Placental calcification: a metastatic process?

Placental calcification commonly increases with gestational age. The mechanism of apatite mineralization probably involves one of three known mechanisms of tissue calcification: physiological (like bone), dystrophic (ischaemia-related) or metastatic (mineralization in a supersaturated environment). This study was designed to determine the mechanism of calcification by examining (1) the mineral content of placental calcifications in comparison to other physiological and pathological apatites, and (2) the expression of bone morphogenetic proteins (BMPs), which are important in physiological calcification, across gestational age. By energy-dispersive x-ray analysis (EDXA), the Ca/P weight ratio for apatitic mineral from mature calcifications was 2.00+/-0.05 (s.e.), which is similar to that for stones formed in a metastatic, supersaturated environment and lower than that observed in physiological calcification. Biologically active BMP, which was determined by bioassay, was demonstrated in mature and postmature placentae. The BMPs PLAB, PDF and related protein INSL-4 were identified by semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR), but their mRNA expression was independent of gestational age (7-41 weeks of gestation). We conclude that (1) the identified BMPs were not related directly to placental calcification, which argues against physiological calcification, and (2) the chemical composition of the apatitic mineral was suggestive of rapid formation in a supersaturated environment, which is consistent with a metastatic mechanism of calcification.

Cell differentiation in vascular calcification.

Ectopic tissue formation is commonly found in calcified atherosclerotic plaques. This suggests that cell differentiation plays an important role in vascular calcification, even though the origin of the cells involved is unclear. Calcifying vascular cells (CVCs), derived from bovine aortic media, have been used as an in vitro model for vascular calcification. CVCs have many characteristics in common with bone cells, but there are also differences suggesting mechanisms that may be applicable to the problem of osteoporosis in the setting of vascular calcification. Matrix GLA protein (MGP) deficient mice develop severe vascular calcification and die prematurely from heart failure and/or aortic rupture. The molecular mechanism of MGP is unknown. It has been hypothesized that MGP acts as a calcification inhibitor by binding calcium, preventing mineral deposition in extracellular fluids near the saturation point for calcium and phosphate. Alternatively, MGP expression may be an attempt to regulate cell differentiation in the vascular wall, possibly by acting as an inhibitor to a factor able to induce cartilage and bone such as bone morphogenetic proteins (BMPs).