Placental calcification: A metastatic process?

Placental calcification commonly increases with gestational age. The mechan ism of apatite mineralization probably involves one of three known mechanis ms of tissue calcification: physiological (like bone), dt strophic (ischaem ia-related) or metastatic (mineralization in a supersaturated environment). This study was designed to determine the mechanism of calcification by exa mining (1) the mineral content of placental calcifications in comparison to other physiological and pathological apatites, and (2) the expression of b one morphogenetic proteins (BMPs), which are important in physiological cal cification, across gestational age. By energy-dispersive x-ray analysis (ED XA), 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 m etastatic, supersaturated environment and lower than that observed in physi ological calcification. Biologically active BMP, which was determined by bi oassay, nas 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 expressio n was independent of gestational age (7-41 weeks of gestation). We conclude that (1) the identified BMPs sere not related directly to placental calcif ication, which argues against physiological calcification, and (2) the chem ical composition of the apatitic mineral was suggestive of rapid formation in a supersaturated environment, which is consistent with a metastatic mech anism of calcification. (C) 2001 Harcourt Publishers Ltd.