Calcification of matrix vesicles in human aortic valve and aortic media

Calcification of human aortic valve and aortic media occurs regularly, increases with age, and is distinctively associated with a zone of lipid accumulation. Ultrastructurally, the accumulated lipids are seen as cellular degradation products derived from senescent and degenerate fibrocytes and smooth muscle cells. The products when deposited in the matrix are morphologically similar to the matrix vesicles described in other calcifying tissues, and serve as the initial site of calcification rather than collagen or elastic fibers. Scattered among the smaller and more typical matrix vesicles, there are seen frequently giant vesicle-like structures measuring several microns in diameter. Many of these large calcified bodies contain needle-shaped, radially arranged apatite crystal deposits. Some of the large calcifying bodies are bounded by folded structures suggesting a membrane component, at times obscured by a more dense floccular osmiophilic deposition. Alcianophilic apparent proteoglycan particles are also adherent to these large calcified bodies. The substance forming the large calcified bodies might be a complex of phospholipids derived from cell membrane and proteoglycan derived from ground substance, this combination possible serving as a nidus for calcification.