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Wednesday, 16 January 2008

Life glue

What causes cells to stick together in the human body rather than simply falling apart?
McKenzie Gibson, Glasgow, UK


Cells are keep together by means of a process called "cell adhesion". Single cells have special molecules in their surface that act as glue among them. These molecules are of three types called cell adhesion molecules (CAMs), substrate adhesion molecules (SAMs) and cell junctional molecules (CJMs). All these molecules help cells to stay together without falling apart. For more information read "Topobiology: an introduction to molecular embryology" by Gerald M. Edelman (Basisc Books, NY 1988).

Dr. Octavio Miramontes
Mexico City
By Anonymous Anonymous on August 15, 2007 7:32 PM  
There is also the extracellular matrix (ECM) which is made up of glycosaminoglycans (protein sugar)and proteins such as collagen. This ECM acts as a frame work for cells to interact and bind with. ECM is particularly important in bone, cartilage and organs.
By Anonymous Anonymous on August 16, 2007 5:13 AM  
The cells actually are wondering the same thing about their constituent molecules, seeing as how the space between the matter dwarfs the actual molecular size. As such they huddle together in groups, in hopes to solve this mystery by combining their collective reasoning.
By Anonymous Anonymous on August 16, 2007 11:37 PM  
I thought this was widely know...that which keeps the universe tied together at the most fundamental level: Duct tape!
By Anonymous Anonymous on August 17, 2007 6:01 PM  
Well for the constituent molecules, their held together, by their intermolecular bonds, hydrogen bonds and (i think) electrostatic forces between different charged parts of the molecule.

Of course the molecules themselves are held together by covalent bonds, bonds created by sharing electrons between atoms.
Here’s what I learnt in college : Cell junctions help bind cells together. They are mostly found in the basal, lateral and apical cell surfaces of epithelial cells, but can be found in other tissues as well. There are 4 kinds doing the job at hand:

A tight junction (zonula occludens) is a belt like junction going all around the apical lateral regions of the cells. The plasma membrane proteins are fused (no extracellular space) so no molecules can pass through them. They are found between the cells that line the digestive tract (prevents enzymes,ions etc from going into the bloodstream),kidney tubules etc.

Adherens Junctions (zonula adherens) reinforces the tight junctions –when tissues are pulled- by anchoring adjacent cells (they are basically transmembrane linker proteins attached to the actin microfilaments of the cytoskeleton to connect the adjacent cells).Also found all around the apical lateral borders of epithelial cells. These are found in the epithelium of our skin.

Desmosomes are circular plaques joined to the neighboring cell’s plaque by linker proteins. These form along the sides of adjacent cells thus anchoring from either side. Bundles of intermediate filaments extend across the cytoplasm and connect to other desmosomes on the opposite side. Thus the epithelium is less likely to tear when pulled at; as the forces are distributed evenly. Cardiac muscles have desmosomes.

Finally Gap Junctions (nexus) are tunnel like structures occurring anywhere on the lateral membranes. At these junctions, the cell membranes are very close and the cells themselves are connected by hollow protein cylinders (connexons) which aid in the movement of ions or water between adjacent cells. These are abundant in cardiac and smooth muscles where they help synchronize contraction.
By Anonymous kiddikat on August 18, 2007 11:54 AM  
Hyaluronic acid
By Anonymous Anonymous on August 18, 2007 4:55 PM  
god does didnt ya know
By Anonymous Anonymous on August 21, 2007 9:56 AM  
The cells are kept together by proteins called cadherins, found on the surfaces of most cell types. There are 30 different types of cadherins, e.g. the VE-cadherin, which is found on the cells that make up blood vessels. Each cadherin only sticks to its own kind, thus making it effective in keeping the same types of cell together to form a tissue while keeping different tissues apart.
By Anonymous Anonymous on August 21, 2007 2:40 PM  
Human cells, like other animal cells will contain particular areas where they may join. These areas are highly concentrated in carbohydrates called the glycocalyx, similar to that of a cell wall in bacteria, fungi and plants. This area is only present on the extra-cellular part, “outside”, of the cell and is where the adhesion sites occur. This will be a collection of proteins and glycoproteins such as cadherins, selectins and even immunoglobulin super family, (IGSF) proteins.

In the case of both IGSF’s and cadherins the interactions will be with those of the same molecule (homophilic interaction). However, selectins will interact with different molecules such as the carbohydrate groups on other surface proteins and this is know as a heterophilic interaction. These interactions may be weak on their own, but as always, strength is in numbers.

Different cells have different patterns of cell adhesion molecules, depending on if the two different cells patterns are similar will decide the strength of the adhesion. It is this which stops certain cells sticking to another cell when they are not supposed to. This allows tissues to form without interference from other unwanted cells.
By Anonymous Oliver Smith on September 06, 2007 1:09 AM  
The following answer was selected and edited by New Scientist staff

Cells in the body are organised in tissues that are held together through a variety of molecular interactions. On the one hand, cells interact with each other. This is a very specific interaction mediated by various families of adhesion molecules called cadherins, neural cell adhesion molecules and intercellular cell adhesion molecules. These are all expressed on the surfaces of cells and anchored in the cytoskeleton of each cell, an arrangement which stabilises and gives strength to interactions between cells.

On the other hand, the body's tissues are not made up solely of cells, but of an intricate network of macromolecules too, called the extracellular matrix. It supports the assembly of cells and is composed of a great variety of polysaccharides and proteins, mainly produced locally by cells called fibroblasts.

These macromolecules are combined into an organised mesh and, depending on the proportions of its components, the matrix can adopt diverse forms adapted to particular functional requirements. For example, it can be calcified and hard as in the bones and teeth, transparent as in the cornea, or elastic and strong as in the tendons. The main components of the matrix, which determine its properties, are fibre-forming proteins that can be structural (collagens and elastin) or adhesive (fibronectin and laminin).

Cells adhere to this complex scaffolding through surface receptors called integrins that are anchored in the cell cytoskeleton and bind to the matrix components. Though integrins are densely packed on the cell surface, they have a relatively low affinity for interacting with the matrix components. This allows the cells to move within the matrix without losing their grip completely, meaning that it is, in effect, a rather flexible glue.

However, the interactions between integrins and matrix components have a deeper purpose than just holding the cells in place. Almost like antennae, they can transmit messages to the cell about the microenvironment to which it needs to adapt, and so influence cell shape, movement and function.

There are of course also cells in the body that remain free. These are the components of the blood: red and white blood cells and platelets that normally float in the bloodstream, delivering oxygen to the tissues and surveying for invading microorganisms and wounds. These cells are capable of attaching themselves to other cells or tissues at will. For example, the encounter of a platelet with a wound activates the integrins of the platelet, enabling it to bind to fibrinogen in the blood vessel and initiate the aggregation process that forms clots and stems bleeding.

Alena Pance, Department of Biochemistry University of Cambridge, UK
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