Glycocalyx

Glycocalyx is a general term referring to extracellular polymeric material produced by some bacteria, epithelia and other cells. The slime on the outside of a fish is considered a glycocalyx. The term was initially applied to the polysaccharide matrix excreted by epithelial cells forming a coating on the surface of epithelial tissue.

Definition

A glycocalyx, literally "sugar coat", is a network of polysaccharides that project from cellular surfaces, e.g. those of bacteria. It serves to protect the bacterium or allows the bacterium to attach itself to inert surfaces (like teeth or rocks; e.g. Streptococcus pneumoniae attaches itself to lung cells), eukaryotes, or other bacteria (their glycocalyxes can fuse to envelop the colony).

Its presence on inert materials (such as metal hardware implanted for fracture fixation or total joint replacement) make it difficult to eradicate deep infections as the bacteria will 'cling' on to the material via the glycocalyx. It is therefore often necessary to completely remove the hardware device in order to fully eradicate a wound infection.

The glycocalyx can be found just outside the cell wall of a bacterium. A distinct, gelatinous glycocalyx is called a capsule, while an irregular, diffuse layer is called a slime layer. Glycocalyx can help protect bacteria from phagocytes. It also helps in the formation of biofilms such as a coating on inert surfaces such as teeth or rocks.

The glycocalyx is also the name given to a specific structure of a mature platelet. The glycocalyx is unique among the cellular components of the blood. It is similar to the bacterial glycocalyx above in that it is made up of glycoproteins and allows the platelet to adhere to surfaces such as collagen of damaged vessels. The glycocalyx appears as a fluffy coat to the outer membrane of platelets and contains many of the receptor proteins that allow cell adhesion. Glycocalyx also appears on the cells lining blood vessels (endothelial cells). Among its established roles are reducing friction to the flow of blood and serving as a barrier for loss of fluid through the vessel wall. In times of inflammation, the endothelial cell glycocalyx is sheared off, to permit attachment of leukocytes and movement of water from microvessels.

The glycocalyx is chemically unique in everyone but identical in monozygote twins, and acts like an identification tag that enables the body to distinguish its own healthy cells from transplanted tissues, invading organisms and diseased cells. Human blood types and transfusion compatibility are determined by glycolipids and glycoproteins.

A glycocalyx can also be found on the apical portion of microvilli within the digestive tract, especially within the small intestine. It consists of glycoproteins that project from the apical plasma membrane of epithelial absorptive cells. It provides additional surface for adsorption and includes enzymes secreted by the absorptive cells that are essential for the final steps of digestion of proteins and sugars.

Functions

Protection: Cushions the plasma membrane and protects it from chemical injury
Immunity to infection: Enables the immune system to recognize and selectively attack foreign organisms
Defense against cancer: Changes in the glycocalyx of cancerous cells enable the immune system to recognize and destroy them
Transplant compatibility: Forms the basis for compatibility of blood transfusions, tissue grafts, and organ transplants
Cell adhesion: Binds cells together so that tissues do not fall apart
Inflammation regulation: Glycocalyx coating on endothelial walls in blood vessels prevents leukocytes from rolling/binding in healthy states
Fertilization: Enables sperm to recognize and bind to eggs
Embryonic development: Guides embryonic cells to their destinations in the body