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ch03.5 - Development of Cells and Tissues

ch03.5 - Development of Cells and Tissues - 98 Essentials...

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begins almost immediately. Inflammation is a gen- eralized (nonspecific) body response that attempts to prevent further injury. The immune response, on the other hand, is extremely specific and mounts a vigorous attack against recognized in- vaders (bacteria, viruses, toxins). These protective responses are considered in detail in Chapter 12. Here we will concentrate on the process of tissue repair itself. Tissue repair, or wound healing, occurs in two major ways: by regeneration and by fibrosis. Regeneration is the replacement of destroyed tissue by the same kind of cells, whereas fibrosis involves repair by dense (fibrous) connective tis- sue, that is, by the formation of scar tissue . Which occurs depends on (1) the type of tissue damaged and (2) the severity of the injury. Generally speaking, clean cuts (incisions) heal much more successfully than ragged tears of the tissue. Tissue injury sets a series of events into motion. The capillaries become very permeable. This allows fluid rich in clotting proteins and other substances to seep into the injured area from the bloodstream. Then leaked clotting proteins construct a clot, which stops the loss of blood, holds the edges of the wound to- gether, and walls off the injured area, prevent- ing bacteria or other harmful substances from spreading to surrounding tissues. Where the clot is exposed to air, it quickly dries and hard- ens, forming a scab. Granulation tissue forms. Granulation tis- sue is a delicate pink tissue composed largely of new capillaries that grow into the damaged area from undamaged blood vessels nearby. 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.
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