Malachite green and phthalocyanine-silver reactions reveal acidic phospholipid involvement in calcification of porcine aortic valves in rat subdermal model F. Ortolani1, L. Petrelli1, S.L. Nori2, G. Gerosa3, M. Spina4 and M. Marchini1 1Department of Medical and Morphological Research, University of Udine, Udine, Italy, 2Institute of Human Anatomy, Catholic University, Rome, Italy,3Institute of Cardiothoracic Surgery, University of Padua, Padua, Italy and 4Department of Experimental Biomedical Sciences, University of Padua, Padua, Italy Offprint requests to: F. Ortolani, Department of Medical and Morphological Research, University of Udine, piazzale Kolbe 3, I-33100, Udine, Italy. Fax: ++39-0432-494201. e-mail: fulvia.ortolani@uniud.it   Summary. Subdermal implant models are helpful in the study of calcification "in vivo" and for testing anticalcific treatments. After implantation of porcine aortic valve leaflets in rat subcutis, we previously found that glutaraldehyde-Cuprolinic blue reactions (GA-CB) at low pH induce favourable tissue unmasking from mineral deposits, and visualize peculiar, electrondense layers that outline the calcifying cells and matrix vesicle-like structures. The layer-forming material seemed to consist of acidic phospholipids because of its anionic nature and differential susceptibility to chemical/enzymatic extractivity. In the present investigation, pre-embedding glutaraldehyde-Malachite green (GA-MG) reactions and subsequent osmium post-fixation were compared with pre-embedding GA-CB reactions, combined with post-embedding von Kossa silver staining (GA-CB-S), to assess whether the layer-forming material is actually composed of acidic phospholipids and exhibits calcium-binding properties. After lowering standard pH, GA-MG reactions also caused sample demineralization and the appearance of pericellular osmium-MG-reactive layers comparable to CB-reactive ones. Moreover, GA-CB-S reactions showed that major silver precipitation was superimposed to the CB-reactive layers, whereas minor metal extra-precipitation occurred at three distinct, additional sites. These results demonstrate that a unique process of cell degeneration occurs in this calcification model, in which acidic phospholipids accumulate at cell surface, replacing cell membrane and acting as major apatite nucleator. However, the overall observations are consistent with the hypothesis that certain phases are common to the various types of normal and/or abnormal calcification. Histol. Histopathol. 18, 1131-1140 (2003) Key words: Aortic Valves, Bioprosthetic Valves, Calcification, Malachite Green, von Kossa DOI: 10.14670/HH-18.1131