Extracellular Matrix

The cells that comprise the organs of our body are embedded within the extracellular material of connective tissues. This material is called the extracellular matrix, and it consists of the protein fibers collagen and elastin (see chapter 2, fig. 2.28), as well as gel-like ground substance. The interstitial fluid referred to previously exists primarily in the hydrated gel of the ground substance.

Although the ground substance seemingly lacks form (is amorphous) when viewed under a microscope, it is actually a

Epithelial membrane

Basal lamina (basement membrane)

Collagenous protein fibers

Elastin protein fibers

Blood capillary

■ Figure 6.1 The extracellular environment. The extracellular environment contains fluid, as interstitial, or tissue, fluid, within a matrix of glycoproteins and proteoglycans. This fluid, derived from blood plasma, provides nutrients and regulatory molecules to the cells. The extracellular environment is supported by collagen and elastin protein fibers, which also form the basal lamina below epithelial membanes.

Epithelial membrane

Basal lamina (basement membrane)

Collagenous protein fibers

Elastin protein fibers

Blood capillary

■ Figure 6.1 The extracellular environment. The extracellular environment contains fluid, as interstitial, or tissue, fluid, within a matrix of glycoproteins and proteoglycans. This fluid, derived from blood plasma, provides nutrients and regulatory molecules to the cells. The extracellular environment is supported by collagen and elastin protein fibers, which also form the basal lamina below epithelial membanes.

Interactions Between Cells and the Extracellular Environment highly functional, complex organization of molecules chemically linked to the extracellular protein fibers of collagen and elastin, as well as to the carbohydrates that cover the outside surface of the cell's plasma membrane (see chapter 3, fig. 3.2). The gel is composed of glycoproteins (proteins with numerous side chains of sugars) and molecules called proteoglycans. These molecules (formerly called mucopolysaccharides) are composed primarily of polysaccharides and have a high content of bound water molecules.

The collagen and elastin fibers have been likened to the reinforcing iron bars in concrete—they provide structural strength to the connective tissues. One type of collagen (there are about fifteen different types known) constitutes the basal lamina (or basement membrane) underlying epithelial membranes (see chapter 1, fig. 1.11). By forming chemical bonds between the carbohydrates on the outside surface of the plasma membrane of the epithelial cells, and the glycoproteins and proteoglycans of the matrix in the connective tissues, the basal lamina helps to wed the epithelium to its underlying connective tissues (fig. 6.1)

There is an important family of enzymes that can break down extracellular matrix proteins. These enzymes are called matrix metalloproteinases (MMPs) because of their need for a zinc ion cofactor. MMPs are required for tissue remodeling (for example, during embryonic development and wound healing), and for migration of phagocytic cells and other white blood cells during the fight against infection. MMPs are secreted as inactive enzymes and then activated extracellularly. They can contribute to disease processes, however, if they are produced or activated inappropriately. For example, cancer cells that become invasive (that metastasize, or spread to different locations) produce active MMPs, which break down the collagen of the basal lamina and allow the cancerous cells to migrate. The destruction of cartilage protein in arthritis may also involve the action of these enzymes, and MMPs have been implicated in the pathogenesis of such neural diseases as multiple sclerosis, Alzheimer's disease, and others. Therefore, scientists are attempting to develop drugs that may be able to treat these and other diseases by selectively blocking different matrix metalloproteinases.

Integrins are a class of glycoproteins that extend from the cytoskeleton within a cell, through its plasma membrane, and into the extracellular matrix. By binding to components within the matrix, they serve as a sort of "glue" (or adhesion molecule) between cells and the extracellular matrix. Moreover, by physically joining the intracellular to the extracellular compartments, they serve to relay signals between these two compartments (or integrate these two compartments—hence the origin of the term integrin.). Interestingly, certain snake venoms slow blood clotting by blocking integrin-binding sites on blood platelets, preventing them from sticking together (see chapter 13 for a discussion of blood clotting).

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Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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