4.6 Tissue Injury and Aging

Tissue Injury and Repair

When tissue gets damaged, the body launches a coordinated response to stop the bleeding, clean up debris, and rebuild what was lost. These repair mechanisms are central to how the body maintains homeostasis, and understanding the sequence of events helps you connect inflammation, healing, and age-related decline.

Signs of Tissue Inflammation

The five cardinal signs of inflammation are a classic set you'll need to know, along with why each one happens:

• Redness (rubor) occurs because blood vessels dilate, increasing blood flow to the injured area. More blood means more oxygen and nutrients for repair.
• Swelling (tumor) results from increased vascular permeability. Fluid leaks out of capillaries into the interstitial space, causing visible enlargement.
• Heat (calor) comes from that same increased blood flow plus heightened metabolic activity as cells ramp up their repair work.
• Pain (dolor) is triggered by inflammatory mediators like prostaglandins and bradykinin stimulating pain receptors. This serves a protective role: it discourages you from using the injured area.
• Loss of function (functio laesa) is the combined effect of tissue damage, swelling, and pain limiting your ability to use the affected area until healing progresses.

Sequential Responses to Tissue Damage

After an injury, the body moves through four overlapping phases in order:

1. Hemostasis stops blood loss. Blood vessels constrict (vasoconstriction), platelets aggregate and activate at the wound site, and the coagulation cascade produces a fibrin clot that seals the break.
2. Inflammation kicks in as vessels dilate and become more permeable. Leukocytes, especially neutrophils first and then monocytes, migrate to the site to phagocytize bacteria and clear debris. Chemical mediators like histamine, prostaglandins, and cytokines coordinate this response.
3. Proliferation and repair rebuild the damaged area. New blood vessels form (angiogenesis), fibroblasts activate and synthesize collagen to reconstruct the extracellular matrix, and epithelial cells migrate across the wound surface (epithelialization) to restore the tissue covering.
4. Remodeling is the final, longest phase. Collagen fibers mature and reorganize along lines of stress, scar tissue strengthens the repaired area, and tissue gradually regains function. This phase can continue for months to years.

Stages of Tissue Repair (Timeline)

These stages overlap with the responses above but emphasize the timing:

• Inflammatory phase begins immediately after injury and lasts up to about 72 hours. The focus is on hemostasis, inflammation, and debris removal to prepare the wound bed.
• Proliferative phase starts around 72 hours post-injury and continues for several weeks. This is when angiogenesis, collagen synthesis, and epithelialization actively rebuild damaged tissue.
• Remodeling phase follows the proliferative phase and can last months to years. Collagen matures, scar tissue forms, and tissue function is gradually restored. The repaired tissue rarely reaches 100% of its original strength.

Aging and Tissue Alterations

Aging affects every tissue type. The changes are gradual, but they add up to slower healing, stiffer tissues, and greater vulnerability to disease.

Effects of Aging on Tissue

• Decreased cell proliferation means cells divide more slowly and less often, so damaged tissue takes longer to repair and is harder to replace.
• Accumulated cellular damage from oxidative stress, DNA mutations, and misfolded proteins builds up over a lifetime. This contributes to tissue dysfunction and raises the risk of age-related diseases.
• Altered extracellular matrix composition is a major factor. Collagen synthesis decreases while existing collagen fibers become increasingly cross-linked. The result is stiffer, less elastic tissues, which is why skin wrinkles and joints lose flexibility.
• Impaired healing and increased disease susceptibility go hand in hand. Older individuals heal more slowly and face higher rates of degenerative conditions like osteoarthritis and osteoporosis, as well as infections.

Cellular and Molecular Changes in Aging

Several specific mechanisms drive tissue aging at the cellular level:

• Cellular senescence is when cells permanently stop dividing but don't die. These senescent cells accumulate in tissues, secreting inflammatory signals that disrupt normal tissue function.
• Telomere shortening happens with each cell division. Telomeres are protective DNA caps on chromosome ends, and once they get too short, the cell can no longer divide. This limits the body's regenerative capacity over time.
• Free radical accumulation causes oxidative stress. Free radicals are highly reactive molecules that damage proteins, lipids, and DNA. The body's antioxidant defenses become less effective with age.
• Mitochondrial dysfunction reduces cellular energy production (ATP output drops) while increasing oxidative damage, creating a cycle that accelerates aging.
• Stem cell exhaustion is the gradual depletion and functional decline of tissue-specific stem cells. Since stem cells are responsible for replenishing damaged tissue, their decline directly impairs tissue regeneration and homeostasis.

Cancerous Alterations in Tissue

Cancer represents a breakdown in the normal controls on cell growth and behavior. Age is a major risk factor because mutations accumulate over time.

• Uncontrolled cell division results from mutations in genes that regulate growth. Proto-oncogenes (which normally promote growth) can become permanently activated, while tumor suppressor genes like p53 and BRCA1 can be inactivated. Cells also evade apoptosis (programmed cell death), allowing damaged cells to keep dividing.
• Altered cell morphology and differentiation means cancer cells lose their specialized shape and function. Normal tissue architecture breaks down as these cells no longer perform their intended roles.
• Angiogenesis supplies the growing tumor. Cancer cells release signals that stimulate new blood vessel formation, providing the oxygen and nutrients needed to sustain rapid division.
• Invasion and metastasis are what make cancer dangerous. Cancer cells break through basement membranes, enter the blood or lymph, and travel to distant organs like the lungs, liver, or bones, where they establish secondary tumors.
• Immune evasion allows cancer cells to avoid detection by the immune system. They use various mechanisms to hide from or suppress immune cells, which is why immunotherapy research focuses on reversing this evasion.