Synoviocytes are specialized cells residing within the synovial membrane that lines joint cavities. This membrane secretes synovial fluid, a lubricating liquid that reduces friction and nourishes articular cartilage. Understanding the characteristics of synoviocytes, both in their native tissue and when found within the fluid, provides insight into the state of the joint.
The Synovium and Its Cellular Components
The inner lining of movable joint capsules is a specialized connective tissue called the synovial membrane, or synovium. The synovium’s inner surface is lined with synoviocytes, which are categorized into two populations.
One population is the macrophage-like synoviocytes, or Type A cells. These cells are derived from hematopoietic monocytes and act as resident macrophages, performing phagocytosis to remove cellular debris and waste from the joint.
The other population is the fibroblast-like synoviocytes, or Type B cells. These are characterized by an abundance of rough endoplasmic reticulum, a feature associated with protein synthesis. Type B synoviocytes are responsible for manufacturing the structural components of the synovial membrane’s matrix and key elements of the synovial fluid.
Synoviocytes’ Contribution to Synovial Fluid
Synoviocytes manage the production of synovial fluid, which begins as an ultrafiltrate of blood plasma from capillaries in the synovium. The synoviocytes then modify this filtrate by secreting specific molecules into the joint space, transforming it into a viscous liquid. Type B synoviocytes are the primary drivers of this synthesis.
They produce hyaluronan, also known as hyaluronic acid, which gives synovial fluid its high viscosity and lubricating properties. This viscosity provides shock absorption during movement, protecting the underlying cartilage from mechanical stress. The amount of fluid in a healthy joint is small; a human knee, for example, contains only about 2 milliliters.
In addition to hyaluronan, Type B synoviocytes secrete a glycoprotein called lubricin. Lubricin molecules coat the surfaces of the articular cartilage, providing boundary lubrication. This action prevents cartilage surfaces from making direct contact during low-speed movements. Other secreted products include proteinases and collagenases, enzymes that help remodel the joint’s matrix components.
Synoviocytes Found Within Synovial Fluid
While synoviocytes form the lining of the synovium, they can also be found floating within the synovial fluid. Normally, the presence of these cells in the fluid results from the natural lifecycle of the synovial membrane. This process involves routine cell turnover, where older cells are shed from the lining to ensure the tissue remains healthy.
The number of synoviocytes in the synovial fluid of a healthy joint is very low. These shed cells are eventually cleared by the phagocytic action of Type A synoviocytes residing within the synovial lining. This baseline presence reflects an equilibrium between cell shedding and removal, which is disrupted during inflammatory or pathological conditions.
Altered Synoviocytes in Synovial Fluid and Joint Disease
In joint diseases, the number and characteristics of synoviocytes within the synovial fluid change, reflecting the underlying pathology. Conditions like rheumatoid arthritis trigger a shift in the synovial environment. The synovial membrane becomes hyperplastic, growing abnormally thick, and the synoviocytes become highly activated. These altered synoviocytes, particularly Type B, are shed into the synovial fluid in greater numbers.
Once in the fluid, these activated synoviocytes contribute directly to the disease process. In rheumatoid arthritis, they release inflammatory cytokines and enzymes that perpetuate the inflammatory response and promote the destruction of cartilage and bone. The ratio of Type A to Type B synoviocytes can also shift, and the fluid becomes crowded with synoviocytes and a large influx of immune cells.
In osteoarthritis, the changes can be less dramatic but are still meaningful. An increase in synoviocyte numbers in the fluid often indicates an inflammatory component of the disease, known as synovitis. These cells may release molecules that contribute to the gradual degradation of cartilage.
In septic arthritis, which is caused by infection, the synovial fluid is filled with bacteria and immune cells. Activated synoviocytes are also present as part of the acute inflammatory response to the pathogen.
Analyzing Synoviocytes from Synovial Fluid for Diagnosis
The analysis of synoviocytes retrieved from synovial fluid offers diagnostic information for clinicians. The procedure to collect this fluid is called arthrocentesis, or joint aspiration, where a needle is inserted into the joint space to withdraw a sample. This fluid is then examined in a laboratory to assess its cellular and biochemical composition.
Examining the number of synoviocytes in the fluid is a primary step. A high synoviocyte count, along with other inflammatory cells, can help differentiate between inflammatory conditions like rheumatoid arthritis and non-inflammatory ones. For example, fluid from an inflammatory joint will have a much higher total cell count than fluid from a healthy or non-inflammatory arthritic joint.
Further microscopic examination can reveal the morphology and activation state of the synoviocytes. Observing activated synoviocytes can support a diagnosis of inflammatory arthritis. These findings, combined with other tests, help clinicians formulate a precise diagnosis and monitor the effectiveness of treatment over time.