TNF and pulmonary fibrosis: Seminal observations regarding common pathogenetic pathways between emphysema and lung fibrosis identified overexpression of key molecules including tumor necrosis factor alpha (TNFa), platelet derived growth factor (PDGF) and metalloproteinases (MMPs) 2, 7, 9 as critical events leading to both alveolar and endothelial cell destruction as well as thickening of the alveolar septa depending on the lung zone [32,33].