Components of Fibrous Connective Tissue

Cells The cells of fibrous connective tissue include the following types:

  • Fibroblasts.9 These are large, flat cells that often appear tapered at the ends and show slender, wispy branches. They produce the fibers and ground substance that form the matrix of the tissue. Fibroblasts that have finished this task and become inactive are called fibrocytes by some histologists.
  • Macrophages.10 These are large phagocytic cells that wander through the connective tissues, where they engulf and destroy bacteria, other foreign particles, and dead or dying cells of our own body, and activate the immune system when they sense foreign matter called antigens. They arise from certain white blood cells called monocytes or from the same stem cells that produce monocytes.
  • Leukocytes,11 or white blood cells (WBCs). WBCs travel briefly in the bloodstream, then crawl out through the capillary walls and spend most of their time in the connective tissues. Most of them are neutrophils, which wander about in search of bacteria. Our mucous membranes often exhibit dense patches of tiny WBCs called lymphocytes, which react against bacteria, toxins, and other foreign agents.
  • Plasma cells. Certain lymphocytes turn into plasma cells when they detect foreign agents. The plasma cells then synthesize disease-fighting proteins called antibodies. Plasma cells are rarely seen except in the walls of the intestines and in inflamed tissue.
  • Mast cells. These cells, found especially alongside blood vessels, secrete a chemical called heparin that inhibits blood clotting, and one called histamine that increases blood flow by dilating blood vessels.
  • Adipocytes (AD-ih-po-sites), or fat cells. These are large rounded cells filled mainly with a droplet of triglyceride, which forces the nucleus and cytoplasm to occupy only a thin layer just beneath the plasma membrane. They appear in small clusters in some fibrous connective tissues. When they dominate an area, the tissue is called adipose tissue.

Fibers Three types of protein fibers are found in fibrous connective tissues:

• Collagenous (col-LADJ-eh-nus) fibers. These fibers, made of collagen, are tough and flexible and resist

9fibro = fiber + blast = producing

© The McGraw-Hill Companies, 2003

Chapter 5 Histology 167

stretching. Collagen is about 25% of the body's protein, the most abundant type. It is the base of such animal products as gelatin, leather, and glue.12 In fresh tissue, collagenous fibers have a glistening white appearance, as seen in tendons and some cuts of meat (fig. 5.13); thus, they are often called white fibers. In tissue sections, collagen forms coarse, wavy bundles, often dyed pink, blue, or green by the most common histological stains. Tendons, ligaments, and the deep layer of the skin (the dermis) are made mainly of collagen. Less visibly, collagen pervades the matrix of cartilage and bone.

12colla = glue + gen = producing

White Fibrous Connective Tissue
Figure 5.13 Tendons of the Hand. The white glistening appearance results from the collagen of which tendons are composed. The bracelet-like band across the wrist is also composed of collagen.

Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

168 Part One Organization of the Body

  • Reticular13 fibers. These are thin collagen fibers coated with glycoprotein. They form a spongelike framework for such organs as the spleen and lymph nodes.
  • Elastic fibers. These are thinner than collagenous fibers, and they branch and rejoin each other along their course. They are made of a protein called elastin, whose coiled structure allows it to stretch and recoil like a rubber band. Elastic fibers account for the ability of the skin, lungs, and arteries to spring back after they are stretched. (Elasticity is not the ability to stretch, but the tendency to recoil when tension is released.) Fresh elastic fibers are yellowish and therefore often called yellow fibers.

Ground Substance Amid the cells and fibers in some tissue sections, there appears to be a lot of empty space. In life, this space is occupied by the featureless ground substance. Ground substance usually has a gelatinous to rubbery consistency resulting from three classes of large molecules: glycosaminoglycans, proteoglycans, and adhesive glycoproteins. It absorbs compressive forces and, like the styrofoam packing in a shipping carton, protects the more delicate cells from mechanical injury.

A glycosaminoglycan (GAG) (gly-COSE-ah-MEE-no-GLY-can) is a long polysaccharide composed of unusual di-saccharides called amino sugars and uronic acid. GAGs are negatively charged and thus tend to attract sodium and potassium ions, which in turn causes the GAGs to absorb and hold water. Thus, GAGs play an important role in regulating the water and electrolyte balance of tissues. The most abundant GAG is chondroitin (con-DRO-ih-tin) sulfate. It is abundant in blood vessels and bones and is responsible for the relative stiffness of cartilage. Some other GAGs that you will read of elsewhere in this book are heparin (an anticoagulant) and hyaluronic (HY-uh-loo-RON-ic) acid. The latter is a gigantic molecule up to 20 ^m long, as large as most cells. It is a viscous, slippery substance that forms a very effective lubricant in the joints and constitutes much of the jellylike vitreous humor of the eyeball.

A proteoglycan is another gigantic molecule. It is shaped somewhat like a test tube brush (fig. 5.14), with the central core composed of protein and the 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, pudding, 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 on the outside. Thus, they create a strong structural bond between cells and extracellular macromolecules and help to hold tissues together.

Hyaluronic acid (a glycosaminoglycan)

Core protein

Chondroitin sulfate (a glycosaminoglycan)

Hyaluronic acid (a glycosaminoglycan)

Core protein

Chondroitin sulfate (a glycosaminoglycan)

Proteoglycan

Figure 5.14 Proteoglycan Molecules Linked to a Hyaluronic Acid Core. The resulting hydrophilic complex is larger than many cells.

Adhesive glycoproteins are protein-carbohydrate complexes that bind plasma membrane proteins to collagen and proteoglycans outside the cell. They bind all the components of a tissue together and mark pathways that guide migrating embryonic cells to their destinations in a tissue.

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