Ultrastructural localization of the 9‐kilodalton vitamin d‐dependent calcium‐binding protein in the murine intraplacental yolk sac

The calcium‐binding protein (CaBP) calbindin has been implicated in the molecular mechanism of placental calcium transfer. Previous light microscopic studies have identified CaBP in visceral (but not parietal) endodermal cells of the yolk sac with the most intense immunocytochemical signal observed in the intraplacental yolk sac. In the present studies, electron microscopy was used to study the localization of CaBP in placenta. Placentas of 17‐day pregnant mice were fixed by perfusion in 0.5% gluteraldehyde, embedded in low‐temperature Lowicryl K4M, and examined in thin section for specific labeling with a polyclonal antiserum. Antibody to CaBP was localized by using protein A‐gold particles which were quantified for subcellular compartmentation by using a Videoplan computer system. A high signal for CaBP was found in the visceral endodermal cells of the intraplacental yolk sac. In these cells, gold particles indicating the location of CaBP were observed over (1) the cytoplasmic matrix where the average number of gold particles per μm2 was 33; (2) the microvilli (17/μm2); (3) the mitochondria (17/μm2); and (4) the nucleus (43/μm2). Sections from antigen‐absorbed controls, by contrast, showed few gold particles: cytosol, 2/μm2; microvilli, 5/μm2; mitochondria, 5/μm2; and nucleus, 4/μm2. Electron‐lucent profiles of the Golgi and endoplasmic reticulum contained no particles in the controls and a low particle count (4/μm2) in the stained sections. Parietal endodermal cells of the intraplacental yolk sac showed a relatively low signal for CaBP compared with the visceral endodermal cells (5 particles/μm2 vs. 39). Only low numbers of gold particles were observed in trophoblasts (6/μm2), lymphocytes (5/μm2), and erythrocytes (5/μm2). These findings indicate that 9 kd CaBP is located predominantly in cytoplasmic matrix, nucleus, and mitochondria within the visceral endodermal cells of the intraplacental yolk sac.