These capillaries are fragile and bleed freely as

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These capillaries are fragile and bleed freely, as when a scab is picked away from a skin wound. Granulation tissue also contains phagocytes that eventually dispose of the blood clot and connective tissue cells (fibro- blasts) that synthesize the building blocks of collagen fibers (scar tissue) to permanently bridge the gap. The surface epithelium regenerates. As the surface epithelium begins to regenerate, it makes its way across the granulation tissue just beneath the scab. The scab soon detaches and the final result is a fully regenerated surface epithelium that covers an underlying area of fibrosis (the scar). The scar is either invisible or visible as a thin white line, depending on the severity of the wound. The ability of the different tissue types to re- generate varies widely. Epithelial tissues such as the skin epidermis and mucous membranes regen- erate beautifully. So, too, do most of the fibrous connective tissues and bone. Skeletal muscle re- generates poorly, if at all, and cardiac muscle and nervous tissue within the brain and spinal cord are replaced largely by scar tissue. Homeostatic Imbalance Scar tissue is strong, but it lacks the flexibility of most normal tissues. Perhaps even more impor- tant is its inability to perform the normal functions of the tissue it replaces. Thus, if scar tissue forms in the wall of the bladder, heart, or another muscular organ, it may severely hamper the functioning of that organ. PART III: DEVELOPMENTAL ASPECTS OF CELLS AND TISSUES We all begin life as a single cell, which divides thousands of times to form our multicellular em- bryonic body. Very early in embryonic development, the cells begin to specialize to form the primary tis- sues, and by birth, most organs are well formed and functioning. The body continues to grow and enlarge by forming new tissue throughout child- hood and adolescence. Cell division is extremely important during the body’s growth period. Most cells (except neu- rons) undergo mitosis until the end of puberty, when adult body size is reached and overall body growth ends. After this time, only certain cells routinely divide for example, cells exposed to abrasion that continually wear away, such as skin and intestinal cells. Liver cells stop dividing; but they retain this ability should some of them die or become damaged and need to be replaced. Still other cell groups (for example, heart muscle and nervous tissue) almost completely lose their abil- ity to divide when they are fully mature; that is, they become amitotic (am ı˘-tot ik). Amitotic tis- sues are severely handicapped by injury because the lost cells cannot be replaced by the same type 98 Essentials of Human Anatomy and Physiology
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