Lining the innermost aspect of the joint capsule is lined by synovium. Synovium is thought to contain three distinct cell populations: phagocytic macrophages, antigen presenting cells, and fibroblast like cells that produce glycosaminoglycans. Synovial fluid is a filtrate of plasma enriched with proteoglycans. The synovial membrane is normally supported by a loose fibrovascular stroma admixed with adipose. The outermost layer, the joint capsule, is composed of dense collagen. Tendons and ligaments look histologically similar and are also composed of very dense collagen. The collagenous fibers in tendons insert directly into bone, and these are called ‘Sharpey’s fibers’. In doing so, the tendon is strongly anchored to the underlying bone. Sharpey’s fibers can easily be visualized under a polarizing microscope.
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