Objective. To characterize morphologically and ultrastructurally using light microscopy, the scanning electron microscopy and transmission electron microscopy the intima synovial of the temporomandibular joint (TMJ) of human fetuses between the 10th and the 38th week of development. Materials and Methods. The TMJ was dissected bilaterally in 37 human fetuses belonging to the Institute of Embryology of the University Complutense of Madrid and of the Federal University of S？o Paulo. Results. The outcome by light microscopy showed the morphology of the TMJ and that the formation of inferior joint cavity precedes the superior joint cavity and the presence of blood vessels in the synovial. Conclusion. By scanning and transmission electron microscopy we observed the presence of two well-defined cell types in the intima layer of synovial of the TMJ of human fetuses, macrophage-like type A cell and fibroblast-like type B cell, and the presence of the a third cell type, defined by the name of intermediate lining cell in the intima layer of the synovial. 1. Introduction The temporomandibular joint (TMJ) is a specialized synovial joint essential for the function of the mammalian jaw. The main components of the TMJ are the mandibular condyle, the mandibular fossa of the temporal bone, the articular disc with collagen fibers interposed with between them, and a synovial [1–5]. Morphologically, the synovial consists of two layers: one cellular intima layer and another for support, the vascular subintima layer, which combines with the articular capsule. The intima layer consists of cells within an amorphous and fiber-free matrix whose width varies approximately from 1 to 4 cells. The subintima layer consists of loose and vascularized connective tissue, with spread out fibroblasts, macrophages, mastocytes, adipose cells, and some elastic fibers that prevent the pleating of the [6–12]. The intima layer has cells with phagocytic ability called macrophage-like type A cell. This same layer also has cells called fibroblast-like B cells, which synthesize proteins, glycoproteins, and proteoglycans [9, 10, 13–16]. There is also a third type of cells not entirely studied yet called intermediate lining cell [6, 17–20]. Among these three cell types there are spaces filled with not very fibrous extracellular matrix and macrophage ground substance. The synovial fluid is formed in the synovial by blood vessel plasma in the subintima layer and travels to the articular space. As it crosses the intima layer, new elements secreted by fibroblast-like type B cells join in and the fluid
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