Placental Fibroblasts Research Articles

Expression of the human PDGF-B gene is regulated by both positively and negatively acting cell type-specific regulatory elements located in the first intron.

Potential cis-acting regulatory elements of the human platelet derived growth factor-B (PDGF-B) gene were identified by DNase I hypersensitive site mapping. The transcription unit was examined for the presence of hypersensitive sites in chromatin DNA isolated from human term placental cytotrophoblasts, human placental fibroblasts, the JEG-3 choriocarcinoma cell line and the U2-OS osteosarcoma cell line. A number of cell type-specific hypersensitive sites were identified, all within the 1st intron. Transient transfection of JEG-3 cells with CAT constructs containing regions of the c-sis 1st intron linked to the basal c-sis promoter identified a cell type-specific positive regulatory activity within the intron, composed of at least two distinct elements. One element appeared to be specific for JEG-3 cells, while the other was also active in U2-OS cells. The overall positive regulatory activity of the 1st intron region was specific for JEG-3 cells, but did not function as a classically defined enhancer, as it was orientation-dependent (unless stably integrated into chromatin DNA). In addition, the activator appears to require interaction with the c-sis promoter, as little or no activation was seen when either the SV40 or human beta-globin promoters were substituted for the c-sis promoter. The 1st intron also contained a negative regulatory element, which was specific for U2-OS cells and silenced an abnormally high basal c-sis promoter activity in these cells. The complexity of the transcriptional control of the PDGF-B gene is discussed.

Cloning and expression of a new pregnancy-specific β1-glycoprotein member

Human pregnancy-specific β 1-glycoproteins (PSGs) are a group of closely related placental glycoproteins and members of the immunoglobulin superfamily. To study possible functional roles of PSG, three type I cDNA ciones which encode PSG in a N-A1-A2-B2-C domain arrangement have been characterized in this laboratory. Now, we report the cloning of a type II cDNA (PSG9) which is 1512 bp and encodes a PSG of 332 amino acids in a N-A1-B2-C domain arrangement. In vitro expression experiments demonstrate that PSG9 encodes a 36 kDa nonglycosylated protein which is processed to a glycosylated protein of 46 kDa. Ribonuclease protection analyses show that PSG mRNAs that share sequence similarity with the PSG9 transcript are expressed in both human placenta and placental fibroblasts.

Human trophoblast interferon: pattern of response to priming and superinduction of purified term trophoblast and choriocarcinoma cells

We recently described a beta interferon response of primary cultures of term human trophoblast exposed to poly(I:C). The response pattern has now been studied further with priming and superinduction both of normal placental cell types and JAR, JEG-3 and BeWo choriocarcinoma cells. Pre-treating placental trophoblast cells, fibroblasts and macrophages with human interferon generally led to increased yields of interferon after poly(I:C) induction, whereas choriocarcinoma cells did not respond to priming. All the cells showed the superinduction phenomenon although to varying degrees. The combination of priming and superinduction conditions led to the production of very high yields of interferon in placental fibroblast cultures. The combined procedure also produced more interferon in macrophage cultures than priming or superinduction alone. Combined superinduction and priming of normal trophoblast did not produce higher yields than those obtainable by superinduction alone. These data might help to provide additional insight into the cellular control mechanisms of sensitivity of normal and malignantly transformed cells to interferon. Furthermore, the large quantity of trophoblast interferon produced by superinduction could be used for studies of its anti-viral and immunomodulatory effects.

7372. Helium-ion damage and nanowire fabrication in GaAs/AlGaAs heterostructures: C M Knoedler, J Appl Phys, 68, 1990, 1129–1137

A mitogenic and plasminogen activator (PA)-inducing activity for endothelial cells has been identified in serum-free culture medium of normal AG 7680 and transformed tumorigenic GM 7373 fetal bovine aortic endothelial (FBAE) cells. The activity binds to heparin-Sepharose and it is quenched by polyclonal antihuman placental basic fibroblast growth factor (bFGF) antibodies. In the serum-free conditioned medium of FBAE cells, the anti-bFGF antiserum recognizes an immunorective Mr 20,000 molecule which co-purifies with the mitogenic and PA-inducing activity on a heparin-Sepharose column. The partially purified Mr 20,000 bFGF-like molecule competes with the typical Mr 18,000 125I-bFGF form for the binding to high-affinity bFGF receptors in intact GM 7373 cells. Immunoprecipitation of biosynthetically labeled GM 7373 cells with anti-bFGF antiserum confirms the presence of a Mr 20,000 bFGF-like molecule in the conditioned medium of these cells and identifies the typical Mr 16,000 and Mr 18,000 bFGF forms and two high-molecular-weight immunoreactive Mr 22,000 and Mr 25,000 bFGF forms in their cell extract. Immunoreactive Mr 20,000 bFGF is detectable also in the conditioned medium of transformed nontumorigenic FBAE GM 7372 cells and of adult bovine aortic endothelial cells, but not in the culture medium of nonendothelial cell types, including rat and mouse fibroblasts, human hepatoma, and human endometrial adenocarcinoma cells. The results indicate that bovine endothelial cells secrete a Mr 20,000 bFGF-like molecule which shares several biological, biochemical, and immunological characteristics with the typical cell-associated Mr 18,000 bFGF.

Interferon Production by Cultured Human Trophoblasts and Choriocarcinoma Cell Lines Induced by Sendai Virus

Human term-placental trophoblasts in primary culture were studied for an interferon (IFN) response when challenged with Sendai virus and compared to three choriocarcinoma cell lines, placental fibroblasts and placental macrophages. Normal trophoblasts were high producers and released both IFN-alpha and IFN-beta. In contrast, one choriocarcinoma cell line was a low producer and all malignant lines produced only IFN-beta. Circulating monocytes produce IFN-alpha but placental macrophages secreted IFN-beta and some IFN-alpha, suggesting that IFN production may be dependent on the stage of differentiation. A role for trophoblast IFNs in protection of the foetus against virus infections is proposed.

Effects of sodium butyrate on the synthesis of human pregnancy-specific beta 1-glycoprotein.

Human pregnancy-specific beta 1-glycoprotein (PS beta G) is a polymorphic placental protein which shows strong sequence similarity with the oncofetal protein, carcinoembryonic antigen. To better understand the role of PS beta G in pregnancy, we examined its synthesis and regulation in placental fibroblasts, which had been shown to express the PS beta G gene. The major placental PS beta G is a 72-kDa glycoprotein, while the major fibroblast PS beta G is a 62-kDa species. Administration of sodium butyrate to these fibroblasts slightly stimulated the synthesis of the 62-kDa species but markedly increased the production of two additional PS beta Gs of 72 and 48 kDa. The similarity between the PS beta Gs synthesized by butyrate-treated fibroblasts and human placenta was confirmed by cell-free protein synthesis. Poly(A)+ RNA from butyrate-treated fibroblasts and placenta directed the synthesis of two polypeptides of 48 and 36 kDa, which form the polypeptide backbone of the 72- and 48-kDa glycoproteins. Moreover, the predicted molecular weights of PS beta Gs encoded by the two types of PS beta G cDNA clones were 48,000 and 36,000. Most PS beta G cDNAs identified to date, including the three cDNAs (PSG16, PSG93, and PSG95) isolated in this laboratory, share strong sequence similarity at the 5' region (designated PSG-5') but differ in sequences at their 3' regions. The PSG-5', PSG93-specific, PSG16/PSG93-3', and PSG95-3' probes, which identify the majority of PS beta G mRNAs, hybridized with three PS beta G mRNAs of 2.3, 2.2, and 1.7 kilobases from placental fibroblasts. Butyrate increased the steady-state levels of all three mRNAs. Ribonuclease protection analysis showed that butyrate increased the PS beta G mRNAs containing the PSG-5' or PSG93-specific sequence to approximately 20% of human placental levels. However, unlike human term placenta, which predominantly expressed PS beta G mRNAs with 3'-sequences similar to PSG16/PSG93, the butyrate-treated fibroblasts expressed roughly equal levels of PS beta G mRNAs with the PSG16/PSG93-3' and PSG95-3' ends. All PS beta G cDNAs identified encode proteins with distinct carboxyl termini, suggesting that the composition of the 72-kDa species in placenta and butyrate-treated fibroblasts is likely to be different. Placental fibroblasts provide a unique model for the study of the mechanisms responsible for the differential expression of the PS beta G gene.

Human placenta contains an epithelial scatter protein

Scatter factor (SF) is a protein produced by cultured fibroblasts which causes epithelia to “scatter” into isolated cells. We found significant scatter activity in vivo in second trimester (but not term) human amniotic fluid and in human placenta. Placental SF was purified 500,000-fold and identified as a protein with Mr 78 kd. Factor scattered Madin-Darby canine kidney and human squamous carcinoma cells at 15 pM. Amino acid sequences from tryptic peptides did not match any known protein. Human placental fibroblasts produced high titers of scatter activity. SF may be involved in development and may enhance carcinoma invasion.

Characterization of a Mr 20,000 basic fibroblast growth factor-like protein secreted by normal and transformed fetal bovine aortic endothelial cells

A mitogenic and plasminogen activator (PA)-inducing activity for endothelial cells has been identified in serum-free culture medium of normal AG 7680 and transformed tumorigenic GM 7373 fetal bovine aortic endothelial (FBAE) cells. The activity binds to heparin-Sepharose and it is quenched by polyclonal antihuman placental basic fibroblast growth factor (bFGF) antibodies. In the serum-free conditioned medium of FBAE cells, the anti-bFGF antiserum recognizes an immunorective M r 20,000 molecule which co-purifies with the mitogenic and PA-inducing activity on a heparin-Sepharose column. The partially purified M r 20,000 bFGF-like molecule competes with the typical M r 18,000 125I-bFGF form for the binding to high-affinity bFGF receptors in intact GM 7373 cells. Immunoprecipitation of biosynthetically labeled GM 7373 cells with anti-bFGF antiserum confirms the presence of a M r 20,000 bFGF-like molecule in the conditioned medium of these cells and identifies the typical M r 16,000 and M r 18,000 bFGF forms and two high-molecular-weight immunoreactive M r 22,000 and M r 25,000 bFGF forms in their cell extract. Immunoreactive M r 20,000 bFGF is detectable also in the conditioned medium of transformed nontumorigenic FBAE GM 7372 cells and of adult bovine aortic endothelial cells, but not in the culture medium of nonendothelial cell types, including rat and mouse fibroblasts, human hepatoma, and human endometrial adenocarcinoma cells. The results indicate that bovine endothelial cells secrete a M r 20,000 bFGF-like molecule which shares several biological, biochemical, and immunological characteristics with the typical cell-associated M r 18,000 bFGF.

Adhesive and degradative properties of human placental cytotrophoblast cells in vitro.

Human fetal development depends on the embryo rapidly gaining access to the maternal circulation. The trophoblast cells that form the fetal portion of the human placenta have solved this problem by transiently exhibiting certain tumor-like properties. Thus, during early pregnancy fetal cytotrophoblast cells invade the uterus and its arterial network. This process peaks during the twelfth week of pregnancy and declines rapidly thereafter, suggesting that the highly specialized, invasive behavior of the cytotrophoblast cells is closely regulated. Since little is known about the actual mechanisms involved, we developed an isolation procedure for cytotrophoblasts from placentas of different gestational ages to study their adhesive and invasive properties in vitro. Cytotrophoblasts isolated from first, second, and third trimester human placentas were plated on the basement membrane-like extracellular matrix produced by the PF HR9 teratocarcinoma cell line. Cells from all trimesters expressed the calcium-dependent cell adhesion molecule cell-CAM 120/80 (E-cadherin) which, in the placenta, is specific for cytotrophoblasts. However, only the first trimester cytotrophoblast cells degraded the matrices on which they were cultured, leaving large gaps in the basement membrane substrates and releasing low molecular mass 3H-labeled matrix components into the medium. No similar degradative activity was observed when second or third trimester cytotrophoblast cells, first trimester human placental fibroblasts, or the human choriocarcinoma cell lines BeWo and JAR were cultured on radiolabeled matrices. To begin to understand the biochemical basis of this degradative behavior, the substrate gel technique was used to analyze the cell-associated and secreted proteinase activities expressed by early, mid, and late gestation cytotrophoblasts. Several gelatin-degrading proteinases were uniquely expressed by early gestation, invasive cytotrophoblasts, and all these activities could be abolished by inhibitors of metalloproteinases. By early second trimester, the time when cytotrophoblast invasion rapidly diminishes in vivo, the proteinase pattern of the cytotrophoblasts was identical to that of term, noninvasive cells. These results are the first evidence suggesting that specialized, temporally regulated metalloproteinases are involved in trophoblast invasion of the uterus. Since the cytotrophoblasts from first trimester and later gestation placentas maintain for several days the temporally regulated degradative behavior displayed in vivo, the short-term cytotrophoblast outgrowth culture system described here should be useful in studying some of the early events in human placen

Human placental microvilli as a source of antigen for the preparation of a polyclonal antibody directed against the LDL receptor

Polyclonal antibodies were prepared by immunization of rabbits with partially purified LDL receptor obtained from human placental microvilli. The antiserum reacted with membranes from human placental microvilli and human fibroblasts, as assessed by immunobinding studies. It also reacted with purified LDL receptors of both origins. The antiserum markedly inhibited 125I-labeled LDL binding to cultured human fibroblasts.

Characterization of Pregnancy-Specific β1-Glycoprotein Synthesized by Human Placental Fibroblasts

We have previously demonstrated that human placental fibroblasts produce a pregnancy-specific beta 1-glycoprotein (PS beta G) immunologically indistinguishable from placental PS beta G. This was confirmed by the immunocytochemical localization of PS beta G in these fibroblasts. In addition, placental fibroblasts contain all three PS beta G mRNAs of 2.3, 2.2, and 1.7 kilobases which hybridize with the three PS beta G cDNAs (PSG16, PSG93, and PSG95) identified, although at 1.4-2.5% of the levels in human term placenta. The major PS beta G species synthesized by placental fibroblasts is a 62K glycopolypeptide formed from a 58K intracellular precursor polypeptide. However, the PS beta G species found in human placenta are one major glycoprotein of 72K and two minor ones of 64K and 54K. Poly(A)+ RNA from placental fibroblasts directed the synthesis of two polypeptides of 48K and 46K (major), whereas, poly(A)+ RNA from human placenta directed the synthesis of higher levels of four polypeptides of 50 K, 48 K (major), 46 K, and 36 K. Thus, the major PS beta G species found in fibroblasts and human placenta differ. The carbohydrate side-chains are essential for the stability of fibroblast PS beta G, because PS beta G synthesis in these fibroblasts could not be detected in the presence of tunicamycin, a protein glycosylation inhibitor which did not affect PS beta G mRNA expression. Our finding that a variant PS beta G species is produced in placental fibroblasts raises the possibility that the authentic placental PS beta G species may have different functions.

Production and characterization of human basic fibroblast growth factor from Escherichia coli.

The cDNA sequence coding for human basic fibroblast growth factor (bFGF) has been cloned downstream of a transcription promoter recognized by the bacteriophage T7 RNA polymerase. Initiation of translation at the fgf gene has been coupled to upstream translation of a fragment of the T7 phi 10 gene. Expression of the fgf gene in this system can lead to an accumulation of approximately 40 mg/liter/A600 unit of bFGF. This material can be purified close to homogeneity from a soluble protein extract on a heparin-Sepharose column. bFGF so obtained has been shown to have bioactivity indistinguishable from human placental fibroblast growth factor in mitogenicity, synthesis of plasminogen activator, and angiogenesis assays.

Generation and characterization of monoclonal antibodies specific for the human IFN-gamma receptor.

Purified preparations of the human IFN-gamma R derived from placental membranes were used to produce receptor-specific murine mAb. Supernatants from growth-positive wells were screened for their ability to block binding of 125I-IFN-gamma to human placental membranes. Ten inhibitory cultures were identified. Two of these (GIR-208 and GIR-301) abrogated all binding of radioligand to either intact placental membranes or soluble, purified IFN-gamma R. Three others (GIR-72, 76 and 94) showed moderate blocking activity (65, 59, and 49%, respectively) whereas the remaining five (GIR-57, 67, 83, 109, and 153) blocked binding to a low but significant extent (20 to 40%). Specificity experiments demonstrated that the antibodies reacted with the receptor and not the ligand (IFN-gamma). None of the antibodies reacted with IFN-gamma by ELISA. Moreover, GIR-208 and GIR-301, but not isotype-matched controls, identified the receptor by Western blot analysis. GIR-208 and GIR-301 also completely abrogated binding of 125I-IFN-gamma to either mononuclear phagocytes (U937) or human fibroblasts (WISH). Competition experiments revealed that GIR-208 and GIR-301 recognized similar epitopes on the IFN-gamma R and that these (or this) epitopes were identical to or linked to the ligand binding site of the receptor. In addition, both antibodies inhibited development of IFN-gamma-dependent anti-viral activity in WISH cells in a dose-dependent fashion. These data thus indicate that the IFN-gamma R expressed on human placental cells, mononuclear phagocytes, and fibroblasts are similar.

Characterization of a plasminogen activator inhibitor induced by glucocorticoids in immature bovine Sertoli cell-enriched cultures.

Plasminogen activation has been shown to be inhibited by the cell-specific production of a number of protease inhibitors belonging to the serine protease inhibitor family. In the bovine testis this inhibitor production is induced by glucocorticoids. Monospecific antibodies raised against the three known classes of plasminogen activator inhibitor were used to identify which type of inhibitor was secreted by bovine Sertoli cell-enriched cultures. Immunoblot analysis and [35S]methionine labelling of newly synthesized proteins revealed that a novel protein with an apparent molecular weight of 49 kDa, which shares antigenic determinants with placental and macrophage PAI and fibroblast protease nexin, is secreted in response to dexamethasone stimulation. This protein was shown by immunoadsorption to be a functionally active inhibitor of both tissue-type and urokinase-type plasminogen activators.

Estrogen synthetase (aromatase) in cultured human term placental cells and neoplastic human trophoblast

Estrogen synthetase (aromatase) is present in large amounts in human term placenta. However, the localization of aromatase within the cellular structure of the placental villus is obscure. By immunocytochemical techniques using antibodies that separately recognize each component of the aromatase cytochrome P-450 enzyme system, the fraction of term placental trophoblast cells in primary culture expressing each aromatase component antigen increased from 20% in fresh mononucleated cells to about 65% for multinucleated giant cells after 72 h. In contrast, about 80% of human choriocarcinoma cells in continuous culture (JAr line) expressed each aromatase component antigen. The fraction of trophoblast cells in primary culture containing human chorionic gonadotropin increased from about 14% in fresh mononucleated cells to about 45% after 72 h and was about 30% in the choriocarcinoma cells. Fibroblast cells in culture, derived from trypsin-treated placental villi, contained aromatase activity, albeit much lower than term placental trophoblast cells. Aromatase specific activity in these placental fibroblasts did not change following growth with dibutyryl cAMP plus theophylline for 72 h.

Purification from a human hepatoma cell line of a basic fibroblast growth factor-like molecule that stimulates capillary endothelial cell plasminogen activator production, DNA synthesis, and migration.

A 17,500-dalton protein which stimulates plasminogen activator production in cultured bovine capillary endothelial cells has been purified from a SK-Hep-1 human hepatoma cell lysate by using heparin affinity chromatography and fast protein-liquid ion exchange chromatography. The purified molecule stimulated plasminogen activator production in a dose-dependent manner between 0.01 and 1 ng/ml. It also stimulated collagenase synthesis, DNA synthesis, and motility in capillary endothelial cells in the same concentration range. This molecule was identified as a basic fibroblast growth factor-like molecule on the basis of its biological activity, its affinity for heparin-Sepharose, and its cross-reactivity with a polyclonal antibody raised against the human placental basic fibroblast growth factor.

Purification from a human hepatoma cell line of a basic fibroblast growth factor-like molecule that stimulates capillary endothelial cell plasminogen activator production, DNA synthesis, and migration.

A 17,500-dalton protein which stimulates plasminogen activator production in cultured bovine capillary endothelial cells has been purified from a SK-Hep-1 human hepatoma cell lysate by using heparin affinity chromatography and fast protein-liquid ion exchange chromatography. The purified molecule stimulated plasminogen activator production in a dose-dependent manner between 0.01 and 1 ng/ml. It also stimulated collagenase synthesis, DNA synthesis, and motility in capillary endothelial cells in the same concentration range. This molecule was identified as a basic fibroblast growth factor-like molecule on the basis of its biological activity, its affinity for heparin-Sepharose, and its cross-reactivity with a polyclonal antibody raised against the human placental basic fibroblast growth factor.

Production of SP 1 by placental fibroblasts in vitro

Cells cultured from first-trimester chorionic villi display round pale nuclei which usually contain several chromocentres. The cytoplasmic membrane shows finger-like processes. Immunohistochemical techniques visualized a cytoskeleton consisting of vimentin, whereas desmin, keratin, factor VII I antigen and protein S-ioo were not found. Electron microscopy showed de novo synthesized extracellular collagen fibres. Obviously the cultured cells are fibroblasts. Within these fibroblasts Schwangerschafts protein i (SP1) can be shown by immunohistochemical methods, particularly in younger cultures prior to confluence. Placental hormones such as human placental lactogen or human chorionic gonadotrophin could be detected in only a few cells, which disappeared from the culture after one or two days. These cells may have a trophoblast origin. From the demonstration ofSP 1 in fibroblasts cultured from placenta we conclude that this protein is not trophoblast-specific but is probably to be found in numerous growing cells.

An Autocrine/Paracrine Role forInsulin-Like Growth Factors inthe Regulation ofHuman Placental Growth*

Tissue derived from preterm (9-19 weeks gestation) and term (38-41 weeks gestation) human placentae were examined for their ability to synthesize and secrete insulin-like growth factors (IGFs) in organ culture. IGF-I was measured by a specific RIA, and IGF-II by a rat placental membrane radioreceptor assay. First, explants of placental tissue were maintained in organ culture. These explants secreted immunoreactive IGF-I (IR-IGF-I). There were no differences in the IR-IGF-I content of media conditioned by term and preterm placentae under these conditions. The similarity of this material to authentic human IGF-I was supported by parallel displacement in a specific RIA and coelution during Sephadex G-50 gel filtration. Second, monolayer cultures of fibroblasts from normal human preterm placentae (15-19 weeks gestation) were established. Confluent monolayers of these fibroblasts secreted IR-IGF-I (3-10 pg/10(5) cells X 40 h). IR-IGF-I secretion was reversibly inhibited by 5.3 microM cycloheximide, suggesting that the IR-IGF-I was the result of de novo protein synthesis. IR-IGF-I secretion was stimulated 5-fold by platelet-derived growth factor (0.6 U/ml). The response of monolayers of placental fibroblasts to IGF-I also was tested. IGF-I stimulated alpha-[3H]aminoisobutyric acid transport in these fibroblasts, with half-maximal stimulation occurring at 2-3 ng/ml. Stimulation of alpha-[3H]aminoisobutyric acid uptake by IGF-I correlated with specific binding of [125I]iodo-IGF-I. Half-maximal inhibition of [125I]iodo-IGF-I binding occurred at 2-3 ng/ml IGF-I. Placental tissue also secreted IGF-II-like activity, as measured by radioreceptor assay. Media conditioned by placental explants contained 15-20 ng/mg protein X 48 h, and media conditioned by placental fibroblasts contained 3-7 ng/10(5) cells X 40 h IGF-II determined by radioreceptor assay. These data support the hypothesis that the human placenta produces IGFs (IGF-II and/or IGF-I) that act locally to regulate placental growth.

External labelling of glycoproteins from first-trimester human placental microvilli.

The brush-border glycoproteins of first-trimester human placentas were investigated by using two external labelling techniques: (1) sequential digestion with neuraminidase and galactose oxidase, followed by reduction with NaB3H4, which 3H-labels terminal galactose and galactosamine residues; and (2) sequential treatment with periodate and NaB3H4, which 3H-labels terminal sialic acid residues. The labelling procedures were performed on intact tissue so that the results would more closely approximate the topography of the brush border in vivo. The microvilli were isolated, subjected to sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, and the [3H]glycoproteins detected by fluorography. Densitometer scans of the fluorograms of the [3H]galactoproteins showed that, under reducing conditions, 90% of the protein-associated radioactivity was incorporated into two glycoproteins. The major [3H]galactoprotein of early placental microvilli had an estimated molecular mass of 92 kDa (desialylated) and migrated as a diffuse band. A minor 180 kDa glycoprotein was less consistently labelled. No change in the apparent molecular mass of either component was detected in the absence of beta-mercaptoethanol, suggesting that the 180 kDa component was not a dimer of the 92 kDa glycoprotein. The remaining 10% the radioactivity was equally distributed among several minor membrane components. Densitometer scans of the fluorograms of the [3H]sialoproteins showed that, under either reducing or non-reducing conditions, 90% of the 3H was preferentially incorporated into the 92-110 kDa region of the gel. Although no distinct bands were visible, the higher-molecular-mass region of this area was always most heavily labelled. A minor 180 kDa glycoprotein was also 3H-labelled. The pattern of brushborder [3H]glycoproteins from first-trimester placentas differed markedly from that of term placental microvilli and from placental fibroblast plasma membranes that were 3H-labelled by identical external labelling techniques. These results indicate that: (1) the glycoprotein determinants of brush-border topography change during pregnancy; (2) within the placenta, the major 92 kDa (desialylated) determinant, which has not been previously described, is unique to the trophoblastic cells.