Notes

The term oro means oral cavity or mouth and facial pertains to the face or structures that make up the face. When we put the two together, orofacial means the mouth and face. This lesson will discuss how the oral cavity and face of a human develop from the beginning to birth. Embryology is the study of the development of an embryo. The embryo development occurs in three stages from the time of fertilization until birth. We will be looking specifically at how the oral cavity, lips, tongue, and the face develop. It all begins with fertilization when the male gamete or sperm fertilizes the female gamete or egg. After this occurs, the zygote begins to travel to the uterus to be implanted; this can take place 24–36 hours after conception. The first two weeks is called the germinal stage. The mass of cells divides over and over, multiplying and doubling with every division. The cells then differentiate and the mass of cells is now known as a blastocyst. The blastocyst attaches to the wall of the uterus. The blastocyst has three primary germ layers or tissue layers: ectoderm, mesoderm, and endoderm. At this time, the sphere shape starts changing shape into a disc shape with the same layer. At the start of the third week, the embryonic period begins and continues through week eight. The embryo divides into three layers that will develop into various body systems. At week four, the neural tube forms; this will become the nervous system. The tube will close to form the brain during this period. Around this same time, the head, eyes, ears, nose, and mouth form. During this stage, the embryo will grow from 1/8 inch to 1 inch in five weeks. The end of the neural tube will start closing and exhibit further growth by developing extensions called processes, or pharyngeal arches. One end of the neural tube will eventually become the head and face and is called the cephalic end or cranial end, and the other end or tail end is called the caudal end. From week nine through birth is considered the fetal period. At the beginning of this stage, the embryo is about 1 inch long, and by 3 months, it is 2^ 1/2 inches in length. At 4 months, the fetus is about 4 inches long. A full-term baby is usually approximately 20 inches at birth. Even though it is amazing how fast the cells multiply, it is even more amazing how small details are carried out like clockwork with great organization.
The development that takes place during weeks two through eight will be discussed first. The three primary germ layers mentioned earlier are continuously developing through all the stages. The ectoderm will always be on the outside of the layers, with the mesoderm in the middle and the endoderm on the inside. Every cell, every tissue, every organ in the human body will be made from these three primary germ layers. The development of each layer of the embryo as well as the development of the oral cavity and face will be covered in this section. The ectoderm forms the oral cavity, the enamel of the teeth, hair, skin, and nails, the oral mucosa of the oral cavity and nose, as well as the nervous tissue. The mesoderm, or middle layer, is extremely diverse and makes up the majority of the major portions of the body. It forms the tissues of the teeth other than the enamel, dermis of the skin, heart, muscles, urogenital system, bones, bone marrow, and blood. The endoderm is responsible for all the internal linings in the body. This thin layer lines the pharynx, lungs, gastrointestinal tract, stomach, vagina, and bladder. At the beginning of the embryonic stage, the cephalic end forms a depression that deepens to an invagination to where the oral cavity will be located. This invagination of ectoderm pushes in and forms the primitive oral cavity and is also known as the stomodeum. The stomodeum becomes the main focal point for the development of the face. The face is formed from five bulges or prominences in the developing embryo. Above the stomodeum is a larger bulge that becomes the frontonasal prominence, or process. The frontonasal prominence is singular and is a collection of tissue from which structures originate. In addition to the frontonasal process, there are two mandibular processes and two maxillary processes. The frontonasal prominence starts growing thicker at the midline. The frontonasal prominence develops into the forehead and bridge of the nose. A larger section splits off and migrates down the center of the face; this additional process is called the median nasal prominence. This section will become the main middle portion of the nose, nasal septum, and tip of nose. As this is growing downward, both sides of the median nasal process start to split off into another long section that grows downward as well. These right and left additional processes are called the right and left lateral nasal prominences. These are thinner and smaller than the median nasal process and are separated at the tip of the nose area by a depression that becomes the olfactory pits or nasal pits. These depressions invaginate further and will become the future nostrils of the nose, or nares. These two lateral nasal processes become the sides of the nose and the side of nostril or ala of the nose. At the end of the median nasal process, another rounded bulge forms called the globular process. This process will make the philtrum of the upper lip and the premaxilla, also called the primary palate or anterior portion of palate. This primary palate is a triangular-shaped piece of the anterior palate and will fuse or join with the next main section that forms the rest of the palate. If this fusion does not happen in time, then a cleft of the palate will occur. All of this growth from the frontal process happens by the seventh week. Week seven is when the philtrum area of the lip fuses with the other section of the upper lip, and if it does not occur then, there will be a cleft of the lip. Clefting can occur on one side (unilaterally), or it can occur on both sides (bilaterally). During this time, there are many other processes growing on the embryo, and two of them are very important in forming the rest of the face. The rest of the face is developed from other main processes called pharyngeal arches. Pharyngeal arch I is also called the mandibular arch. It is found immediately below the stomodeum and it begins developing at the third week. Pharyngeal arch I will be responsible for forming the majority of the rest of the face and mouth. Soon after it forms, it will split into two distinct processes that are named the maxillary prominence (process) and the mandibular prominence (process). The maxillary prominence grows along on the sides of the face with the stomodeum separating the right and left sides. The mandibular prominence will form the mandible and all associated structures and tissues in that area. The mandibular bone, floor of the mouth, the chin, the anterior two-thirds of the tongue, and all glands in that area are developed from the mandibular process of the first pharyngeal arch. In the same respect, the two parts of the maxillary prominence will develop above the mandible and form the maxillary bone, sides of the upper lip, sinuses, posterior sides of the hard palate, and all tissues and glands associated in that area of the face. The maxillary prominences will form the alveolar ridge of the maxilla where the teeth from the canine to the third molar will sit. This part of the palate is also called the secondary palate and it will fuse with the primary palate in the premaxilla that formed from the globular process. Pharyngeal arch I will also form the four pairs of the muscles of mastication and the nerves and blood vessels to all these areas of the face. The posterior third of the base of the tongue and the muscles of facial expression are formed by pharyngeal arch II, also known as the hyoid arch, which is located beneath pharyngeal arch I. Pharyngeal arches III, IV, and VI are mainly involved in forming the neck and throat. A small portion of the base of the tongue is formed by pharyngeal arch III. The posterior of the tongue then fuses with the anterior portion of the tongue, which is formed by the first pharyngeal arch. The second pharyngeal arch does not contribute to the tongue. All of the swallowing mechanism of the throat and the muscles that move the neck, hyoid bone, ligaments, and the lymph glands in that area are made from these arches. In week eight, immediately following the fusion of the lip, the palate starts to fuse. Between weeks eight and twelve, the fusion of the hard palate takes place. The maxillary process has two shelves, or lateral processes, that come off the side where the majority of the palate will form. The premaxilla, or primary palate from the globular process, fuses with the lateral process from the maxillary process that is making the right and left sides of the palate. If this fusion is disturbed or does not occur, there will be a cleft palate unilaterally or bilaterally. If it occurs closer to week eight, the cleft will be located more anteriorly and appear more severe than if it occurs closer to week twelve. If clefting occurs closer to week twelve, there may be a smaller cleft of the soft palate or uvula. While the oral cavity and the alveolar bone are being formed and the palate is fusing in weeks eight through twelve, the developing bone of the maxilla and mandible are also forming tooth germs. At approximately six to seven weeks in utero, odontogenesis, or tooth development, begins. The process starts inside the stomodeum, with the invagination of the oral ectoderm layer into the underlying mesenchyme of the embryonic mesoderm. The process starts here and continues until about age 21 when the third molar has developed and erupts into the oral cavity of an adult. The oral ectoderm that overlies the developing maxilla and mandible is the origin of the dental lamina. On the developing maxillary and mandibular arches, the oral ectoderm starts growing rapidly and pushing into the underlying embryonic tissue known as mesenchyme. This dental lamina grows into both dental arches and then in 10 specific areas where the primary teeth will form. The dental lamina is responsible for the formation of the enamel organ. The primary teeth start to develop at weeks six though seven, and the permanent teeth start developing at about week 17 in utero. Both sets of teeth are developing simultaneously before the fetus has developed to half-term. There are four stages in tooth development. Each of the first three stages is named according to the shape of the enamel organ (bud, cap, and bell stages). The last stage, apposition, is when the tooth tissues are being deposited. It is important to note that the beginning of tooth development only pertains to the enamel of the crown. The ectoderm will become the enamel of the tooth with the dentin, pulp, cementum, alveolar bone, and the periodontal ligament (PDL) developing from mesenchymal tissue. The crown of the tooth is formed from the enamel organ. It develops first and shapes itself depending on what type of tooth it will become. During the first stage of tooth development, the dental lamina begins to bulge out in areas in both arches. This is called the bud stage because it is very similar in appearance to a bud of a flower on the branch of a tree. Therefore, on a deciduous or primary dentition, 10 growths or buds on each arch are apparent; these later become the primary teeth. The permanent teeth develop in a similar manner. Each arch has 16 buds developing into one tooth each. The last three molars in each quadrant develop behind the primary dentition. The branch of the tree is the dental lamina that pushed into the mesenchyme with the bud stage of a tooth at the end. In this first stage, the cells are not differentiated but are proliferating or multiplying rapidly. This stage will continue until the shape of the bud starts to change. The second stage is the cap stage. This is named because the bud becomes shaped like a cap that is curved on top and concave underneath. This gives it the cap-like shape. The cells at this stage are still proliferating and are also beginning to histodifferentiate into certain cells that will make the enamel organ of the tooth. The cap-shaped enamel organ sits over the dental papilla. The dental papilla is the collection of mesenchymal cells discussed earlier. The cap-like shape will begin to grow into the size of the tooth it will become. The incisor will be smaller overall when compared to the size of a molar. At this point of development, there are three layers of cells that make up the framework of the enamel organ. The outside top two-thirds of the enamel organ are bordered with cells called the outer enamel epithelium (OEE). These cells will begin to shape the enamel organ. The bottom third of the enamel organ is a concave area and is layered with cells called the inner enamel epithelium (IEE). This layer will eventually become the ameloblasts that form the enamel of the tooth. The third layer of cells that are present in the cap stage is the stellate reticulum (SR). This layer helps hold the shape of the enamel organ and the crown of the developing tooth. The third stage of tooth development is the bell stage and is so named because of the shape of the enamel organ. The cells inside the enamel organ become fully differentiated to be able to start the next stage of laying down enamel in the appositional stage. The laying down of enamel is known as amelogenesis. This is the last stage before the tooth tissues are laid down. There are four distinct layers in the bell stage. The stratum intermedium (SI) is the last developed layer. The SI layer is adjacent to the IEE and is not very thick, but plays an important role in nourishing the inner enamel epithelium (IEE). The SI layer appears around the fourteenth week in utero. When the IEE cells are maturing into ameloblasts and forming enamel, they need to have energy and nourishment. It is believed that the SI layer nourishes the ameloblasts, though the exact function of the cells in the SI layer is unknown. These four layers form the crown of the tooth and prepare the enamel organ for its final and most important stage, the appositional stage. It is in this stage that certain anomalies can occur. A supernumerary or extra tooth may form at this time. An odontoma or extra calcified tissue may form at this time as well. The enamel organ now has four distinct layers of cells that have developed: the outer enamel epithelium, the inner enamel epithelium, the stellate reticulum, and the stratum intermedium. The four layers of the enamel organ are ready to make enamel and go into the final stage of tooth development. The last stage of tooth development is called the appositional stage and begins when the tooth tissues start to be laid down. During this stage, enamel, dentin, and cementum are laid down in increments. Predentin is the first tissue laid down. This signals the ameloblasts to secrete the amelogenin from the Tome’s process located at the apical side of the cell adjacent to the layer of predentin. Tome’s process guides the enamel matrix into place. Next, hydroxyapatite crystals are deposited into the amelogenin, it becomes mineralized, and is now called enamel. Enamel is laid down in increments just like dentin. As each increment of amelogenin is produced, it then becomes mineralized and another layer of amelogenin is laid down. The enamel is deposited in layers that form keyhole-shaped cells that are called enamel rods or prisms once the enamel is hardened. These rods, which are not visible to the naked eye, are four micrometers in diameter, have variable lengths, and are shaped in the pattern of a fish. Their location in the enamel is such that the head is surrounded by the tails of two other enamel rods. Hunter Schreger bands are visible in enamel under a microscope. These are alternating light and dark bands caused by the arrangement of enamel prisms at right angles to each other. This arrangement strengthens the enamel and prevents it from cracking. The substance surrounding the inner portion, the rod core, of each enamel rod is the interprismatic substance. Of these substances, the enamel rods are hardest, and the interprismatic substance is the weakest. This repeating process creates lines, like rings of a tree called Striae of Retzius, and can be seen under a microscope. The enamel rods are keyhole shaped and can be compared to bricks. The mortar is the interprismatic substance (interred substance) that holds the enamel rods together. Both the enamel rods and the interprismatic substance make up the structure of the tooth enamel. As the enamel hardens to its 96% inorganic and 4% organic composition, it becomes the hardest substance in the body. It cannot repair itself, so once it is damaged, decayed, or worn down, it is gone. Enamel is thinner at the area on the tooth where the enamel and the cementum meet; this allows the yellower dentin under it to show through. Enamel is whiter at the chewing surfaces and cusp tips where it is thicker. Fluoride during development helps make enamel stronger; however, too much fluoride during development can make the enamel gray, brownish, or striped in bluish-gray colors. The strength and hardness of enamel make it resist wear; it is durable and smooth, which allows it to be self-cleansing. All of these tissues are very vulnerable at this stage. If the pregnant female takes an antibiotic such as tetracycline, the enamel may be permanently stained. If the mother experiences an illness at this stage, the enamel may not form properly. Enamel dysplasia results in a reduction in the normal levels of enamel. Dysplasia usually is caused by enamel hypoplasia or enamel malformation or by enamel hypocalcification, which is a decrease in the hardness of the enamel. Both of these abnormalities can also take place during this stage of gestation. The dental papilla develops adjacent to the lower half of the enamel organ at the same time that the enamel organ is forming. It starts out as a collection of mesenchyme that changes into specialized cells called odontoblasts that will secrete and mineralize the dentin of the tooth. Predentin is formed before the dentin. The formation of predentin sends a signal to the IEE to differentiate into ameloblasts that will secrete the enamel matrix. The predentin is laid down in increments and then hardened before the next layer of predentin is laid down. These layers can be identified under a microscope and look like rings on a tree; they are called imbrication lines of von Ebner. Contour lines of Owen are accentuated lines of von Ebner that demonstrate a disturbance in the formation of the matrix or mineralization. The dentin will mineralize to its 70% inorganic and 30% organic level. The process of dentinogenesis continues until it fills the crown portion of the tooth and will continue into the root after the crown portion is almost complete. The odontoblasts migrate apically as they lay down dentin, and the ameloblasts migrate coronally as they lay down enamel in the crown of the tooth. Dentin is the second-hardest substance in the body and the second-hardest tooth tissue. Dentin is more yellow in color than enamel. Dentin makes up the bulk of the tooth structure. It is located beneath the enamel in the crown and the cementum in the root. One of the major differences between dentin and enamel is that dentin can repair itself to a certain extent and continues to grow throughout the life of the tooth. The structure of dentin is very different from enamel in that it has odontoblasts living inside the calcified tissue. These cells have long processes or tails that start out at the junction of the dentin and enamel (DEJ) or junction of the dentin and cementum (DCJ) when the pre-dentin is first laid down. As the dentin grows thicker, then the odontoblastic process grows longer and longer with the main body of the odontoblast located within the pulp cavity. These processes extend into the dentin and because they are living and not hardened tooth structure, they become encircled with the hard dentin and end up in tunnel-like channels called dentinal tubules, which contain dentinal fluid. These processes can conduct sensitivity into the tooth’s pulp by way of these tubules. If the enamel or cementum is ever broken or worn away, it can expose the dentin and make the tooth very sensitive. This can be a very common problem, and dentistry has several remedies for covering up these tubules to decrease sensitivity. Dentinogenesis continues throughout the life of a tooth. The dentin that is laid down in different stages of a tooth’s life is identified with different terms. Once the predentin is calcified, and before the tooth erupts into the oral cavity, this newly formed dentin is also called primary dentin. The layer of primary dentin closest to the enamel is known as mantle dentin. The amount of dentin that forms at this time is the bulk of the dentin for that tooth. The dentin that forms once a tooth erupts into the oral cavity after the root of a tooth has fully formed falls in the category of secondary dentin. Secondary dentin forms slowly throughout the life of the tooth at the expense of the pulp cavity. As a person gets older, the pulp chamber gets smaller and smaller until there is very little pulp tissue left in the tooth. This is one reason why many older patients do not have much dental sensitivity. Another type of dentin, tertiary dentin, is called reparative dentin and it forms in response to trauma. This trauma can be caused by invading decay, a cavity, or occlusal pressures that can damage the tooth. When a tooth has a threat of decay, then the pulp lays down a thicker layer of dentin under that part of the tooth where the caries is advancing. The reparative dentin is less organized and laid down faster than the secondary dentin. This dentin can often give a tooth a little more time before the pulp is invaded with the carious lesion. Having radiographs and routine dental care can help the dental team discover decay earlier and remove it in a timely manner so the tooth can be restored. Dentin also differs from area to area and is not uniform throughout. Peritubular dentin is the dentin that creates the wall of the dentinal tubule. Dentin found between the tubules is called intertubular dentin. The first predentin that is formed and matures within the tooth is called mantle dentin. The layer of dentin that surrounds the pulp is called circumpulpal dentin. Surrounding the dental papilla and the enamel organ is the dental sac, the third and final part of the tooth germ. This layer of cells is derived from the mesenchyme and encircles the dental papilla and enamel organ. The dental sac will eventually form the cementum that covers the root of the tooth. Cementum is formed by cells called cementoblasts; cementoclasts are involved in root resorption. Cementum is a thin layer of hard tissue found only on the outside of the root. Cementum is 55% organic and 45% inorganic, which makes it slightly harder than alveolar bone. The main function of cementum is to serve as an attachment of the tooth to the alveolar bone by way of the PDL that surrounds the root. The dental sac is also responsible for the formation of the periodontal ligament or PDL surrounding the root of the tooth as well as the alveolar bone in which they sit. There are two types of cementum found on the root of the tooth. Acellular or primary cementum is a very thin layer that covers the entire root, and after it is laid down and mineralized, the cells, called cementoblasts, stop producing cementum. The second type, cellular cementum or secondary cementum, is found at the apical third of the root tip and will continue to slowly be deposited throughout the life of the tooth. When we start to see how the rest of the tooth develops, we start at the edges of the enamel organ’s inner enamel epithelium (IEE) and outer enamel epithelium (OEE) cells at the CEJ of the tooth. The OEE cells around the CEJ finish their role by becoming the epithelial attachment at the base of the sulcus. These two layers stop forming the crown of the tooth, but the tooth is not finished forming yet. The cells of the dental papilla have been forming dentin inside the crown and will continue to form dentin in the root.After the enamel of the crown has been formed, cells from the inner and outer enamel epithelium in the area of the cervical loop will continue to function. These two layers of cells elongate and now comprise Hertwig’s epithelial root sheath, which will form and shape the roots. The cells that were the IEE will now produce the intermediate cementum on the surface of the root dentin as it continues to be deposited by the odontoblasts of the dental papilla. Cells from the dental sac will differentiate and change into cementoblasts that will form secondary cementum on the intermediate cementum on the root surface. Other cells from the dental sac will form the periodontal ligaments that suspend the root in the alveolar bone, which is formed by cells of the dental sac. The two layers of cells of Hertwig’s root sheath give the odontoblasts the matrix to start to lay down predentin. The IEE cells from Hertwig’s root sheath lay down cementoid, and the DCJ or dentinocemental junction is formed. The cementoid is then mineralized as hydroxyapatite crystals are added to the matrix in the same manner that the dentin and enamel were formed. Hertwig’s epithelial root sheath continues to elongate and shape the roots through cementogenesis. The process of the root formation can take up to four years after a tooth has erupted into the oral cavity. The apex of the root is not completed until a few years after the tooth erupts. Once the tooth has erupted and the apex is completed a few years later, Hertwig’s epithelial root sheath has completed its function. Normally these cells are resorbed by the body. Sometimes Hertwig’s epithelial root sheath is not totally resorbed and cells can remain within the periodontal ligament space. These remaining cells can become trapped and are known as the epithelial rests of Malassez. It is believed that these cells can proliferate to form the lining of cysts or can become calcified within the PDL. The process of odontogenesis occurs for every tooth in both the primary and the permanent dentitions. After the primary tooth germs develop, there are successional laminae that form slightly lingual to the primary teeth. These successional laminae will become the permanent tooth germs to replace the primary teeth. These succedaneous or permanent tooth germs grow lingual to the primary tooth germ. The same process occurs with the non-succedaneous molars, but instead of the laminae extending from the primary tooth, it is a distal extension of the original dental lamina posterior to the primary molars. The pulp of the tooth is the vital part of the tooth. It is located at the central portion of the tooth in the crown portions and in the roots of teeth. The part of the pulp in the crowns is known as the coronal pulp. The part of the pulp in the roots is known as the radicular pulp. The radicular pulp is continuous with the apical foramen. The part of the pulp that extends into the tips of each tooth is known as a pulp horn. The pulp basically follows the shape of the tooth in which it is contained. The pulp is the nerve center of each tooth and contains living connective tissue as well as odontoblasts, fibroblasts, osteoblasts, and osteoclasts. The pulp also contains nerves and blood vessels that enter the pulp chamber through the apical foramen. Similar to dentin, the pulp is formed from the dental papilla. When the dentin forms around the dental papilla, the central tissues are the pulp of each tooth. As with any tissue development, there can be some types of variations that can occur. Sometimes these tissues do not mineralize completely and leave empty areas, or they may fill in too densely, and other times some cells can be found in the wrong areas. The last part of this section describes some of these artifacts that can be found in the enamel, dentin, or cementum. There are two primary parts of a tooth: the crown and the root. The part of the tooth that you can see when you smile is called the crown of the tooth. The crowns are shaped differently depending on the type of tooth. They are covered on the outside by the strongest tissue of the body—enamel—which protects the tooth. The root is the part of the tooth that you cannot see located beneath the gingiva. The root is embedded in the bone of the maxilla and mandible, which securely holds the tooth in the jaw. A tooth can have a single, bifurcated, or trifurcated root. The end of the root tip is called the apex or apical end. At the very end of the root apex is a hole or foramen where the blood vessels and nerves enter and exit the tooth and make it a vital, living part of our mouth. Where the crown meets the root is the cervical area or cervix of the tooth. It is also where the enamel of the crown and the cementum of the root meet. This is known as the cementoenamel junction, or CEJ. There is usually a raised line here called the cervical line. When looking at the CEJ, there are three possibilities of a junction between the cementum and enamel. The cementum will overlap the enamel slightly 60% of the time and meet the enamel exactly 30% of the time. That leaves 10% of the time when there is a small gap between the enamel and cementum. When this gap occurs, exposed dentinal tubules in the area can be very sensitive since that area is not protected. Inside the tooth’s hard structure are two other junctions. Where the enamel meets the dentin on the inside of the crown is the dentoenamel junction, or the DEJ. In the root, the area where the cementum meets the dentin is called the dentocemental junction of CDJ.The periodontium is composed of the structures that surround and support your teeth. It is made up of four major structures: the alveolar process, cementum, periodontal ligament (PDL), and gingiva. There are two divisions of the periodontium: the attachment unit and gingival unit. The attachment unit (apparatus) is composed of the alveolar process, cementum, and periodontal ligament. The maxilla and mandible are the tooth-bearing bones of the alveolar process. The alveolar process provides the bony support for the roots of the teeth, attachment for the gums that protect the teeth and bone, and attachment points for muscles involved in jaw and tongue movement. The alveolar process is a thick ridge on the inferior surface of the maxilla and superior surface of the mandible called the cortical plates. It is divided into the alveolar bone proper and the supporting alveolar bone. Alveolar bone is formed by osteoblasts. The cells that remodel and resorb bone are called osteoclasts. The alveolar bone proper is composed of compact bone that lines the alveolus and makes up the alveolar crests and interdental septum. The interdental septum separates each tooth socket. If the tooth has multiple roots, the bone that separates the roots is identified as the interradicular septum.The alveolus or alveolar socket, is a cavity within the alveolar process that holds the tooth. Lining the alveolus is very dense compact bone called the lamina dura. The PDL is attached to the lamina dura and the cementum of the tooth to secure the tooth in the jaw. The lamina dura contains numerous holes where canals pass from the alveolar bone into the periodontal ligament. The alveolus does not actually contact the root because the periodontal ligament suspends it in place. A healthy alveolar crest is the highest point of the alveolar ridge and joins the facial and lingual cortical plates. It is located approximately 1.5 to 2 mm apical to the CEJ. Interdental bone or septa are plates of bone that separate the individual alveolus, and the interradicular septum or bone separates roots of multi-rooted teeth. The outer border of the supporting alveolar bone consists of plates of compact bone or cortical plates on both the facial and lingual surfaces of the alveolar bone. The cortical plates are 1.5 to 3 mm thick over the posterior teeth and are thinner over the anterior teeth. Cancellous bone, also referred to as trabecular bone or spongy bone, is located between the alveolar bone proper and the cortical plates Cancellous bone forms a lattice-like, spongy, porous bone tissue made of open spaces connected by flat planes of bone known as trabeculae. Red bone marrow, blood vessels, and connective tissue fill the spaces to carry out regenerative and regular functions of the bone. Inside the trabeculae are three types of bone cells: osteoblasts, osteocytes, and osteoclasts. Their function is formative, and they make new tissues for self-repair of damaged tissue. The alveolar bone function is to support, maintain, and retain the tooth in the arch. The alveolar process contains nerves to receive and transmit stimuli and blood supply to provide moisture and nutrients to nourish the bone and teeth. The PDL fibers are embedded into the cementum of the tooth and lamina dura of the alveolar bone, securely attaching the tooth in the alveolus. The periodontal ligament is also known as the periodontal membrane. The periodontal ligament occupies the space between the cementum of the root and the alveolar bone that lines the alveolus or tooth socket. This ligament is wider at the cervix (CEJ) and at the apex and narrow between these points. The main components of the PDL include collagen fibers that create fiber bundles that help literally attach the tooth to the bone. The periodontal ligament also includes a variety of cells such as fibroblasts, cementoblasts, osteoblasts, nerves, blood vessels, and lymphatic vessels. It is derived from the dental sac in embryonic development. The PDL is a layer of fibrous connective tissue that serves as a cushion between the tooth and the bone. Its main function is to have a supportive role and hold the tooth in the alveolus. It is composed of rubber band–like tissue of fibers that suspend the tooth in the alveolus or tooth socket. The periodontal ligament has a sensory function also that transmits tactile sense, pressure, and pain. It can act as a suspensory mechanism that keeps the root and bone from abrading away, and it also acts as a shock absorber. The PDL fibers cushion any impact between tooth and bone when pressure is exerted or when biting or chewing. Because it acts as a cushion, the PDL allows a slight movement of the tooth when chewing, and the tooth is not set directly against bone. Other functions of the PDL are formative; it carries blood supply to the periodontium for nutrition and remodeling in which it provides cells involved in the formation and resorption of the cementum and bone and also of itself. It also allows some degree of movement of the tooth and allows it to slightly tip, rotate, or be compressed. It also helps keep a constant mesial drift of the tooth. The PDL has what are called principal fibers that are groups of fibers which are orientated to give the tooth optimal resistance to all types of functional loading patterns. The PDL principal fiber groups are named according to their location in respect to the tooth’s root or how they are oriented along the root surface. There are five groups of PDL fibers that belong to the alveolodental ligament fibers group, and one group that belongs to the interdental or transseptal fibers group. There are other fibers and unusual calcifications besides the principle fibers that can be found in the periodontal ligament. Sharpey’s fibers are one of the other types of fibers found in the PDL. These are thicker, more fibrous, collagen-type fibers with sharper ends that embed into the cementum and bone across the PDL at right angles. Cementicles are small calcifications of cementum that are trapped within the PDL. They are not attached to the bone, but sometimes can be attached to the cementum of the tooth. Gingiva is the part of the oral mucosa covering the alveolar bone that supports the teeth. Healthy gingival tissues are very important for a healthy mouth. The gingival unit is composed of epithelial tissue that is attached to the alveolar bone and surrounds the neck of the teeth, providing a seal around the base of the teeth to prevent the entry of bacteria carrying plaque and calculus. Thus, when healthy, it presents an effective barrier to the barrage of periodontal insults to deeper periodontal tissue. When the gingiva is not healthy, it can provide a gateway for infection to advance into the deeper tissue of the periodontium, leading to periodontal disease and a loss of teeth. If we could look inside the gingival tissues under a microscope, we would see that five fiber groups make up the gingival unit. The overall function of these fibers is to resist gingival displacement