DEVELOPMENT OF THE SQUAMOSO-MANDIBULAR ARTICULATION IN THE RAT

¹ Departments of Orthodontia and Oral Pathology, University of Illinois Dental School, Chicago, Ill.

This histologic investigation is based on complete serial frontal sections through the squamoso-mandibular articulation of 31 normal and 30 ia rats ranging from 16 days insemination age to 30 days after birth. The ia rats are a mutation in which bone resorption does not occur and the pattern of bone and cartilage formation can be accurately traced.

This study showed that the condylar cartilage differentiates in the body of the mandible at 17 days insemination age and grows backward toward the zygomatic process of the squamosal bone. In the mesenchyme which separates the cartilage from the bone, a superior and an inferior slit appear at 19 and 20 days insemination age respectively. At 21 days insemination age, the anlage of the capsular ligament appears and thus at this age the squamoso-mandibular articulation differentiates.

In the articular disc which at this time consists only of fusiform cells, some collagen fibers appear at 2 days after birth and their number progressively increases with age. Their general orientation is anterior-posterior. In the central part of the articulation, the disc is narrowest while medially, laterally, and especially, anteriorly and posteriorly, it widens. In the latter two areas, it contains blood vessels and fat cells but elsewhere it is avascular. The synovial cavities are lined partly by flattened and partly by cuboidal epithelium. At 15 days after birth, villi begin to form and extend into the anterior and posterior portions of the synovial cavities. These villi have a vascularized core and their number and size increases with age. The articular fossa is formed by the zygomatic process and the inferior half of the squama of the squamosal bone. Generally rapid bone apposition occurs on the inferior border of the zygomatic process and the depth of the fossa and resorption occurs on the surfaces of the fossa which are superolateral and superomedial to the condylar cartilage. Thus the fossa widens while its depth remains more or less constant. The direction of the growth of the condylar cartilage is first predominantly backward but later it shifts to a more upward and outward direction. These directions of growth are related to the development and growth of teeth. The destruction of condylar cartilage and its replacement by bone begins at 19 days insemination age and progressively reduces the cartilaginous part of the condylar process. Modeling resorption of the neck of the condylar process begins at 15 days after birth. The capsular ligament of the squamoso-mandibular articulation of the rat is not well marked. It extends from the posterior half of the lateral rim of the fossa to the condylar neck. In the anterior part of the lateral surface of the articulation, the masseter muscle and medially the external pterygoid act as the capsule.