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Orban's Oral Histology & Embryology, 13/e
 

Orban's Oral Histology & Embryology, 13/e

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First published in 1944, Orban’s Oral Histology and Embryology has become the classic text for successive generations of dental students. The thirteenth edition, while retaining the same ...

First published in 1944, Orban’s Oral Histology and Embryology has become the classic text for successive generations of dental students. The thirteenth edition, while retaining the same fundamentals and lucid writing style, reflects the current advances and latest curriculum offered in Indian universities. This new edition all the chapters have been extensively revised and also includes a summary at the end of every chapter and review questions for the benefit of the students.

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    Orban's Oral Histology & Embryology, 13/e Orban's Oral Histology & Embryology, 13/e Document Transcript

    • Chapter |1| An Overview of Oral Tissues The oral cavity contains a variety of hard tissues and soft tissues. pulp are derivatives of dental papilla while cementum, periodontal The hard tissues are the bones of the jaws and the tooth. The ligament and alveolar bone, are all derivatives of dental follicle. soft tissues include the lining mucosa of the mouth and the The cells that form these tissues have their names ending in salivary glands. blast. Thus, ameloblast produces enamel, odontoblast dentin, The tooth consists of crown and root. That part of the tooth cementoblast, cementum and osteoblast bone. These synthesiz- visible in the mouth is called clinical crown; the extent of ing cells have all the features of a protein secreting cell—well which increases with age and disease. The root portion of the developed ribosomes and a rough endoplasmic reticulum (ER), tooth is not visible in the mouth in health. The tooth is sus- Golgi apparatus, mitochondria and a vesicular nucleus, which pended in the sockets of the alveolar bone by the periodontal is often polarized. The cells that resorb the tissues have their ligament. The anatomical crown is covered by enamel and the names ending in ‘clast’. Thus, osteoclast resorbs bone, cemen- root by the cementum. Periodontium is the term given to sup- toclast, cementum and odontoclast resorbs all the dental tissues. porting tissues of the tooth. They include the cementum, peri- The ‘clast’ cells have a similar morphology in being multinucle- odontal ligament and the alveolar bone. The innermost portion ated giant cells. Their ultra structural features include numerous of the crown and root is occupied by soft tissue, the pulp. The lysosomes and ingested vacuoles. dentin occupies the region between the pulp and enamel in the Dentin is the first hard tissue of the tooth to form. Enamel crown, and between pulp and cementum in the root. starts its formation after the first layer of dentin has formed. The enamel formation is from its junction with dentin outwards, first in the cuspal/incisal and later in the cervical regions. Dentin formation is similar, but from the dentinoenamel junction, the DEVELOPMENT OF TOOTH formation is pulpward. Cementum formation occurs after the root form, size, shape and number of roots is outlined by the epithelial The tooth is formed from the ectoderm and ectomesenchyme. root sheath and dentin is laid down in these regions. Formation The enamel is derived from the enamel organ which is differen- of enamel, dentin and cementum takes place as a daily event in tiated from the primitive oral epithelium lining the stomo- phases or in increments, and hence they show incremental deum (primitive oral cavity). Epithelial mesenchymal interactions lines. In dentin and cementum formation, a layer of uncalcified take place to determine the shape of the tooth and the differen- matrix forms first, followed by its mineralization. While in tiation of the formative cells of the tooth and the timing of enamel formation enamel matrix is calcified, but its matura- their secretion. The ectomesenchymal cells which are closer to tion or complete mineralization occurs as a secondary event. the inner margins of the enamel organ differentiate into dental Mineralization occurs as a result of supersaturation of calcium papilla and the ectomesenchymal cells closer to the outer mar- and phosphorus in the tissue fluid. The formative cells concen- gins of the enamel organ become dental follicle. Dentin and trate the minerals from calcium phosphate (apatite) and secreteChapter-01.indd 1 6/15/2011 11:17:26 AM
    • 2 Orban’s Oral Histology and Embryology them into the organic matrix, in relation to specific substances like collagen, which act as attractants or nucleators for miner- PULP alization. The mechanism of mineralization is quite similar in all the hard tissues of tooth and in bone. The pulp, the only soft tissue of the tooth, is a loose connective tissue enclosed by the dentin. The pulp responds to any stimuli by pain. Pulp contains the odontoblast. Odontoblasts are ter- minally differentiated cells, and in the event of their injury and ENAMEL death, they are replaced from the pool of undifferentiated ecto- mesenchymal cells in the pulp. The pulp is continuous with the The enamel is the hardest tissue in the human body. It is the periodontal ligament through the apical foramen or through only ectodermal derivative of the tooth. Inorganic constituents the lateral canals in the root. Pulp also contains defense cells. account for 96% by weight and they are mainly calcium phos- The average volume of the pulp is about 0.02 cm3. phate in the form of hydroxyapatite crystals. These apatite crys- tals are arranged in the form of rods. All other hard tissues of the body, dentin, cementum and bone also have hydroxyapatite CEMENTUM as the principal inorganic constituent. Hydroxyapatite crystals differ in size and shape; those of the enamel are hexagonal and The cementum is comparable to bone in its proportion of inor- longest. Enamel is the only hard tissue, which does not have ganic to organic constituents and to similarities in its structure. collagen in its organic matrix. The enamel present in the fully The cementum is thinnest at its junction with the enamel and formed crown has no viable cells, as the cells forming it—the thickest at the apex. The cementum gives attachment to the ameloblast degenerates, once enamel formation is over. Therefore, periodontal ligament fibers. Cementum forms throughout life, all the enamel is formed before eruption. This is of clinical impor- so as to keep the tooth in functional position. Cementum also tance as enamel lost, after tooth has erupted, due to wear and forms as a repair tissue and in excessive amounts due to low tear or due to dental caries, cannot be formed again. Enamel, grade irritants. lacks not only formative cells but also vessels and nerves. This The cells that form the cementum; the cementoblast lines makes the tooth painless and no blood oozes out when enamel the cemental surface. Uncalcified cementum is usually seen, is drilled while making a cavity for filling. as the most superficial layer of cementum. The cells within the cementum, the cementocytes are enclosed in a lacuna and its process in the canaliculi, similar to that seen in bone, but in a far less complex network. Cementocytes presence is limited DENTIN to certain regions. The regions of cementum containing cells are called cellular cementum and the regions without it, are The dentin forms the bulk of the tooth. It consists of dentinal known as the acellular cementum. The acellular cementum is tubules, which contains the cytoplasmic process of the odonto- concerned with the function of anchorage to the teeth and the cel- blasts. The tubules are laid in the calcified matrix—the walls of lular cementum is concerned with adaptation, i.e. to keep the the tubules are more calcified than the region between the tooth in the functional position. Like dentin, cementum forms tubules. The apatite crystals in the matrix are plate like and throughout life, and is also avascular and noninnervated. shorter, when compared to enamel. The number of tubules near the pulp are broader and closer and they usually have a sinusoi- dal course, with branches, all along and at their terminus at the dentinoenamel or cementodentinal junction. The junction PERIODONTAL LIGAMENT between enamel and dentin is scalloped to give mechanical retention to the enamel. Dentin is avascular. Nerves are present The periodontal ligament is a fibrous connective tissue, which in the inner dentin only. Therefore, when dentin is exposed, by anchors the tooth to the alveolar bone. The collagen fibers of loss of enamel and stimulated, a pain-like sensation called sen- the periodontal ligament penetrate the alveolar bone and sitivity is experienced. The dentin forms throughout life with- cementum. They have a wavy course. The periodontal ligament out any stimulation or as a reaction to an irritant. The cells that has the formative cells of bone and cementum, i.e. osteoblast and form the dentin—the odontoblast lies in the pulp, near its bor- cementoblast in addition to fibroblast and resorptive cells—the der with dentin. Thus, dentin protects the pulp and the pulp osteoclast. Cementoclasts are very rarely seen as cemental resorp- nourishes the dentin. Though dentin and pulp are different tion is not seen in health. Fibroblast, also functions as a resorptive tissues they function as one unit. cell. Thus, with the presence of both formative and resorptiveChapter-01.indd 2 6/15/2011 11:17:27 AM
    • An Overview of Oral Tissues 3 cells of bone, cementum and connective tissue, and along with the wavy nature of the fibers, the periodontal ligament is able ERUPTION AND SHEDDING OF TEETH to adjust itself to the constant change in the position of teeth, and also maintains its width. The periodontal fibers connect all The eruption of teeth is a highly programed event. The teeth the teeth in the arch to keep them together and also attach the developing within the bony crypt initially undergo bodily and gingiva to the tooth. The periodontal ligament nourishes the eccentric movements and finally by axial movement make its cementum. The presence of proprioceptive nerve endings pro- appearance in the oral cavity. At that time, the roots are about vides the tactile sensation to the tooth and excessive pressure on half to two thirds complete. Just before the tooth makes its the tooth is prevented by pain originating from the pain recep- appearance in the oral cavity the epithelium covering it, fuses tors in the periodontal ligament. with the oral epithelium. The tooth then cuts through the degenerated fused epithelium, so that eruption of teeth is a bloodless event. Root growth, fluid pressure at the apex of the erupting teeth and dental follicle cells ALVEOLAR BONE contractile force are all shown to be involved in the eruption mechanism. The bony crypt forms and resorbs suitably to adjust Alveolar bone is the alveolar process of the jaws that forms and to the growing tooth germ and later to its eruptive movements. supports the sockets for the teeth. They develop during the The deciduous teeth are replaced by permanent successor teeth eruption of the teeth and disappear after the tooth is extracted as an adaptation to the growth of jaws and due to the increased or lost. The basic structure of the alveolar bone is very similar masticatory force of the masticatory muscles, in the process of to the bone found elsewhere, except for the presence of imma- shedding. The permanent successor teeth during the eruptive ture bundle bone amidst the compact bone lining the sockets movement cause pressure on the roots of deciduous teeth and for the teeth. The buccal and lingual plates of compact bone induce resorption of the roots. The odontoclast, which has a enclose the cancellous bone. The arrangement and the density similar morphology to osteoclast and participates in this event, of the cancellous bone varies in the upper and lower jaws and has the capacity to resorb, all dental hard tissues. is related to the masticatory load, the tooth receives. The ability of bone, but not cementum, to form under tension and resorb under pressure makes orthodontic treatment possible. ORAL MUCOSA The mucosa lining the mouth is continuous anteriorly with the skin of the lip at the vermilion zone and with the pharyngeal TEMPOROMANDIBULAR JOINT mucosa posteriorly. Thus, the oral mucosa and GI tract mucosa are continuous. The integrity of the mucosa is interrupted by This only movable bilateral joint of the skull has a movable the teeth to which it is attached. The oral mucosa is attached to fibrous articular disk separating the joint cavity. The fibrous the underlying bone or muscle by a loose connective tissue, layer that lines the articular surface is continuous with the peri- called submucosa. The mucosa is firmly attached to the perios- osteum of the bones. The fibrous capsule, which covers the teum of hard palate and to the alveolar process (gingiva). The joint, is lined by the synovial membrane. The joint movement mucosa in these regions is a functional adaptation to mastica- is intimately related to the presence or absence of teeth and tion, hence, they are referred to as masticatory mucosa. Elsewhere, to their function. except in the dorsum of tongue, the mucosa is loosely attached as an adaptation to allow the mucosa to stretch. The mucosa in these regions is referred to as lining mucosa. The stratified squamous epithelium varies in thickness and is either kerati- MAXILLARY SINUS nized as in masticatory mucosa or non-keratinized as in lining mucosa. The submucosa is prominent in the lining and is nearly The maxillary posterior teeth are related to the maxillary sinus absent in the masticatory mucosa. The cells that have the abil- in that, they have a common nerve supply and that their roots ity to produce keratin, called keratinocytes, undergo matura- are often separated by a thin plate of bone. Injuries to the lining tional changes and finally desquamate. The non-keratinocytes, and extension of infection from the apex of roots are often do not undergo these changes, and they are concerned either encountered in clinical practice. Developing maxillary canine with immune function (Langerhans cells) or melanin production teeth are found close to the sinus. Pseudostratified ciliated (melanocytes). The mucosa that attaches to the tooth is unique, columnar epithelium lines the maxillary sinus. thin and permeable. The fluid that oozes through this liningChapter-01.indd 3 6/15/2011 11:17:27 AM
    • 4 Orban’s Oral Histology and Embryology into the crevice around the tooth is called gingival fluid. It aids in defense against entry of bacteria, through this epithelium. STUDY OF ORAL TISSUES The mucous of the dorsum of tongue, is called specialized mucosa because it has the taste buds in the papillae. For light microscopic examination, the tissues have to be made thin and stained, so that the structures can be appreci- ated. The teeth (and bone) can be ground or can be decalcified before making them into thin slices. In the first method, all SALIVARY GLANDS hard tissues can be studied. In the second method, all the hard tissues except enamel, pulp and periodontal ligament can The major salivary glands (parotid, submandibular and sub- be studied. Soft tissues of the mouth require a similar prepara- lingual) and the minor salivary glands present in the submu- tion as soft tissues of other parts of the body for microscopic cosa, everywhere in the oral cavity except in gingivae and examination. anterior part of the hard palate; secrete serous, mucosa or mixed For traditional light microscopic examination, the tissues salivary secretion, into the oral cavity by a system of ducts. The have to be made into thin sections and differentially stained by acini, which are production centers of salivary secretion, are of utilizing the variations they exhibit in their biochemical and two types—the serous and the mucous acini. They vary in size immunological properties. There are various histochemical, and shape and also in the mode of secretion. The composition enzyme-histochemical, immunohistochemical, immunofluo- and physical properties of saliva differ between mucous and rescent techniques developed to enhance tissue characteristics. serous secretions. The ducts, act not merely as passageways for Apart from light microscopy, tissues can be examined using saliva, but also modify the salivary secretion with regard to electron microscope, fluorescent microscope, confocal laser quantity and electrolytes. The ducts, which vary in their struc- scanning microscope and autoradiography techniques for bet- ture from having a simple epithelial lining to a stratified squa- ter recognition of cellular details, functions and the series of mous epithelial lining, show functional modifications. events that take place within them.Chapter-01.indd 4 6/15/2011 11:17:27 AM