THE ANATOMICAL RECORD 263:314 –325 (2001) Prenatal Development of the Human Mandible SUK KEUN LEE,1 YEON SOOK KIM,1 HEE SOO OH,2 KYU HO YANG,2 EUN CHEOL KIM,3 AND JE GEUN CHI4* 1 Department of Oral Pathology, Kangnung National University College of Dentistry, Seoul, Korea 2 Department of Pedodontics, Chonnam National University Dental College, Seoul, Korea 3 Department of Oral Pathology, Wonkwang University Dental College, Seoul, Korea 4 Department of Pathology, Seoul National University College of Medicine, Chongno-gu, Seoul, Korea ABSTRACT In an effort to better understand the interrelationship of the growth and development pattern of the mandible and condyle, a sequential growth pattern of human mandibles in 38 embryos and 111 fetuses were examined by serial histological sections and soft X-ray views. The basic growth pattern of the mandibular body and condyle appeared in week 7 of fertilization. Histologically, the embryonal mandible originated from primary intramembranous ossification in the fibrous mesenchymal tissue around the Meckel cartilage. From this initial ossification, the ramifying trabecular bones developed forward, backward and upward, to form the symphysis, mandibular body, and coronoid process, respectively. We named this initial ossification site of embryonal mandible as the mandibular primary growth center (MdPGC). During week 8 of fertilization, the trabecular bone of the mandibular body grew rapidly to form muscular attachments to the masseter, temporalis, and pterygoid muscles. The mandible was then rapidly separated from the Meckel cartilage and formed a condyle blastema at the posterior end of linear mandibular trabeculae. The condyle blastema, attached to the upper part of pterygoid muscle, grew backward and upward and concurrent endochondral ossification resulted in the formation of the condyle. From week 14 of fertilization, the growth of conical structure of condyle became apparent on histological and radiological examinations. The mandibular body showed a conspicuous radiating trabecular growth pattern centered at the MdPGC, located around the apical area of deciduous first molar. The condyle growth showed characteristic conical structure and abundant hematopoietic tissue in the marrow. The growth of the proximal end of condyle was also approximated to the MdPGC on radiograms. Taken together, we hypothesized that the MdPGC has an important morphogenetic affect for the development of the human mandible, providing a growth center for the trabecular bone of mandibular body and also indicating the initial growth of endochondral ossification of the condyle. Anat Rec 263:314 –325, 2001. © 2001 Wiley-Liss, Inc. Key words: mandible growth; condyle growth; mandibular primary growth center; human; fetus The mandible, derived from the first branchial arch mesenchyme, remains one of the most debated topics in the morphogenesis of oro-facial structure. The mandible, comparable to long bone, is movable and antagonistic to the maxilla with the control of masticatory, facial expression, and some suprahyoid muscles (Azeredo et al., 1996; Bareggi et al., 1995; Lee et al., 1992). Anatomically, the mandible is connected to the temporal bone through the temporomandibular joint, innervated by a mandibular branch of the trigeminal nerve and serves important functions such as mastication, deglutition, and speech. Through the outcome of phylogenetic © 2001 WILEY-LISS, INC. evolution it is likely that the mandible has evolved into more complex regulatory development via different Grant sponsor: Ministry of Health and Welfare, Republic of Korea; Grant number: HMP-98-M-4-0048. *Correspondence to: Je Geun Chi, MD, Department of Pathology, Seoul National University College of Medicine, 28 Yongondong, Chongno-gu, Seoul 110-799, Korea. E-mail: email@example.com Received 22 August 2000; Accepted 15 February 2001 Published online 00 Month 2001 DEVELOPMENTAL PATTERN OF HUMAN MANDIBLE 315 Fig. 1. Measurements of prenatal mandibular growth. a, b: soft X-ray view of 24-week-old fetus, a; lateral view, b; vertical view, c, d: scheme of (a) and (b). Co, condyle head; Go, Gonion; Al, alveolar bone; Lb, lower border of mandible; MdPGC, mandibular primary growth center. pathways, i.e., muscular, alveolar, neural, and articular parts (Goret-Nicaise and Dhem, 1984; Jakobsen et al., 1991; Padwa et al., 1998). Previous studies on mandibular development were focused mainly on the growth of condyle and symphysis (Bareggi et al., 1995; Ben-Ami et al., 1992; Berraquero et al., 1995; Bjork and Skieller, 1983; Kjaer, 1978b; Morimoto et al., 1987; Orliaguet et al., 1993b). A study on mandibular growth in an early human fetal development (weeks 8 –14) revealed the mandibular ramus grew faster than the body, both in length and height; the greatest growth rate was found in the height of ramus; and the mandibular growth patterns differed significantly from those of successive developmental periods (Bareggi et al., 1995). Many authors had emphasized the importance of growth of the Meckel cartilage (Bhaskar et al., 1953), condylar head in mandibular growth (Kjaer 1978a; Morimoto et al., 1987; Shibata et al., 1996; Xu et al., 1983). A precise description of the prenatal human mandibular growth and developmental pattern, however, has not been reported. The purpose of this study is to investigate a sequential growth pattern of the prenatal human mandible using radiological and histological methods. This study is intended to show how morphogenetic evidence of the prenatal mandible relates to the developmental mechanism and functional structure of the human mandible. MATERIALS AND METHODS Thirty-eight normally developed embryos and 111 fetuses were obtained from the Department of Pathology, Seoul National University Hospital after thorough gross and microscopic examinations. Gestational age of each embryo and fetus was deduced from the crown-rump length or maternal records. The 38 embryos aged from 5– 8 weeks of fertilization (six at 5 weeks old; 19 at 6 weeks old; eight at 7 weeks old; and five at 8 weeks old, respectively). Embryos were fixed in 10% buffered formalin, embedded in paraffin, serially sectioned in 4 m thickness on sagittal, transverse, or horizontal planes, and stained with hematoxylin and eosin. Twenty-three fetal heads developed early, ranging from week 9 to week 15 of fertilization (six at 9 weeks old; five at 10 weeks old; one at 12 weeks old; four at 13 weeks old; four at 14 weeks old; and three at 15 weeks old, respectively). Fetal heads were fixed in 10% buffered formalin, decalcified in 10% EDTA, pH 7.0, embedded in paraffin, and serially sectioned on frontal and horizontal planes in 4 m thickness and stained with hematoxylin and eosin. The later-developed fetal mandibles (from 17 to 40 weeks of gestation) were removed and fixed in 10% buffered formalin. Removed mandibles were radiographed on lateral and vertical views using Faxitron (Hewlett Packard, Corvallis, OR) and soft X-ray film (Fuji, Tokyo, Japan). The specimens were decalcified in 5% nitric acid, embedded in paraffin, and longitudinal and cross sections of the mandibles were made in 4 m thickness and stained with hematoxylin and eosin. A point of concentric radiopacity at the apical area of deciduous first molar, from which linear trabecular bones radiate to all directions of the mandible, was named as the mandibular primary growth center (MdPGC). For the statistical analysis, five measurements of the fetal mandible were made on the lateral and vertical view: 1) the length of condyle growth was measured from MdPGC to condyle head (Co); 2) the length of anterior mandibular growth was measured from MdPGC to symphysis; 3) the length of posterior mandibular body growth was measured from MdPGC to mandibular angle (Go); 4) the length of anterior mandibular height growth was measured from upper border of alveolar (Al) bone to lower border (Lb) of mandible through the MdPGC; and 5) the length of posterior mandibular height growth was measured from Go to Co. The gonial angles formed by lower and posterior borderlines of the mandible at the mandibular angle were also measured (Fig. 1). RESULTS Growth of Mandibular Body In the middle of week 5 of fertilization (Streeter stage 16), a pair of Meckel cartilage appeared in the center of mandibular arch along with the growth of mandibular 316 LEE ET AL. TABLE 1. Incremental growth of mandibular measurements of human fetus on radiogram Fertilization age (week) Cases (n ⫽ 111) Co-MdPGC (mm) Co-Go (mm) MdPGC-Go (mm) MdPGC-Sym (mm) Al-Lb (mm) Gonial angle 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 1 2 2 1 4 5 7 4 3 2 3 6 3 4 4 6 10 5 5 4 3 6 4 4 10 3 14.1 15.10.5 16.90.1 17.2 18.32.0 17.90.9 20.90.6 23.01.7 23.32.3 24.91.2 25.74.5 24.81.7 26.01.8 27.81.7 27.02.6 28.61.9 29.62.6 30.80.6 31.22.0 32.32.0 32.91.4 35.53.3 35.12.9 35.92.2 35.12.9 39.32.6 4.5 6.50.0 6.90.1 7.0 7.50.6 8.40.4 9.90.5 11.10.4 11.00.5 11.60.0 11.80.7 12.00.3 11.90.2 12.30.2 13.10.6 13.10.8 14.30.6 14.20.3 14.80.8 15.80.6 17.00.5 17.30.6 17.00.8 17.80.6 18.21.2 19.80.3 8.0 9.50.7 9.50.7 10.0 10.50.4 11.00.3 12.41.3 12.91.0 12.80.3 14.50.7 15.20.8 14.60.5 16.20.8 17.81.0 16.60.8 17.70.8 18.21.0 18.30.3 17.40.5 18.10.3 18.71.2 20.01.1 20.31.3 19.91.2 21.21.4 22.30.6 — 5.90.4 5.30.5 6.4 18.104.22.168 8.01.1 8.21.0 8.50.4 9.10.0 9.50.9 9.31.6 9.21.6 10.40.3 12.11.6 12.72.0 11.51.3 13.22.4 13.01.2 11.92.1 10.51.5 15.52.2 14.22.5 13.30.9 15.40.5 13.90.9 15.51.6 4.0 4.30.4 4.80.4 5.0 5.00.7 5.8 7.40.6 7.50.4 7.30.6 8.0 9.01.0 7.90.4 8.80.3 9.30.3 9.90.6 9.60.5 10.70.8 11.10.2 10.70.4 11.60.4 11.30.6 12.00.7 12.50.6 12.40.5 13.40.8 12.80.8 150 1484 1484 — 1483 1423 1474 1448 1400 1450 1445 1465 1464 1467 1466 1416 1403 1414 1397 1447 1423 1438 1418 1404 1367 1395 nerves and vessels to form the hyaline cartilaginous tissue and thick perichondral fibrous mesenchyme. From the middle of week 6 of fertilization (Streeter stage 19), the mandibular ossification appeared as intramembranous bony apposition in close approximation to the Meckel cartilage. The initial intramembranous ossification of the mandible began at the facial fibrous mesenchyme around the Meckel cartilage, with a direct contact (Fig. 5a), or an encirclement with the Meckel cartilage in contrast to the other long bones. In week 7 of fertilization (Streeter stage 21), the linear trabeculae of mandible developed anteroposteriorly from the initial ossification of the mandible. Serial sections revealed that these linear trabeculae branched toward the future mandibular symphysis, alveolar bone, mandibular body, and coronoid process. At this time, the genioglossus muscle (Fig. 5b,c) was tightly attached to the lower side of the anterior portion of Meckel cartilage, whereas the primordium of masticatory muscle was located around the middle portion of Meckel cartilage, i.e., masseter as well as temporalis muscles on the facial side and pterygoid muscle on the lingual side of Meckel cartilage respectively. As the ossification of mandible progressed into the week 7– 8 of fertilization (Streeter stage 22–23), the muscular attachment of the genioglossus muscle gradually changed from Meckel cartilage into the anterior portion of linear trabeculae of mandibular symphysis (Fig. 5b). The primordia of masticatory muscles (Fig. 6a), i.e., masseter, temporalis, and pterygoid muscles, departed from the Meckel cartilage and repositioned around the linear trabeculae of mandible. Late in week 9 of fertilization, the serial sections of masseter and temporalis muscles showed muscular attachment to the buccal side of mandibular body and coronoid process, respectively. The pterygoid muscle (Fig. 6c), which had been primarily located on the lingual side of Meckel cartilage, was far from the lingual side of mandibular body, because the mandibular body was gradually shifted toward the facial direction. Simultaneously, the pterygoid muscle gradually moved toward the posterior portion of mandibular body and was divided mesially and laterally respectively; the former attached to the lingual side of posterior mandibular body and the latter attached to the posterior end of linear trabeculae of mandible, but not to Meckel cartilage. Serial sections of the embryonic jaw also revealed that the thinned fibrous mesenchyme around Meckel cartilage was traced to the thickened periosteal mesenchyme of mandible. Subsequently, the condyle blastema appeared with the condensation of cellular mesenchyme at the posterior end of linear trabeculae of the mandible with an attachment to the lateral pterygoid muscle. In week 10 of fertilization, the mandibular ossification advanced to form an anatomical structure of the lower jaw including the mandibular angle (Fig. 7a), coronoid process (Fig. 7a), and symphysis (Fig. 5e). By this time, the lower part of genioglossus muscle was attached to the lower portion of mandibular symphysis, while the upper part of genioglossus muscle was still attached to the anterior portion of Meckel cartilage. In week 11 of fertilization, as the anteroposterior growth of the mandible increased with multilayered bony trabeculae, then the upper part of genioglossus muscle was almost detached from Meckel cartilage. The site of initial intramembranous ossification of the mandible (Fig. 5d), however, remained approximate to the Meckel cartilage (Fig. 5d,f,g). In week 12 of fertilization, most of the genioglossus muscle was attached to the lower portion of anterior DEVELOPMENTAL PATTERN OF HUMAN MANDIBLE Fig. 2. Growth pattern of prenatal mandible by soft X-ray view, lateral view of mandible. a: 16-week-old fetus; b: 17-week-old fetus; c: 18-week-old fetus; d: 20-week-old fetus; e: 25-week-old fetus; f: 30-week-old fetus; g: 34-week-old fetus; h: 38-week-old fetus. 317 318 LEE ET AL. Fig. 3. Incremental growth of representative measurements of prenatal human mandible. Co-MdPGC, length from condyle head to mandibular primary growth center; MdPGCGo, length from mandibular primary growth center to Gonion; Co-Go, length from condyle head to Gonion; MdPGC-Sym, length from mandibular primary growth center to mandibular symphysis; Al-Lb, length from alveolar bone to lower mandibular border. Fig. 4. Changes of gonial angle during fetal period. Fig. 5. Relationship between Meckel cartilage and mandible during morphogenetic stage of human embryos. a: Six weeks old, mandible (Md) appears at facial side of Meckel cartilage (Mc) (HE, ⫻40). Tg, tongue; DL, dental lamina. b: Eight weeks old, lingual side of Meckel cartilage is not covered with intramembranous ossification of mandible, note the genioglossus muscle (Gg) attached tightly both on the lower side of anterior portion of Meckel cartilage and on the portion of mandibular symphysis (HE, ⫻20). Mx, maxilla; UL, upper lip; LL, lower lip. c: High magnification of (b), intramembranous ossification of mandible is closely associated with the fibrous mesenchyme of Meckel cartilage (HE, ⫻70). d: Eleven weeks old, mandibular ossification continuously grows outwardly from Meckel cartilage with a direct contact (HE, ⫻70), square; site of initial intramembranous ossification. e: Ten weeks old, frontal section, newly formed mandibular arch is larger than Meckel cartilage arch (HE, ⫻60). Sym, symphysis. f: Eleven weeks old, horizontal section of mandible shows the initial ossification site of mandible that was named as mandibular primary growth center (MdPGC) (HE, ⫻10). SM, submandibular gland; Hy, hyoid cartilage. g: High magnification of (f), the MdPGC showed ramifying trabecular structure, the portion of Meckel cartilage approximated by MdPGC was rapidly resolved (arrows) (HE, ⫻60). h: Twelve weeks old, sagittal section showing longitudinal alignment of Meckel cartilage and linear trabecular bone of mandible (⫻5). TG, trigeminal ganglion; SM, submandibular gland; IE, inner ear organ; Ey, Eye. i: High magnification of (h), the linear trabeculae of mandible was approximated to the anterior portion of Meckel cartilage (⫻70). To, tooth germ. j: Ten weeks old, hyalin cartilage with intact cellular morphology (inlet, ⫻1,000) (HE, ⫻200). k: Twelve weeks old, the chondrocytes were swollen and some of them disappeared (arrows) (HE, ⫻400). l: Twenty weeks old, the Meckel cartilage was shrunken and separated from mandible (HE, ⫻200). m: high magnification of (l), peripheral chondrocytes were gradually resolved (arrows) (HE, ⫻1,000). DEVELOPMENTAL PATTERN OF HUMAN MANDIBLE Figure 5. 319 320 LEE ET AL. mandible to form the median symphyseal structure. As a result, Meckel cartilage became completely detached from the linguo-mandibular architecture (Fig. 5f,g) and rapidly decreased in size (Fig. 5h,i). At this stage, the Meckel cartilage became atrophic and its perichondral fibrous mesenchyme remained thin, and its hyaline cartilage tissue also showed degenerative changes of chondrocytes, i.e., enlarged empty lacunae without nuclei or pyknotic/karyorrhectic nuclei (Fig. 5j,k). The cartilage matrix and chondrocytes of the Meckel cartilage were gradually shrunken and finally resolved with infiltration of tissue macrophages into the perichondral fibrous tissue. The Meckel cartilage, however, showed no endochondral ossification until later in fetal life (Fig. 5l,m). From week 12 of fertilization, the intramembranous bony ossification was active at the periphery of ramifying trabeculae of the mandibular body, coronoid process and symphysis. The central trabecular bone became thick and sclerotic (Fig. 5h,i). During weeks 13–15 of fertilization, the mandible grew as multilayered trabeculae radiating from the primary ossification site of the embryonic mandible, namely the mandibular primary growth center (MdPGC). From week 16 of fertilization, the radiating trabeculae of mandible could easily be demonstrated by a soft X-ray view. Thereafter, the radiating trabecular bones from the MdPGC corroborate the mandibular body growth during later in fetal life (Fig. 2). The radiological dimensions of MdPGC-Sym and MdPGC-Go, which represent the growth of anterior and posterior body of mandible respectively, showed similar incremental growth rates during the fetal period. The incremental growth rate of Go-Co, representing the posterior mandibular height, was higher than that of Al-Lb, representing the anterior mandibular height. The incremental growth rate of Al-Lb was similar to those of MdPGC-Sym and MdPGC-Go during the fetal period (Fig. 3). The gonial angle was measured about 146 –148° in an early fetal period, and decreased to 141–143° until late in fetal period (Fig. 4). Growth of Condyle In the early week 7 of fertilization (Streeter stage 21), a group of cellular mesenchymal tissue was formed around the posterior end of linear trabeculae of the mandible (Fig. 6a,b). In serial sections, this cellular mesenchymal tissue was traced to the fibrous mesenchyme around the Meckel cartilage (Fig. 6c). A branch of pterygoid muscle was clearly associated with the condensed mesenchyme late in week 7 of fertilization. Early in week 8 of fertilization (Streeter stage 23), the posterior end of linear trabeculae of the mandible showed an increased osteoblastic hyperplasia and was well surrounded by the condensed mesenchyme that produced a condyle blastema, to which the lateral pterygoid muscle was attached. From early in week 9 of fertilization, however, the blastema of the condyle produced a cartilaginous tissue forming a condylar head at the posterior end of linear trabeculae of the mandible. The condyle grew rapidly with the bony deposition of endochondral ossification. As the condyle was elongated upward and laterally, a part of pterygoid muscle moved along with the condyle head. Simultaneously, the pterygoid muscle was divided into mesial (internal) and lateral (external) groups (Fig. 6d,e). The mesial pterygoid muscle remained at the mesial side (lingual or internal side) of the mandibular body, while the lateral pterygoid muscle moved continuously upward and laterally (externally) in concert with the rapid condyle head growth. Expansive growth of the cartilaginous condyle head produced a conical bony structure, which was in contrast to the adjacent mandibular body growth on radiograms and histological sections. In week 12 of fertilization, the conical shaped condyle was elongated toward the temporal squama to form the temporomandibular joint. Thereafter, the condyle grew in a characteristic conical shape. The condyle, composed of a distally thickened cartilaginous cap and proximally thinned apex, converged toward the MdPGC (Fig. 7a–f), where bundles of vessels and nerves were located (Fig. 7g). The conical condyle contained abundant hematopoietic cells in its marrow space, and formed a curve along the angulation from the ramus to the mandibular body as it’s growth advanced (Fig. 7h,i). The amount of incremental growth of the conical condyle (CoMdPGC), however, was highest in the representative anatomical dimensions of the human mandible during the fetal period (Fig. 3). DISCUSSION We observed that mandibular ossification started from the mandibular primary growth center (MdPGC), and that the mandibular growth pattern was characterized by intramembranous ossification of the mandibular body and endochondral ossification of the condyle. In our previous study, we explored the growth pattern of human prenatal maxillae and confirmed a pair of maxillary primary growth centers (MxPGC). The MxPGC showed the characteristic radiating, trabecular patterns by both the histological and radiological observations (Lee et al., 1992). It was suggested that the MxPGC is an initial ossification site of the maxilla. The MxPGC was an important anatomical landmark to analyze the stress-bearing maxillary structure, and remained as a sclerotic trabecular bone containing channels of nerve bundles and vessels later in fetal life, while major growth sites of the maxilla were at the distal ends of trabecular bones that radiated from the MxPGC. In this study we found a similar growth pattern in the mandibular development of human fetuses. During the developmental stages of the mandible, its primary growth center (MdPGC) was detected as a primary site of intramembranous ossification around the middle portion of the embryonal jaw. The MdPGC became the central part of the mandibular body, which appeared as a sclerotic focus of radiating trabeculae of the mandibular body Fig. 6. a– d: Condyle growth. e– g: Cross section of mandibular body. a: Seven weeks old, condyle blastema (CB) developed from the posterior end of linear trabeculae of mandible (HE, ⫻40). Tp, temporalis muscle; MN, mandibular nerve; LPt, lateral pterygoid muscle; MPt, mesial pterygoid muscle; Ms, masseter muscle. b: High magnification of (a), the condyle blastema consists of active osteoblastic deposition (arrow) and abundant mesenchymal condensation (HE, ⫻400). c: Eleven weeks old. d: Twelve weeks old. e: High magnification of (d), condyle blastema (CB) attached by lateral pterygoid muscle (LPt) grew toward temporal bone (Te), note upper lateral pterygoid muscle (ULPt) and lower lateral pterygoid muscle (LLPt) (HE, ⫻40). f: Sixteen weeks old, cross section of mandible at first deciduous molar area (To), retrogressive Meckel cartilage (Mc) is remained at the lingual side of mandible (Md) (HE, ⫻40). g: High magnification of (f), the Meckel cartilage (Mc) has no direct connection to mandibular ossification (HE, ⫻200). h: Twenty weeks old, cross section of mandible, the Meckel cartilage (Mc) is rudimentary and almost isolated from the mandible (Md) (HE, ⫻40). DEVELOPMENTAL PATTERN OF HUMAN MANDIBLE Figure 6. 321 322 LEE ET AL. shown on radiograms taken later in fetal life, whereas major growth sites of the mandible were at the distal ends of trabecular bones radiated from MdPGC. The sequential development of the human mandible started from the middle of week 5 of fertilization, with the formation of core cartilage in mandibular swelling i.e., Meckel cartilage, and the mandible grew actively to form a mandibular arch protuberance. Three stages of Streeter’s development appeared particularly important during the mandibular development: stage 16 (appearance of Meckel cartilage), stage 20 (beginning of membranous ossification), and stage 23 (end of the human embryonic period, week 8) (Orliaguet et al., 1993a). Many authors presumed that the Meckel cartilage, the first branchial arch cartilage, had no relationship to the processes of mandibular ossification (Merida-Velasco et al., 1993; Orliaguet et al., 1993b, 1994; Rodriguez-Vazquez et al., 1997a,b; Tomo et al., 1997). Unlike the long bones, Meckel cartilage entirely regressed during the later fetal period (Ellis and Carlson, 1986). In this study, however, we observed the primary intramembranous ossification of embryonal mandible developed in Streeter stage 19, earlier than the ossification of long bones usually found at Streeter stage 20 (Orliaguet et al., 1993a). We found that the intramembranous ossification as well as the condensed cellular mesenchyme of the condylar blastema was closely associated with a portion of perichondral fibrous tissue of the Meckel cartilage. Because the primary intramembranous ossification of the mandible greatly affects the following histomorphogenetic processes of the whole mandible (Bareggi et al., 1995; Berraquero et al., 1995; Orliaguet et al., 1993b, 1994; Rodriguez-Vazquez et al., 1997b; Tomo et al., 1997), we accentuate the primary intramembranous ossification and named it as the mandibular primary growth center (MdPGC). The MdPGC was approximated to the middle portion but lateral in position of the Meckel cartilage in the early embryonal period. Then, the trabecular bones originating from the MdPGC grew out rapidly toward the facial side, losing the relationship to the Meckel cartilage. These findings imply an important role of Meckel cartilage for the initial ossification of the mandible. We have also observed that the primary intramembranous ossification of the embryonic mandible did not encircle the Meckel cartilage the same as long bones but rather dislocated gradually to the facial side apart from the Meckel cartilage. It was also noted that the human Meckel cartilage did not undergo endochondral ossification unlike the core cartilages of long bones, although some animals showed calcification of the Meckel cartilage during the fetal period (Ishizeki et al., 1999; Tomo et al., 1997; Yamazaki et al., 1997). In the serial sections of human embryonic mandibles, however, we observed that the ossifying mandible and its attached muscles were detached from Meckel cartilage and dislocated outwardly as the lingual growth was advanced to fill the stomodeal cavity and to influence the mandibular movement. Thus, we hypothesize that early mandibular movement by the masseter and suprahyoid muscles may influence the premature dislocation of the primary mandible from Meckel cartilage in the early embryonic period. From the serial sections of human embryos we also observed that the genioglossus muscle was attached to the perichondral fibrous tissue of Meckel cartilage in the early week 6 of fertilization. The genioglossus muscle was successively reattached to the inferior portion of mandibular symphysis at 12 week of fertilization. Other muscles, such as masticatory, mylohyoid, etc., were not attached but were positioned around the perichondral fibrous tissue of Meckel cartilage during weeks 6 –7 of fertilization. When the intramembranous ossification of the mandible advanced to form multilayered linear trabeculae, the masticatory and mylohyoid muscles were attached tightly to the outgrowing mandible rather than Meckel cartilage during weeks 8 –9 of fertilization. Although the direct histogenetic effect of Meckel cartilage on the embryonal induction of mandible remains unclear, we presume that the Meckel cartilage plays an important role to integrate the formation of human mandible, which was evolutionarily adapted to provide increased arch size and mobility. The question of “What influences the transition of the mandibular core skeleton from Meckel cartilage into mandible?” remained unanswered. It was suggested that it may depend on the early mouth opening movement, primarily induced by tongue musculature which matured quite early in orofacial structures (Bresin et al., 1999; Kang et al., 1992; Kiliaridis and Katsaros, 1998; Lee et al., 1990; Lightfoot and German 1998; Ogutcen-Toller and Juniper, 1993; Radlanski et al., 1999; Robertson and Bankier 1999; Sato et al., 1994). It was also reported that it may be influenced by mandibular movement in the human embryo beginning around week 8 of fertilization, when the temporomandibular joint is yet to be formed (Hall 1982a,b; Kjaer, 1997; Ouchi et al., 1998). Although the mechanism of an early mouth movement is unclear, it is apparent that the masticatory muscles do not induce the early embryonic mandibular movement at this stage because of their immaturity. We presume that the tongue movements directly induce the early mandibular movement, because Meckel cartilage, a primary skeleton of the mandible during weeks 5–7 of fertilization, was tightly attached to the genioglossus muscle. We also observed, however, that the primordia of the masseter, temporalis, and pterygoid muscles became attached to the newly formed mandible in the late week 8 of fertilization. This finding may imply that the early mouth opening movement causes the primordia of the masseter, temporalis, and pterygoid muscles relocate from the Meckel cartilage to the newly formed mandible moving along with tongue movement. Thus, we believe that the mandible supported by masticatory and tongue muscles would be able to control the development of the lower jaw as a new articulation without the influence of Meckel cartilage from approximately week 8 of fertilization. The present study also indicates that the characteristic structure of the mandibular body exhibits a radiating trabecular pattern from the MdPGC that is closely related to the attachment of surrounding muscles. The pulling force of associated muscles may induce continuous appositional growth of intramembranous ossification on the periosteal side, rather than in the MdPGC, which is no longer proliferative later in fetal life. We suggest that the MdPGC is a primary ossification site of the fetal mandible, forming a rigid center of the mandibular structure. Serial sections of fetal mandibles showed that the linear trabeculae of the mandibular body were focused at the center of the MdPGC. In week 12 of fertilization, however, the architecture of the mandibular body was almost complete with the characteristic shapes of the mandibular body, coronoid process, mandibular angle, and symphysis. From week 15 to 16 of fertilization, the growth of mandibular Fig. 7. Growth pattern of mandibular body and conical condyle. a: Twenty weeks old, longitudinal section of mandible, conical condyle growth (C) is characteristic (HE, ⫻5). CP, coronoid process; MA, mandibular angle; Sy, symphysis; ToD, deciduous first molar tooth germ; ToE, deciduous second molar tooth germ. b: Twenty-five weeks old, tube like condyle growth is conspicuously demarcated (arrows) (HE, ⫻7). EA, external auditory meatus. c: Twenty-seven weeks old, rapid growth of conical condyle (arrows) (HE, ⫻10). d: Twenty-eight weeks old, conical condyle growth is clearly distinguished by its trabecular pattern (arrows) (HE, ⫻10). e: Thirty-five weeks old, conical condyle remained as sclerotic bone (*) (HE, ⫻10), ToC, deciduous canine tooth germ; To6, permanent first molar tooth germ. f: High magnification of asterisk (*) area of (e), still the trabecular structure of conical condyle growth (arrows) is different from that of mandibular body growth (HE, ⫻40). g: High magnification of arrow area of (e), dilated vessel (V) and thick sclerotic trabecular bone around the MdPGC (HE, ⫻200). h: Thirty weeks old, cross section of mandible at deciduous second molar (ToE) area, the conical condyle growth is demarcated by arrows, and it contains hematopoietic tissue in the center (HE, ⫻10). i: Thirty weeks old, longitudinal section of mandible, hematopoietic tissue (*) is abundant in the marrow space in the conical condyle growth (arrows) (HE, ⫻100). body and condyle was clearly distinguished by radiography. The MdPGC clearly showed a radiating trabecular pattern originating from the apical area of the deciduous first molar tooth germ. This growth pattern of the mandibular body became most conspicuous during weeks 20 –25 of gestation. The MdPGC was conspicuously detected near the apical area of the first deciduous molar tooth germ. Numerous linear bony trabeculae originating from the MdPGC grew peripherally, extending to the coronoid process, mandibular angle, symphyseal area, and even to the alveolar ridge (Fig. 8). Later in the fetal period, from week 30 of fertilization, the image of the radiating trabecular pattern was gradually overlapped with the image of tooth germs and peripheral cortical bone consolidated by muscular attachments. A morphological study on the developing lateral pterygoid muscle and its relationships to the temporomandibular joint and Meckel cartilage indicated that all of temporomandibular joint structures and lateral pterygoid muscle assumed their adult shapes by week 14 of fetal life. At this stage, the lateral pterygoid muscle formed a complex structure with several aponeuroses dividing the muscle into three main parts: superior, inferomedial, and inferoanterior (Ogutcen-Toller and Juniper, 1993). This means that the muscular forces arising from mandibular movement directly influence the growth of the condyle and temporomandibular joint simultaneously. Thus, in this study we observed that the lateral pterygoid muscle was primarily attached to the condyle blastema tissue and became elongated through rapid condylar growth during weeks 8 –10 of fertilization. This may imply that the lateral pterygoid muscle guides the conical condyle to form the temporomandibular joint. These data, however, suggest that the mandibular movement primarily controlled by the genioglossus muscle in the early embryonic period could affect the growth of the mandibular body and the condyle. Premature mandibular movement occurred at least 2 weeks earlier than the temporomandibular joint movement and stimulated the adaptational growth of the mandibular body and condyle. Thereafter, condyle growth was highly accelerated to form its conical structure and became independent of mandibular body growth. The incremental growth of the mandibular dimension on the radiogram showed well-harmonized growth curves between the growth rate of mandibular body and condyle during the fetal period. The incremental growths of MdPGC-Sym, MdPGC-Go, and Al-Lb represent the pattern of mandibular body growth and the incremental growths of MdPGC-Co and Co-Go represent the pattern of condyle growth. The former group, however, showed a slightly reduced growth curve compared with that of the latter group. This may imply that condylar growth is much accelerated compared with those of the mandibular body. The slight reduction in gonial angle during the fetal period may also indicate increased growth of the condyle more in a vertical than a horizontal direction. These findings are concurrent with previous concepts of the mandibular development and growth (Baccetti et al., 1997; Bareggi et al., 1995; Buschang et al., 1999; Keith, 1982; Kjaer 1978a,b; Radlanski et al., 1999; Ronning, 1995). In summary, we studied the sequential growth of the human fetal mandible and found that radiating trabeculae of the mandibular body focused into a primary growth center, MdPGC. From the MdPGC, the mandibular development was divided into two distinctive growth patterns Fig. 8. Representative scheme for mandibular body growth (upper) and condyle growth (lower). of the mandibular body and condyle, as shown in Figure 8. We suggest that the MdPGC is an important anatomical landmark from which we can measure the growth directions or amounts of the mandible and that the MdPGC has an important morphogenetic implication for the development of human mandible, providing a growth center for the trabecular bone of the mandibular body and also indicating an initial growth of endochondral ossification of the condyle. ACKNOWLEDGMENTS We would like to express our sincere appreciation to the devoted donors of human materials, who made it possible to perform this study through the legally approved procedure from the center of Congenital Malformation, Seoul, Korea. We are very thankful to Dr. Soo Il Chung and Dr. Yoo Mie Chung for their kind and critical review of the manuscript. LITERATURE CITED Azeredo RA, Watanabe I, Liberti EA, Semprini M. 1996. The arrangement of the trabecular bone in the vestibular surface of the human fetus mandible. I. A scanning electron microscopy study (1). Bull Assoc Anat (Nancy) 80:7–12. Baccetti T, Franchi L, McNamara JA Jr, Tollaro I. 1997. Early dentofacial features of Class II malocclusion: a longitudinal study from the deciduous through the mixed dentition. Am J Orthod Dentofacial Orthop 111:502–509. Bareggi R, Sandrucci MA, Baldini G, Grill V, Zweyer M, Narducci P. 1995. Mandibular growth rates in human fetal development. Arch Oral Biol 40:119 –125. Ben-Ami Y, Lewinson D, Silbermann M. 1992. Structural characterization of the mandibular condyle in human fetuses: light and electron microscopy studies. Acta Anat (Basel) 145:79 – 87. DEVELOPMENTAL PATTERN OF HUMAN MANDIBLE Berraquero R, Palacios J, Gamallo C, de la Rosa P, Rodriguez JI. 1995. Prenatal growth of the human mandibular condylar cartilage. Am J Orthod Dentofacial Orthop 108:194 –200. Bhaskar SN, Weinmann JP, Schour I. 1953. Role of Meckels cartilage in the development and growth of the rat mandible. J Dent Res 32:398 – 410. Bjork A, Skieller V. 1983. Normal and abnormal growth of the mandible. A synthesis of longitudinal cephalometric implant studies over a period of 25 years. Eur J Orthod 5:1– 46. Bresin A, Kiliaridis S, Strid KG. 1999. Effect of masticatory function on the internal bone structure in the mandible of the growing rat. Eur J Oral Sci 107:35– 44. Buschang PH, Santos-Pinto A, Demirjian A. 1999. Incremental growth charts for condylar growth between 6 and 16 years of age. Eur J Orthod 21:167–173. Ellis Ed, Carlson DS. 1986. Histologic comparison of the costochondral, sternoclavicular, and temporomandibular joints during growth in Macaca mulatta. J Oral Maxillofac Surg 44:312–321. Goret-Nicaise M, Dhem A. 1984. The mandibular body of the human fetus. Histologic analysis of the basilar part. Anat Embryol 169: 231–236. Hall BK. 1982a. How is mandibular growth controlled during development and evolution? J Craniofac Genet Dev Biol 2:45– 49. Hall BK. 1982b. Mandibular morphogenesis and craniofacial malformations. J Craniofac Genet Dev Biol 2:309 –322. Ishizeki K, Saito H, Shinagawa T, Fujiwara N, Nawa T. 1999. Histochemical and immunohistochemical analysis of the mechanism of calcification of Meckels cartilage during mandible development in rodents. J Anat 194:265–277. Jakobsen J, Jorgensen JB, Kjaer I. 1991. Tooth and bone development in a Danish medieval mandible with unilateral absence of the mandibular canal. Am J Phys Anthropol 85:15–23. Kang YK, Lee SK, Chi JG. 1992. Maxillo-mandibular development in cerebrocostomandibular syndrome. Pediatr Pathol 12:717–724. Keith DA. 1982. Development of the human temporomandibular joint. Br J Oral Surg 20:217–224. Kiliaridis S, Katsaros C. 1998. The effects of myotonic dystrophy and Duchenne muscular dystrophy on the orofacial muscles and dentofacial morphology. Acta Odontol Scand 56:369 –374. Kjaer I. 1978a. Histochemical and radiologic studies of the human fetal mandibular condyle. Scand J Dent Res 86:279 –299. Kjaer I. 1978b. Relation between symphyseal and condylar developmental stages in the human fetus. Scand J Dent Res 86:500 –502. Kjaer I. 1997. Mandibular movements during elevation and fusion of palatal shelves evaluated from the course of Meckels cartilage. J Craniofac Genet Dev Biol 17:80 – 85. Lee SK, Kim YS, Lim CY, Chi JG. 1992. Prenatal growth pattern of the human maxilla. Acta Anat (Basel) 145:1–10. Lee SK, Lim CY, Chi JG. 1990. Development and growth of tongue in Korean Fetuses. Korean J Path 24:358 –374. Lightfoot PS, German RZ. 1998. The effects of muscular dystrophy on craniofacial growth in mice: a study of heterochrony and ontogenetic allometry. J Morphol 235:1–16. Merida-Velasco JA, Sanchez-Montesinos I, Espin-Ferra J, GarciaGarcia JD, Roldan-Schilling V. 1993. Developmental differences in 325 the ossification process of the human corpus and ramus mandibulae. Anat Rec 235:319 –324. Morimoto K, Hashimoto N, Suetsugu T. 1987. Prenatal developmental process of human temporomandibular joint. J Prosthet Dent 57:723–730. Ogutcen-Toller M, Juniper RP. 1993. The embryologic development of the human lateral pterygoid muscle and its relationships with the temporomandibular joint disc and Meckels cartilage. J Oral Maxillofac Surg 51:772–779. Orliaguet T, Darcha C, Dechelotte P, Vanneuville G. 1994. Meckels cartilage in the human embryo and fetus. Anat Rec 238:491– 497. Orliaguet T, Dechelotte P, Scheye T, Vanneuville G. 1993a. Relations between Meckels cartilage and the morphogenesis of the mandible in the human embryo. Surg Radiol Anat 15:41– 46. Orliaguet T, Dechelotte P, Scheye T, Vanneuville G. 1993b. The relationship between Meckels cartilage and the development of the human fetal mandible. Surg Radiol Anat 15:113–118. Ouchi Y, Abe S, Sun-Ki R, Agematsu H, Watanabe H, Ide Y. 1998. Attachment of the sphenomandibular ligament to bone during intrauterine embryo development for the control of mandibular movement. Bull Tokyo Dent Coll 39:91–94. Padwa BL, Mulliken JB, Maghen A, Kaban LB. 1998. Midfacial growth after costochondral graft construction of the mandibular ramus in hemifacial microsomia. J Oral Maxillofac Surg 56:122– 128. Radlanski RJ, Lieck S, Bontschev NE. 1999. Development of the human temporomandibular joint. Computer-aided 3D-reconstructions. Eur J Oral Sci 107:25–34. Robertson SP, Bankier A. 1999. Oromandibular limb hypogenesis complex (Hanhart syndrome): a severe adult phenotype [letter]. Am J Med Genet 83:427– 429. Rodriguez-Vazquez JF, Merida-Velasco JR, Arraez-Aybar LA, Jimenez-Collado J. 1997a. A duplicated Meckels cartilage in a human fetus. Anat Embryol (Berl) 195:497–502. Rodriguez-Vazquez JF, Merida-Velasco JR, Merida-Velasco JA, Sanchez-Montesinos I, Espin-Ferra J, Jimenez-Collado J. 1997b. Development of Meckels cartilage in the symphyseal region in man. Anat Rec 249:249 –254. Ronning O. 1995. Basicranial synchondroses and the mandibular condyle in craniofacial growth. Acta Odontol Scand 53:162–166. Sato I, Ishikawa H, Shimada K, Ezure H, Sato T. 1994. Morphology and analysis of the development of the human temporomandibular joint and masticatory muscle. Acta Anat (Basel) 149:55– 62. Shibata S, Suzuki S, Tengan T, Ishii M, Kuroda T. 1996. A histological study of the developing condylar cartilage of the fetal mouse mandible using coronal sections. Arch Oral Biol 41:47–54. Tomo S, Ogita M, Tomo I. 1997. Development of mandibular cartilages in the rat. Anat Rec 249:233–239. Xu Y, Liu J, Yang M. 1983. [A study of the prenatal growth and development of the temporomandibular joint (TMJ)]. Ssu Chuan I Hsueh Yuan Hsueh Pao 14:1–7. Yamazaki K, Suda N, Kuroda T. 1997. Immunohistochemical localization of parathyroid hormone-related protein in developing mouse Meckels cartilage and mandible. Arch Oral Biol 42:787–794.