Extracellular matrix (ECM) deposition in acute and chronic muscle regeneration. Acute injury to healthy muscle produces rapid and controlled inflammation that removes dead and damaged myofibers, and promotes replacement of the injured muscle. However, in conditions of chronic injury, as occurs in the muscular dystrophies, chronic inflammatory events result in the excessive accumulation of ECM components, which inhibit myogenic repair and lead to muscle being replaced by fibrotic/scar tissue. (Top) Tibialis anterior muscles of mice were injected with cardiotoxin and samples were taken at different stages of the regeneration process. A representative sample showing the inflammatory phase, characterized by a transient increase in collagen deposition, and subsequently the resolving phase of healing, with progressive recovery of the normal tissue morphology (hematoxylin and eosin). (Bottom) Evolution of the morphological changes seen in the diaphragm of mdx dystrophic mice with disease progression, leading to heterogeneity in fiber size and increased collagen deposition between the altered myofibers. Bars = 50 μm.