High resolution ultrasound imaging of the mouse placenta during development revealed highly echogenic foci localized near the materno-placental interface in early gestation and, near term, in the placental labyrinth (the exchange region of the placenta). Echogenic foci and calcium deposits identified in histological sections using Alizarin red staining showed similar localization and changes with gestation. Calcium deposits caused the echogenic foci because incubating uteri in a decalcifying solution eliminated both the deposits and echogenic foci. Transmission electron microscopy, X-ray microanalysis, and electron diffraction were used to show that deposits were calcium hydroxyapatite crystals. Calcium deposits were extensive and densely packed at days 7.5-9.5 of gestation at the border between the maternal decidua and the fetal trophoblast giant cells of ectoplacental cone. After the formation of the chorio-allantoic placenta (approximately day 10.5), calcification deposits appeared larger and more rarefied but were still localized at the border between the maternal decidua and the fetal trophoblast giant cells of the placenta. Calcification deposits were not observed in the labyrinthine region of the mouse placenta until > or = day 15.5 (day 18.5 is full term). We conclude that deposits of calcium hydroxyapatite crystals in the mouse placenta are detectable by high resolution ultrasound imaging. These deposits provide an ultrasound detectable marker of the maternal-placental interface that is particularly prominent during the establishment of the chorio-allantoic placenta between days 7.5 and 9.5 of gestation.
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