ch03.5 - Development of Cells and Tissues

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

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begins almost immediately. Infammation is a gen- eralized (nonspeciFc) body response that attempts to prevent ±urther injury. The immune response, on the other hand, is extremely speciFc 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 o± tissue repair itsel±. Tissue repair, or wound healing, occurs in two major ways: by regeneration and by Fbrosis. Regeneration is the replacement o± destroyed tissue by the same kind o± cells, whereas fbrosis involves repair by dense (Fbrous) connective tis- sue, that is, by the ±ormation o± scar tissue . Which occurs depends on (1) the type o± tissue damaged and (2) the severity o± the injury. Generally speaking, clean cuts (incisions) heal much more success±ully than ragged tears o± the tissue. Tissue injury sets a series o± events into motion. The capillaries become very permeable. This allows fuid rich in clotting proteins and other substances to seep into the injured area ±rom the bloodstream. Then leaked clotting proteins construct a clot, which stops the loss o± blood, holds the edges o± the wound to- gether, and walls o±± the injured area, prevent- ing bacteria or other harm±ul substances ±rom spreading to surrounding tissues. Where the clot is exposed to air, it quickly dries and hard- ens, ±orming a scab. Granulation tissue Forms. Granulation tis- sue is a delicate pink tissue composed largely o± new capillaries that grow into the damaged area ±rom undamaged blood vessels nearby. These capillaries are ±ragile and bleed ±reely, as when a scab is picked away ±rom a skin wound. Granulation tissue also contains phagocytes that eventually dispose o± the blood clot and connective tissue cells (Fbro- blasts) that synthesize the building blocks o± collagen ±ibers (scar tissue) to permanently bridge the gap. The surFace epithelium regenerates. As the sur±ace epithelium begins to regenerate, it makes its way across the granulation tissue just beneath the scab. The scab soon detaches and the Fnal result is a ±ully regenerated sur±ace epithelium that covers an underlying area o± Fbrosis (the scar). The scar is either invisible or visible as a thin white line, depending on the severity o± the wound. The ability o± the di±±erent tissue types to re- generate varies widely. Epithelial tissues such as the skin epidermis and mucous membranes regen- erate beauti±ully. So, too, do most o± the Fbrous connective tissues and bone. Skeletal muscle re- generates poorly, i± 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 fexibility 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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This note was uploaded on 01/31/2011 for the course BIO 210 taught by Professor S during the Spring '10 term at University of Toronto- Toronto.

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

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