The synovium is the innermost portion of capsular ligament of synovial joints. It is commonly seen as a multilayered complex comprising several layers (intima, subintima, and superficial vascular plexus). Besides its function as immunologic microenvironment, the syno-vium allows disconnection between adjacent moving structures. Moreover, it is responsible for synovial fluid synthesis, a dialysate of plasma containing high-molecular weighted polysaccharides, known as hyaluronan. The synovial fluid maintains a fluid film between cartilage surfaces under load, which allows hydrodynamic lubrication. Synovial villi, which provide versatile deformability during movement, become more common with age and may compensate for the growing inelasticity and increasingly fibrous character of the subintima.
Investigations of radiohumeral joints showed that the villous pattern changes as a result of aging. Dependent on the location within the joint, the villi even may display a hard plicate pattern in the adult (Isogai et al., 2001).
Morphological studies of the aging synovium revealed an overall loss of synovium lining cells. In detail, synovial intimal fibroblast numbers decreased with advancing age, whereas there was a relative increase in synovial intimal macrophages. Moreover, a fibrosis of the intimal layer caused by collagen accumulation was described, as well as a decreased vascularity (Jilani et al., 1986; Pasquali-Ronchetti et al., 1992).
The composition of the lubricating synovial fluid is also subjected to age-dependent alterations. The concentrations of chondroitin sulfate and hyaluronic acid were measured and were found to vary with age. Their values were highest between 20 and 30 years of age, and thereafter they showed a tendency to decrease (Nakayama et al., 2002). Similar results were described for hydroxyproline and glycosaminoglycan levels, as these macromolecules were reduced with increasing age (van den Boom et al., 2004).
Interestingly, the synovial tissues harbor multipotent mesenchymal stem cells. These cells could be induced to differentiate to the chondrocyte, osteocyte, and adipocyte lineages. The multipotent mesenchymal stem cells showed only limited senescence; neither extensive expansion in monolayer culture, nor donor age or cryopreservation affected their multilineage potential (De Bari et al., 2001). These findings suggest the presence of active synovial mesenchymal stem cells even in elderly individuals. To determine their physiological function, however, further research is needed.
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