Porth's Essentials of Pathophysiology, 4e

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Cell and Tissue Function

U N I T 1

SUMMARY CONCEPTS (continued)

Healing by ConnectiveTissue Repair The primary objective of the healing process is to fill the gap created by tissue destruction and to restore the structural continuity of the injured part. Tissue regen- eration refers to the restoration of injured tissue to its normal structure and function by proliferation of adja- cent surviving cells. As discussed earlier in the chapter, cell renewal occurs continuously in labile tissues such as gastrointestinal epithelium and skin. Regeneration can also occur in parenchymal organs with stable cell popu- lations but, with the exception of the liver, is usually a limited process. It should be pointed out that extensive regeneration can occur only if the residual tissue is struc- turally and functionally intact. If the tissue is damaged by infection or inflammation, regeneration is incomplete and accomplished by replacement with scar tissue. When regeneration cannot occur, healing by replace- ment with a connective (fibrous) tissue occurs, a process that terminates in scar formation . The term fibrosis is often used to describe the extensive deposition of collagen that occurs in organs that are incapable of regeneration. 2 When this occurs, fibrous tissue grows into the area of damage, converting it to a mass of fibrous tissue, a process called organization . 1,2 It can also occur in serous cavities (pleura, peritoneum) when excessive exudate accumulates and can- not be cleared. In the pericardium, fibroblasts secrete and organize collagen within fibrous strands, thereby binding the visceral and parietal pericardia together 1 (Fig. 4-4). Fibrous strands sometimes become organized within the peritoneal cavity following abdominal surgery or perito- nitis. These strands of collagen, called adhesions, can trap loops of bowel and cause obstruction. 1 Phases of Repair Repair by connective tissue deposition can be divided into three phases: (1) hemostasis, angiogenesis, and ingrowth of granulation tissue; (2) emigration of fibroblasts and ■■ Tissue regeneration and repair are mediated by growth factors that increase cell size, cell division, and cell differentiation, and enhance the production of specialized extracellular matrix (ECM) proteins by cells such as fibroblasts. Among its many functions, the ECM aids in the regulation of cell proliferation, movement, and differentiation during wound repair by providing a foundation for cell adhesion and a reservoir for growth factors. those that are unable to regenerate. Stem cells are undifferentiated cells of continuously dividing tissues that have the capacity to generate multiple cell types.

deposition of extracellular matrix; and (3) matura- tion and reorganization of the fibrous tissue (remodel- ing). 1,2,10–16 It usually begins within 24 hours of injury and is evidenced by the migration of fibroblasts and the induction of fibroblast and endothelial cell proliferation. 2 By 3 to 5 days, a special type of tissue called granulation tissue is apparent. 2 The granulation tissue then progres- sively accumulates connective tissue, eventually resulting in the formation of a scar, which is then remodeled. Angiogenesis and Ingrowth of Granulation Tissue Granulation tissue is a glistening red, moist connective tissue that fills the injured area while necrotic debris is removed 1,2 (Fig. 4-5). It is composed of newly formed capillaries, proliferating fibroblasts, and residual inflam- matory cells. The development of granulation tissue involves the growth of new capillaries (angiogenesis). Angiogenesis is a tightly regulated process that includes migration of endothelial cells to the site of tissue injury, Repair, regeneration, and fibrosis. In: Rubin R, Strayer DS, eds. Rubin’s Pathology: Clinicopathologic Foundations of Medicine. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams &Wilkins; 2012:94.) FIGURE 4-4. Organized strands of collagen (arrows) in constrictive pericarditis. (From Sephal GC, Davidson JM.