Histology 101: Synovial Joints, a Possible Explanation to their Susceptibility to Infection

A synovial joint consists of articulating bone surfaces that are covered with hyaline cartilage (aka articular cartilage) separated with a fluid filled space surrounded by a joint capsule. The lining of the joint space is covered with a synovial membrane. Synovial membranes are composed of cuboidal collagenous cells called synoviocytes, they function to secrete synovial fluid that aid lubrication.

This anatomical set-up seems pretty safe, a dense layer of cells surrounds the underlying vasculature to protect from infection. Well, not quite... in most epithelium, such as our stratified squamous epithelium of our skin, there are tight junctions and basal lamina that aid in keeping propagators of inflammation out. However, the layer of synoviocytes do not come with this protective shield. This fact has lead scientists to group synoviocytes separate from the tradition epithelium cell type.

What's the connection to arthritis? One of the many causes of arthritis is inflammation that can be caused by a septic infection that can eventually make its way to the synovial fluid of joints. Since there are no tight junctions or basal lamina within the synovium, bacteria have the ability to enter the highly vasculature tissue and initiate inflammatory processes. The body's reaction to joint inflammation often leads to joint destruction and therefore arthritis.

The important role of tight junctions and the basal lamina can be seen in the comparison between epithelium and synoviocytes and their ability to react to Staphylococcus aureus. Staphylococcus aureus is a very common bacteria strain seen within the mucus membranes of the respiratory tract and basically any outer surface of our body. Within the mucus membranes, for example, the bacteria are essentially trapped outside the body. However, within the synovial membrane, Staphylococcus aureus can easily enter. Staphylococcus aureus is the most commone infection seen within infectious or septic arthritis.

Young, Barbara, James Lowe, Alan Stevens, and John Heath. Wheater's Functional Histology, a Text and Colour Atlas. 5th ed. Churchill Livingstone Elsevier.

"Septic Arthritis." MayoClinic.Com. 31 July 2006. Mayo Clinic. http://www.mayoclinic.com/health/bone-and-joint-infections/
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