Healing I: Introduction

Tissue healing occurs through three pathways: resolution, regeneration, and replacement. Resolution is the simplest form, where minimal damage allows cells to fully recover without scarring, as seen in mild sunburns. Regeneration replaces lost cells with structurally and functionally identical new cells, common in tissues like the liver. Replacement occurs when cells cannot divide, resulting in fibrous scar tissue instead of functional tissue restoration.

Resolution represents the most complete healing form, occurring when injury is minimal and tissue architecture remains largely intact. Damaged cells recover their normal structure and function through intracellular repair mechanisms without residual scarring. Once inflammation subsides, the tissue returns to its pre-injury state. This contrasts with regeneration and replacement, which involve more extensive cellular changes and potential permanent alterations.

The liver contains hepatocytes that can re-enter the cell cycle under growth factor influence, allowing regeneration of lost tissue. Cardiomyocytes in the heart, however, have little or no capacity for division. After a myocardial infarction, dead cardiomyocytes are replaced by fibrous scar tissue rather than new functional cells, resulting in permanent functional loss despite structural stability.

Following a myocardial infarction, necrotic cardiomyocytes are removed by macrophages and replaced by granulation tissue rich in fibroblasts and new capillaries. Over time, this tissue matures into dense collagenous scar. Although the fibrous matrix maintains structural stability, it lacks the contractile and conductive properties of the original myocardium, resulting in permanent functional loss.

Growth factors such as hepatocyte growth factor and transforming growth factor-alpha stimulate hepatocytes to re-enter the cell cycle during liver regeneration. These signaling molecules coordinate cellular proliferation to restore hepatic mass. However, when the extracellular matrix framework is disrupted or damage is extensive, regeneration may be incomplete, and scar formation often accompanies the repair process.

The extent and nature of tissue damage, combined with the tissue's cellular capacity, determine whether healing occurs by resolution, regeneration, or replacement. Tissues with labile or stable cell populations capable of proliferation, like the liver, typically heal through regeneration. Tissues composed of permanent cells with no division capacity, such as cardiac muscle and neurons, heal through replacement by fibrosis.

Scar tissue formed during replacement healing consists of dense collagenous fibers that provide structural stability but lack the specialized functions of original tissue. In the heart, fibrous scar tissue cannot contract or conduct electrical impulses like cardiomyocytes. This functional deficit is permanent, distinguishing replacement healing from resolution, where tissue returns to its pre-injury state.