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Fibroblasts differentiate into myofibroblasts that organize and represent the predominant cell of fibroinflammatory buds. Inflammatory cells and fibrin are progressively replaced by aggregated fibroblasts/myofibroblasts intermixed with a loose connective matrix tissue rich in collagen (especially collagen III) and fibronectin. This process, resembling that of cutaneous wound healing, is similarly reversible, without significant sequelae. It is likely that the relative preservation of the alveolar basal laminae is crucial in determining the reversibility of the lesions. Although COP and secondary OP appear very similar, the microvascular density and the density of collagen fibers within intraalveolar air spaces may be higher in secondary OP than in COP.