Paper - The development of the first branchial arch in man and the fate of Meckel's cartilage
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Richany SF. Bast TH. and Anson BJ. The development of the first branchial arch in man and the fate of Meckel's cartilage. (1956) Q Bull Northwest Univ Med Sch. 30(4):331-55. PMID: 13408429.
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- 1 The Development of the First Branchial Arch in Man and the Fate of Meckel's cartilage
- 1.1 Introduction
- 1.2 Materials and Methods
- 1.3 Observations and Discussion
- 1.3.1 8 mm and 14 mm embryos
- 1.3.2 17 mm embryo
- 1.3.3 28 mm fetus
- 1.3.4 36 mm fetus
- 1.3.5 43 mm fetus
- 1.3.6 50-mm fetus
- 1.3.7 55 mm fetus
- 1.3.8 62 mm and 70 mm fetuses
- 1.3.9 103 mm fetus
- 1.3.10 115 and 120 mm fetuses
- 1.3.11 160 mm and 161 mm fetuses
- 1.3.12 203 mm fetus
- 1.3.13 222 mm fetus
- 1.3.14 310 mm fetus
- 1.4 Conclusions
- 1.5 Development of the Mandible
- 1.6 Fate of Meckel’s Cartilage
- 1.7 Bibliography
The Development of the First Branchial Arch in Man and the Fate of Meckel's cartilage
Shafik F. Richany, Ph.D., M.D., Theodore H. Bast, Ph.D. and Barry J. Anson, Ph.D.
Contribution from the Department of Anatomy of the University of Wisconsin and the Department of Anatomy of Northwestern University Medical School (No. 627 from the latter).
- This study was made possible through the financial aid of the Central Bureau of Research of the American Otological Society and of the Wisconsin Research Foundation. The cost of regular engraving of all but four of the sixteen plates of illustrations and the entire special cost of reetching and outlining halftones was met by the use of these funds. Expenditures for art, photographic and technical services were likewise defrayed from the sources named.
- The photomicrographs were taken by Homer Montague, labeled by Lucille Cassell Innes; the drawings of the reconstructions were executed by "Jean McConnell; the reconstructions were prepared by Shafik I’. Richany.
- The paper upon which this article is largely based, was read at the Sixty-ninth Annual Session of the American Association of Anatomists, Marquette University School of Medicine, Milwaukee, Wisconsin, April 4 to 6, 192 . (Abstract in the Anatomical Record, vol. 124, No.2, issue February, 1956. Received for publication, October easter Bulletin of Ebbets baat Us niversity Medical School, vol. 30 no. 2, pp. 235 to 249, 1
- Pertinent articles have already Baan Gtads in an earlier issue of this journal (vol. 29, no. 1, pp. 21-36, 1955), and in the Annals of Otology, Rhinology and Laryngology (vol. 64, no. 3, pp. 802- 824, 1955)
In the preceding issue of this journal, the authors reported on the development of Reichert’s cartilage (of the second branchial arch), in particular on the part which it plays in the formation of a canal for the facial nerve. Ossicular and ligamentous derivatives of the second arch of the branchial skeleton were discussed in other publications in the series.”
Now the authors’ concern is with the developmental stages and adult derivatives of Meckel’s cartilage (of the first branchial arch). Like Reichert’s cartilage, Meckel’s bar gives rise to ligaments; and like the former, in the opinion of some investigators, Meckel’s cartilage contributes to the ossicular apparatus; both are transitorily important to the formation of the skull—the one serving as part of the anterior wall of the primordial facial canal, the other becoming partially enveloped by bone of the medial alveolar wall of the mandible.
The exact relation of Meckel’s cartilage to the lower jaw and auditory ossicles has been the subject of a voluminous and controversial literature. From this considerable accumulation, certain representative articles will be discussed briefly. Interest in the nature and significance of the first branchial arch began with Meckel’s study (1825) of the human mandible and two cartilaginous symmetrical bars on the inner surfaces of the embryonic lower jaw. However, somewhat earlier (1819) Serres, the French anatomist, observed these cartilaginous bars and designated them the ‘‘maxillaire inferieure temporaire.”” Serres, Meckel, Magitot and Robin (1826), Strelzoff (1873), and Stieda (1875) regarded the extra-tympanic portion of Meckel’s cartilage as a transitory or rudimentary structure destined to disappear in early fetal life. On the other hand, Reichert (1827), Callender (1869),  Semmer (1872), Masquelin (1878), and Koelliker (1882), believed that the anterior portion of Meckel’s cartilage enters into the ossification of the lower jaw, while the rest of Meckel’s bar disappears. Fawcett (1905), Gaupp (1905), Dieulafe et Herpin (1906), and Low (1910) stated that the mandible ossifies from one center, Meckel’s cartilage, on the contrary, being ossified on its anterior portion by invasion from the surrounding membrane bone (the remainder undergoing atrophy).
Other views may be summarized as follows: the mandible ossifies from five centers which unite to form a single bony element (Macalister,) ;each mandibular half ossifies from a main center, three others (coronoid, condyloid, and angular) being complementary (Cruveilhier,); each lateral half ossifies from six centers, all of which unite in late fetal life (Ramboud and Renault), (Bland and Sutton) ; cartilage is present in the condyloid process, but not in the coronoid (Fawcett. and Low!’),
It will become apparent that several aspects require further investigation; among these are the following: (a) the compound nature and the varied types of ossification of the lower jaw; (b) the acquisition of the complementary and accessory nuclei of growth, their origin and histological character; (c) the formation of the condyloid and coronoid processes during the period of development of the temperomandibular joint; (d) the ultimate fate of Meckel’s cartilage.
The course of morphogenesis will be followed from the stage of the 17-mm. (7-week) embryo to that of the 310-mm. (34-week) fetus, that is, through approximately 7 intrauterine months.
Materials and Methods
Like its predecessor, this report is based upon a critical reexamination of many of the series of sections in the otological collection at the University of Wisconsin. Reconstructions were preprepared from series of the following stages: 8-mm., 14-mm., 17-mm., 28-mm.., 43-mm., 50-mm., 70-mm., 103-mm., and 160-mm. fetuses (CR length). The reconstruction of the 160-mm. specimen was prepared at a magnification of 10 diameters, those of the other stages, at 20 diameters; all of which were drawn fullsize, reduced in reproduction (as recorded in the legends of figs. 14, 15, and 16). The following stages are represented by photomicrographs: 17-mm., 28-mm., 36-mm., 50-mm., 62-mm., 70-mm., 103mm., 115-mm., 120-mm., 160-mm., 161mm., 203-mm., 222-mm., and 310-mm. fetuses (figs. 1 through 13). In most instances, the photomicrographs are arranged in sets (as fig. 1 to fig. 13). The introductory photomicrographs (for example, fig. la, fig. 2a, fig. 6a) are topographical; the accompanying pictures record histological detail (examples being figs. 1b and 1c, 2b and 2c, 6b and 6c, 11b and 11d). All but one of the lowermagnification figures were taken at 35 times natural size; the exception, fig. 11c, being magnified 16 times. Those in the high-magnification category were photomicrographed at these enlargements: at 75 diameters, figs. 2b and 2c, 4a and 4b, 5b, 8, 9b, 10a and 10b, 13b and 13¢; at 100 diameters, figs. 11b and 11d; at 160 diameters, figs. 1b and 1c; at 170 diameters, figs. 6b and 6c, 7b. All photomicrographs were reduced in reproduction.
Observations and Discussion
8 mm and 14 mm embryos
At the 8-mm. (5-week) stage there is no definite condensation of the mesenchyme to indicate the skeletal elements of the visceral arches. At the 14-mm. (6-week) stage there is a marked cellular condensation in the region of the ossicles and Meckel’s cartilage but this is still diffuse and no difference can be detected between what may become cartilage or muscle.
17 mm embryo
At this 7-week stage, the cellular condensation in the region of Meckel’s and Reichert’s cartilages has differentiated at least in part into precartilage and in some small areas into young cartilage. In the region of the ossicles such metamorphosis is not yet complete. The paired Meckel’s bars are separated from each other by an appreciable interval at their anterior extremities, the hiatus marking the site of the future symphysis of the lower jaw (fig. la). It is important to record that Meckel’s bar is one of the earliest skeletal structures to be identified in cartilage.
Figs. 1a to 1c, Photomicrographs from otological series showing types of early bone-formation in the human fetus. Transverse section. Embryo of 17 mm. (CR length); Wisconsin (Bardeen) series 10 (slide 11, line 5, section 2). Fig. la, X 35; Figs. 1b and 1c, X 160.
Fig. la. Typical example of early formation of membrane bone in the mandible (upper part of section) and early atypical bone-formation in the clavicle (lower portion of section). The blocked areas are enlarged in Figs. 1b and 1c.
Fig. 1b. Detail of structure of membrane bone in the developing mandible (the area at the wpper left of Fig. 1a). Membrane bone is arising directly from mesenchyma, independent of Meckel’s cartilage. Fig. Ic. Detailed histology of ossification in the clavicle (the area at the lower right of Fig. 1a). Atypical bone arises from a preskeletal tissue in which the cells, somewhat resembling those of young cartilage, are transformed into bone.
When most of the skeletal elements in the embryonic body are identifiable in either a cartilaginous or precartilaginous state, two sites of membranous ossification are already present in the 17-mm. stage. One of the two is related to the anterior aspect of Meckel’s cartilage, situated entirely outside of its perichondrium and lateral to the future mandibular symphysis; the other belongs to the clavicle (figs. la and 1c).
Both the mandible and the clavicle share the distinction of being the earliest skeletal elements to be formed in bone. Their histological texture is fundamentally similar (figs. 1b and Ice).
Ossification in the mandible follows the classical pattern of membrane bone formation: a preosseous change takes place in the connective tissue; this involves the production of osteoblasts which are arranged in rays with interlacing fibrous and protoplasmic processes. This precocious change in the matrix produces a tissue which may be described as osteoid. The change is then followed by deposition of bone (fig. 1b).
Histological observations on the early site of ossification in the clavicle raises a question as to whether it is of cartilaginous or membranous origin or both. Some of the constituent cells are of enlarged mesenchymal type, lodged in cartilagelike lacunae. In the matrix between these cells, mineral salts have been already deposited (fig. 1c). Confusion reflected in the literature centers around this preskeletal tissue. The question still remains, is this a peculiar type of preosseous tissue, or is it precartilaginous? It would appear to be neither a typical form of membranous ossification nor one of cartilage-bone formation; rather, it seems to be a precursor or preskeletal tissue which can develop in either direction. It is of interest that similar tissue is produced at a later stage in the coronoid and condyloid processes, the ramus, the body, the alveolar margin, and the symphysis of the mandible (figs. 5a and 5b, 6a to 6c, 7a and 7b, 8, 10a and 10b). In the case of the condyloid process, this preskeletal tissue, in the early stage, changes directly into bone; however, in later stages, it passes into cartilage which is then changed into cartilage-bone.
A comparative histological study of these two early stages of ossification, in the mandible and clavicle, reveals apparently identical cells, but varying intercellular content. The cells in both appear to be osteoblasts, differing from them in the absence of encapsulating lacunae or haloes in the mandibular center of ossification (while such lacunae are characteristic of the clavicular ossification). In both instances, the cells remain when ossification takes place.
28 mm fetus
In this fetus of 8 weeks, the lamellae of bone, which originally began to ossify at the level of the mental foramina, have attained relatively considerable size. The sheet-like bone extends along the lateral surface of the anterior two-thirds of Meckel’s cartilage, thus constituting the outer and part of the inner alveolar walls (fig. 2a). Due to rapid progress of ossification at the original site, a more solid mass of bone is produced; it appears as a stout, shell-like process which overlies a knob-like protuberance, elevated from the ventrolateral part of Meckel’s cartilage (fig. 2a, at arrow). From this protuberance a thin lamella extends posteriorly for a short distance along the lateral aspect of Meckel’s cartilage, marking the beginning of the bony medial or inner wall of the alveolar groove. The outlet of the mental nerve has already become a true foramen (fig. 2a). The distal part of Meckel’s cartilage shows no preosseous changes. The coronoid and the condyloid eminences both consist of an osteoid lamina within a well-marked condensation of osteoblasts. Meckel’s cartilage is completely separate from the mandible proximal to the point of the knob-like enlargement.
Figs. 2a to 2c. Photomicrographs demonstrating the early relationships of the ossifying mandible to Meckel’s cartilage and the precocious change which accompanies the ossification of the anterior part of the mandible. The knob-like thickening of Meckel’s cartilage (at the arrow in each figure) meets the alveolar wall of the osseous mandible. Fig. 2a, fetus of 28 mm. (8-2/7 wks.); Wis. ser. 158 (sl. 3, line 1, sect. 1). Fig. 2b, fetus of 36 mm. (8-6/7 wks.);Wis. ser. 175 (sl. 5, line 1, sect. 1). Fig. 2c, fetus of 50 mm. (10 wks.); Wis. ser. 194 (sl. 2, line 1, sect. 3). Fig. 2a, X 32; Figs. 2b and 2c, X 68.
Fig. 2a. Lateral to Meckel’s cartilage the mental nerve is emerging from the sulcus between the alveolar walls; passing through the mental foramen it will continue toward the symphysis of the mandible.
Fig. 2b. In a specimen approximately one-half week older, the cartilage cells in the thickening (indicated by the arrow) are enlarging preparatory to the formation of an ossification center.
Fig. 2c. One week later (in the 10-week stage) the ossification center is being invaded by osteogenic buds from the surrounding osseous mandible. At the symphysis, near the tips of Meckel’s cartilage, proliferation of connective tissue is taking place. The tissue of the inner alveolar wall serves as the perichondral bone around the ossification center of Meckel’s cartilage.
At this stage another intramembranous ossification center occurs in the outer part of the perichondrium of Meckel’s cartilage at its proximal end (that is, near the malleus). This center will later unite with the malleus as the anterior process (described by the present authors in an earlier issue of this journal.”*)
36 mm fetus
In a fetus of approximately 9 weeks an early osseous change is taking place in the knob-like thickening at the anterolateral part of Meckel’s cartilage (fig. 2b). The cartilage cells underlying the abutting portion of the mandible show initial signs of enlargement with accompanying calcification of the matrix.
43 mm fetus
As demonstrated by reconstruction (fig. 14a), the nonossifying portion of Meckel’s cartilage appears as a cylindrical bar, terminating in an enlarged, upturned mass (the hamulus) anteriorly and continuous posteriorly with the malleus (fig. 14a). The process of osteogenesis of the mandible has continued to advance, the two eminences (coronoid and condyloid) being well formed in membrane bone—however, with no indication as yet of further tissue change toward a preskeletal type. Meckel’s cartilage, although somewhat removed from the ramus and the two processes of the mandible, lies closely applied thereto along the medial side of the inner wall of the dental groove. In the anterior part the cartilage makes contact with the osseous mandible, where osteogenic tissue is about to invade the knob-like enlargement. This change is evident in the 50mm. stage (fig. 2c). Tooth sockets for the canine teeth are beginning to form in membrane bone.
“Richany, 8S. F., Anson, B. J. and Bast, T. H.: The Development and Adult Structure of the Malleus, Incus and Stapes. Quarterly Bulletin of Northwestern University Medical School, vol. 28, no. 1, pp. 17-45, 1954.
This stage marks the beginning of changes which prepare the lower jaw for rapid growth. The first is the acquisition of an ossification center at the anterior portion of Meckel’s cartilage (fig. 2c, at arrow). Here osteogenic excavation into the originally calcified matrix is a well-marked feature. This zone of calcification is characteristic of the endochondral type of ossification. The hyaline cartilage at this site is undergoing degenerative changes, pyknosis and hypertrophy; consequently, the distended cartilage lacunae are opened and penetrated by vascular buds which convey periosteal osteoblasts. This zone of activity is the sole ossification center for Meckel’s cartilage. The second characteristic change is the production of preskeletal tissue in the condyloid process. This preskeletal tissue undergoes an unusual type of histogenesis at the posterolateral aspect of the condyloid process; the constituent cells possess an essentially embryonic character, intermediate in nature between chondroblast and osteoblast. The condyloid process, as a consequence, has a varied structure: posteriorly, at the growing point, the cells are mesenchymal in character (fig. 7a at upper left of condyle) ; in the middle, where the preskeletal tissue occurs. the cells are partly stellate or rounded and vacuoled (fig. 7b, upper part); anteriorly they are typical osteoblasts which have arranged themselves in osteogenic rays with a matrix osteoid in character (fig. 7b, lower part). The last-named feature represents an early stage in the formation of membrane bone. The rounded or vacuolated cells seem to blend with the osteoid zone.
Fig. 3. Photomicrograph showing the relationship of the ossifying distal extremities of Meckel’s cartilages to the halves of the bony mandible and to their area of symphysis (arrow indicates the point of junction of the ossifying and the nonossifying segments of Meckel’s cartilage). The connective tissue of the symphysis apparently contributes to growth of the tips of mandible and of Meckel’s cartilage and also gives rise to accessory skeletal elements (at *). Fetus of 70 mm. (11-4/? wks.); Wis. ser. 184 (sl. 41, line 2, sect. 4), X38.
The coronoid process, unlike the condyloid, still consists of membrane bone (fig. 7a). As demonstrated by reconstruction, Meckel’s cartilage is more flattened anteriorly and in a closer approximity to the inner alveolar wall—actually, in a sulcusof considerable depth (fig. 14b). Posteriorly, Meckel’s cartilage remains removed from the ramus of the mandible by an appreciable distance; it passes between the two pterygoid muscles to continue as the malleus in the tympanic cavity. The two branches of the mandibular nerve (inferior alveolar and lingual) course along its lateral and medial aspects, respectively. The chorda tympani meets the lingual nerve on the an terior surface just beyond its point of emergence from the tympanic cavity (fig. 14b).
55 mm fetus
At the site of the original ossification center the process of endochondral excavation in Meckel’s cartilage has extended anteriorly and a little posteriorly. The cartilage cells show atrophic changes. The matrix is markedly calcified, a change which keeps pace with the progress of penetration by osteogenic tissue. The cartilage cells are being destroyed, and the resulting spaces are concurrently being filled with osteogenic tissue and scattered spicules of endochondral bone.
62 mm and 70 mm fetuses
Within the eleventh week of intrauterine life, Meckel’s cartilage shows, in addition to histological progress in the ossifying area, a gross change in its shape; Meckel’s bar develops a marked convex curvature directed outward—perhaps as a result of increase-in. width of the anterior part of the jaw (figs. 14c and 14d). The mechanism of excavation is similar to that which takes place in a long bone, except for absence of an epiphyseal center. At their anterior extremities the two bars are almost contiguous; they share a common, intervening, perichondrium; at the tips of the cartilage the cells are in a prechondrocytic stage. In the region of its ossification center, Meckel’s cartilage is now being transformed into bone; deposition of endochrondral bone is under way.
The coronoid eminence, unlike the condyloid, is still composed of membrane bone. The preskeletal tissue, now partly ossified but originally formed in the condyle, has become elongated so as to extend in wedge-like shape into the ramus (fig. 14c at dark area). Other similar masses appear separately near the symphysis. As previously mentioned, the constituent cells of this preskeletal tissue are precursors of either cartilage or typical membrane bone. In the early stage of development the cells show a gradual transition to bone, a circumstance which suggests that some of them have been transformed into osteocytes. The matrix is Osseous in appearance, but the cellular structure simulates that of cartilage (figs. 6a and 6b). In later stages the tissue in the condylar region differentiates further, to form true hyaline cartilage (see figs. 8 and 9a). This indeterminate cytological stage suggests that the undifferentiated cells may become either cartilage or bone, depending on the “environment” under which they are growing. Those nearest the blood supply have been observed to differentiate directly into bone; others, more remote, may be the forerunners of cartilage.
Meckel’s cartilage shows no continuity with this preskeletal tissue, although each bar lies deeply in a furrow in the membrane bone of the alveolar wall; anteriorly the chondral bar disappears in the substance of the body of the mandible (at the arrows in figs. 14c and 14d).
Meckel’s cartilage almost all of the calcified matrix has been excavated and transformed into endochondral bone. However, the tip is still cartilaginous; it protrudes into the symphysis.
The condyloid process shows a gradual transition, posterior to anterior, from the fibroblastic type of cells to a preskeletal tissue (which resembles cartilage) and then to bone (compare fig. 8).
103 mm fetus
A three-week period of growth (between the 70-mm. and 103mm. stages) results in considerable growth of both the mandible and Meckel’s cartilage, in more intimate relation between the two, in further change in conformation of the cartilage and in the appearance of a new type of tissue in the mandible (figs. 15a and 15b).
The medial alveolar wall is now deeply suleate, the furrow accommodating Meckel’s cartilage along the mandibular ramus and body (figs. 15b and 15c). Near the symphysis, however, the free tipof the bar turns away from the jaw in a sinuous curve (half-encircled by the arrow in fig. 15a; seen from below, at arrow in fig. 15b). Preskeletal tissue is present in the condyloid and coronoid processes, and in the form of elongate plaques continued from the condyle along both surfaces of the ramus.
This stage in the histogenesis of the condyloid processes, as already indicated, shows the mandible elongated in part by growth or multiplication of the fibroblasts at the proximal or articular end of the mandible so that the segment which was condyloid process in the 50-mm. fetus becomes, in the 103-mm. specimen, part of the ramus; the condyle is newly produced by the proliferating tissue at the proximal end. The transformation of the growing fibroblasts into mandibular bone differs at the various stages (progression depicted in figs. 7, 8 and 9).
Up to the 103 mm stage, fibroblasts changed from preskeletal tissue directly into bone (figs. 7a and 7b). In the 103 mm. fetus, the process is more complicated, in that the preskeletal tissue gives rise to a tissue which resembles cartilage and whose matrix stains like that of the calcified matrix of cartilage (fig. 8 at *); between this and the bone which was directly produced by the skeletal tissue (fig. 7), there is a layer of young cartilage. And so it is, that in this 103-mm. stage the preskeletal tissue begins to give rise to cartilage instead of bone—as was the case in earlier stages (the successive steps being shown by the succession the tissues followed from top to bottom in fig. 8).
Figs. 4a and 4b. Photomicrographs of sections showing the differing histological changes which take place concurrently within Meckel’s cartilage. Fig. 4a, fetus 115 mm. (15+ wks.); Wis. ser. 150 (sl. 65, line 2, sect. 2). Fig. 4b, fetus of 120 mm. (16 wks.); Wis. ser. 196 (sl. 22, line 2, sect. 2). Figs. 4a and 4b, X75.
Meckel’s cartilage at these stages possesses a prominent hook-like bend which serves to mark the line of separation of the ossifying from the nonossifying portions (division at arrows). Proximally, Meckel’s cartilage retains its chondral character; distally, the cartilage is undergoing ossification.
Figs. 5a and 5b. Photomicrographs from a fetal series showing types of tissue in, and closely related to, Meckel’s cartilage. The area blocked in Fig. 5a is shown at higher magnification in Fig. 5b. Figs. 5a and 5b, fetus of 115 mm. (15 + wks.); Wis. ser. 150 (sl. 68, line 1, sect. 3). Fig. 5a, X35; Fig. 5b, X75.
Marked proliferative activity of the fibroblasts at the symphysis menti gives rise, variously, to the cartilaginous tip of Meckel’s bar, to accessory cartilages, to preskeletal tissue and to bone of an atypical variety.
Figs. 6a to 6c. Photomicrographs from fetal series showing types of early tissue in the area of the developing mandible. Figs. Ga and 6b. Fetus of 62 mm. (11+ wks.); Wis. ser. 160 (sl. 10, line 1, sect. 3). Fig. 6c, fetus of 103 mm. (14+ wks.); Wis. ser. 166 (sl. 9, line 1, sect. 1). Fig. 6b pictures, at higher magnification, the area blocked in Fig. 6a. Fig. 6a, X33; Figs. 6b and 6c, X 160.
Figs. 6a and 6b. Photomicrographs showing a representative portion of the mandibular ramus, (in relation to Meckel’s cartilage) in which the preskeletal tissue (derived from fibroblasts) is being converted into atypical bone in one portion of the mandible, removed on the opposite surface in the presence of osteoclasts.
Fig. 6c. Preskeletal tissue at the front of the zygomatic process which will either become bone or be removed through the action of osteoclasts.
In the succeeding stages, the growing fibroblasts at the posterior or proximal end of the condyloid process will gradually be transformed into cartilage directly, that is, without passage through the intermediate condition of a preskeletal tissue. This is the case in the 160-mm. fetus (fig. 9a). Thus, the permanent condyle will consist of the cartilage-bone, whereas the core of the ramus is bone produced by direct ossification of the preskeletal tissue (fig. 15a, dark area). The ramus around this core is membrane bone.
The coronoid process lags well behind the condyloid in the production of preskeletal tissue. The latter tissue first appears in the condyloid process at about the 84-mm. stage; in the 103-mm. specimen, the coronoid process contains preskeletal tissue. This tissue is also present at the root of the zygomatic process, where it is being removed by osteoclasts (fig. 6c).
115 and 120 mm fetuses
The anterior part of Meckel’s cartilage is now completely transformed into bone and is incorporated into the anterior portion of the body of the mandible. The protruding tips, however, show young proliferating cartilage cells lodged in the fibrous portion of the symphysis, as well as accessory cartilage masses; the latter, although occupying this area, is neither connected with Meckel’s bar nor derived from preskeletal tissue (figs. 5a and 5b). The same condition exists in a 160-mm. specimen (figs. 16a to 16c). The anterior part of Meckel’s cartilage, which does not ossify, takes a sharp hook-like bend as it enters the mandible. The tip of the hook gradually becomes detached from the ossified part (fig. 4a). In the 120-mm. fetus the nature of this difference is even clearer, because the nonossifying part of Meckel’s cartilage at this stage has become detached from its ossified anterior segment (fig. 4b at arrow). Being thus detached, the greater part of Meckel’s cartilage takes no part whatsoever in the formation of the osseous mandible. However, the intratympanic part is associated with the anterior process of the malleus and itself becomes both the malleus and the incus. Histologically, the tissues are much the same as the constituent elements of the 130-mm. fetus.
160 mm and 161 mm fetuses
In the 5-week period between the 103-mm. and 160-mm. stages, the length of the osseous mandible has virtually doubled. The sulcus for Meckel’s cartilage has undergone further deepening, and substantially adult relationship has been established between the inferior alveolar nerve, its branches and the mandible (figs. 16a and 16b). Preskeletal tissue has appeared at and near the symphysis (figs. 16a to 16c). Fibroelastic tissue is also present in the cleft between the mandibular halves; here also is found the nodular remnant of the hamulus of Meckel’s cartilage (fig. 16¢).
The portion of Meckel’s cartilage which remains unossified extends anteriorly to a point (internally) just distal to the level (externally) of the mental foramen (fig. 16c). Further distally, the anterior ossified portion is completely integrated with the body of the mandible. The unossified portion of the cartilage begins to show deorganization along its proximal part, where the branchial bar is broadly continuous with the malleus (figs. lla and 11b). This charge is preparatory to the conversion of the tissue of Meckel’s cartilage into that of the anterior ligament of the malleus (compare figs. lle and 11b). The anterior process of the malleus until now an independent bone, is beginning to unite with the malleus.
The condyloid process of the mandible presents interesting features of ossification. The intact perichondrium is broken down and osteogenic tissue has excavated the calcified cartilage of its growing end (fig. 9a). At a lower level in the condyloid process cartilage is being replaced by bone marrow and bony spicules of endochondral and intrachondrial types (fig. 9b). Ossification takes place from a lower to a higher level, a cireumstance which permits further growth, through continuous addition of new cartilage at the proximal end, until adult size is attained.
Figs. 7a and 7b. Photomicrographs depicting an early stage in the formation of the condyloid and coronoid processes of the mandible. Figs. 7a and 7b, fetus of 50 mm. (10 wks.); Wis. ser. 194 (sl. 8, line 2, sect. 4). Fig. 7a, X 35; Fig. 7b (of the area blocked in Fig. 7a), X 170.
Fig. 7a. In the 10-week fetus the coronoid process is already formed in membrane bone. The condyloid process, on the contrary, is in an early stage of development. Fig. 7b. In the posterior, growing, part of the condyloid process preskeletal tissue has been formed from fibroblasts; anteriorly, in the older portion, the preskeletal tissue is being converted into tissue of osteoid type. The next step in the process of ossification is pictured in Fig. 8.
Fig. 8. Photomicrograph of a section through the condyloid process in a fetus of 103 mm, (14+ wks.); Wis. ser. 166, sl. 15, sect. 1). X71. Within a 4-week period, elapsed since the stage depicted in Fig. 7, bone has appeared in the condyloid process. In the developmentally advanced portion (the lower portion of the figure) preskeletal tissue is passing into cartilage, the matrix of which shows evidence of calcification (at *). The calcified cartilage is continuous with a rim of young cartilage which intervenes between the calcifying tissue and a newly-formed ossifying stratum. The latter is succeeded by an osteogenic tissue derived from periosteum. The next definitive step in ossification is demonstrated by Fig. 9.
Figs. 9a and 9b. Photomicrographs depicting later stages in the development of the condyloid process. Fetus of 160 mm. (19+ wks.); Wis. ser. 165 (sl. 1, sect. 2). Fig. 9a, X 35; Fig. 9b, X 75. Fig. 9a. Less than 6 weeks later (as compared with the stage shown in Fig. 8), the area of calcifying cartilage (marked by * in Fig. 8) has now become a center of endochondral ossification. Differing from the anterior portion, the posterior growing end demonstrates direct transformation of perichondrium into cartilage.
Fig. 9b. A still later stage in the production of endochondral bone is seen in this area of the condyle (inferior to that shown in Fig. 9a). The cartilage, with cells in now enlarged lacunae, is being removed through osteogenic excavation. Intrachondrial bone is produced concurrently; and upon the newly-formed spicules, endochondral layers are being applied.
The coronoid process contains preskeletal tissue of the type which was observed in the 103-mm. fetus (fig. 10b). On the lateral side, the coronoid process shows beginning excavation by osteoclasts, whereas medially it continues to bone. This same type of tissue has also been noted at the symphysis and on the inner and outer alveolar walls.
Grossly, the mandible is thickened and the trough is deepened and widened to make way for the developing tooth sockets (figs. 16 and 16b). In the depth of the gutter between the alveolar walls lies the inferior alveolar nerve. Posteriorly, the medial alveolar wall overarches the inferior alveolar nerve forming a short canal, the posterior orifice of which is partially bounded by the lingula. The relation of the inferior alveolar and lingual nerves to Meckel’s cartilage is altered by the growth of the inner alveolar wall; the former nerve occupies the canal, whereas the latter nerve and Meckel’s cartilage lie on its medial aspect. The mylohyoid nerve, a branch of the inferior alveolar, retains its relationship to Meckel’s cartilage, traversing the sulcus formed by Meckel’s cartilage on the medial side of the inner alveolar wall.
203 mm fetus
In the 23-week specimen the primitive type of bone derived from the preskeletal tissue in the coronoid process has now been partly replaced by bone marrow. ‘True cartilage cells are present on which primary endochondral bone is deposited. Similar remnants of cartilage have been observed in the mandibular ramus at the same stage.
Meckel’s cartilage, to a striking degree, is undergoing deorganization all along its length (figs. 12a and 12b, 13a and 13b), a change which is even more marked in the extratympanic portion of the cartilage between the lingula of the mandible and its point of emergence from the tympanic cavity. This segment, with the exception of an island of altered cartilage matrix, is transformed into a ligament (fig. 12a). Anteriorly the cartilage persists for a longer time, as it does also at the malleolar extremity (fig. 12b, compare figs. lla and 11b).
222 mm fetus
In the 25-week specimen Meckel’s cartilage is greatly altered at its malleolar extremity, few cartilage cells now remaining within it (figs. 11le and 11d).
310 mm fetus
In the fetus of 34weeks all of Meckel’s cartilage has undergone deorganization. The cells, having reverted to fibroblastic character, have given rise to a ligamentous structure in portions of which a wavy amorphous band of collagenous material still exists (fig. 13c). The ligament extends from the malleus through the Glasserian fissure to an attachment on the lingula of the mandible. With the further progress of ossification, the spine of the sphenoid bone will invade the path of the ligament at the point where the latter leaves the petrotympanic (Glasserian) fissure; as a result, the ligament becomes attached to the fibrous portion of periosteum which lines the fissure. The original ligament thereupon becomes two ligaments: the proximal part extends from the malleus to the fissure as the anterior suspensory ligament of the malleus; the remainder becomes the sphenomandibular ligament. Further anteriorly the deorganized Meckel’s cartilage is an adjunct to the fibrous periosteum on the inner surface of the mandible.
onoid process of the mandible (compare Fig. 7a). Fetus of 103 mm. (14+ wks.); Wis. ser. 166 (Fig. 10a, sl. 12, sect. 3; Fig. 10b, sl. 15, sect. 1). X75.
Fig. 10a. At the growing tip of the coronoid process, surrounding fibroblasts are undergoing differentiation into preskeletal tissue.
Fig. 10b. Near the base of the coronoid process (at a level inferior to that shown in Fig. 10a), the central part is still composed of tissue of preskeletal type, whereas, at the periphery, the tissue has changed into atypical bone which is being surrounded by membrane bone.
he incus, malleus and Meckel’s cartilage, showing a crucial step in the retrogressive histological change in the cartilage and the neck of the malleus. Figs. 11a and 11b, fetus of 161 mm. (19— wks.), Wis. ser. 23 (sl. 62, line 1, sect, 3); Figs. 11¢ and 11d, fetus of 222 mm. (25 wks.), Wis. ser. 46 (sl. 56, line 1, sect. 2). Figs. 11a X 34; Fig. 11c, X 16; Figs. 11b and 11d, X 100.
Fig. 11a. In the malleus, while bone spreads from the single ossification center in the head and neck toward Meckel’s cartilage, the latter element is undergoing retrograde change.
Fig. 11b. Histological detail of the area blocked in Fig. 11a, demonstrating the advance of osteogenesis in the malleus and the concurrent retrogressive modification of Meckel’s cartilage (in the course of its conversion into ligamentous tissue.)
Fig. 11c. The malleus and incus of the 25-week fetus have acquired an outer (perichondral) shell of bone, which is complete except where the ossicles articulate and where one of them becomes related to the ligament in the petrotympanic (Glasserian) fissure.
Fig. 11d. Histological detail of the area blocked in Fig. 11c. Meckel’s cartilage is present only as a remnant, the tissue having been transformed into that of the suspensory ligament of the malleus. The residual portion of the degenerating Meckel’s cartilage is surrounded by tissue of the ligament.
Figs. 12a and 12b. Photomicrographs showing remnants of Meckel’s cartilage and its derivatives in a fetus of 203 mm. (23 wks.);Wis. ser. 200 (Fig. 12a, sl. 19, sect. 1; Fig. 12b, sl. 35, sect. 4). X 35. Fig. 12a. The sphenomandibular ligament, a product of Meckel’s cartilage, contains a remnant of the latter. The attachment of the ligament to the lingula of the mandible appears in the lower part of the illustration.
Fig. 12b. Demonstrating the deorganization of Meckel’s cartilage into fibrous tissue which has hecome adjacent to the inner periosteum of the mandible.
At the point in the petrotympanic fissure where the chorda tympani and the original Meckel’s cartilage emerged, there exists an iter or a bony canal, which was first described by Huguier. This canal lodges the anterior process of the malleus (formerly termed the processus gracilis), and also provides an exit for the chorda tympani. The anterior opening of this canal is situated beneath a small nodule of bone on the outer circumference of the tympanic ring. The opening remains identifiable in many skulls as a bony process projecting over the exit of the chorda tympani and into the nonarticular portion of the petrotympanic fissure on the posterior wall of the mandibular fossa.
The development of the mandible is closely associated with that of the malleus and incus, since they differentiate from the cartilaginous or membranous portions of the first visceral arch. Meckel’s cartilages represent the temporary skeleton of this arch. These two symmetrical cartilaginous bars, in early fetal life, describe a gothi¢ arcade, or parabolic arch; they serve as-the model and remain the sole guide in the early morphogenesis of the lower jaw, and each gives rise to two of the auditory ossicles. In the development and fate of Meckel’s cartilage three main regions require consideration: the distal portion, which becomes incorporated into the anterior part, or tip, of the mandible; the middle portion, which gives rise to the sphenomandibular ligament and contributes to the formation of the fibrous periosteum which lines the mylohyoid groove of the mandible; and the proximal, or intratympanic portion, which differentiates into the malleus, the incus and the anterior malleolar ligament.
Figs. 13a and 13c. Photomicrographs showing advanced stages of deorganization within Meckel’s
Figs. 13a and 13b, fetus of 203 mm. (23 wks.); 200 (sl. 9, sect. 1). Fig. 13c, fetus of 310 mm. (34 wks.); Wis. ser. 68 (sl. 83, sect. 4). Fig.
Fig. 13a. The proximal extremity of Meckel’s cartilage, although generally in an advanced stage of metamorphosis, still harbors some relatively unaltered cartilage. Fig. 13b. An area (blocked in Fig. 13a) in which cartilage is undergoing retrogressive change.
(zone marked by arrows.)
Fig. 13c. An area, in a fetus 10 weeks older, in which a collagen-like, almost amorphous, remnant of Meckel’s cartilage (outlined in ink) persists in the upper part of the extratympanic portion.
Development of the Mandible
The earliest ossification centers for the mandible occur between the 15-mm. and 17-mm. stages, in a position just ventrolateral to, and near the tips of, Meckel’s cartilages. The two intramembranous ossification centers (one for each side) are the primary, independent and sole centers for the osteogenesis of the mandible. Within a period of 2 weeks (in the 28mm. stage) ossification extends from each center anteriorly and posteriorly along the anterolateral two-thirds of the cartilaginous bars of the lower jaw, to form two shelves which forecast the future outer and inner alveolar walls of the dental groove. This shell of bone, near the point of its origin, is in contact with a knob-like thickening on Meckel’s cartilage. This protuberance marks the spot of initial ossification in Meckel’s cartilage. In the 36-mm. fetus the cartilage cells in this thickening enlarge, and the matrix contains mineral deposits. In the 50-mm. stage osteogenic buds from the adjacent membrane bone invade the cartilage, which, in being rapidly transformed into bone, helps to form the anterior part of the mandible. Such conversion to bone in the distal part of Meckel’s cartilage is well advanced in the 120-mm. fetus except at the symphysis, where, still in a proliferative stage, it continues to produce new cartilage cells. This transformation of the tip of the bar into bone is similar to that in other long bones, except for the fact that no epiphyseal center develops. At this 120-mm. stage the remainder of Meckel’s cartilage, which does not ossify, becomes disconnected from the ossified segment. Owing to rapid growth and to concurrent transformation into bone, the distal part of Meckel’s bar becomes markedly arched; at the same time the tip of the nonossifying part of the cartilaginous bar, near the point where it separates from the ossified part, has become bent into a hook-like form. From this point posteriorly, Meckel’s cartilage takes no part in the formation of the mandible.
Figs. 14a to 14d. Drawings of reconstructions of Meckel’s cartilage and the mandible in three developmental stages. Figs. 14a to 14¢ show the reconstruction as seen in superolateral view; in Fig. 14d, the reconstruction of the 70-mm. stage is viewed from below. Fetal specimens of 43 mm. (9 wks.), 50 mm. (10 wks.) and 70 mm. (11 wks.). Wis. ser. 197, 194 and 184, respecttively. X 6.6.
Fig. 14a. At this early stage (9 weeks), the distal extremity of the cartilaginous arch reaches the symphysis, there to take the form of an upturned enlargement, the so-called hamulus, situated on the internal aspect of the mandible. The mandible is already formed in bone except in the region of the condyloid process. Distally, Meckel’s cartilage is separated from the alveolar sulcus by the inner alveolar wall; proximally, on the contrary, where the wall has not yet been formed, the cartilage occupies the area of the future sulcus.
Fig. 14b. One week later (that is, in the fetus of 10 weeks), Meckel’s cartilage is partially enveloped by bone of the inner (medial) alveolar plate of the mandible; it has begun to assume an S-shaped form. The inferior alveolar (dental) nerve lies lateral to Meckel’s cartilage, whereas the lingual nerve is situated medial thereto. The internal pterygoid muscle is medial to these several structures.
Figs. 14¢ and 14d. Within the space of another week (at the fetal stage of 11 weeks) the distal portion of Meckel’s cartilage has become incorporated in the osseous mandible, into the substance of which it disappears (at arrows). The sigmoid configuration of the free portion of the cartilage bar is now a striking feature. Fibrous tissue and cartilaginous remnants are present at the symphsis. Preskeletal tissue (shown as darkened zone) forms a large fraction of the condyloid processes, but has not appeared in the coronoid process.
Figs. 15a to 15c. Drawings of a reconstruction of the mandible and Meckel’s cartilage in a fetus of 103 mm. (14+ wks.); Wis. ser. 166. Fig. 15a, like Figs 14a to 14c, shows the reconstruction as seen from above and from the right side; Figs. 15b and 15c are segments of the left jaw seen in inferomedial view. X6.6.
Fig. 15a. Accompanying rapid growth, in the course of which the length of the mandible is increased by one and one-half times in a 3-week period, the medial alveolar wall has grown caudalward to place Meckel’s cartilage on the medial aspect of the newly-formed wall. The distal extremity of the nonossifying Meckel’s cartilage shows a hook-like bend (encircled by an arrow). The ossified segment on each side is now integrated with the distal portion of a mandibular half. At or near the symphysis menti accessory cartilages or preskeletal nodules are commonly present. Preskeletal tissue has appeared in the coronoid process (depicted as darkened areas); similar tissue has spread distalward from its original site along the lateral surface of the ramus. The medial alveolar wall has grown caudalward to place Meckel’s cartilage on the medial aspect of the newly-formed plate of bone.
Fig. 15b. The nonossifying segment of Meckel’s cartilage (terminating at the point indicated by the arrow) is now lodged in a deep furrow in the mandible.
Fig. 15c. The inner alveolar wall now separates Meckel’s cartilage from the alveolar sulcus (at *).
Figs. 16a to 16c. Drawings of Meckel’s cartilage, the mandible and related nerves in a fetus of 160 mm. (19+ wks.). Fig. 16a, the entire reconstruction seen from above and from the right side (view similar to those of Figs. 14a to 14c and Fig. 15a); Fig. 16b, segment at the symphysis view from above and in front; Fig. 16c, tho sane portion seen from below.
Fig. 16a. The inner alveolar wall has now not only separated Meckel’s cartilage from the inferior alveolar nerve, but also, by uniting with the outer wall, has formed a canal for transmission of the nerve. Meckel’s cartilage occupies the sulcus on the medial surface of the mandible—a sulcus which comes to accommodate the mylohyoid nerve. Owing to growth of the body of the mandible, Meckel’s cartilage has been drawn into a relatively straight rod. X 3.3.
Figs. 16b and 16c. Near the symphysis, Meckel’s cartilage has become a part of the ossified mandible; at the symphysis, accessory cartilage persists (Fig. 16c). In the latter position, proliferating fibroblasts occupy the space between the mandibular halves, and preskeletal tissue occurs on the superior and medial aspects of the mandible (Figs. 16b and 16c). This tissue is like that already described for the condyloid and coronoid processes.
The condyloid and coronoid processes of the mandible ossify along the lateral side of Meckel’s cartilage, but remain independent thereof. These parts develop rapidly, so that by the 43-mm. stage their adult form is forecast. The inner alveolar wall of the ramus, which lagged behind the outer wall in development, is now more definite; it lies in close proximity to the lateral margin of Meckel’s cartilage. With further growth, the inner alveolar wall partly invests Meckel’s cartilage, so that the latter comes to lie in a sulcus of the former. The sulcus later becomes the mylohyoid groove.
In the early phase of development the branches of the mandibular nerve (the inferior alveolar and the lingual) flank, respectively, the lateral and medial sides of Meckel’s cartilage. The chorda tympani shares with Meckel’s cartilage the same outlet from the tympanic cavity. Just beyond its exit the nerve joins the lingual branch of the mandibular nerve in the anteromedial border of the extratympanic part of Meckel’s cartilage. With further development, Meckel’s cartilage lengthens and the bony wall of the alveolar gutter increases in extent. These progressive steps in growth are traceable in fetuses of 70, 108, 115, and 160 mm. The dental gutter is gradually deepened. The inner alveolar bony wall terminates posteriorly in the lingula; overarching the upper surface of Meckel’s cartilage, it separates the latter from the inferior dental nerve. In the 160-mm. stage the dental gutter remains open; however, the inner alveolar ridge curls over the nerve and completes the closure to form the dental canal.
The histological composition of the ossifying mandible presents a compound type of osseous tissue, consisting of membrane bone and cartilage-bone of endochondral and intrachondral varieties. The primary ossification center of the mandible, already present in the 17-mm. stage, represents a classical type of membranous ossification. Osteoblasts are arranged in osteogenic rays, within an osteoid matrix—a condition preparatory to deposition of bone. In addition, a preskeletal tissue occurs which may differentiate either into cartilage or membrane bone. The sites of this tissue are the symphysis of the mandible, the anterior margins of the tooth sockets, and the condyloid and coronoid processes. The tissue in the condylar area presents a transition from osteoblasts anteriorly to fibroblasts posteriorly. The cells in the intermediate stage contain a vacuolated cytoplasm and nuclei of various shapes. The matrix is equally indeterminate, except on the anterior portion, where it has already become osteoid in character. As the condyloid process elongates, this tissue becomes wedged in the growing ramus and so remains in continuity with the condyle. In later fetal stages the same tissue actually differentiates into cartilage in the condylar area. That part of the ramus which originally was the condylar base undergoes a peculiar differentiation similar to that occurring in the clavicle and the coronoid process. Some of this tissue differentiates to bone directly; other parts are removed by osteoclasts and replaced by bone, while some continue to differentiate into cartilage (for example, in the condyle, around which primary membrane bone is deposited). Direct removal of preskeletal tissue by osteoclasts takes place in the remus of the mandible of the same fetal age. The growing condyle, following the 70-mm. stage, continues to be preformed in cartilage until adult size is attained; it then undergoes ossification in a manner typical of many other bones. RICHANY ET AL.—FIRST BRANCHIAL ARCH
The preskeletal tissue seems to arise in areas where there is a rapid proliferation and deorganization of cells possessing an osteogenic property, and where there is a paucity of blood supply. If the blood supply is poor, these cells differentiate to cartilage; if the blood supply is adequate, they contribute to bone. This difference is demonstrated in the ramus of the mandible, where the tissue is continued into that of the condyloid process (the former differing from the latter, in that it seldom reaches the cartilaginous stage).
To recapitulate, the bony mandible is made up of a composite of osseous tissue: in the tip of the mandible from the symphysis to the mental foramen it is a compound of the original membrane bone and cartilage bone derived from the distal portion of Meckel’s cartilage; generally, the remainder of the mandible is of membranous origin. However, at or near the symphysis, in the posterior part of the ramus, and in the condyloid and coronoid processes a modified preskeletal tissue is formed. Some of this tissue passes directly into bone; in other areas, especially at the articular portion of the condyloid process, it gives rise to cartilage which is then transformed into cartilagebone.
Fate of Meckel’s Cartilage
Meckel’s cartilages constitute the primordia of the mandibular skeleton. They are the earliest elements to be formed in cartilage. Almost all skeletal structures which have been preformed in cartilage ultimately ossify. Some, however, (for example, the tracheal, nasal, and articular cartilages) retain their cartilaginous character. Meckel’s cartilage belongs in a class by itself: while originally a cartilage, most of which undergoes deorganization to become a fibrous tissue, the distal and proximal extremities undergo ossification like that of long bones. The two ossicles, malleus and incus, are derived from the proximal end; the anterior portion of the body of the mandible comes in part from the distal end of Meckel’s cartilage. The remaining, or intermediate part of Meckel’s cartilage never ossifies; it is destined to undergo a peculiar type of histological deorganization in late fetal life. This process involves the segment of Meckel’s cartilage from the point of its detachment from the ossifying portion, along the mandible, through the petrotympanic fissure to the zone of its continuity with the malleus. The early histological changes are more marked peripherally than centrally. The cells lose their identity and tend to be fibroblastic in character. Ultimately this altered fabric, with the exception of certain amorphous remnants of cartilage, becomes surrounded by a fibrous tissue. Thus the whole intermediate portion of Meckel’s cartilage is converted into a fibrous band which enters the same canal, leading into the tympanic cavity, which Meckel’s cartilage (with the chorda tympani and the developing anterior process of the malleus) has already produced in the medial portion of the head of the tympanic ring. This canal, an iter in the petrotympanic fissure, is marked on the outer circumference of the bony ring; and it remains prominent in many fetal skulls on the posterior wall of the nonarticulating surface of the mandibular fossa in the petrotympanic fissure. The ligamentous band eventually gives rise to three separate structures.
The intratympanic portion of the originally single band becomes the anterior ligament of the malleus; the ligament overlies the anterior process of the malleus and anchors this anteriorly placed eminence to the periosteum of the petrotympanic fissure. It will be recalled that the anterior process of the malleus is a membrane bone which arose along the malleolar extremity of the intratympanic segment of Meckel’s cartilage, and in complete independence of the latter branchial derivative.
As ossification progresses, the spine of the sphenoid bone presses into the path of the ligamentous band and becomes attached thereto at the point where the ligament emerges from the petrotympanic fissure. Passing peripherally, the ligament acquires an attachment to the lingula of the mandible, thus becoming the sphenomandibular ligament.
Further distalward the fibrous band contributes to the periosteum lining the mylohyoid groove, a sulcus which was originally occupied by Meckel’s cartilage.
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Cite this page: Hill, M.A. (2019, August 24) Embryology Paper - The development of the first branchial arch in man and the fate of Meckel's cartilage. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_development_of_the_first_branchial_arch_in_man_and_the_fate_of_Meckel%27s_cartilage
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