Figure 1  Sagittal sections through Carnegie stage 22 (right) and 23 (left) human embryos showing the formation of the palate. At stage 22 the tongue occupies the common oronasal cavity, whereas at stage 23 the tongue is depressed and the forming palate intervenes between the oral and nasal cavities.
Figure 2  Coronal sections through Carnegie stage 22 (left) and 23 (right) human embryos showing the formation of the palate. The palatal shelves are independent on either side of the tongue and depress it as the shelves extend toward the midline.
Figure 3  Human embryo at 62 days. The entire embryo has been stained with alizarin red, which preferentially stains mineralized bone, to show the membranous bones forming in association with the skull and face.
Figure 4  Rat embryo at 16 days stained with alizarin red (to show bone) and alcian blue (to show cartilage). No ossification is detectable at this stage.
Figure 5  Rat embryo at 17 days stained with alizarin red (to show bone) and alcian blue (to show cartilage). Note the membranous bone formation lateral to Meckel’s cartilage and the formation of the condylar cartilage.
Figure 6  Rat embryo at 19 days. Mandible stained with alizarin red (to show bone) and alcian blue (to show cartilage) showing the primary (Meckel’s) and secondary (condylar, coronoid, angular, and alveolar) cartilages associated with the membranous bone forming the body of the mandible.
Figure 7  Coronal section through a developing embryo. A number of developmental features are apparent. The tongue is easily identified. Note Meckel’s cartilage and intramembranous ossification of the forming mandible lateral to it. The forming mandible is growing alveolar plates to surround a bud stage tooth germ. In the developing upper jaw, a cap stage can be seen.
Figure 8  Late cap stage of tooth development. The forming tooth is undergoing histodifferentiation, and the dental papilla and dental follicle can be clearly distinguished. Note the enamel knot.
Figure 9  Bud stage of tooth development. Note the accumulation of ectomesenchymal cells around the epithelial bud.
Figure 10  High-power view of a developing tooth bud.
Figure 11  Early cap stage of tooth development. The enamel organ is expanding, but folding is just beginning.
Figure 12  Early cap stage of tooth development. The enamel organ is undergoing histodifferentiation, and the dental papilla and dental follicle can be clearly distinguished. Note the enamel knot.
Figure 13  Late cap stage of tooth development. The enamel organ has folded, producing a concavity into which the ectomesenchymal cells of the papilla accumulate. Intramembranous bone formation has initiated around the developing tooth crown.
Figure 14  High magnification of a late cap stage tooth organ. Note the difference in structural appearance between the inner and outer dental epithelia and the concentration of ectomesenchymal cells facing the inner dental epithelium.
Figure 15  Cap stage tooth bud. The enamel knot is an accumulation of epithelial cells on the internal aspect of the inner dental epithelium.
Figure 16  The enamel cord is a condensation of epithelial cells that extends from the inner to the outer dental epithelium.
Figure 17  Bell stage of tooth development.  The primary tooth has acquired its final shape but not its final size.
Figure 18  High-power view of the four layers of the enamel organ. (Courtesy P. Tambasco de Oliveira.)
Figure 19  Bell stage of tooth development. Both dentinogenesis and amelogenesis have begun (dentin, pink; enamel, purple) at the cusp tip. Note collapse of the stellate reticulum where mineralized matrix has been formed.
Figure 20  Bell stage of tooth development. Both dentinogenesis and amelogenesis have begun (dentin, pink; enamel, purple) at the cusp tip. Crown pattern formation has folded the inner dental epithelium upward to bring the ameloblasts close to the blood vessels situated outside the outer dental epithelium—the so-called collapse of the stellate reticulum.
Figure 21  Forming root. Hertwig epithelial root sheath (HERS) is present at the leading root edge.
Figure 22  Body of the mandible. The outer layers consist of compact bone, between which is a supporting network of trabecular bone.
Figure 23  Phase-contrast micrograph of woven bone. Note the random organization of the refringent collagen fibrils.
Figure 24  Ground section of lamellar bone. The osteon represents the basic structural unit. It consists of concentric lamellae that form a cylinder of bone with a vascular canal, the Haversian canal, at its center.
Figure 25  Lamellar bone visualized by phase-contrast microscopy. Note the concentric lamellae that form the osteon.
Figure 26  Interstitial lamellae (fragments of preexisting concentric lamellae) are interspersed between osteons, and circumferential lamellae enclose the outer and inner aspects of bone.
Figure 27  Osteocytes are entrapped within bone. These cells reside in lacunae, and their processes in interconnecting canaliculi (arrows) form an extensive network.
Figure 28  High magnification of an osteon showing its concentric lamellae, the centrally located Haversian canal, and the osteocyte lacunae interspersed among the lamellae.
Figure 29  Intramembranous bone formation in the region of the future mandible; plump-looking osteoblasts line forming bone surfaces.
Figure 30  The presence of numerous osteoclasts associated with this trabecula of embryonic bone suggests that it is being turned over rapidly.
Figure 31  Trabecular bone. All the cell types associated with bone can be recognized in this micrograph.
Figure 32  Osteocytes are usually found within the calcified matrix but can also be present within osteoid (asterisks).
Figure 33  Histochemical detection of tartrate-resistant acid phosphatase activity, a marker for osteoclasts in the primary spongiosa of the rat tibia growth plate.
Figure 34  Human osteoclasts stained for tartrate-resistant acid phosphatase. Note the presence of multiple nuclei in these cells.
Figure 35  Endochondral ossification in the rat tibia growth plate. The section is stained with von Kossa's stain to reveal mineral distribution (black deposits).
Figure 36  Mineral deposition starts in the calcification zone of the growth plate.
Figure 37  High magnification of mineralized cartilage in the growth plate. Note that mineral (black deposits) is only present in the longitudinal septae.
Figure 38  Light micrograph of intramembranous bone formation in the rat calvarium. The first step is condensation of ectomesenchymal cells between the skin and developing brain.
Figure 39  As intramembranous bone formation progresses, ectomesenchymal cells differentiate into osteoblasts that form woven cancellous bone.
Figure 40  Cross section of the cartilage model of a digit. Before the start of intramembranous bone formation, alkaline phosphatase activity appears in the connective tissue surrounding the anlage (perichondrium) and periosteum in areas of vascular invasion.
Figure 41  Vascular invasion of the cartilage model of a digit. Alkaline phosphatase–positive cells from the periosteum accompany the vascular elements and accumulate at the center of ossification.
Figure 42  The center of ossification of the cartilage model of a digit in longitudinal section. Alkaline phosphatase activity is present both in the center of ossification and the periosteum.
Figure 43  Section showing the hard and soft tissues of the tooth.
Figure 44  Tooth bud at the stage when both enamel and dentin formation begins.
Figure 45  Incisal tip of a tooth just before the start of the enamel layer formation.
Figure 46  A and B, Early crown stage of tooth development. Dentin (D) and enamel (E) have begun to form at the crest of the folded inner dental epithelium (incisal tip). There is a reduction in the amount of stellate reticulum (SR) in the region where matrix deposition has occurred. Note the developmental gradient in cell differentiation from the tip toward the cervical portion of the tooth crown. Am, Ameloblasts; Od, odontoblasts; ODE, outer dental epithelium; SI, stratum intermedium; PD, predentin.
Figure 47  Secretory stage amelogenesis. Tomes’ processes jut into enamel and in certain species in a "picket fence" appearance. The line at the base of the ameloblasts represents the proximal cell web (pcw), and that at the apex, the distal cell web (dcw).
Figure 48  Histologic section of a decalcified tooth along the slope of the cusp showing an incisal to cervical gradient in enamel maturation. As maturation progresses, enamel matrix is lost and mineral content increases. Almost mature enamel (top right) appears whitish because mineral has been removed and there is very little matrix left in this area. Note the striae of Retzius and morphology of maturation stage ameloblasts (no Tomes’ process).
Figure 49  Maturation stage of amelogenesis. A, Smooth-ended ameloblasts. Note that the three other layers of enamel organ have amalgamated together to form a highly infolded and vascular layer, the papillary layer. Ameloblasts undergo modulation, a process by which their apexes alternate between a smooth-ended border (A) and a ruffle-ended border (B).
Figure 50  Once enamel is completely mature, the enamel organ forms the reduced dental epithelium. At this stage, ameloblasts are no longer distinguishable.
Figure 51  Mature enamel. In decalcified preparation, the fully calcified enamel is completely removed, leaving behind the space it occupied. Note also that the enamel organ on the cuspal aspect of the tooth has reorganized into a reduced dental epithelium in which individual cell layers cannot be distinguished.
Figure 52  Indicator dyes can be used to detect regional variations in pH along the maturing enamel of rat incisors that correspond to the modulation cycle of ameloblasts. (Courtesy C. E. Smith.)
Figure 53  Ground section viewed by contrast-phase microscopy. In a longitudinal section of the tooth, the striae of Retzius are seen as a series of dark lines extending from the dentinoenamel junction toward the tooth surface and capping its tip.
Figure 54  Striae of Retzius manifest on the surface of the tooth as a series of grooves called perikymata.
Figure 55  Striae of Retzius manifest on the surface of the tooth as a series of wavelike grooves, or Hunter-Schreger bands, as seen in a decalcified section of maturing enamel.
Figure 56  Transmitted light image of ground section showing the alternating orientation of a group of rods in the region of Hunter-Schreger bands.
Figure 57  Phase-contrast microscopic image of the longitudinal ground section of a calcified tooth.
Figure 58  Transmitted light image of cross-sectional ground section of a tooth  showing a lamella and concentric lines/bands representing the striae of Retzius.
Figure 59  Ground section of a tooth showing the disposition of striae of Retzius and of enamel tufts at the dentinoenamel junction.
Figure 60  High magnification of the dentinoenamel junction.
Figure 61  Enamel tufts resemble tufts of grass in ground section.
Figure 62  Ground sections permit ready visualization of the scalloped appearance of the dentinoenamel junction. Also note the complex trajectory of the enamel rods in the inner enamel.
Figure 63  Enamel spindles represent odontoblast processes trapped in enamel.
Figure 64  Odontoblast differentiation and initial dentin formation. An acellular zone separates the undifferentiated cells of the dental papilla from the differentiating ameloblasts. The preodontoblasts gradually develop into tall and polarized cells with their nucleus away from the matrix; they deposit at the interface with ameloblasts.
Figure 65  Higher magnification of the acellular zone separating differentiating odontoblasts and ameloblasts.
Figure 66  The first secretory products of odontoblasts accumulate as an unmineralized layer, predentin, that gradually mineralizes to form mantle dentin.
Figure 67  Thicker fibers of collagen (arrowheads) originate from between odontoblasts and extend into the forming mantle predentin. These fibers are referred to as von Korff’s fibers.
Figure 68  Mineralization foci appear in the initial matrix deposited by odontoblasts. These foci eventually grow and coalesce to form the mantle dentin.
Figure 69  Primary dentin. Odontoblasts border the pulp chamber and line the predentin surface. Below the odontoblasts is a cell-free zone followed by a cell-rich zone.
Figure 70  Ground section of dentin stained to demonstrate dentin phosphophoryn (mauve). Note its absence from mantle and reparative dentin. (Courtesy Takagi Y, Sasaki S: J Oral Pathol 15:463, 1986.) 
Figure 71  Undemineralized section of the mature dentin-pulp complex. The vascularity of the pulp is evident. The cell-free zone of Weil can be clearly seen beneath the odontoblast layer.
Figure 72  Tooth section stained to demonstrate the nerves of the pulp. Note the plexus beneath the odontoblast layer.
Figure 73  Histologic preparation illustrating the transformation of predentin into mineralized dentin along a linear mineralization front (arrows).
Figure 74  Globular mineralization results in an irregular mineralization front (arrows) at the predentin-dentin interface.
Figure 75  Odontoblasts have apical processes that remain in the matrix they form.
Figure 76  Silver-stained section illustrating the globular nature of the mineralization front.
Figure 77  Interglobular dentin represents unmineralized matrix regions resulting from imperfect globular mineralization.
Figure 78  Ground section of dentin showing the dentinal tubules in which the odontoblast processes run.
Figure 79  The junction between primary and secondary dentin is characterized by a change in the direction of dentinal tubules (arrowheads).
Figure 80  Cellular cementum. Cementocytes have extensive cell processes that point toward the root surface. This calcified ground section shows the lacunae and canaliculi that accommodate the cementocyte bodies and processes, respectively.
Figure 81  Pulp stone (false). Note the concentric layers of matrix and the absence of cells. Its proximity to dentin surface suggests that it may eventually become embedded in it (attached pulp stone).
Figure 82  Ectopic calcification. Illustrated here is a “free” pulp stone; it is not attached to dentin. Note the concentric layering of its matrix, which reflects a phasic growth pattern. (Courtesy P. Tambasco de Oliveira.)
Figure 83  A tooth and mandible cut in the sagittal plane. (Courtesy P. Tambasco de Oliveira.)
Figure 84  Histologic preparation illustrating the tissues supporting and investing the tooth. These consist of cementum, periodontal ligament, alveolar bone, and that part of the gingiva facing the tooth.
Figure 85  The undersurface of a developing root. The apical foramen is still widely open and will close when root formation is completed.
Figure 86  Forming root. Hertwig epithelial root sheath is present only on the advancing root edge. 
Figure 87  Histologic section through an elongating root. The apical foramen is widely open and delimited by a slight inward inflection of Hertwig epithelial root sheath, called the diaphragm. 
Figure 88  Histologic section of the advancing root edge in a rat molar during acellular extrinsic fiber cementum (AEFC) formation. In the rat, Hertwig epithelial root sheath (HERS) is still present when radicular dentin calcifies.
Figure 89  Periodontal tissues. Note the presence of epithelial cell rests of Malassez along the root surface.
Figure 90  Remnants of Hertwig epithelial root sheath persist in the periodontal ligament (PDL) as clusters of cells called epithelial rests of Malassez (ERM). Cb, Cementoblast; Cc, cementocyte; CIFC, cellular intrinsic fiber cementum.
Figure 91  Light micrograph of a porcine forming root. Epithelial cell rests of Malassez (ERM) are present in the periodontal ligament (PDL) close to the surface of acellular extrinsic fiber cementum (AEFC) and sometimes appear as relatively long strands of cells.
Figure 92  In a tangential section to the tooth surface, the epithelial cell rests of Malassez appear to form a network. 
Figure 93  Epithelial cell rests of Malassez. Note the abundance of heterochromatin in the nuclei.
Figure 94  Collagen fiber bundles (arrows) pass between cementoblasts and insert into cementum.
Figure 95  Section of a human tooth extracted for orthodontic reasons. The periodontal ligament had been destroyed during extraction, but cementoblasts and fiber fringes extending between them are still visible.
Figure 96  In ground sections, the granular layer of Tomes’ process (GLT) appears as a region containing dark polymorphic structures at the interface between dentin and cementum, here of the acellular extrinsic fiber variety (AEFC).
Figure 97  Ground section showing the transition between acellular extrinsic (AEFC) and cellular intrinsic (CIFC) fiber cementum. Note the lacunae occupied by cementoblasts in cellular cementum.
Figure 98  In some cases, cementum overlaps enamel at its cervical extremity. This cementum is generally of the acellular afibrillar variety; in a porcine specimen, the overlapping cementum is of the cellular variety, particular to this animal species.
Figure 99  Phase-contrast image of the cementoenamel junction. In some cases, cementum and enamel do not abut, leaving a region of exposed dentin (arrow) between them that may lead to tooth sensitivity. (Courtesy P. Tambasco de Oliveira.)
Figure 100  Phase-contrast image of cementoenamel junction. In the majority of cases cementum and enamel meet end to end (arrow) along the cervical margin of the crown.  (Courtesy P. Tambasco de Oliveira.)
Figure 101  Low-power view of the tooth support tissues.
Figure 102  Periodontal tissues. Note the longitudinal lines in cementum resulting from its appositional deposition.
Figure 103  The periodontal ligament is a highly cellular and vascular connective tissue.
Figure 104  A, Histologic section of periodontal tissues examined by transmitted light. AEFC, Acellular extrinsic fiber cementum. B, Same section examined by polarized light, which allows for readily seen striations in the cementum layer and the lamellar organization of the bone. (Courtesy P. Tambasco de Oliveira.)
Figure 105  Acellular extrinsic fiber cementum (AEFC). Some histologic stains allow visualization of both a longitudinal layering (successive layers of cementum) and a fibrous fringe at the surface of the cementum.
Figure 106  Appositional growth lines in cellular intrinsic fiber cementum (CIFC).
Figure 107  Histologic section stained to highlight the fibrous component of the periodontal ligament.
Figure 108  Specially prepared section to demonstrate oxytalan fibers in the periodontal ligament.
Figure 109  Histologic preparation of alveolar bone examined by transmitted light microscopy. Periodontal ligament fiber bundles insert into the bone lining the alveolar socket, giving it the name bundle bone. The inserted fibers are referred to as Sharpey’s fibers. (Courtesy P. Tambasco De Oliveira.)
Figure 110  Root resorption. The lost dentin has essentially been replaced by cellular cementum, on top of which acellular cementum has formed.
Figure 111  Periodontium. The surface of the alveolar bone shows many osteoclasts, indicating that it is undergoing remodeling.
Figure 112  The transseptal ligament (part of the gingival ligament) is situated just below the junctional epithelium and extends from the cementum of one tooth, over the alveolar crest, to the cementum of an adjacent tooth.
Figure 113  Micrograph of the globular organization of salivary glands.
Figure 114  Lobule of a salivary gland showing the presence of both serous and mucous acini.
Figure 115  Salivary gland section stained to demonstrate mucous acini.
Figure 116  Seromucous demilunes capping mucous acini in the sublingual gland.
Figure 117  Salivary gland immunostained to demonstrate actin in the contractile myoepithelial cells.
Figure 118  Parotid gland. Connective tissue septae divide the serous acini into lobules. 
Figure 119  Higher-magnification view of parotid gland lobule.
Figure 120  Submandibular gland. This mixed gland contains both serous and some mucous acini.
Figure 121  Higher magnification of the submandibular gland in Figure 120. Striated ducts and mucous acini with serous demilunes can be seen.
Figure 122  Monkey mandible in sagittal section showing the two primary molars in function and the partially erupted first molar. Note the position of the permanent premolar tooth germs between the resorbing roots of the deciduous molars.
Figure 123  The gubernacular canal overlying the crypt of an erupting permanent incisor. The canal is filled with soft connective tissue and an epithelial strand, which is the remnant of the dental lamina.
Figure 124  Odontoclasts resorbing dentin.
Figure 125  Erupting molar. Both the reduced dental epithelium, overlying the enamel space in this demineralized section, and the oral epithelium have begun to proliferate into the intervening connective tissue as it breaks down.
Figure 126  Erupting tooth. The reduced dental organ and overlying tissues reorganize as the tooth is about to protrude into the oral cavity. 
Figure 127  Erupting molar. Fusion of the reduced dental epithelium and the oral epithelium has occurred to form the beginning of an epithelial-lined canal.
Figure 128  The mucogingival junction (arrows) can be readily seen in a healthy dentition.
Figure 129  The junctional epithelium attaches the gingiva to the tooth surface.
Figure 130  The dentogingival junction. Junctional, sulcular, and keratinized gingival epithelium can all be distinguished.
Figure 131  Higher magnification of the gingiva. Note the various fiber groups in the gingival ligament.
Figure 132  Free gingiva. The sulcular epithelium is not keratinized, whereas that of the exposed gingival surface is keratinized.
Figure 133  The interdental papilla is the part of the gingiva that fills the space between two adjacent teeth.
Figure 134  Attached gingiva. Note the thick layer of keratin.
Figure 135  Mucogingival junction. Keratinized gingiva (right) and nonkeratinized mucosa (left) are shown.
Figure 136  Masticatory mucosa covering the hard palate.
Figure 137  Attached gingiva. This masticatory mucosa has no distinct submucosa. The collagen fibers of the lamina propria attach directly and firmly to the periosteum of the alveolar bone.
Figure 138  Palate. The lamina propria consists of a dense connective tissue. Fat can be found in some regions of the submucosa.
Figure 139  Sagittal section through the tongue. The dorsal surface is covered by a specialized keratinized and nonkeratinized mucosa, whereas the ventral surface shows a thinner, nonkeratinized epithelium. Filiform papillae cover the entire anterior part of the tongue.
Figure 140  Specialized mucosa of the tongue. Filiform papillae cover the anterior dorsal portion of the tongue.
Figure 141  Low-power view of a circumvallate papilla.  The papilla is surrounded by a deep circular groove into which open the ducts of minor salivary glands.
Figure 142  Taste buds line the lateral walls of circumvallate papillae.
Figure 143  Specialized mucosa. Taste bud at the junction of the hard and soft palates.
Figure 144  Keratinized epithelium of the skin just below the vermillion zone of the lip. Sebaceous glands and hair follicles are present within the dermis.
Figure 145  Block dissection of a human temporomandibular joint.
Figure 146  Section through the rat temporomandibular articulation.
Figure 147  Temporomandibular articulation. The condylar cartilage undergoes typical endochondral ossification.
Figure 148  Lateral view of bones of the temporomandibular joint. (From Liebgott B: The anatomical basis of dentistry, ed 2, St. Louis, 2001, Mosby.) 
Figure 149  Mandibular fossa and articular eminence of temporal bone. A, Lateral aspect. B, Inferior aspect of the base of the skull. (From Liebgott B: The anatomical basis of dentistry, ed 2, St. Louis, 2001, Mosby.)	
Figure 150  Internal features of the temporomandibular joint. A, Sagittal section. B, Coronal section. (From Liebgott B: The anatomical basis of dentistry, ed 2, St. Louis, 2001, Mosby.)