MMP9 and atrial fibrillation: MMPs, a family of proteinases that digest the extracellular matrix, play important roles in tissue remodeling processes such as angiogenesis, tissue repair, and cancer metastasis.[23–26] MMP-2 destroys type IV collagen (a component of the basement membrane) and initiates angiogenesis together with membrane-type MMPs and MMP-9.[27,28] Moreover, MMP-2 is associated with pathologic retinal neovascularization.[29,30] If a fetus exposed to high MMP-2 in AF is born prematurely, the retina may be sensitive and may be involved in the formation of abnormal new blood vessels in the immature retina.