Combined Raman Microscopy and Transmission Electron Microscopy shows the co-existence of whitlockite crystals and carbonated hydroxyapatite-mineralized collagen fibrils in human calcified aortic valves

Abstract

Aortic valve stenosis (AS), the leading valvular disease in aging populations, is driven by complex extracellular matrix (ECM) modifications and progressive calcification. Despite its clinical significance, the molecular mechanisms underlying AS remain poorly understood, limiting therapeutic options to invasive valve replacement. This study employs Raman microscopy, a label-free imaging technique, and its integration with transmission electron microscopy (TEM) to elucidate the biochemical and ultrastructural changes in stenotic aortic valves. High-resolution spectroscopic imaging revealed distinct extracellular matrix modification across different regions, involving elastin degradation, cholesterol deposition, and mineral formation, comprising not only hydroxyapatite (cHAp) but also whitlockite. Elastin-rich domains associated with cHAp deposition exhibited cross-link degradation, while collagen matrices supported mineralized phases with varying mineral-to-matrix ratios that, in heavily mineralized regions, went far above those of mature human bone. For the first time we demonstrated whitlockite as a mineral deposit in calcified aortic valves, in areas with different degrees of calcification. This implies that this Mg containing mineral, which has been considered a precursor to cHAp in pathological calcification, forms independently, challenging prevailing models of calcification. The combination of Raman and TEM showed how bone-like cHAp mineralized collagen matrix in later stages engulfs the initial non-physiological whitlockite deposits. This correlative multimodal approach advances our understanding of AS by capturing spatially resolved chemical and structural dynamics at the nanoscale. The findings highlight Raman microscopys potential for probing calcification mechanisms across diverse tissue types and suggest its role in identifying novel therapeutic targets. This study underscores the value of integrative imaging methodologies in unraveling complex pathological processes and advancing patient care.