Book - Oral Histology and Embryology (1944) 8
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Orban B. Oral Histology and Embryology (1944) The C.V. Mosby Company, St. Louis.
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Chapter VIII - Maxilla and Mandible (Alveolar Process)
1. Development of Maxilla and Mandible
In the beginning of the second month of fetal life the skull consists of three parts: The chondrocranium, which is cartilaginous, comprises the base of the skull with the otic and nasal capsules; the desmocranium, which is membranous, forms the lateral walls and roof of the brain case; the appendicular or visceral part of the skull consists of the cartilaginous skeletal rods of the branehial arches.
The bones of the skull develop either by endochondral ossification, replacing the cartilage, or by intramembranous ossification in the mesenchyme. Intramembranous bone may develop in close proximity to cartilaginous parts of the skull, or directly in the desmocranium, the membranous capsule of the brain (Fig. 151).
The endochondral bones are the bones of the base of the skull: ethmoidal bone; inferior concha (turbinate bone); body, lesser wings, basal part of greater wings, and lateral lamella of pterygoid process of the sphenoid bone; petrosal part of temporal bone; basilar, lateral, and lower part of squamous portion of occipital bone. The following bones develop in the desmocranium: frontal bones; parietal bones; squamous and tympanic parts of temporal bone; parts of the greater wings, and medial lamina of pterygoid process of sphenoid bone; upper part of squamous portion of occipital bone. All the bones of the upper face develop by intramembranous ossification; most of them close to the cartilage of the nasal capsule. The mandible develops as intramembranous bone, lateral to the cartilage of the mandibular arch. This cartilage, Mecke1’s cartilage, is in its proximal parts the primordium for two of the auditory ossicles: incus (anvil) and malleus (hammer). The third auditory ossicle, the stapes (stirrup), develops from the proximal part of the skeleton in the second branchial arch which then gives rise to the styloid process, stylohyoid ligament and part of the hyoid bone which is completed by the derivatives of the third arch. The fourth and fifth arches form the skeleton of the larynx.
First draft submitted by Harry Sicher and Joseph P. Welnmanu.
Fig. 151. Reconstruction of the sl_£u1l ot a. human embryo 80 mm. long. gtarfilagez green. Intramembranous bones: punk. Endochondral bones: white. (Sxeher and Tandlerfl) A. Right lateral view. B. Left lateral view after removal of left int:-amernbranous bones.
The human maxillary bone is formed, on either side, from the union of two bones, premaxilla and maxilla, which remain separate in most other mammals. In man the two bones begin to fuse at the end of the second month of fetal life. The line of fusion is indicated in young individuals by the intermaxillary (incisive) suture on the hard palate.
Fig. 152. Development of the mandible as intramembranous hone lateral to Meckers cartilage (human embryo 45 mm. long).
The maxilla proper develops from ‘one center of ossification which appears in the sixth week. The bone is then‘ situated on the lateral side of the cartilaginous nasal capsule and forms the wall of the nasal cavity when the cartilage has disappeared. The prernaxilla, or os incisivum, has two independent centers of ossification. Ultimately, it forms that part of the maxilla which icarriesthe two incisors, the anterior part of the palatine process, the rim of the piriform aperture, and part of the frontal processfi“
Fig. 153. Development of mandibular symphysis. A. Newborn infant: symphysis wide open: mental ossicle (roentgenogram). B. Child 9 months: symphysis partly closed; mental ossicies fused to mandible (roentgenogram). C. Frontal section through mandibular symphysis of newborn infant. Connective tissue in midline is transformed into cartilage on either side which is later replaced by bone.
The mandible makes its appearance as a bilateral structure in the sixth week of fetal life and is a thin plate of bone lateral to, and at some distance from, Meckel’s cartilage (Fig. 152). The latter is a cylindrical rod of cartilage; its proximal end (close to the base of the skull) is continuous with the hammer, and is in contact with the anvil. Its distal end (at the midline) is bent upwards and is in contact with the cartilage of the other side (Fig. 151). The greater part of Meckel’s cartilage disappears without contributing to the formation of the bone of the mandible. Only a small part of the cartilage, some distance from the midline, is the site of endochondral ossification. Here, it calcifies and is invaded and destroyed by connective tissue and replaced by bone. Throughout fetal life the mandible is a paired bone the two halves of which are joined in the midline by fibrocartilage. This synchondrosis is called mandibular symphysis. The cartilage at the symphysis is not derived from Meckel’s cartilage but differentiates from the connective tissue in the midline. In it small irregular bones, known as the mental ossicles, develop and, at the end of the first year, fuse with the mandibular body. At the same time the two halves of the mandible unite by ossification of the symphysial fibrocartilage (Fig. 153).
2. Development of the Alveolar Process
Near the end of the second month of fetal life, the bones of maxilla and mandible form a groove which is open towards the surface of the oral cavity (Fig. 152). In a later stage the tooth germs are contained in this groove which also includes the dental nerves and vessels. Gradually, bony septa develop between the adjacent tooth germs, and much later the primitive mandibular canal is separated from the dental crypts by a horizontal plate of bone.
An alveolar process in the strict sense of the word develops only during the eruption of the teeth. It is important to realize that during growth part of the alveolar process is gradually incorporated into the maxillary or mandibular body while it grows at a fairly rapid rate at its free borders. During the period of rapid growth, a special tissue may develop at the alveolar crest. Since this tissue combines characteristics of cartilage and bone, it is called ch/(android bone (Fig. 154).
3. Structure of the Alveolab. Process
The alveolar process may be defined as that part of the maxilla and mandible which forms and supports the sockets of the teeth (Fig. 155). Anatomically, no distinct boundary exists between the body of the maxilla or mandible and their respective alveolar processes. In some places the alveolar process is fused with and partly masked by bone which is not functionally related to the teeth. In the anterior part of the maxilla the palatine process fuses with the alveolar process. In the posterior part of the mandible the oblique line is superimposed upon the bone of the alveolar process (Figs. 155, D and E’ ) . As a result of its adaptation to function, two parts of the alveolar process can be distinguished. The first consists of a thin lamella of bone which surrounds the root of the tooth and gives attachment to principal fibers of the periodontal membrane. This is the alveolar bone proper. The
Fig‘. 154. Vex-tical growth of mandible at alveolar crest. Formation of chontlrold bone which is replaced by typical bone.
second part is the bone which surrounds the alveolar bone and gives support to the socket. This has been called supporting bone.” The latter, in turn, consists of two parts: the compact bone (cortical plate) forming the vestibular and oral plates of the alveolar processes, and the spongy bone between these plates and the alveolar bone proper (Fig. 155). The cortical plates, continuous with the compact layers of maxillary and mandibular body, are generally much thinner in the maxilla than in the mandible. They are thickest in the bicuspid and molar region of the lower jaw, especially on the buccal side. In the maxilla the outer cortical plate is perforated by many small openings through which blood and lymph vessels pass. In the lower jaw the cortical bone of the alveolar process is dense and, occasionally, shows small foramina. In the region of the anterior teeth of both jaws the supporting bone is, usually, very thin. No spongy bone is found here and the cortical plate is fused with the alveolar bone proper (Figs. 155, B and C).
Fig. 155. Gross relations of alveolar processes:
A. Horizontal section through upper alveolar process.
B. Labiollngual section through upper lateral incisor.
C. Labiolingual section through lower cuspid.
D. Labiolingual section through lower second molar.
E. Labiolingual section through lower third molar. (Sicher and Tandlerfl)
Fig. 156. Diagrammatic illustration of the relation between the cemento-enamel junction of adjacent teeth and the shape of the crests of the alveolar septa. (Ritchey, B., and Orban,
Fig. 157A. Appositional growth of mandible by formation of circumferential lamellae.
Ehetshe are replaced by Haversian bone; remnants of circumferential lamellae in the ep .
The outline of the crest of the intraalveolar septa, as they appear in the roentgenogram, are dependent upon the position of the adjacent teeth. In a healthy mouth the distance between the cemento-enamel junction and the free border of the alveolar bone proper is fairly constant. In consequence the alveolar crest is often oblique if the neighboring teeth are inclined. In the majority of individuals the inclination is most pronounced in the premolar and molar region, the teeth being tipped mesially. Then, the ceniento-enamel junction of the mesial tooth is situated in a more occlusal plane than that of the distal tooth and the alveolar crest, therefore, slopes distally (Fig. 156).
The interdental and interradicular septa contain the perforating canals of Zuckerkandl and Hiischfeld, which house the interdental and interradicular arteries, veins, lymph vessels and nerves.
Fig. 157B. Bundle bone and I-Iaversian bone on the distal alveolar wall (silver impregnation).
Histologically, the cortical plates consist of longitudinal lamellae and Haversian systems (Fig. 157A) . In the lower jaw circumferential or basic lamellae reach from the body of the mandible into the cortical plates. The trabeculae of the spongy bone of the alveolar process are placed in the direction of the stresses to which it is subjected as a result of mastication (Fig. 158). The functional adaptation of this spongy bone is particularly evident between the alveoli of molars where the trabeculae show a parallel horizontal arrangement. From the apical part of the socket of lower molars trabeculae are, sometimes, seen radiating in a slightly distal direction. These trabeculae are less prominent in the upper jaw, because of the proximity of the nasal cavity and maxillary sinus. The marrow spaces in the alveolar process may contain hemopoietic, but, usually, contain fatty marrow. In the condyloid process, angle of mandible, maxillary tuberosity, and other foci cellular marrow is frequently found, even in adults.
Fig. 158. Suppox-ting trabeculae between alveoli. A. Roentgenogram or a. mandible.
B. Meslodistal section through mandibular molars showing alveolar bone proper and supporting bone.
The alveolar bone proper which forms the inner wall of the socket (Fig. 158) is perforated by many openings which carry branches of the interalveolar nerves and blood vessels into the periodontal membrane (see chapter on Periodontal Membrane). It is called cribriform plate or lamina dura ; the latter term refers to the dense appearance of the alveolar bone proper in roentgenograms. The alveolar bone proper consists partly of lamellated, partly of bundle bone. The lamellae of the lamellated bone are arranged roughly parallel to the surface of the adjacent marrow spaces, others form Haversian systems. Bundle bone is that bone to which the principal fibers of the periodontal membrane are anchored. The term bundle bone was chosen because the bundles of the principal fibers continue into the bone as Sharpey’s fibers. The bundle bone is characterized by the scarcity of the fibrils in the intercellular substance. These fibrils, moreover, are all arranged at right angles to the Sharpey’s fibers. The bundle bone appears much lighter in preparations stained with silver than lamellated bone because of the reduced number of fibrils (Fig. 157B). Because all the fibrils run in the same direction, the bundle bone is not lamellated.
4. Physiologic Changes in the Alveolar Process
The internal structure of bone is adapted to mechanical stresses. It changes continuously during growth and alteration of functional stresses. In the jaws this change takes place according to the growth, eruption, wear and loss of teeth. With advancing age parts of the bone and the osteocytes lose their vitality, and regeneration has to follow. All these processes are made possible only by a coordination of destructive and formative activities. Specialized cells, the osteoclasts, have the function of eliminating overaged bony tissue or bone which is no longer adapted to mechanical stresses.
Osteoclasts are multinucleated giant cells (Fig. 159, A). The number of nuclei in one cell may rise to a dozen or more. However, it is to be noted that, occasionally, uninuclear osteoclasts are found. The nuclei are vesicular, showing a prominent nucleolus and little chromatin. The cell body is irregularly oval or club-shaped and may show many branching processes. In general, osteoclasts are found in bay-like grooves in the bone which are called Howship’s lacunae; they are hollowed out by the activity of the osteoclasts. The cytoplasm which is in contact with the bone is distinctly striated. These striations have been explained as the expression of resorptive activity of these cells. The osteoclasts seem to produce a proteolytic enzyme which destroys or dissolves the organic constituents of the bone matrix. The mineral salts thus liberated are removed in the tissue fluid or ingested by macrophages. A decaleification of bone during life has often been claimed but has never been demonstrated.
Osteoclasts differentiate from young fibroblasts or undifferentiated mescnchymal cells probably by division of nuclei without the usual division of the cytoplasm. Some investigators believe that the osteoclasts may arise by fusion of osteoblasts, others believe that they difierentiate from endothelial cells of capillaries. The stimulus which leads to the diiferentiation of mesenchymal cells into osteoblasts or osteoclasts is not known. Osteoclastic resorption of bone is partly genetically patterned, partly functionally determined. Also overaged bone seems to stimulate the differentiation of osteoclasts‘ possibly by chemical changes that are the consequence of degeneration and final necrosis of the osteocytes.
New bone is produced by the activity of osteoblasts (Fig. 159, B). These cells also difierentiate from fibroblasts or the undifferentiated mesenchymal cells of the connective tissue. Functioning osteoblasts are arranged along the surface of the growing bone in a continuous layer, similar in appearance to a cuboidal epithelium.
Fig. 159. Resorption and apposition of bone. A. Osteoclasts in Howship’s lacunae B. Osteoblasts al 11 a ho tr b la. La. 1 1 fomafion (high magngflmflolxbe. a ecu yer o osteo 1! tissue as a sign of bone
The osteoblasts are said to produce the bone matrix by secretion. The matrix is at first devoid of mineral salts. At this stage, it is termed osteoid tissue. It is still undecided Whether the fibrils of the matrix are connective tissue fibrils which become embedded in the substance of the matrix, or whether the fibrils difierentiate in the primarily amorphous matrix. If a certain amount of matrix is produced some of the osteoblasts become embedded in the matrix, and are known as osteocytes. Normally, the organic matrix calcifies immediately after formation?’ 1‘ The ratio of organic and inorganic substance in dry bone is approximately 1 :2. (See Table in chapter on Enamel for complete data.)
5. Internal Reconstruction of Bone
The bone in the alveolar process is identical to bone elsewhere in the body and is in a constant state of flux. During the growth of jaw bones, bone is deposited on the outer surfaces of the cortical plates.. The changes are most readily observed in the mandible, with its thick cortical layer of compact bone. Here bone is deposited in the shape of basic or circumferential lamellae (Fig. 157A). When the lamellae reach a certain thickness they are replaced from the inside by Haversian bone. This is a reconstruction in accordance with the functional and nutritional demands of the bone. In the Haversian canals, closest to the surface, osteoclasts differentiate and resorb the Haversian lamellae, and part of the circumferential lamellae. After a time the resorption ceases and new bone is apposed onto the old. The scalloped outline of Howship’s lacunae which turn their convexity toward the old bone remains visible as a darkly stained cementing line, sometimes called the “reversal” line. This is in contrast to those cementing lines which seem to correspond to a rest period in an otherwise continuous process of bone apposition; they are called resting lines (Fig. 157A). Resting and reversal lines are found between layers of bone of varying age.
Another type of internal reconstruction is the replacement of compact bone by spongy bone. This can be observed following the growth of the bone when the compact outer layer has expanded to a certain extent. The process of destruction can be observed from a section through bone, by noting the remnants of partially destroyed Haversian systems, or partially destroyed basic lamellae which form the interstitial lamellae of the compact bone.
Wherever a muscle, tendon, ligament, or periodontal membrane is attached to the surface of bone, Sharpey’s fibers can be seen penetrating the basic lamellae. During replacement of the latter by Haversian systems fragments of bone containing Sharpey’s fibers remain in the deeper layers. Thus, the presence of interstitial lamellae containing Sharpey’s fibers indicates the former level of the surface.“
Alterations in the structure of the alveolar bone are of great importance in connection with the physiologic movements of the teeth. Thee movements are directed mesio-occlusally. At the alveolar fundus the continual apposition of bone can be recognized by resting lines separating parallel lavers of bundle bone; when the bundle bone has reached a cer tain thickness it is partly resorbed from the marrow spaces and then replaced by Haversian bone or trabeculae. The presence of bundle bone indicates the level at which the alveolar fundus was previously situated. During the mesial drift of a tooth, bone is apposed on the distal, and resorbed on the mesial, alveolar wall (Fig. 160). The distal Wall is made up almost entirely of bundle bone. However, the osteoclasts in the adjacent marrow spaces remove part of the bundle bone when it reaches a certain thickness. In its place lamellated bone is laid down (Fig. 157B).
Fig. 160. Meslel drift. .4. Appositlon of bundle bone on the distal alveolar wall. 3. Resorption of bone on the mesial alveolar wall. (Weinmann.”)
On the mesial alveolar wall of a drifting tooth signs of active resorption are observed by the presence of Ho\vship’s lacunae containing osteoclasts (Fig. 160). Bundle bone on this side is present in relatively few areas. When found, it usually forms merely a thin layer (Fig. 161). This is due to the fact that the mesial drift of a tooth occurs in a rocking motion. Thus, resorption does not involve the entire mesial surface of the alveolus at one and the same time. Moreover, periods of resorption alternate with periods of rest and repair. It is during these rest periods that bundle bone is formed, and detached periodontal membrane fibers are again secured. It is this alternating action that stabilizes the periodontal membrane attachment on that side of the tooth. Islands of bundle bone are separated from the lamellated bone by reversal lines which turn their convexities toward the lamellated bone (Fig. 161).
wan istin fly or simple lamellated bone; islands of Fig‘ 161 bfiilhslglbghgecfigghoringcggisnd giuoé-s of the periodontal membrane.
6. Clinical Considerations
Bone is one of the hardest tissues of the human body. Nevertheless, bone is, biologically, a highly plastic tissue. Where bone is covered by a vascularized connective tissue, it is exceedingly sensitive to pressure whereas tension acts, generally, as a stimulus to the production of new bone. It is this biologic plasticity which enables the orthodontist to move teeth Without disrupting their relations to the alveolar bone. Bone is resorbed on the side of pressure, and apposed on the side of 1361131011; thus allowing the entire alveolus to shift with the tooth.
The adaptation of bone structure to functional stresses is quantitative as Well as qualitative, namely, decreased function leads to a decrease in the bulk of the bone substance. This can be observed in the supporting bone of teeth which have lost their antagonists.’ Here, the spongy bone around the alveolus shows marked rarefication: the bone trabeculae are less numerous and very thin (Fig. 162). The alveolar bone proper, however, is generally well preserved because it continues to receive some stimuli by the tension exerted upon it via principal fibers of the periodontal membrane. A similar distinction in the behavior of alveolar and supporting bone can be seen in certain endocrine disturbances and vitamin deficienciesli 2 (Fig. 163).
Fig. 162. Osteoporosis of alveolar process caused by inactivity or the tooth which has no antagonist Labiolingual sections through upper molars of the same individual: 4. Disappearance of bony trabeculae after loss of function; plane or mesiobuccal root; alveolar bone proper remains intact. 3. Normal spongy bone in the plane of mesiobuccal root or functioning tooth.
The independence of the growth mechanisms of the upper and lower jaws accounts for their frequent variations in relative size. Trauma or inflammatory processes can destroy the condylar growth center of the mandible on one or both sides. I-Iyperfunction of the hypophysis, leading to acromegaly, causes a characteristic overgrowth of the mandible, even at a time when sutural growth has ceased.“ The maxillary growth in such cases is confined to bone apposition on the surfaces, because a general enlargement of the upper face is impossible.
During healing of fractures or extraction wounds, an embryonic type of bone is formed which only later is replaced by mature bone. The embryonic bone, immature or coarse fibrillar bone, is characterized by the great number, great size, and irregular arrangement of the osteocytes and the irregular course of its fibrils. The greater number of cells and the reduced volume of calcified intercellular substance renders this immature bone more radiolucent than mature bone. This explains why bony callus cannot be seen in roentgenograms at a time when histologic examination of a fracture reveals a well-developed union between the fragments and why a socket after an extraction wound appears to be empty at a time when it is almost filled with immature bone. The visibility in radiograms lags two to three weeks behind actual formation of new bone.
Fig. 163. Dlference in reaction of alveolar bone and supporting bone: A. Normal bone structure in bifurcation of dog's tooth. B. Osteoporosis of supporting bone in bifurcation ot dog’s tooth. Dog fed on diet deficient in nicotinic acid. Alveolar bone proper intact. (Similar conditions could be produced by other dietary deficiencies.) (Courtesy H. Becks,’ University of California.)
References
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