Definition
The joint mucosa (synonym: synovialis and synovial membrane) lines the joint capsule, tendon sheaths and bursae from the inside. Its primary function is the production of synovial fluid, which nourishes the cartilages and is supposed to reduce friction. In addition, some of the cells of the joint mucosa also have a phagocytizing effect, which means that these cells can absorb tissue debris and microorganisms and dissolve them by means of enzymes.
Anatomy
The anatomy of the synovium is different from the other internal body cavities, which are continuously covered with either endothelium, epithelium or mesothelium, which are derived from all three cotyledons of embryonic development. This is different for the synovial mucosa or synovial membrane. It is lined by so-called synoviocytes, which are not epithelial cells and originate from the mesenchymal connective tissue, i.e. embryonic connective tissue.
The mesenchyme originates from the mesodermal cotyledon. The mesenchyme develops into all the connective tissue that the body possesses. Therefore, it is also the origin for joint capsules and their inner lining.
The joint mucosa consists of two layers: The intima and the vascular subintima. The intima consists of the synoviocytes and amorphous extracellular matrix. The synoviocytes do not have a continuous basement membrane, which is only formed in pieces, and are divided into type A and type B.
Type A cells have a phagocytizing function; they take up and break down cell and tissue debris. It is assumed that they originate from mononuclear cells of the blood and are local macrophages, comparable to the Kupffer cells of the liver. Type B synoviocytes are the producing cells, which produce a mixture of hyaluronan, collagen and fibronectin and release it into the environment and articular cavity.
They are therefore the main producers of the synovia (“synovial fluid“), which serves to feed the cartilage and reduce friction in the joint. In order to perform this task, which requires the production of many proteins, type B synoviocytes have a particularly large amount of rough endoplasmic reticulum, which is very important for the production of secretory proteins. In general, synoviocytes have a very polymorphic appearance and can be flattened, cubic, spindle-shaped or even macrophage-like.
Below this intimate layer, which consists of a few cell layers, the so-called subintima is found. This consists of a loose connective tissue, which is very well supplied with blood. It contains other cells such as fat cells, macrophages and fibroblasts. This layer forms a kind of buffer layer between the intimate synoviocytes and the taut and relatively hard connective tissue of the joint capsule. It should also be noted here that a synovial membrane is also formed in bursae and tendon sheaths and is also responsible for the production of synovial fluid, although its anatomical structure is slightly different.
All articles in this series:
