Paper - The Development of the Human Mandibular Joint

Embryology - 16 Apr 2024        Expand to Translate
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Paper - The Development of the Human Mandibular Joint

The Development of the Human Mandibular Joint

By N. B. B. Symons

Dental School, University of St Andrews, Dundee

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Introduction

The mandibular joint has long been of interest in that phylogenetically it represents a new joint replacing the older one between the articulare (malleus) and the quadrate (incus). In this connexion considerable controversy has arisen over the formation of the articular disc and its relation to the lateral pterygoid muscle. Furthermore, the mode of formation of the mandibular joint is somewhat different from that of most other synovial joints.

In the literature, so far as I have found, the only previous workers who studied the development of the human mandibular joint by means of a series of specimens are Kjellberg (1904), Vinogradoff (1910) and Mundaca (194-8) The earliest stage described by any of these three workers is one of 35 mm. 0.1:. length.

A considerable period before there is any sign of a joint-cavity the chief elements of the mandibular joint have been mapped out. This is particularly true with regard to the condylar process of the mandible. Even as early as the 22 mm. stage in the human embryo there is some indication of its form.

Material and Methods

The materials used in this work were one embryo of 22 mm. C.R. length and a series of foetuses of 80, 84, 40, 48, 57 (two specimens), 60, 65, 70 (two specimens), 95, 150, 180 mm. (two specimens) C.R. length and one of full term. The material was fixed in formalin, dehydrated and cleared by routine methods and embedded in paraffin. One of the 57 mm. specimens, the 180 mm. and the full-term specimens, however, were treated by a double embedding method.

Observations

22 mm stage

A considerable amount of bone-formation has taken place and formed a plate on the lateral side of Meckel’s cartilage; this plate is in the substance of, and is surrounded by, that mesodermal condensation which outlines the mandible and precedes the formation of bone. The plate of bone is confined to the region of what approximately corresponds to the future body of the mandible, but the mesodermal condensation can be traced further backwards, always, of course, on the lateral side of Meckel’s cartilage and the associated branches of the mandibular nerve. The condensation admittedly becomes gradually much less sharply defined but is still distinguishable from the surrounding tissue. Finally, some distance beyond the limit of bone-formation, the lateral pterygoid muscle can be seen running into and outlining the terminal part of the mesodermal condensation. This is the first indication of the condylar process in my series (Pl. 1, fig. 1). Meckel’s cartilage with the inferior dental and lingual nerves on its upper surface lies medial to this rudimentary condylar area and inferior to the lateral pterygoid muscle.

30 mm stage

The mesodermal condensation of the mandible is now everywhere clearly defined. In a section, the head of the condyle appears as a distinct oval-shaped mass to which the fibres of the lateral pterygoid are attached. Above and lateral to the condyle is an oval condensation, in the centre of which is a small area of bone-formation. This is the first indication of the temporal element of the articulation in my series. Already the outermost cells of the condylar condensation form a distinct layer and the temporal condensation shows the same feature but to a less marked extent. This is the first indication of the future layer of fibrous tissue which will cover the articular surfaces of the condyle and the temporal bone.

34 mm stage

The tissue forming the condylar process has been invaded by an extension of the membrane bone from the ramus. A considerable proportion of the condylar neck and the anterior half of the head are occupied by this membrane bone, but there is only a slight indication of it in the posterior half of the head. In the early stages, a coronal section through the condyle head shows the zygomatic arch, since the condyle is well ventral to the temporal region compared with its definitive position (Pl. 1, fig. 2). There is a wide interval between the condylar and zygomatic condensations, which is filled with a loosely formed tissue apart from a dense strip lying close to the condyle. A few fibres of the lateral pterygoid muscle can be traced into the medial side of this strip of tissue. In the previous stage there is already an indication of this narrow condensation, also connected to the lateral pterygoid.

Traced farther back to the point where the condyle is decreasing in size, the zygomatic condensation changes its oval shape and bends medially as a horizontal shelf forming the anterior root of the zygoma and the roof of the future mandibular joint. Some bone had been formed in this condensation and it is continuous with the bone forming in the temporal region lateral to the middle ear. The distinction of the superficial layer of the condylar and zygomatic condensations noted at the previous stage has become more marked and the fibrillar element of this layer is now apparent.

40 mm stage

Very few changes, except for a general increase in size, are to be seen compared with the preceding stage. There is a greater amount of membrane bone-formation in both the head of the condyle and the zygomatic condensation. There is the same arrangement of a wide area of loosely formed tissue and a narrow strip of denser tissue between the zygoma and the condyle, the future intra-articular region. On the medial side the connexion of this narrow strip with the lateral pterygoid is maintained (Pl. 1, fig. 3).

48 mm stage

The secondary cartilage of the condyle is seen for the first time at this stage. It appears as a broad fringe of tissue at, and continuous with, the tip and lateral side of the core of membrane bone in the condyle, and is covered superficially by a thick fibrous layer, the deeper cells of which are continuously adding to the cartilage. At this stage the cartilage consists of large round cells which, in the one direction, can be traced in series with the cells of the fibrous-tissue covering which gives origin to them, and in the other direction are incorporated in the lacunae of the adjacent bone; that is, there is a continuous series from cells that are fibroblasts, through this cartilage-like tissue, to cells that are osteoblasts. The intercellular substance of the cartilage in the direction towards the fibrous-tissue covering it becomes less in amount and is continuous with its fibrillar network, while in the other direction it merges with the intercellular element of the bone.

57 mm stage

The condylar cartilage is now in the form of a cone whose tapering end extends forwards into the ramus of the mandible. The innermost cells of the fibrous-tissue layer outlining the condyle have become more definitely flattened and spindle- shaped, and are arranged with their long axes parallel to the condyle surface. These cells, traced towards the centre of the condyle, gradually become rounded and merge into the area of cartilage (Pl. 1, fig. 4). Towards the anterior end of the condylar cartilage the cell—spaces have become larger, and the intercellular material appears as more sharply defined bars between the cells, as if it had been compressed.

At this stage there is the first sign of joint-cavity formation. This appears as a thinning-out of the loose tissue on either side of the strip of dense tissue above the condyle (Pl. 1, fig. 4). It is now obvious that this strip of tissue is the future articular disc; like the superficial layers of the condylar and zygomatic-temporal condensa- tions, it is still quite cellular. A similar picture was found in another foetus of the same size. There is already a certain indication of a capsule as, from the lateral side of the zygomatic-temporal condensation, a narrow extension passes downwards to link up with the lateral margin of the future articular disc and reach towards the lateral side of the condyle.

60 mm stage

Joint-cavity formation is somewhat advanced compared with the preceding stage. The strip of dense tissue above the head of the condyle is now readily identifiable with the articular disc as the lower joint cavity has appeared as a distinct cleft. This cleft-like lower joint-cavity is limited to the central area of the condyle. The formation of an upper j oint-cavity does not appear to have progressed so far.

65 mm stage

The condyle has greatly increased in size and is completely occupied by cartilage, apart from the area of proliferation of the cells of the fibrous-tissue covering. This area is now confined to the articular surface of the condyle, except for a noticeable extension into the lateral half of the condyle head. The larger size of the cartilage cells and the compressed appearance of the intercellular material, first seen at the 57 mm. stage, now affect the bulk of the cartilage, and, in the older more anterior part of the tissue, the cells show some degenerative changes, being shrunken and distorted.

Joint-cavity formation is considerably advanced, particularly in the lower joint-

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Cite this page: Hill, M.A. (2024, April 16) Embryology Paper - The Development of the Human Mandibular Joint. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_Development_of_the_Human_Mandibular_Joint

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