and twelfth weeks, and according to Moore and Lavelle (1974), between the tenth and twelfth weeks.
Formation of the bony mandible begins in weeks 6-7 lateral to Meckel’s cartilage in both halves of the face. A double anlage of Meckel’s cartilage is extremely rare (Rodriguez-Vazquez et al. 1997). Its effect on embryonic development is unknown. By about the twelfth week the two palatal processes have united at the midline to complete the separation of the oral and nasal cavities. At the same time, bony anlagen of the maxilla form in the region of the future infraorbital foramina. These spread rapidly in a horizontal direction and progressively fill the space between the oral cavity and the eyes. When the crown-rump length (CRL) is approximately 76 mm (weeks 10-12), the anlagen of the maxillary bone, the zygomatic bone, and the temporal bone come into contact with one another. Ossification of the base of the cranium and of the facial portion of the skull follows in a strict, genetically determined sequence (Bach-Petersen et al. 1994). First to ossify is the mandible, followed by the maxilla, medial alar process of the sphenoid bone, frontal bone, zygomatic bone, zygomatic arch, squamous part of the occipital bone, greater wing of the sphenoid bone, tympanic bone, condyles of the occipital bone, lesser wing of the sphenoid bone, and finally the dorsolateral portion of the sphenoid bone.
In an embryo with a CRL of approximately 53 mm the coronoid process and the condylar process can already be clearly distinguished from one another. The biconcave form of the articular disk becomes apparent at a CRL of 83 mm. In histological preparations, fibers of the pterygoid muscle can also be seen streaming in quite early (Radlanski et al. 1994). At this stage the superior belly of the lateral pterygoid muscle inserts at the middle and central third of the disk and the lower belly inserts at the condyle (Merida-Velasco et al. 1993). At a CRL of 95 mm all structures of the temporomandibular joint can be clearly identified and thereafter undergo no essential change other than an increase in size (Bontschev 1996).
Embryology of the Temporomandibular Joint and the Muscles of Mastication
During the development of the temporomandibular joint the articular fossa is the first structure to become recognizable. This occurs during weeks 7-8 (Burdi 1992). It First appears as a concentration of mesenchymal cells over an area of tissue that later differentiates into disk and capsule. Between the tenth and eleventh weeks the fossa begins to ossify. Development of the cortical layer and the bony trabeculae is more rapid in the fossa than in the condyle. The fossa develops first as a protrusion on the original site of the zygomatic arch and grows in a medial-anterior direction (Lieck 1997). At the same time the articular eminence begins to develop. The condyle, at first cartilaginous, develops between the tenth and eleventh weeks from an accumulation of mesenchymal cells lateral to Meckel’s cartilage (Burdi 1992). Enchondral ossification progresses apically, creating a bony fusion with the body of the mandible. After the fifteenth week the chondrocytes have differentiated enough so that the cartilage already exhibits the typical postnatal organization of structure (Perry et al. 1985), and from the twentieth prenatal week onward only the superficial portion of the process consists of cartilage.
Joint development
18 Tenth week
A histological section in the frontal plane showing the condylar process (1) and Meckel’s cartilage (2) at the tenth week of embryonic development. The condylar process is rounded over and surrounded by a layer of especially dense mesenchyme (arrows). It lies lateral to Meckel’s cartilage. The fast-growing dorsocranial portion of the accumulation of cartilage cells creates the distinctive shape of the condyle.
19 Eleventh week
Above: A human temporomandibular joint in the frontal plane at the eleventh week of development. This represents the same area shown in Figure 19 only 10 days further along. The condylar process is beginning to ossify (arrows). At this time the swallowing reflex is also developing and is accompanied by the formation of secondary cartilage (n the temporomandibular joint (Lakars 1995). Contributed by R. Wurgaft Dreiman Below: Sagittal section of a temporomandibular joint at the same stage of development. Above the condyle (1) is a distinct concentration of mesenchymal cells (arrows). At its inferior region the mesenchymal thickening is already beginning to detach from the condyle as the lower joint space forms. During this time the first collagen fibers of the disk become visible and increase greatly in number until the twelfth week. Contributed by R.j. Radlanski
The articular disk can first be identified after 7.5 weeks in utero as a horizontal concentration of mesenchymal cells (Burdi 1992). Between weeks 19 and 20 its typical fibrocartilaginous structure is already evident.
The joint capsule first appears between weeks 9 and 11 as thin striations around the future joint region (Burdi 1992). After 17 weeks the capsule is clearly demarcated, and after 26 weeks all of its cellular and synovial parts are completely differentiated.
In weeks 9-10 the lateral pterygoid muscle is recognizable with its superior head inserting on the disk and capsule and its inferior head inserting on the condyle. Fibers of the masseter and temporal muscles also insert on the disk (Merida Velasco et al. 1993).
During the tenth week the first blood vessels become organized around the joint. The disk has small blood vessels only at its periphery and is itself avascular (Valenza et al. 1993). Branches of the trigeminal and auriculotemporal nerves are clearly visible in the twelfth week (Furstman 1963). The numerous nerve endings that can still be seen in the disk in the twentieth week diminish rapidly so that after birth the disk is no longer innervated (Ramieri et al. 1996).
20 Fourteenth week
Sagittal section of a human disk-condyle complex. A distinct joint space has now formed between the condyle (1) and the disk (2). Above the disk the temporal blastema begins to split away to form the upper joint space (arrows). The cartilage of the condyle is increasingly replaced by bone the original cartilage remain in the neck of the condyle until past puberty.
21 Sixteenth week
Horizontal section of a human temporomandibular joint during the sixteenth week of embryonic development. Insertion of the lateral pterygoid muscle (1) onto the condyle (2) can be clearly identified. In agreement with reports in the literature (Ögütcen-Toller and juniper 1994. Ögütcen-Toller 1995), the discomaleolar ligament (arrows) runs from the joint capsule through the tympanosquamosal fissure to the malleus (3) as an extension of the muscle.
22 Eighteenth week
Frontal section through a human temporomandibular joint in the eighteenth week of embryonic development. The fossa (1), disk (2) and condyle (3) are completely developed and from now on will experience joint capsule (arrows) can also be clearly identified. The cartilaginous condyle will ossify further. Distribution of cartilage at this stage indicates that future growth will be primarily in the laterosuperior direction. Contributed by R. Wurgaft Dreiman
Development of the Upper and Lower Joint Spaces
