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These U-shaped bands-dental laminae-follow the curves of the primitive jaws. Bud Stage of Tooth Development Each dental lamina develops 10 centers of proliferation from which swellings-tooth buds (tooth germs)-grow into the underlying mesenchyme. The tooth buds for permanent teeth that have deciduous predecessors begin to appear at approximately 10 weeks from deep continuations of the dental lamina (see. The permanent molars have no deciduous predecessors and develop as buds from posterior extensions of the dental laminae (horizontal bands). The tooth buds for the permanent teeth appear at different times, mostly during the fetal period. Cap Stage of Tooth Development As each tooth bud is invaginated by mesenchyme-the primordium of the dental papilla and dental follicle-the bud becomes cap shaped. The ectodermal part of the developing tooth, the enamel organ, eventually produces enamel. The internal part of each cap-shaped tooth, the dental papilla, is the primordium of dentine and the dental pulp. The outer cell layer of the enamel organ is the outer enamel epithelium, and the inner cell layer lining the papilla is the inner enamel epithelium (see. The central core of loosely arranged cells between the layers of enamel epithelium is the enamel reticulum (stellate reticulum). As the enamel organ and dental papilla of the tooth develop, the mesenchyme surrounding the developing tooth condenses to form the dental sac (dental follicle), a vascularized capsular structure (see. The periodontal ligament is the fibrous connective tissue that surrounds the root of the tooth, attaching it to the alveolar bone (see. Bell Stage of Tooth Development As the enamel organ differentiates, the developing tooth assumes the shape of a bell. The mesenchymal cells in the dental papilla adjacent to the internal enamel epithelium differentiate into odontoblasts, which produce predentine and deposit it adjacent to the epithelium. As the dentine thickens, the odontoblasts regress toward the center of the dental papilla; however, their fingerlike cytoplasmic processes-odontoblastic processes (Tomes processes)-remain embedded in the dentine (see. The color of the translucent enamel is based on the thickness and color of the underlying dentine. Cells of the inner enamel epithelium differentiate into ameloblasts under the influence of the odontoblast, which produce enamel in the form of prisms (rods) over the dentine. As the enamel increases, the ameloblasts migrate toward the outer enamel epithelium. Enamel and dentine formation begins at the cusp (tip) of the tooth and progresses toward the future root. D, At 10 weeks, showing the early bell stage of a deciduous tooth and the bud stage of a permanent tooth. Note that the connection (dental lamina) of the tooth to the oral epithelium is degenerating. I, Section through a developing tooth showing ameloblasts (enamel producers) and odontoblasts (dentine producers). The root of the tooth begins to develop after dentine and enamel formation are well advanced. The inner and outer enamel epithelia come together in the neck of the tooth (cementoenamel junction), where they form a fold, the epithelial root sheath (see. The odontoblasts adjacent to the epithelial root sheath form dentine that is continuous with that of the crown. As the dentine increases, it reduces the pulp cavity to a narrow root canal through which the vessels and nerves pass (see. The inner cells of the dental sac differentiate into cementoblasts, which produce cement that is restricted to the root. Cement is deposited over the dentine of the root and meets the enamel at the neck of the tooth. As the teeth develop and the jaws ossify, the outer cells of the dental sac also become active in bone formation. The tooth is held in its alveolus (bony socket) by the strong periodontal ligament, a derivative of the dental sac (see.

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Depending on the disability and its severity, various positioning methods may be helpful for increasing visibility into the mouth and reducing excessive movement. American Dental Association: Evidence-based clinical recommendations on the prescription of dietary fluoride supplements for caries prevention, J Am Dent Assoc 141: 1480-1489, 2010. Brudevold F, Naujoks R: Caries-preventive fluoride treatment of the individual, Caries Res 12(suppl l):52-64, 1978. In Treatment eff ects in the national preventive dentistry demonstration program, Santa Monica, Calif, 1984, Rand Corporation, pp 19-4 1. Seppa 1, Lepponen T, Hausen H: Fluoride varnish versus acidulated phosphate fluoride gel: a 3-year clinical trial, Caries Res 28:327-330, 1995. Dye A, Tan S, Smith V et al: Trends in oral health status: United States, 1988-1994 and 1 999-2004, Vital Health Stat 1 l (248):1-92, 2007. American Dental Association Council on Scientific Affairs: Professionally applied topical fluoride: evidence-based clinical recommendations, J Am Dent Assoc 1 37: 1 1 5 1-1 1 59, 2006. American Academy of Pediatric Dentistry: Guideline on the role of dental prophylaxis in pediatric dentistry, Pediatr Dent 33(special issue}: 15 1-152, 20 1 1. Sizes, shapes, colors, timers, and motivational characters are widely available from all manu facturers. The attention span of a typical 3- to 6-year old is short, and with at least 100 surfaces on primary teeth to clean, they require parental assistance and supervision. Power toothbrushes have been shown to be as effective as manual toothbrushes and have the additional feature of being fun to use (Figure greatly increased. A small amount of dentifrice should be placed on the brush and the child instructed to expectorate when brushing is completed. Large amounts of dentifrice are not indicated, and studies have shown that preschoolers swallow large amounts of dentifrice that may contribute to development of fluorosis. Therefore compliance is Good hygiene practice suggests that mouth hygiene care be performed after meals. When they are unable to do so, a thorough swishing of the mouth with water is recommended. At bedtime, mouth care is especially important because of the reduction in saliva production at night with an increase in acid production. Toward the end of this period the pre school child begins to lose the primary teeth. The areas of exfoliation may be painful and the gingiva may be swollen, leading to discomfort. During these times the parent must assist the child daily to maintain the habits established earlier and to eliminate additional inflammation around exfoliating teeth. This chapter is intended to identify commonly used materials in pediatric dentistry and provide information that applies specifically to their use. Many mate rials are available, and in many cases, clinical considerations will dictate the choice of the appropriate material. Table 20-1 identifies the most commonly used materials in pediatric restorative dentistry and the relevant clinical considerations. Bases and Liners the use of bases and liners is important in pediatric den tistry. Bases and liners are available to reduce marginal microleakage from the restoration and to prevent sensitivity to the underlying tooth structure. Traditionally, preparations of calcium hydroxide, zinc oxide-eugenol, and zinc phos phate were the materials of choice. A catalyst paste containing calcium hydroxide, zinc oxide, and zinc stearate in ethylene toluene sulfonamide reacts with a base paste containing calcium tungstate, calcium phosphate, and zinc oxide in glycol salicylate to form an amorphous calcium disalicylate. Studies have shown that calcium hydroxide "softens" under amalgam and resin-based com posite restorations. As hydrolysis occurs, occlu sal forces cause gingival displacement of the restoration, leading to discrepancies and breakdown at the restoration margin. Visible light-cured calcium hydroxide preparations have demonstrated clinical success3 and may be less suscep tible to hydrolysis. When calcium hydroxide is used, a less soluble high-strength base may be placed to overlie the calcium hydroxide. The rosin increases fracture resistance and the zinc acetate is effective in accelerating the reaction rate.

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The attack may last for a varying period of time and may recur at short intervals. Deafness is sensorineural in type, fluctuating, usually unilateral and progressive. As the disease progresses the deafness becomes more pronounced and speech discrimination worsens. Recent studies have shown a spontaneous remission rate of upto 71 per cent of cases within 8 years of diagnosis. The hearing loss is more for the lower frequencies in the early stages of the disease. Vestibular function test: Spontaneous nystagmus is absent except during an attack. Pure tone audiometry and speech audiometry are done after intervals of one hour for 2 to 3 hours and compared with pretest records. Improvement in hearing signifies an endolymphatic hydrops in about 60 percent of cases. The test is contraindicated in patients with cardiac and renal diseases as well as in diabetics. Strong reassurance and stressing the nonfatal nature of the disorder is necessary. The following drugs are commonly used- prochlorperazine (Stemetil) 15 to 75 mg daily, orally or by injections; promethazine (Avomine, Phenergan); chlorpromazine (Largactil 25 mg thrice daily); or dimenhydrinate (Dramamine). Sometimes the stellate ganglion block during an acute attack helps to relieve the symptoms. Dietetic therapy: It is suggested that low salt and limited water intake reduces the hydrops. Favourable effects have resulted from the administration of nicotinic acid and vitamin A and D. Diuretic therapy: Diuretics like acetazolamide have been used on the assumption that these drugs will reduce the hydrops. Vasodilators: Such drugs have been used with an idea that they relieve the angiospastic vascular changes in the endarterial distribution of the labyrinthine artery. Streptomycin therapy: Previously large doses of streptomycin were used particularly in bilateral cases to inducel labyrinthine damage (Chemical Labyrinthectomy). Cervical sympathectomy: the operation is thought to correct the microcirculatory fault in the labyrinth and thus relieve the symptoms. Myringotomy with grommet insertion: the exact mode of action is not clearly known. Operations on the endolymphatic sac: the aim of the operation is to decompress and/or drain the sac (shunt operation) so that adequate absorption of endolymph occurs with resultant relief of the hydrops. Vestibular neurectomy: this involves selective section of the vestibular division of the eighth nerve, particularly in cases with intractable vertigo but with a good hearing. Labyrinth destruction: Total destruction of the labyrinth (labyrinthectomy) may be a last resort for cases with intractable symptoms and poor hearing levels. This is accomplished by selective destruction of the vestibular end organs in the labyrinth without damaging the cochlea or facial nerve. Cryosurgical methods: A cryoprobe is placed on the semicircular canal or promontory and subnormal temperature achieved which causes destruction of the adjacent labyrinthine tissue. Ultrasound: Ultrasonic vibrations are passed to the semicircular canal by an applicator through the mastoid route. Eighth nerve tumour (acoustic neuroma): Differentiation of this condition is difficult particularly in the early stages when it only gives otological symptoms. X-ray studies of internal auditory meatus and other tests like myelography, and scanning help in the final diagnosis. Vestibular neuronitis: this disease is characterised by vertigo of sudden onset, sometimes occurring in small epidemics, often with a recent history of upper respiratory tract infection. Benign positional vertigo: the patient complains of recurring attacks of vertigo which are induced by change in position. Neurological examination is normal, hearing is unaffected and caloric tests are also usually normal. The only consistent physical finding is the vertigo and nystagmus that develop when the head is placed in a particular position.

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Thus, when the margins of the metal crown nearly approximate the greatest diameter of the tooth, the spaces are small enough so that the metal can be adapted closely to the tooth. The margins of the trimmed crown should approximate the shape of the gingival crest around the tooth. As you look at the marginal gingiva around the first to second primary molar, as well as from buccal to it is traced from distal to mesial. If you can picture the letter S on its side and stretched out somewhat, and if a tooth crown is placed on top of this curved line, the term "stretched out S" can be used to describe the contour. The proximal 2 1 - 1 7, contours of the lingual marginal gingiva of all first primary molars resemble "smiles" (Figure contours of almost all primary teeth "frown" (Figure B) because the shortest occlusocervical heights are about midpoint buccolingually. The margins of the finished, trimmed steel crown consist of a series of curves or arcs as determined by the marginal gingiva of the tooth being restored. However, there are several areas of consideration as follows: Prepare occlusal reduction of one tooth completely before beginning occlusal reduction of the other tooth. When reduction of two teeth is performed simultaneously, the tendency is to underreduce both. A Insufficient proximal reduction is a common problem when adjacent crowns are placed. Contact between adja cent proximal surfaces should be broken, producing an approximately l. Note the contour in the region of the mesiobuccal bulge of the first primary molar. B, the proximal gingival contour of primary molars h as been described as a frown because the shortest occlusocervical heights are about m idpoint buccolingually. As described in an B Both crowns should be trimmed, contoured, and prepared for cementation simultaneously. It is generally best to begin placement and cementation of the more distal tooth first. Most importantly, however, the sequence of place ment of crowns for cementation should follow the same sequence as that when the crowns were placed for final fitting. Sometimes crowns will seat quite easily in one placement sequence and will seat with great difficulty if the sequence is altered. When this happens, the crown required to fit over the buc colingual dimension will be too wide mesiodistally to be placed and a crown selected to fit the mesiodistal space will be too small in circumference. This adjust ment is accomplished by grasping the marginal ridges of the crown with Howe utility pliers and squeezing it, thereby reducing the mesiodistal dimension. Considerable recon touring of proximal, buccal, and lingual walls of the crown with the no. Contouring and crimping pliers are necessary to apply the appropriate gingival adapta tion. Keeping the principles of crown length and marginal shape in mind will ensure optimal adaptation and clinical success of the crown. When a choice exists between a Class I I amalgam and a crown i n a child younger than 4 years, the l ikelihood of fai l u re of the amalgam is approximately twice that of the crown. If the clinician will not see the child regu larly or home care will not be sufficiently supervised to ensure compliance, an additional advantage of crowns is the preventive aspect that ful l coverage provides. I n the caries-prone child or the child for whom recal l and long-term follow-u p will be lacking, this restoration provides protection from recurrent caries. When these restorations fail, replacement with another amalgam/composite is usua l ly i m possible because recur rent caries removal and redefi nition of the preparation further weaken the tooth. A crown (and m a ny times a pulpotomy, depending on recurrent caries involvement) is required for retreatment. Cost-compa rison studies of restorations in primary teeth have shown that a malgam replaced by a crown is the most costlyY Third-party payers are requiri ng increased accou ntability for the cost-effectiveness of the outcomes of our treatment. This then becomes part of the multifactorial decision-making process we use to select a restoration, and the expectation for long-term success m ust be considered. Because it is frequently l u m ped with cast crowns in the minds of many den tists, however, it is viewed as a n aggressive restorative a pproach and its indications for use in the primary dentition are poorly u nderstood. For m any practitioners, the decision to restore a tooth with a crown denotes excessive tooth removal, great expense, and a genera l feeli ng of havin g com promised the tooth. Rarely do dental students have time to assimilate the differences in treatment planning for the developin g, changing primary denti tion compared with the more static nature of the permanent dentition.

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Blocking: We index the instances based on their objects in two knowledge bases respectively, and then select the instances which contain the same keys as candidate instance pairs. We limit the number of pairs to be compared by this step, which significantly improve the efficiency of the system. Multi-strategy: We implement several matchers in our instance matching system, we can execute these matchers in parallel and then aggregate the result according to the characteristics of the source ontologies. For exmaple, there are a number of cross-lingual links between two different language versions 210 of Wikipedia. To make full use of these data, we formalize the instance matching as a binary classification problem, and use the reference mappings to train a classifier, which will determine whether an instance pair is equivalent or not. It presents a novel multi-strategy method to be fit for different kinds of ontology and employs a learning-based approach to get instance alignments in multilingual environments. Preprocess: the system begins with Preprocess, which loads the ontologies and parameters into system. In the meantime, preprocessor can get some meta data about the two ontologies, which will be used in the later processes, Predicate alignment and Matcher choosing 2. Predicate Alignment: In this process, we will get the alignments of the predicates between the two ontologies. Matcher choosing: the system will choose the most suitable one or more matchers according to the meta data of the ontologies. Candidate Pairs Generation: In this step, we get candidate pairs when the instances have the same literal objects on some discriminatory predicates. In case of unsupervised method, we calculate similarities between two instances on each property, and then we aggregate these similarities according to the degree of identifying obtained in step 1. On the contrary, we conduct a supervised method when there exist reference alignments. Then we construct a similarity vector for each pairs and train a logistic regression model [7]. For each candidate instance pair, we use this model to determine whether it is equivalent or not. Validation: We will evaluate the alignment result on Precision, Recall and F1Measure if there is validation data set. We also calculate some information of each predicate, in order to obtain the degree of identifying of predicates which will be used in similarity aggregation. Predicate Alignment: the predicates can express rich semantics, and there exist one-to-one, one-to-many, or many-to-many relationships among these predicates. It is apparent that we should get the alignments of the predicates before we calculate the similarity of instances. Blocking: this step aims to pick a relatively small set of candidate pairs from all pairs. Due to the large scale of knowledge bases, it is impossible to calculate matching scores of all instance pairs. This method may reduce the recall slightly, but it also reduce the scale of computation significantly. If the range of predicates is similar, the label-based approach will play a key role in the matching process. Unsupervised method: we use a object-based method to get alignments, it is defined as follows: p fpn (i1, i2) = Sim(Oi1n, Oi2n) p (1) p where i1 and i2 are instances from two data sets respectively. Sim(Oi1n, Oi2n) represent the similarity of object values between these two instances on property pn and its corresponding property pn. For example, we use Levenshtein distance for type:text and indicator function for type:int. Supervised method: In equation 2, the weight wi is determined by meta-data of ontology or manual. Intuitively, it could be improved by a learning-based method if we have some existing alignments. So, basically, we formulate this instance matching problem as a binary classification problem. Thus, we can use a sigmoid function to compute the probability i=1 that instances i1 is equivalent with i2. The assumption in this model is that we can use the machine learning method to determine which property is more important for instance matching problem.

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The effectiveness of these procedures is controversial, especially for conditions such as irreversible pulpitis/-9 the long buccal nerve supplies the molar buccal gingivae and may provide accessory innervation to the teeth. A small quantity of solution is deposited in the mucobuccal fold at a point distal and buccal to the most posterior molar (Figure 28-8). Some operators advocate the use of a periodontal liga ment injection for anesthetizing singular teeth. Insert needle in the mucobuccal fold at a point distal and buccal to the most posterior molar. Remove the needle and apply pressure to the area with 2 x 2 inch gauze for hemostasis. These responses range from dizziness, blurred limits of local anesthetics are exceeded in the presence of a n d death. The primary effect of local anesthetics on the heart is that of myocardial depression. I I Local complications may include mastica oxygen supplementation, ventilatory support, and possible hospitalization. One of the advan Wand handpiece is used in conjunction with it (Milestone tage of the system is the thin, wandlike syringe that appears restorative procedure. Some evidence sug childP anesthetic at a relatively slow rate and constant pressure by similar to and is held like a pen. A foot control delivers the gests this system may appear less threatening to a young Vibratory devices that attach to dental syringes have the area, thus reducing the time of soft tissue paresthesia. One study has indicated that there is a reduction of thought to facilitate the clearance of local anesthetic from 55% to on the gate control theory of pain and possibly auditory also been introduced. Future studies may provide evidence for their efficacy in minimizing injection discomfort. Electronic dental anesthesia is based primarily on the concept of hyperpolarization of excitable tissue; however, it has not been well accepted in pediatric dentistry despite evidence that children may prefer electronic dental anesthesia to traditionally delivered local anesthesia. The purpose of this section is to present basic techniques and surgical principles needed to perform oral surgical proce dures safely and competently on children and adolescents. Their reduced size more easily allows placement in the smaller oral cavity of the child patient. The smaller working ends (beaks) more closely adapt to the anatomy of the primary teeth. The choice of the proper instrumentation can also depend on special considerations unique to the child and the ado lescent. The use of cow horn mandibular forceps is contra indicated for primary teeth, owing to the potential for injury to the developing premolars. Great care must also be given to the routine use of elevators and forceps adjacent to large restorations such as chrome crowns and especially restora tions adjacent to erupting single-rooted teeth that may easily become dislodged with the slightest force. Important considerations in caring for the child patient include the following: 1. Being fully capable of managing emergency situations when they occur addition to the medical preoperative evaluation, it is important to perform a thorough dental preoperative evaluation, which includes taking appropriate preoperative radiographs. These often include two or more periapical radiographs of the same area to determine buccal, lingual, facial, or palatal relationships of impacted teeth. Another preoperative consideration is the future need for space main tenance as a result of the premature loss of primary teeth. Failure to provide immediate space maintenance may allow for the mesial migration of permanent first molars after premature primary molar loss. However, most pediatric dentists and oral and maxillofacial surgeons prefer the smaller pediatric extraction forceps, such as the no. It is essential that the dentist take the time to describe the ensuing procedure completely and accurately to the child. The extraction appointment should always begin with proper topical and local anesthesia, with consideration given to oral, intravenous, or nitrous oxide sedation on an individual basis (see Chapter 8). The choice of proper local anesthesia/sedation/general anesthesia tech nique depends on the psychological constitution of the child and the extent and nature of the surgical procedure. The appropriate local anesthetic technique for each type of tooth is described earlier in this chapter. Several steps of the extraction procedure should be per formed with every extraction.

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Inferiorly, at the midline of the anterior nasal aperture, the right Chapter 6 Osteology 69 and left maxillae fuse, forming a small, bony, nipplelike structure, the anterior nasal spine. Posteriorly, the nasal cavity extends to the posterior nasal aperture, or choanae, where, similarly, the horizontal plates of the palatine bones fuse in the midline to form the posterior nasal spines. The nasal cavity is divided in the midline into right and left halves by the nasal septum, composed of the perpendicular plate of the ethmoid anteriorly and superiorly and the vomer bone inferiorly and posteriorly. The sphenoid, maxillae, and palatine bones also make minor contributions to the bony nasal septum. The floor of each nasal cavity is formed by the horizontal plate of the palatine bone posteriorly and by the palatine process of the maxilla anteriorly. The incisive canals are located at the junction of the vomer with the anterior-most portion of the palatine process of each maxilla. These canals transmit the descending septal arteries and the nasopalatine nerves, which course along on both sides of the nasal septum. The two incisive canals open on the oral palatal surface of the maxillae in the midline just posterior to the interproximal aspect of the central incisors, at the incisive foramina housed in the incisive fossa. The lateral wall of the nasal cavity is rather complex because it contains foramina communicating with the sinuses, meatuses (which form air passages in an anteroposterior direction), and their overlying turbinate bones, known as conchae. Several bones, listed in an anteroposterior direction, participate in the formation of the lateral wall: the maxilla, lacrimal, ethmoid, and palatine bones; the medial pterygoid plate of the sphenoid; and the inferior nasal concha. The ethmoid bone has turbinate bones, the superior and middle conchae, protruding into the nasal cavity. The superior meatus extends as far as the middle concha, and it communicates with the posterior ethmoid air cells. The space below and deep to the middle nasal concha and superior to the inferior nasal concha is the middle meatus. This meatus communicates indirectly with the anterior ethmoidal air cells, directly or indirectly with the frontal sinus, and with the maxillary sinus via the opening (ostium) of the maxillary sinus. The space lateral and inferior to the inferior nasal concha is the inferior meatus, which extends as far inferiorly as the floor of the nasal cavity. The portion of the face between the inferior rim of the orbit and the upper teeth is formed primarily by the maxillae. Lateral to this is the suture between the zygomatic process of the maxilla and the maxillary process of the zygoma, with the two processes contributing to the bony cheek prominence (Figs 6-1, 6-2, and 6-6). The inferior-most aspects of the two maxillae house the 16 maxillary teeth, forming the upper dental arch. Each maxilla contains a central and a lateral incisor and a canine, whose single root forms a prominent tuberosity on the maxilla, known as the canine eminence. Medial to the canine eminence is a fossa superior to the two incisors, the incisive fossa, and a similar fossa located lateral to the canine eminence, known as the canine fossa (see. Teeth of this arch articulate with those of the mandible, the only bone of the skull that possesses the capacity to move. The right and left halves of the mandible each contain a central and lateral incisor and a canine, whose single root is demarcated on the mandible as the canine eminence. At the level of the second premolar of the mandible is the mental foramen, through which the mental nerve and vessels exit the mandibular canal. Occasionally, a line indicating the mental symphysis may be observed in the midline inferior to the interdental septum between the two central incisors, extending through the mental protuberance or point of the chin. This represents the line of fusion of the right and left halves of the mandible during embryogenesis. The oblique line, the angle, and the anterior border of the mandible are also evident from this view. The lateral view displays the cranial vault with some of the sutures between the various bones making up the cranium, some of the bones of the face, and the bones forming the zygomatic arch. The face is formed by 14 bones: the paired nasal bones, zygoma, lacrimal bones, maxillae, palatine bones, inferior nasal conchae, and the singular vomer and mandible.

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Seven pairs of ribs (1-7)-true ribs-attach through their own cartilages to the sternum. Five pairs of ribs (8-12)-false ribs-attach to the sternum through the cartilage of another rib or ribs. The last two pairs of ribs (11 and 12)-floating ribs-do not attach to the sternum. Development of the Sternum A pair of vertical mesenchymal bands, sternal bars, develop ventrolaterally in the body wall. They fuse craniocaudally in the median plane to form cartilaginous models of the manubrium, sternebrae (segments of the sternal body), and xiphoid process. Centers of ossification appear craniocaudally in the sternum before birth, except that for the xiphoid process, which appears during childhood. Development of the Cranium the cranium (skull) develops from mesenchyme around the developing brain. Later, endochondral ossification of the chondrocranium forms the bones in the base of the cranium. The ossification pattern of these bones has a definite sequence, beginning with the occipital bone, body of sphenoid, and ethmoid bone. The parachordal cartilage, or basal plate, forms around the cranial end of the notochord (see. This cartilaginous mass contributes to the base of the occipital bone; later, extensions grow around the cranial end of the spinal cord and form the boundaries of the foramen magnum (see. The hypophysial cartilage forms around the developing pituitary gland (Latin, hypophysis cerebri) and fuses to form the body of the sphenoid bone. The trabeculae cranii fuse to form the body of the ethmoid bone, and the ala orbitalis forms the lesser wing of the sphenoid bone. Otic capsules develop around the otic vesicles, the primordia of the internal ears (see Chapter 18), and form the petrous and mastoid parts of the temporal bone. Nasal capsules develop around the nasal sacs (see Chapter 9) and contribute to the formation of the ethmoid bone. Membranous Neurocranium Intramembranous ossification occurs in the mesenchyme at the sides and top of the brain, forming the calvaria (cranial vault). During fetal life, the flat bones of the calvaria are separated by dense connective tissue membranes that form fibrous joints, the sutures. The softness of the bones and their loose connections at the sutures enable the calvaria to undergo changes of shape during birth, called molding. During molding of the fetal cranium (adaptation of fetal head to the pelvic cavity during birth), the frontal bones become flat, the occipital bone is drawn out, and one parietal bone slightly overrides the other one. Cartilaginous Viscerocranium Most mesenchyme in the head region is derived from the neural crest. Neural crest cells migrate into the pharyngeal arches and form the bones and connective tissue of craniofacial structures. Homeobox (Hox) genes regulate the migration and subsequent differentiation of the neural crest cells, which are crucial for the complex patterning of the head and face. These parts of the fetal cranium are derived from the cartilaginous skeleton of the first two pairs of pharyngeal arches (see Chapter 9). The dorsal end of the first pharyngeal arch cartilage forms two middle ear bones, the malleus and incus. The dorsal end of the second pharyngeal arch cartilage forms the stapes of the middle ear and the styloid process of the temporal bone. Its ventral end ossifies to form the lesser horn (Latin, cornu) and superior part of the body of the hyoid bone. The third, fourth, and sixth pharyngeal arch cartilages form only in the ventral parts of the arches. The third arch cartilages give rise to the greater horns and the inferior part of the body of the hyoid bone. The fourth pharyngeal arch cartilages fuse to form the laryngeal cartilages, except for the epiglottis (see Chapter 9). Membranous Viscerocranium Intramembranous ossification occurs in the maxillary prominence of the first pharyngeal arch (see Chapter 8) and subsequently forms the squamous temporal, maxillary, and zygomatic bones. The mesenchyme in the mandibular prominence of the first pharyngeal arch condenses around its cartilage and undergoes intramembranous ossification to form the mandible.

References:

  • https://www.who.int/genomics/publications/GTS-MedicalGeneticServices-oct06.pdf
  • https://www.supremecourt.gov/opinions/19pdf/17-1618_hfci.pdf
  • https://imccs.org/wp-content/uploads/2020/02/World-Climate-Security-Report-2020_2_13.pdf
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