Structural features of the articular disc of the rat temporomandibular joint: Fibrocartilage or dense fibrous lamina?

The structure of the temporomandibular joint (TMJ disc) articular disc as a dense fibrous lamina or fibrocartilage is an ongoing debate. To clarify this, we investigated TMJ disc cells in 3- to 20-week-old rats from the perspectives of general histology, ultrastructure, immunohistochemistry, and in situ hybridization. Sections from paraffin-embedded TMJ discs were immunohistochemically stained using antibodies against aggrecan and collagen type II. Expression of Acan and Col2a1 mRNA was analyzed by in situ hybridization using antisense cRNA probes. Other samples were analyzed by transmission electron microscopy. Chondrocyte-like cells were visualized on the most medial aspect of the anterior band at 20 weeks. Transmission electron microscopy revealed that these cells were structurally similar to chondrocytes (termed "morphological chondrocytes") but with less abundant rough endoplasmic reticulum and low expression of Acan and Col2a1 mRNA. Immunoreactivity for aggrecan and collagen type II was sparse in the extracellular matrix, but the areas of positivity were narrow. Immunoreactivity for hypoxia-inducible factor-1α was detected in the morphological chondrocytes and CD31-positive capillaries did not enter the disc, indicating an enhanced hypoxic environment in this region. The presence of morphological chondrocytes in the articular disc region indicates an adaptation to the hypoxic environment rather than increased synthesis of cartilaginous matrix synthesis; hence, the rat TMJ disc can be regarded, in general, as a dense fibrous lamina rather than consisting of fibrocartilage.