In some people, especially dark-skinned adults, healing skin wounds exhibit excessive fibrosis, producing raised, shiny scars called keloids (fig. 5.35). Keloids extend beyond the boundaries of the original wound and tend to return even if they are surgically removed. Keloids may result from the excessive secretion of a fibroblast-stimulating growth factor by macrophages and platelets. They occur most often on the upper trunk and earlobes. Some tribespeople practice scarification-scratching or cutting the skin to induce keloid formation as a way of decorating the body.
- Figure 5.35 A Keloid of the Earlobe. This scar resulted from piercing the ear for earrings.
Figure 5.34 illustrates the following stages in the healing of a cut in the skin, where both regeneration and fibrosis are involved:
- Severed blood vessels bleed into the cut. Mast cells and cells damaged by the cut release histamine, which dilates blood vessels, increases blood flow to the area, and makes blood capillaries more permeable. Blood plasma seeps into the wound, carrying antibodies, clotting proteins, and blood cells.
- A blood clot forms in the tissue, loosely knitting the edges of the cut together and interfering with the spread of pathogens from the site of injury into healthy tissues. The surface of the blood clot dries and hardens in the air, forming a scab that temporarily seals the wound and blocks infection.
Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
186 Part One Organization of the Body
Beneath the scab, macrophages begin to clean up tissue debris by phagocytizing and digesting it.
- New blood capillaries sprout from nearby vessels and grow into the wound. The deeper portions of the clot become infiltrated by capillaries and fibroblasts and transform into a soft mass called granulation tissue. Macrophages remove the blood clot while fibroblasts deposit new collagen to replace it. This fibroblastic (reconstructive) phase of repair begins 3 to 4 days after the injury and lasts up to 2 weeks.
- Surface epithelial cells around the wound multiply and migrate into the wounded area, beneath the scab. The scab loosens and eventually falls off, and the epithelium grows thicker. Thus, the epithelium regenerates while the underlying connective tissue undergoes fibrosis, or scarring. Capillaries withdraw from the area as fibrosis progresses. The scar tissue may or may not show through the epithelium, depending on the severity of the wound. The wound may exhibit a depressed area at first, but this is often filled in by continued fibrosis and remodeling from below, until the scar becomes unnoticeable. This remodeling (maturation) phase of tissue repair begins several weeks after injury and may last as long as 2 years.
Before You Go On
Answer the following questions to test your understanding of the preceding section:
- Distinguish between differentiation and metaplasia.
- Tissues can grow through an increase in cell size or cell number. What are the respective terms for these two kinds of growth?
- Distinguish between atrophy, necrosis, and apoptosis, and describe a circumstance under which each of these forms of tissue loss may occur.
- Distinguish between regeneration and fibrosis. Which process restores normal cellular function? What good is the other process if it does not restore function?
Insight 5.4 Clinical Application
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Acne is a name that is famous in its own right, but for all of the wrong reasons. Most teenagers know, and dread, the very word, as it so prevalently wrecks havoc on their faces throughout their adolescent years.
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