The synovial membrane (also known as the synovial stratum, synovium or stratum synoviale) is a specialized

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

that lines the inner surface of capsules of

synovial joint A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articul ...

s and

tendon sheath A tendon sheath is a layer of synovial membrane around a tendon. It permits the tendon to stretch and not adhere to the surrounding fascia. It has two layers: * synovial sheath A synovial sheath is one of the two membranes of a tendon sheath wh ...

. It makes direct contact with the fibrous membrane on the outside surface and with the

synovial fluid Synovial fluid, also called synovia, elp 1/sup> is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its egg white–like consistency, the principal role of synovial fluid is to reduce friction between the articul ...

lubricant on the inside surface. In contact with the synovial fluid at the tissue surface are many rounded

macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...

-like synovial cells (type A) and also type B cells, which are also known as fibroblast-like synoviocytes (FLS). Type A cells maintain the synovial fluid by removing wear-and-tear debris. As for the FLS, they produce

hyaluronan Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglyca ...

, as well as other extracellular components in the synovial fluid.

Structure

The synovial membrane is variable but often has two layers: * The outer layer, or subintima, can be of almost any type of connective tissue –

fibrous Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporat ...

(dense collagenous type), adipose (fatty; e.g. in intra-articular fat pads) or areolar (loose collagenous type). * The inner layer (in contact with synovial fluid), or

intima The tunica intima (New Latin "inner coat"), or intima for short, is the innermost tunica (layer) of an artery or vein. It is made up of one layer of endothelial cells and is supported by an internal elastic lamina. The endothelial cells are in ...

, consists of a sheet of cells thinner than a piece of paper. Where the underlying subintima is loose, the intima sits on a pliable membrane, giving rise to the term ''synovial membrane''. This membrane, together with the cells of the intima, provides something like an inner tube, sealing the synovial fluid from the surrounding tissue (effectively stopping the joints from being squeezed dry when subject to impact, such as running). Just beneath the intima, most synovium has a dense net of fenestrated small

blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide awa ...

s that provide

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excre ...

s not only for synovium but also for the avascular cartilage. In any one position, much of the cartilage is close enough to get nutrition directly from the synovium. Some areas of cartilage have to obtain nutrients indirectly and may do so either from diffusion through cartilage or possibly by 'stirring' of synovial fluid. The surface of synovium may be flat or may be covered with finger-like projections or villi, which, it is presumed, help to allow the soft tissue to change shape as the joint surfaces move one on another. The synovial fluid can be thought of as a specialised fluid form of synovial extracellular matrix rather than a secretion in the usual sense. The fluid is

transudative Transudate is extravascular fluid with low protein content and a low specific gravity (< 1.012). It has low nucleated cell counts (less than 500 to 1000 /microliter) and the primary cell types are This is especially important since it is the major source of metabolic support for articular cartilage. Under normal conditions synovial fluid contain <100/mL of leucocytes in which majority are monocytes.

Synovial cells

The intimal cells are of two types, fibroblast-like synoviocytes or type B cells and

-like synovial cells. Surface cells have no basement membrane or junctional complexes denoting an epithelium despite superficial resemblance. * The fibroblast-like synoviocytes (derived from

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every ...

) manufacture a long-chain sugar polymer called

(hence rich in

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ...

); which makes the synovial fluid "ropy"-like egg-white, together with a molecule called

lubricin Proteoglycan 4 or lubricin is a proteoglycan that in humans is encoded by the ''PRG4'' gene. It acts as a joint/ boundary lubricant. Function Lubricin is present in synovial fluid and on the surface (superficial layer) of articular cartilag ...

, which lubricates the joint surfaces. The water of synovial fluid is not secreted as such but is effectively trapped in the joint space by the hyaluronan. * The macrophage-like synovial cells (derived from monocytes in blood) are responsible for the removal of undesirable substances from the synovial fluid (hence are rich in

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles i ...

). It accounts for approximately 25% of cells lining the synovium.

Mechanics

Although a biological joint can resemble a man-made joint in being a

hinge A hinge is a mechanical bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation: all other ...

or a ball and socket, the engineering problems that nature must solve are very different because the joint works within an almost completely solid structure, with no wheels or nuts and bolts. In general, the bearing surfaces of manmade joints interlock, as in a hinge. This is rare for biological joints (although the badger's jaw interlocks). More often the surfaces are held together by cord-like

ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal l ...

s. Virtually all the space between

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...

s, ligaments,

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...

s, and

cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck ...

is filled with pliable solid tissue. The fluid-filled gap is at most only a twentieth of a millimetre thick. This means that synovium has certain jobs to do. These may include: # Providing a plane of separation, or disconnection, between solid tissues so that movement can occur with minimum bending of solid components. If this separation is lost, as in a 'frozen shoulder', the joint cannot move. # Providing a packing that can change shape in whatever way is needed to allow the bearing surfaces to move on each other. # Controlling the volume of fluid in the cavity so that it is just enough to allow the solid components to move over each other freely. This volume is normally so small that the joint is under slight suction.

Pathology

Synovium can become irritated and thickened ( synovitis) in conditions such as

osteoarthritis Osteoarthritis (OA) is a type of degenerative joint disease that results from breakdown of joint cartilage and underlying bone which affects 1 in 7 adults in the United States. It is believed to be the fourth leading cause of disability in the ...

, Ross River virus or

rheumatoid arthritis Rheumatoid arthritis (RA) is a long-term autoimmune disorder that primarily affects joints. It typically results in warm, swollen, and painful joints. Pain and stiffness often worsen following rest. Most commonly, the wrist and hands are inv ...

(RA). The fibroblast-like synoviocytes (FLS) play a key role in the pathogenesis of RA, and the aggressive phenotype of FLS in RA and the effect these cells have on the microenvironment in the joint can be summarized into hallmarks that distinguish them from healthy FLS. These hallmark features of FLS in RA are divided into seven cell-intrinsic hallmarks (such as reduced apoptosis and impaired contact inhibition) and four cell-extrinsic hallmarks (such as their ability to recruit and stimulate immune cells). In general, inflamed synovium is accompanied by extra

recruitment (as well as the existing type A cells),

fibroblast A fibroblast is a type of biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework ( stroma) for animal tissues, and plays a critical role in wound healing. Fibroblasts are the most common cells ...

proliferation and an influx of inflammatory cells including

lymphocytes A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic ada ...

monocytes Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also inf ...

and plasma cells. When this happens, the synovium can interfere with the normal functioning of the joint. Excessive thickened synovium, filled with cells and fibrotic collagenous tissue, can physically restrict joint movement. The synovial fibroblasts may make smaller hyaluronan so it is a less effective lubricant of the cartilage surfaces. Under stimulation from invading inflammatory cells, the synovial cells may also produce

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

s (

proteinase A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...

s) that can digest the

extracellular matrix. Fragments of

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...

can then further irritate the synovium.

Etymology and pronunciation

The word ''synovium'' is related to the word ''synovia'' in its

sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system re ...

meaning "

". The latter was coined by

Paracelsus Paracelsus (; ; 1493 – 24 September 1541), born Theophrastus von Hohenheim (full name Philippus Aureolus Theophrastus Bombastus von Hohenheim), was a Swiss physician, alchemist, lay theologian, and philosopher of the German Renaissance. He ...

. More information is given at '' Synovial fluid § Etymology and pronunciation''.

References