The extracellular matrix (ECM) of first-trimester human decidua was examined with indirect immunofluorescence using affinity-purified antibodies to human collagen types I, III, IV, V, laminin, and fibronectin. In addition, the validity of the classification "mesenchymal-epithelioid" for differentiated decidual cells was addressed using antibodies to the intermediate filament proteins, vimentin, a mesenchymal marker, and keratin, an epithelial marker. Biosynthesis of extracellular matrix components was examined by radiolabeling of decidual explants in culture with 3H-proline, followed by immunoprecipitations of synthesized proteins with collagen type-specific antibodies. Immunofluorescence showed decidual cells are embedded in an extensive network of collagen types I and III, and intracytoplasmic staining suggested synthesis of these collagens by the decidual cells. Collagen type IV and laminin localized in the external lamina which surrounds the differentiated decidual cell, and some fluorescence was evident in the peripheral cytoplasm. Immunoreactive collagen type V was observed in close association with the external lamina and in the peridecidual matrix. Fibronectin localized throughout the decidual ECM and in fibrillar and punctuate patterns in the decidual cell cytoplasm. Differentiated decidual cells retained a "mesenchymal" intermediate filament cytoskeleton containing an abundance of vimentin filaments, but very few, if any, keratin filaments. Collagen types I, III, V, and to a lesser extent, IV, were immunoprecipitated from the medium of decidual explants after 24 hours of culture, demonstrating in vitro synthesis and secretion of these collagens by first trimester human decidua.

A distinct ultrastructural feature of human decidual cells is the presence of membrane-bound secretory bodies, 0.3-0.5 micron in diameter, located within club-shaped processes at the cell periphery. These secretory bodies contain 30-60 nm electron-dense granules. Using specific antibody and the protein A-gold technique, we examined the localization of heparan sulfate proteoglycan in human decidual cells. Morphometric analysis of gold particles in cellular compartments was performed with a Zeiss Videoplan computer system. Immuno-gold staining was present in the decidual cell cytoplasm and the extracellular space, especially in the zone of the external lamina. Gold particles, indicating the locale of heparan sulfate proteoglycan, were concentrated over the electron-dense granular material within decidual secretory bodies contained in club-shaped processes at the cell periphery. Immunolabeling of placental fibrinoid was also observed. This report provides the first identification of a specific molecular constituent of decidual secretory bodies and indicates a role for these structures in secretion of the peri-decidual cell extracellular matrix.

Proteins synthesized and secreted by first trimester decidua in primary culture were identified. Explants were cultured for 24 h, in RPMI-1640 or Dulbecco's Modified Eagles Medium containing the radioactive amino acid 35S-methionine or 3H-proline. Electron microscopy of explants before and after 24 h of culture demonstrated the relative purity of the decidua, maintenance of cell integrity, and ultrastructural features indicative of active protein synthesis and secretion. Proteins synthesized and secreted by the explants into the medium were analyzed by fluorography of one-dimensional polyacrylamide gels in the presence of sodium dodecyl sulfate. By comparison of radiolabeled proteins from four women, eight 35S-methionine-labeled bands of 78, 70, 60, 50, 43, 34, 25, and 23 kDa were identified as common decidual peptides. A comparison of autoradiographs of the medium from decidual cultures to decidual cell homogenates showed that seven of these peptides were enriched in the culture medium. When labeled peptides from fibroblast cultures were compared to labeled proteins from decidual cultures each of the common decidual peptides (except the 70 kDa protein) occurred only in the decidual culture medium. Comparison of 3H-proline and 35S-methionine-labeled decidual proteins revealed that the 78, 70, 60, 50, and 34 kDa proteins were of similar fluorographic intensity when labeled with the two different amino acids. The 43, 25, and 23 kDa proteins appeared to contain more methionine, and proteins at 36, 20, 13, and 12 kDa were proline-rich, but contained less methionine. The seven decidual explant-specific, 35S-methionine-labeled secreted proteins were concentrated and purified by preparative gel electrophoresis, and antisera were generated to four of the putative decidual secretory proteins.