Justin Groody
Dr. Ballard
BIO-415
December 4, 2014
Histology of the Human Tooth Teeth in the human mouth may just look like pieces of bone mass shooting out of the gum line, but they are actually much more complex. They function for the mechanical breakdown of food, and are essential for the formation of speech. Humans throughout their existence have needed different sizes and shapes of teeth to adapt to their diet. Humanity’s’ ancestors consumed less meat and coarser vegetable material, whereas modern day humans consume more meat as a whole. The consumption of an herbivorous diet required large broad teeth necessary to grind the plant material. The modern day mouth consists of a combination of smaller herbivorous teeth, the premolars and molars, and sharper incisors and canine used for the ripping apart of tough meat products (Bigham). Human’s mouths contain a variety of different shaped teeth that look very diverse on the surface, but they actually contain the same components. Under their tough exterior are the living portions of the tooth, the nerves and blood vessels. These components lead to the classification of teeth as individual organs (Bougalis). The nerves are responsible for the sensations of hot, cold, or pain and the blood vessels nourish the tissue. Each tooth can be broken down into three main regions, the crown, neck, and root or roots depending on the location in the mouth. The crown is the visible portion in the mouth and its size and shape changes drastically between the types of teeth. The neck is the meeting place of the crown and root or in anatomical terms, where the enamel and cementum fuse, also called the cement-enamel junction (CEJ). Roots are
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Cementummmmmm embedded into the maxilla or mandible and are responsible for anchoring the teeth with the help of the periodontal ligament. Roots contribute to two thirds of the overall length of the tooth and some teeth such as the molars, contain multiple roots depending on location in the mouth (Bougalis).

Figure 1. Basic structure of the tooth (Wikipedia). The four main parts of the tooth that make up the three regions are the enamel, dentin, cementum, and pulp, each with its own structure and function. The enamel is the outermost layer of the crown and is the hardest of all the parts and is the most mineralized and resembles bone tissue (WebMD). Unlike bone, enamel is 96% inorganic hydroxyapatite, making it much harder and denser. The other 4% is mostly water. Enamel is composed of structural units called enamel rods. These rods are produced during development of the tooth in the gum before eruption into the mouth occurs. Once in the mouth the enamel contains no cells, vasculature, or nervous tissue. Prior to eruption, the enamel rods are produced by cells called ameloblasts, tall columnar ectodermally derived epithelial cells (Histology Guide). The Tomes process of the ameloblast secretes substances in a merocrine fashion that form the enamel rod. The secretory products form from the cusps of the developing tooth, downward toward the base of the cusps. After the ameloblasts terminate secretion, the cells shorten and condense being held together by numerous desomosomes. Following formation, the enamel must mature. During this phase, the composition shifts from 30% inorganic and 30% organic (the other 40% is water), to 95% inorganic and 1% organic. The organic portion is nearly all protein. The protein portion is not collagen which is found in bone tissue however it consists of amelogenins and enamelins. These two proteins are poorly understood but they are believed to be the framework of the enamel. Following maturation, the normal adult enamel is remarkably dense due to the tightly packed enamel rods. One rod has a “key-hole” shaped appearance and this shape allows rods to wedge together for an optimal fit making enamel the hardest substance in the human body. The rounded heads of the rods face upwards toward the cusp tip or the tooth and the tail end faces downward toward the cervical margin (Kentucky). Observing a mature tooth in a slide will show cross-striations throughout the enamel. Each striation exhibits the amount of enamel produced per day during odontogenesis (Tooth formation) (WebMD). Closer to the boarder of dentin and enamel, these regular striations become irregular and interwoven and is named the gnarled enamel. This occurs due to the orientation of the enamel rods in multiple directions. The striations throughout alternate irregularly between lighter and darker bands. This is due to the normal and hypocalcification of the enamel rods (Kentuky).

Figure 2 and 3. Left. ‘A’ is the layer of ameloblasts and ‘B’ is the newly formed enamel in a developing tooth. Right. ‘B’ is the line separating the dentin and enamel of a mature tooth. Each indention in the line represents where an ameloblast once was. ‘C’ represents the striations of calcified and hypocalcified enamel (Kentucky). Similar to enamel, Dentin is a bone-like structure that makes up most of the mature tooth. It functions to immediately cover and protect the living portion of the tooth, the pulp. Dentin is less dense than enamel only containing 70% inorganic materials, but is still much harder than bone (Bougalis). Unlike enamel, dentin is very vulnerable to mechanical decay, however, it is considered a living tissue and can repair itself to some extent over time. Formation of dentin is completed by a simple columnar layer of cells called odontobasts that lie on the inner portion of dentin at the boarder or dentin and dental pulp. Dentin is acellular and avascular but contains some nerve endings for temperature and pain sensation. Odontoblast processes span the entire length of dentin, however, the cell bodies are not located in the dentin itself. The small canals in which the processes, called tubuli, are located can be seen under a microscope and are the identifying feature of dentin. The secretory product of the odontoblast is referred to as predentin, and is easily discernable in H&E staining due to light pink color. Dentin can be subcategorized. Primary dentin consists of mantle dentin, that is adjacent to the enamel, and circumpulpal dentin, which is more centrally located. Secondary dentin is formed during the functional life of the tooth and slowly intrudes into the pulp chambers throughout life. The last category is tertiary dentin (reparative dentin), which only forms as a result of a pulpal response to trauma. Tertiary dentin is more irregular in structure due to the appearance of odontoblast cell bodies, making tertiary dentin resemble bone closely (Dentin).
Figure 4. Diagram of the layers of dentin (StudyBlue). On the inner side of dentin is cementum, the outermost layer of the root portion of the tooth. Cementum is less dense than dentin and contains less inorganic material. Unlike dentin and enamel, cementum is only made up of 45% hydroxyapatite and its organic portion is composed of collagen. It serves to immediately protect the root and anchor the tooth into either the mandible or the maxilla. The periodontal ligament is responsible for the connection of cementum and the jaw bone (Bougalis). Cementum is visible on a slide as a slightly basophilic layer and can be discerned from dentin due to the absence of tubuli. Cementum is formed by fibroblasts in the dental follicle, prior to maturation, that migrate to the distal ends of the root and differentiate into cementoblasts. As it matures, cementoblasts become cementocytes that are fixed in lacunae which have canaliculi. Unlike the lacunae and canaliculi in bone, these canaliculi contain cementocytic processes that lead to the periodontal ligament and contain no nerves. Repair of cementum can occur throughout an individual’s lifetime, however, it is slow like dentin repair. Cementoblasts that do not become Figure 5. Layer of cementum trapped in the lacunae migrate towards and line the showing cementocyte containing periodontal ligament. These cementoblasts can add new lacunae and canaliculi (Cementum). layers to cementum if there is proper stimuli from trauma (Cementum). The final, innermost layer of the tooth is the soft living tissue, the dental pulp. The pulp is located in the pulp cavity of the crown of the tooth and in the root canals. It is classified as loose connective tissue because it contains nerves, collagen, blood vessels, fibroblasts, odontoblasts, macrophages, and lymphocytes. Pulp functions to sense temperature and pain, nourish the surrounding tissue, and form dentin and cementum during development (Bougalis). Dental pulp can be separated into coronal pulp and radicular pulp. Coronal pulp is located in the pulp cavity of the crown. Pseudostratifies columnar odontoblasts line the outside of the coronal pulp during development and change into a simple cuboidal epithelial layer following maturation. Radicular pulp is located in the root. Similar to coronal pulp, odontoblasts line the outside of the pulp, however, their shape is simple columnar. The epithelial root sheath is present in the developing tooth and aids in the production of the root. It is located between pulp and dentin and as the tooth matures, it begins to breakdown. Over time, the breakdown of the epithelial root sheath allows cementum to fuse with dentin (Kentucky).
Figure 6. A-Radicular Pulp. B-Odontoblasts. C-Dentin. D-Epithelial Root Sheath. E-Predentin (Kentucky). The human tooth is a complex organ that is essential for not only mechanical breakdown of food, but also proper speech. Being the hardest structure in the human body, teeth require a meticulous and flawless development in order to survive constant mechanical pressure and stress. The enamel, dentin, cementum, and pulp all have vital roles in the development, maintenance, and repair human’s teeth in a very specific manner, making teeth a remarkable organ.

Works Cited
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Bougalis, Costas. "The Parts of a Tooth." Parts of a Tooth. 1 Jan. 2010. Web. 21 Nov. 2014
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"Flashcards - Dentin - Is Dentin Replaced as You Age? Are Odontoblasts Part of the Dentin? StudyBlue. StudyBlue. 1 Jan. 2014. Web. 23 Nov. 2014.
"Oral Histology." Digital Lecture on the Web. University of Kentuky. Web. 21 Nov. 2014.
Sahara, N. "A Histological Study of the Exfoliation of Human Deciduous Teeth." Journal of Dental Research 72.3 (2013): 643-40. Sage Journals. Web. 21 Nov. 2014.
"The Mouth." Histology Guide. University of Leeds. Web. 21 Nov. 2014.
"The Teeth (Human Anatomy): Diagram, Names, Number, and Conditions." WebMD. WebMD, 1 Jan. 2013. Web. 21 Nov. 2014.
Wikipedia. Wikimedia Foundation. Web. 23 Nov. 2014.