Tissue Repair

Two repair routes. After injury, the body restores tissue by regeneration, fibrosis, or a combination:

• Regeneration replaces damaged cells with the same cell type, restoring normal function. It requires cells that can still divide.
• Fibrosis fills the gap with scar tissue (dense collagen made by fibroblasts). Scar is strong but is not the original tissue, so it does not perform the lost tissue's special function.

Why tissues differ. A tissue's repair capacity depends on how well its cells divide:

• Tissues with actively dividing cells — epithelium (skin, gut lining) and bone — regenerate well.
• Tissues with limited division — most smooth muscle and some others — regenerate modestly.
• Tissues whose cells rarely or never divide — cardiac muscle and nervous tissue (CNS) — repair mainly by scarring, which is why heart attacks leave non-contracting scar and why brain/spinal injuries often cause permanent deficits. This connects directly to the G0 resting state and the poor regeneration of neurons and cardiac muscle noted earlier.

Steps of wound healing. For a typical skin wound, healing proceeds in overlapping stages:

1. Hemostasis (bleeding control). Blood vessels constrict and a clot forms, sealing the wound; a scab develops at the surface.
2. Inflammation. Blood vessels dilate and become leaky, bringing immune cells (like neutrophils and macrophages) to clean up debris and fight infection. The classic signs — redness, heat, swelling, and pain — appear here.
3. Proliferation (rebuilding). New tissue is built: fibroblasts lay down collagen, and new capillaries grow, forming pink granulation tissue. Epithelial cells migrate across to cover the surface.
4. Maturation (remodeling). Collagen is reorganized and strengthened over weeks to months, and the scar shrinks and fades. The repaired area regains much, though often not all, of its original strength.

When healing goes wrong. Healing can be delayed or impaired by infection, poor blood supply (as in peripheral artery disease), diabetes (which harms circulation and immune function), poor nutrition (especially low protein and vitamin C, needed for collagen), advanced age, and certain medications (like corticosteroids). Excessive scarring can also cause problems, such as keloids or internal adhesions.

How it works

Deciding the repair outcome:

1. Can the tissue's cells divide well? → regeneration (function restored).
2. Do they divide poorly (heart, CNS)? → fibrosis/scar (function reduced).
3. Wound healing order: hemostasis → inflammation → proliferation → maturation.

Comparisons

Common confusions

• Regeneration vs fibrosis. Regeneration restores the original functioning tissue; fibrosis makes a non-functional scar.
• Inflammation is part of normal healing, not just a problem — it clears debris and infection.
• Granulation tissue is good. The pink, bumpy new tissue is a sign of healing, not infection.
• Scars aren't as strong or functional as original tissue, even when fully healed.

Memory aids

• Healing order: "Hungry Iguanas Prefer Mangoes" → Hemostasis, Inflammation, Proliferation, Maturation.
• Regeneration = "regenerate the real thing"; Fibrosis = "fills with scar."
• Heart and brain "scar, don't restore."

Quick review

• The body repairs tissue by regeneration (same tissue, function restored) or fibrosis (scar, structural only).
• Regeneration works well in dividing tissues (epithelium, bone); cardiac muscle and CNS repair by scarring, causing permanent loss.
• Wound healing steps: hemostasis → inflammation → proliferation (granulation) → maturation.
• Infection, poor circulation, diabetes, and poor nutrition impair healing.