Light and electron microscopic morphology of the temporomandibular joint in growing and mature crab-eating monkeys (Macaca fascicularis): the condylar articular layer.

In an attempt to establish maturational alterations in the morphology of the articular tissue layer, mandibular condyles of four immature and four mature male monkeys (Macaca fascicularis) were studied using light microscopy as well as scanning and transmission electron microscopy. Specimens were fixed in situ by perfusion in the presence of ruthenium red to stabilize proteoglycans. Preparations intended for observation in the scanning electron microscope were first dehydrated and sputtered for the examination of articular surfaces, and afterwards treated with trypsin to expose the spatial arrangement of collagen fibrils. Gross anatomical relations between joint components indicated that the anterior and central, but not the posterior region of the condylar articular surface can be subject to compressional load. Load-bearing and non-load-bearing regions differed with respect to the morphology of the articular layer. Load-bearing surfaces were covered by a prominent articular surface lamina similar to that observed on articular cartilage. This lamina seemed to constitute an integral part of the articular layer, distinct from the lining of synovial fluid, and to be composed largely of proteoglycans. It was unaffected by maturation. The subjacent, load-bearing articular layer differed markedly in structure, both from articular cartilage, and between immature and mature animals. Articular cells of immature animals were classified as fibroblastlike, but unlike typical fibroblasts, were surrounded by a thin, often incomplete halo of fibril-free pericellular matrix, presumably consisting of proteoglycans. In mature animals, articular cells closely resembled chondrocytes, but exhibited prominent nuclear fibrous laminae, which usually are found only in fibroblasts. Thus, the load-bearing part of the articular layer seems to undergo a maturation-dependent metaplastic conversion, from a dense connective tissue with some features of fibrocartilage, to a fibrocartilage-like tissue containing chondrocyte-like cells with some features of fibroblasts. This conversion might reflect an adaptation to a maturation-associated increase in articular stress.