Progression of coronary calcification. Non-decalcified arterial segments (A and B) and decalcified segments (C to J) were serially cut for the microscopic assessment. A shows pathologic intimal thickening (PIT) characterized by lipid pool (LP) that lacks smooth muscle cells (SMCs) (negative for α-smooth muscle actin [α-SMA]) and shows the presence of apoptotic SMCs which can be identified by prominent basement membrane which stains positive with periodic acid Schiff (PAS) and the arrows point to in the high-power image (top right corner). Early microcalcification (≥0.5 μm, typically <15 μm in diameter) likely results from SMC apoptosis and calcification is detected by von Kossa staining within the LP (corresponding with a boxed area in the Movat image) where bone related proteins such as osteoprotegerin (OPG), osteopontin (OPN), and matrix Gla protein (MGP) are detected. Early necrotic core (NC) (B) not only lacks SMCs but is infiltrated by macrophages which eventually undergo apoptosis and calcification, which is observed as punctate (≥15 μm) areas of calcification. The microcalcification in early NC show variable amounts of staining for macrophage CD68 antigen; however, von Kossa staining clearly shows relatively larger punctate areas of calcification resulting from macrophage cell death within the NC as compared to microcalcification of dying SMCs. These calcified macrophages show co-localization of bone related proteins. Substantial amount of macrophage calcification can be observed in early NC (C) but the degree of calcification in NC typically increases towards the medial wall where fragmented calcifications can be seen (D). Microcalcification resulting from macrophage or SMC deaths can also be detected within a thin-fibrous cap and may be associated with plaque rupture (E). Calcification generally progress into the surrounding area of the NC (F), which leads to the development of sheets of calcification where both collagen matrix (G) and necrotic core itself are calcified (H). Nodular calcification may occur within the plaque in the absence of luminal thrombus and is characterized by breaks in calcified plates with fragments of calcium separated by fibrin (I). Ossification may occur at the edge of an area of calcification especially in nodular calcification (J). Immunohistochemical stainings in A and B were performed with the use of antibodies for CD68 (dilution 1:800; Dako, Carpinteria, CA), α-SMA (dilution 1:400, Dako), OPG (dilution 1:50, Novus Biologicals, LLC, Littleton, CO), OPN (dilution 1:400, generously provided by Larry W. Fisher, PhD, National Institute of Health, Bethesda, MD), and MGP (dilution 1:200, Enzo Life Science, Farmingdale, NY), respectively.