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
The upper and lower joint spaces arise through the formation of multiple small splits in the dense mesenchyme from which the condyle, disk, and fossa arose previously.
The lower joint space appears first at about the tenth week (50-65 mm CRL), but later the upper joint space overtakes it in its development (Burdi 1992). At first the space is extensively compartmentalized, and it is only later that the individual cavities merge (Bontschev 1996). The lower joint space lies close to the embryonic condyle.
The upper joint space appears after about the twelfth week (60-70 mm CRL) and spreads posteriorly and medially over Meckel’s cartilage with its contour corresponding to that of the future fossa. After week 13 the lower joint space is already well formed as the upper joint space continues to take shape. From its beginning, the upper joint space has fewer individual islands of space and grows more rapidly than the lower joint space. After week 14 both joint spaces are completely formed. During weeks 16-22 the lumens of the chambers become adapted to the contours of the surrounding rounding bone. The fibrocartilaginous articular disk develops from the concentrated mesenchyme between the two joint spaces. The articular disk is not visible until the CRL is 70 mm. Even before formation of the joint spaces the disk is already thinner at its center than at the periphery and this leads to its final biconcave form (Bontschew 1996). The peripheral portions are not sharply demarcated from the surrounding loose mesenchyme. In fetuses with a CRL of 240 mm, the mesenchymal tissue changes into dense fibrous connective tissue. At this stage the peripheral region has a greater blood supply than the central region. According to Moffet (1957), compression of the disk between the temporal bone and the condyle results in an avascular central zone. At the beginning of its development the disk lies closer to