Anatomic Pathology / PATHOLOGY OF THE SYNOVIUM Pathology of the Synovium

Synovium is specialized mesenchymal tissue that is essential for the appropriate function of the locomotor apparatus. It is the site for a series of pathologic processes that are characteristic, and in some cases specific, to this distinctive tissue. In this article, the normal microscopic anatomy of synovium is briefly reviewed. Synovial proliferative disorders, including pigmented villonodular synovitis, giant cell tumor of tendon sheath, hemosiderotic synovitis, and fatty infiltration of the synovial membrane are discussed. Additionally, the subjects of intrasynovial cartilaginous lesions (primary and secondary synovial chondromatosis) and crystal deposition diseases are reviewed. Finally, the response of synovial tissues to implanted foreign materials that are used in large and small joint arthroplasty are described. Phylogenetically, synovium is one of the newer attributes of the vertebrate locomotor apparatus. The first synovial joints developed in the piscine jaw of ancestors of modern lungfish by an evolutionary process that modified preexisting fibrous and cartilaginous joints, which were the predominant articulation of the early seaand land-dwelling vertebrates.1 Embryologically, synovium is derived from specialized mesoderm termed the interzonal mesenchyme ❚Image 1❚.1 This differentiates from the primitive mesenchyme that lies between developing mobile cartilaginous skeletal elements. Fully developed mature connective tissue retains the capacity to differentiate into synovium as exemplified by the development of adventitial bursae and pseudarthroses and the synovial reaction to fixed foreign implanted material such as breast implants. As Gray’s Anatomy eloquently proposes, “...once a region of pliant connective tissue is established between rigid skeletal elements [or foreign material], their ultimate separation by a synovial cavity appears to be an evolutional probability.”1