Attract sodium and potassium ions which in turn cause

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attract sodium and potassium ions, which in turn cause the GAGs to absorb and retain water. Thus, GAGs play an important role in regulating the water and electrolyte balance of tissues. The most common GAG is chondroitinsulfate(con-DRO-ih-tin). It is abundant in blood vessels andbones and gives cartilage its relative stiffness. Other GAGs that you will read of in this book are heparin(an anticoagulant) and hyaluronicacid(HY-uh-loo-RON-ic). The latter is a gigantic molecule up to 20 μm long, as large as most cells. It is a viscous, slippery substance that forms a lubricant in thejoints and constitutes much of the jellylike vitreous bodyof the eyeball. A proteoglycanis another gigantic molecule. It is shaped somewhat like a bottle brush, with a central core of protein and bristlelike outgrowths composed of GAGs. The entire proteoglycan may be attached to hyaluronic acid, thus forming an enormous molecular complex. Proteoglycans form thick colloids similar to those of gravy, gelatin, and glue. This gel slows the spread of pathogenic organisms through the tissues. Some proteoglycans are embedded in the plasma membranes of cells, attached to the cytoskeleton on the inside and to other extracellular molecules in the matrix. They create a strong structural bond between cells and extracellularmacromolecules and help to hold tissues together.Adhesive glycoproteinsare protein–carbohydrate complexes that bind plasma membrane proteins to extracellular collagen and proteoglycans. They bind the components of a tissue together and mark paths that guide migrating embryonic cells to their destinations in a tissue.Types of Fibrous Connective TissueFibrous connective tissue is divided into two broad categories according to the relative abundance of fiber:looseanddense connective tissue.In loose connective tissue, much of the space is occupied by ground substance, whichdissolves out of the tissue during histological fixation and leaves empty space in prepared tissue sections. The loose connective tissues we will discuss are areolarand reticulartissue(table 5.4). In dense connective tissue,fiber occupies more space than the cells and ground substance, and appears closely packed in tissue sections. We will discuss two types: dense regularand dense irregular connective tissue10
Areolar tissue15(AIR-ee-OH-lur) exhibits loosely organized fibers, abundantblood vessels, and a lot of seemingly empty space. It possesses all six of the aforementioned cell types. Its fibers run in random directions and are mostly collagenous, but elastic and reticular fibers are also present. Areolar tissue is highly variable in appearance. In many serous membranes, it looks like figure 5.14, but in the skin and mucous membranes, it is more compact(see fig. 5.8) and sometimes difficult to distinguish from dense irregular connective tissue. Some advice on how to tell them apart is given after the discussion of dense irregular connective tissue.Areolar tissue is found in tissue sections from almost every part of the body.

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