Hypothetical scheme of the main pathogenic events in IPF/UIP. Unknown insults provoke multiple microscopic foci of epithelial damage and stimulation. Activated alveolar epithelial cells release factors inducing fibroblast migration and proliferation and changes in cell phenotype. In the microenvironment of the lesion, myofibroblasts can induce epithelial cell apoptosis and basement membrane disruption, thus contributing to abnormal re-epithelialization and perpetuation of a vicious circle. Finally, fibroblasts/myofibroblasts secrete excessive amounts of extracellular matrix components and a disequilibrium between MMPs and TIMPs means that matrix degradation does not occur. The final result is an aberrant remodeling of the lung parenchyma.