Synovium
Encyclopedia
Synovial membrane is the soft tissue
found between the articular capsule (joint capsule) and the joint cavity of synovial joint
s.
The word "synovium" is related to the word "synovia" (synovial fluid
), which is the clear, viscid, lubricating fluid secreted by synovial membranes. The word "synvovia" or "sinovia" was coined by Paracelsus
, and may have been derived from the Greek word "syn" ("with") and the Latin word "ovum" ("egg") because the synovial fluid
in joints that have a cavity between the bearing surfaces is similar to egg white
.
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).
The intimal cells are of two types, fibroblast
s and macrophage
s, both of which are different in certain respects from similar cells in other tissues.
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.
Just beneath the intima, most synovium has a dense net of small blood vessel
s that provide nutrient
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 direct from 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.
in being a hinge
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
s. Virtually all the space between muscle
s, ligament
s, bone
s, and cartilage
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 rather unexpected jobs to do. These may include:
. When this happens, the synovium can become a danger to the bearing surface structure in a variety of ways. Excess synovial fluid weeping from inflamed synovium can provide a barrier to diffusion of nutrients to cartilage. The synovial cells may also use up nutrients so that the glucose
level in the tissue is almost zero. These factors may lead to starvation and death of cartilage cells. Synovial cells may also produce enzyme
s that can digest the cartilage surface, although it is not clear that these will damage cartilage with healthy cells.
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
found between the articular capsule (joint capsule) and the joint cavity of synovial joint
Synovial joint
A Synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones....
s.
The word "synovium" is related to the word "synovia" (synovial fluid
Synovial fluid
Synovial fluid is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its yolk-like consistency , the principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement.-Overview:The inner membrane of synovial joints...
), which is the clear, viscid, lubricating fluid secreted by synovial membranes. The word "synvovia" or "sinovia" was coined by Paracelsus
Paracelsus
Paracelsus was a German-Swiss Renaissance physician, botanist, alchemist, astrologer, and general occultist....
, and may have been derived from the Greek word "syn" ("with") and the Latin word "ovum" ("egg") because the synovial fluid
Synovial fluid
Synovial fluid is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its yolk-like consistency , the principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement.-Overview:The inner membrane of synovial joints...
in joints that have a cavity between the bearing surfaces is similar to egg white
Egg white
Egg white is the common name for the clear liquid contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms around either fertilized or unfertilized egg yolks...
.
Structure
Synovium is very variable but often has two layers.- The outer layer, or subintima, can be of almost any type: fibrous, fatty or loosely "areolar"Loose connective tissueLoose connective tissue is extremely loose. It is a category of connective tissue which includes areolar tissue, reticular tissue, and adipose tissue....
.
- The inner layer, or intima, consists of a sheet of cellCell (biology)The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s thinner than a piece of paper.
Where the underlying subintima is loose, the intima sits on a pliable membrane
Biological membrane
A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...
, 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).
The intimal cells are of two types, fibroblast
Fibroblast
A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the structural framework for animal tissues, and plays a critical role in wound healing...
s and macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...
s, both of which are different in certain respects from similar cells in other tissues.
- The fibroblasts manufacture a long-chain sugar polymer called hyaluronanHyaluronanHyaluronan is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues...
, which makes the synovial fluid "ropy"-like egg-white, together with a molecule called lubricin, 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 macrophages are responsible for the removal of undesirable substances from the 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.
Just beneath the intima, most synovium has a dense net of small blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s that provide nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
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 direct from 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.
Mechanics
Although a biological joint can resemble a man-made jointReplacement joint
Replacement arthroplasty [from Greek arthron, joint, limb, articulate, + -plassein, to form, mould, forge, feign, make an image of], or joint replacement surgery, is a procedure of orthopedic surgery in which the arthritic or dysfunctional joint surface is replaced with an orthopaedic prosthesis...
in being a hinge
Hinge
A hinge is a type of 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. Hinges may be made of flexible material or of moving components...
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
Badger
Badgers are short-legged omnivores in the weasel family, Mustelidae. There are nine species of badger, in three subfamilies : Melinae , Mellivorinae , and Taxideinae...
's jaw interlocks).
More often the surfaces are held together by cord-like ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...
s. Virtually all the space between muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s, ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...
s, bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s, and cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
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 rather unexpected 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 in conditions such as rheumatoid arthritisRheumatoid arthritis
Rheumatoid arthritis is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks synovial joints. The process produces an inflammatory response of the synovium secondary to hyperplasia of synovial cells, excess synovial fluid, and the development...
. When this happens, the synovium can become a danger to the bearing surface structure in a variety of ways. Excess synovial fluid weeping from inflamed synovium can provide a barrier to diffusion of nutrients to cartilage. The synovial cells may also use up nutrients so that the glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
level in the tissue is almost zero. These factors may lead to starvation and death of cartilage cells. Synovial cells may also produce enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s that can digest the cartilage surface, although it is not clear that these will damage cartilage with healthy cells.