. Also, there is the resonance of the dental lamina, which persists as epithelial pearls or eye lens within the jaw, these are called as rest of serres.  

Stages of Tooth Development

These stages of development are called the morphologic stage, this is named as such because of the appearance.

1. Bud Stage

Below the dental lamina, we have ectomesenchyme or connective tissue. The epithelium of this dental lamina is separated from the ectomesenchyme by the basement membrane. Here we have certain swelling in the basement membrane at 10 points, as we have 10 teeth in each arch. We have round ovoid swellings. which arise at basement membranes at 10 different points, this point corresponds to the future positions of deciduous teeth. These points are the tooth buds when the development will start to proliferate more rapidly than the other portions. In the bud stage that starts at the 8th week of intrauterine life, the localized epithelial swellings or the dental placodes proliferate and invaginate further into the ectomesenchyme and occupies a shape of a bud. The cellular density of the ectomesenchyme also increases and it becomes densely packed around the epithelial bud. Here at this stage names are changed, the tooth bud here is now called enamel organ because it gives rise to the enamel of the tooth. The condensed ectomesenchyme is called the dental papilla. IT gives rise to the future dentin and pulp of the tooth. The dental lamina is short and alose to oral epithelium during this stage. 

2. Cap Stage

As the developmental process enters this stage which starts in the 9th week, the enamel organ drags along with its part of the dental lamina and occupies the shape of the cap sitting on the condensed ball of the dental papilla. The cells lying on the concave region of the cap-shaped enamel organ become short columnar and are now called the inner enamel epithelium. While lying at the convex region becomes short cuboidal and is now called the outer enamel epithelium. The cells that are present between the inner and the outer enamel epithelium secretes glycosaminoglycans into their extracellular compartment. These molecules of glycosaminoglycans are hydrophilic as a result they pull water from their surroundings into the enamel organ, causing the cells to force apart. As a result, the cells become star-shaped and are now called the stellate reticulum but still, the cells remain connected with each other and the underlying inner enamel epithelium through their desmosomal junctions. The successional dental lamina of the permanent tooth arises at the cap stage of tooth development and it gives rise to its permanent successors. The cells of ectomesenchyme surrounding the enamel organ and the dental papilla are now called the dental follicle of the dental sac. The dental follicle will give rise to periodontal tissues of the tooth. Together the enamel organ, dental papilla, and the dental follicle make up the tooth germ.    

3. Bell Stage

This stage is been further divided into2 stages which are the early bell stage and the late bell stage. The early bell stage is characterized by 2 important events called the histodifferentiation and the morphodifferentiation. In histodifferentiation, the cells acquire their distinctive phenotypes. Cells of the dental papilla will differentiate into an odontoblast with cytoplasmic extinction. The short columnar cells of the inner enamel epithelium will differentiate into long columnar cells of ameloblast. In morphodifferentiation the crown form is determined, which means that if it is a molar tooth the enamel organ will assume the shape of the molar crown or if tooth germ belongs to an incisor the enamel organ assumes the shape of the incisor crown.  These infoldings occur as a result of differential rates of mitosis and the differences in their cell differentiation times.