Bcl-2 expression as a novel immunohistochemical marker for ruptured tubal ectopic pregnancy

Elisabeth Kucera1,4, Franz König2, Stefan Tangl3, Karl Grosschmidt3, Christian Kainz1 and Gerhard Sliutz1

1 Department of Gynaecology and Obstetrics, University of Vienna, Waehringer-Guertel 18–20, 2 Department of Medical Statistics, University of Vienna and 3 Bone and Biomaterials Research Laboratory, Institute for Histology and Embryology, University of Vienna, Schwarzspanierstrasse 17, 1090 Vienna, Austria


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
Programmed cell death by apoptosis occurs in fetal and maternal tissues during early pregnancy and plays an important role during implantation, decidualization, and in fetal development. In the regulation of apoptosis, bcl-2 is one of the central controlling genes, and acts by protecting the cell against apoptosis. It is postulated that invasiveness of ectopic trophoblast towards and through the muscularis zone of the tubal wall consequently leading to tubal rupture might be due to disturbed regulation of apoptosis. By means of immunohistochemistry and a computerized image analysis, bcl-2 immunostaining was localized and quantified in 36 randomly selected paraffin-embedded ectopic trophoblast tissue specimens collected from women undergoing surgery for ruptured (n = 18) and non-ruptured (n = 18) tubal ectopic pregnancies. Immunostaining was found in the villi syncytiotrophoblast in all patients, while the percentage of positive bcl-2 immunostained area (%PA) (P = 0.0009) and staining intensity (P = 0.0042) were consistently greater in the group of ruptured ectopic pregnancies. Including the variables %PA and saturation into a logistic regression model for a probability threshold of 0.5 (<0.5 = non-ruptured ectopic pregnancy, >0.5 = ruptured ectopic pregnancy) to identify tubal rupture, a sensitivity and specificity of 94.4% were found. It is suggested that elevated bcl-2 immunostaining in the syncytiotrophoblast layer reflects unlimited cell survival of ectopic trophoblast and could lead to the establishment of a circulating marker for tubal rupture.

Key words: Bcl-2/ectopic pregnancy/immunohistochemistry/tubal rupture


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
Apoptosis is a process of programmed cell death, which is responsible for the elimination of aged cells that have lost their functional importance in maintaining appropriate cell numbers in adult organs and tissues. The typical morphological signs of apoptosis (cellular shrinkage, membrane blebbing, nuclear condensation and fragmentation) are the results of a biochemical cascade of events, which is under the intrinsic control of individual cells (Wyllie et al., 1980Go; Huppertz et al., 1999Go). The cell remains without any inflammatory response, while the apoptotic bodies are phagocytosed by neighbouring cells and macrophages (Thompson et al., 1994Go). Apoptotic cell death requires the co-ordination of gene-directed energy-dependent biological processes, and thus is comparable with proliferation and differentiation (Cotter et al., 1990Go; Hengartner, 1997Go). Apoptosis is not only part of the normal physiology in a variety of organs, but obviously also plays an important role in the development of the feto–maternal interface during early pregnancy and in the morphogenesis of embryonic tissues (Thompson, 1994Go; Lea et al., 1997Go).

In the regulation of apoptosis the bcl-2 protein favours cell survival and thus inhibits apoptosis, while in contrast, other members of the bcl-2 family such as bax and bcl-xs appear to be proapototic. During the apoptosis cascade, the proapoptotic proteins of the bcl-2 family regulate execution caspase activation and thus initiate irreversible progression of apoptosis, a process known as a `point of no return' (Huppertz et al., 1999Go). Bcl-2 is a 25 kDa protein, which is localized at the inner mitochondrial membrane, the endoplasmic reticulum, the nuclear envelope and the plasma membrane, and is often topographically restricted to long-lived or proliferating cell zones (Hockenbery et al., 1990Go). While bcl-2 acts by protecting the cell against apoptosis, high bcl-2 expression extends cell survival and proliferation (Wyllie, 1994Go; Marzioni et al., 1998Go).

Bcl-2 has already been immunolocalized in the syncytiotrophoblast of the chorionic villi in human placenta from the first to the third trimester; undisturbed regulation of apoptosis has been supposed to be of importance for implantation, early pregnancy, and the maintenance of trophoblast mass during later pregnancy, while disturbed regulation of apoptosis has been suggested to be associated with pregnancy failure (Sakuragi et al., 1994Go; Marx et al., 1999Go; Toki et al., 1999Go). While bcl-2 was immunolocalized at the maternal–fetal interface in healthy and failing pregnancies, it was shown that bcl-2 immunostaining was greater in viable pregnancies (Lea et al., 1997Go, 1999Go). Tissue obtained from ectopic pregnancy has always been proposed as an excellent model to identify the mechanism of trophoblast invasion although, according to recent evidence (Marx et al., 1999Go) in ectopic pregnancy, fewer apoptotic cell bodies were present as compared with eutopic pregnancy. Unlike in normal pregnancy, apoptosis has been shown to occur in the villi but not in the decidua of tubal ectopic pregnancies (Kokawa et al., 1998bGo), which suggests that the implantation site might affect the occurrence of apoptotic changes in early pregnancy of humans. As apoptosis seems to be intensified in cases of spontaneous abortion compared with normal pregnancies (Lea et al., 1997Go; Kokawa et al., 1998aGo), it is postulated that invasiveness of ectopic trophoblast towards the muscularis zone of the tubal wall, consequently leading to tubal rupture, might also be due to disturbed regulation of apoptosis, such as high antiapoptotic activity and, thus, unlimited cell survival and proliferation of ectopic trophoblast.

This preliminary study was designed to investigate the quantitative immunolocalization of bcl-2 and thus the regulation of apoptosis during first-trimester ruptured and non-ruptured human tubal ectopic pregnancies. We wanted to find out whether bcl-2 protein expression was disturbed in cases of ruptured tubal pregnancies. Knowledge about bcl-2 expression in ruptured and non-ruptured tubal ectopic pregnancies was thought to be a first step in the establishment of a circulating marker for tubal rupture in the future. As efforts towards the identification of a prognostic marker for tubal rupture have already been made, though mainly on retrospective evaluation of patient's characteristics (Job et al., 1999Go; Mol et al., 1999Go), the aim was to elucidate this most threatening complication of ectopic pregnancy on the immunohistochemical level.


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
The expression of bcl-2 protein was evaluated in paraffin-embedded tissue specimens of 18 patients with ruptured and 18 patients with non-ruptured first-trimester tubal ectopic pregnancies by means of immunohistochemistry using monoclonal mouse anti-human bcl-2 oncoprotein antibodies (Clone 124, Dako No. 0887).

The study was carried out in accordance with the 1975 Helsinki Declaration on Human Experimentation and approved by the Ethics Committee.

Immunohistochemistry
Serial sections (5 µm thick) of paraffin-embedded specimens were cut and then mounted on glass slides. All specimens were cut in one sequence and stained in a single reaction to ensure standard thickness and comparability of colour development. The sections were deparaffinized using xylene (Merck, Germany) and rehydrated in graded alcohol series (100 to 70%). Then they were treated with 0.3% hydrogen peroxide for 20 min to block endogenous peroxidase activity. Afterwards, the slides were placed in a citrate buffer (Antigen Retrieval Citra; Bio Genex, San Ramon, CA, USA) and heated three times for 5 min in the microwave oven (HM 146; Elektra Bregenz, Austria) set to decreasing levels of power (750, 650, 500 W). Bcl-2 expression was detected by using a standard immunohistochemical procedure, incubating the slides at 4°C overnight with monoclonal mouse anti-human bcl-2 [clone 124, isotype immunoglobulin (Ig) G1; Dako, Glostrup, Denmark] diluted 1:40 in phosphate buffered saline (PBS) containing 1% bovine serum albumin (BSA, Dako). After two washes with PBS, the slides were incubated with secondary biotinylated rabbit anti-mouse antibodies (Dako) for 30 min. Then they were exposed to avidin-biotin-peroxidase complex (Dako). Sections were stained with Fast Red Chromogen (Bio Genex) for 10 min and then washed twice with distilled water for 5 min. Counterstaining was performed with haematoxylin for 10 s before mounting. The positive control slides were prepared from tonsillar tissue and lymphocytes. For negative controls the mouse antibody HK 119/7M (Bio Genex) was used.

Evaluation of immunostaining by a computerized image analysis
All sections were investigated by light microscopy (Nikon, Mikrophot-FXA, x20 objective). The images were scanned with a video-camera (Hamamatsu, Colour Chilled 3CCD, Hamamatsu City, Japan), and afterwards saved with the Raster OPS-Videograbbercard for McIntosh Power PC 8100. Finally, transformation to a digitized image (Adobe Photoshop 4.0, Mountain View, CA, USA) for computerized image analysis (Lucia 32 G/VGA-Version 4.11 for Optoteam Image Analysis System; Laboratory Imaging, Ltd, Nikon Optoteam, http://www.lim.cz.) and quantification of bcl-2 immunostaining were performed.

First, the syncytiotrophoblast layer of the chorionic villi was identified manually by tracing the lines on each colour image with a computer mouse (Figure 1aGo: non-ruptured tubal ectopic pregnancy, Figure 1bGo: ruptured tubal ectopic pregnancy). The resulting binary image was measured by splitting the red immunostaining of bcl-2 into red-green-blue (RGB) signals. For RGB signals thresholds were determined: red (range 115–180), green (range 95–160) and blue (range 100–155). The measuring frame was 0.45 mm2. To assess bcl-2 immunostaining objectively in the syncytiotrophoblast, five randomly selected fields of vision were measured on the basis of good tissue morphology in the 36 specimens of tubal ectopic pregnancies. For measuring of the five fields of vision, one sample site in the upper left, another one in the upper right, the next two sites right and left at the bottom of the sample, and another one in the middle of the sample were selected. It is suggested that with the selection of five fields of vision, a representative survey regarding bcl-2 immunostaining was obtained.



View larger version (154K):
[in this window]
[in a new window]
 
Figure 1. (a) Chorionic villi of non-ruptured tubal ectopic pregnancy after transformation to a digitized image for computerized image analysis. The resulting binary image of red bcl-2 immunostaining in the villi syncytiotrophoblast was measured by splitting the red colour into red-green-blue signals. (b) Chorionic villi of ruptured tubal ectopic pregnancy after transformation to a digitized image for computerized image analysis. The resulting binary image of red bcl-2 immunostaining in the villi syncytiotrophoblast is measured by splitting the red colour into red-green-blue signals.

 
The mean percentages of positive stained area (%PA) and standard deviations (SD) of all measured frames were then determined in the 36 specimens, in addition to the mean hue (H), i.e. the main wavelength of the colour, the mean saturation (S) which depicts the purity of the whiteness in the colour, and the mean intensity (I), the quantity of total brightness of all measured frames in the 36 immunohistochemically stained specimens. After quantification of bcl-2 immunostaining, the findings in the 18/36 tissue specimens of those patients who were found to have tubal rupture were compared with the findings in the 18/36 tissue specimens of those patients who had no tubal rupture at surgery.

Observations were made of signs of apoptosis in the samples morphologically: cellular shrinkage, nuclear condensation and fragmentation were clearly diagnosed by use of light microscopy, although differences between ruptured and non-ruptured samples could only be suspected.

Statistics
Differences regarding patient and specimen specific characteristics such as quantified staining intensity between ruptured and non-ruptured tubal ectopic pregnancies were compared using the Wilcoxon two-sample test. P-values of 0.05 were considered to be statistically significant.

Pearson correlation coefficients were performed for correlation analyses between bcl-2 immunostaining, such as the mean percentage (±SD) of %PA, the variables of the HSI system, and patient characteristics.

The method of stepwise logistic regression was used to evaluate the possible role of bcl-2 immunostaining in ruptured and non-ruptured tubal ectopic pregnancies. The variables age, gestational weeks, size of adnexal mass measured by transvaginal sonography, human chorionic gonadotrophin (ß-HCG), and the variables regarding bcl-2 immunostaining, such as %PA and the variables of the HSI system, were considered as independent variables to model the probability of rupture of the tube. Calculations were done using the SAS statistical software system (SAS Institute Inc., Cary, N.C., USA).


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
As in previous study groups (Lea et al., 1997Go; Marzioni et al., 1998Go), positive immunostaining for bcl-2 was observed only in the syncytiotrophoblast layer of the chorionic villi of all 36 specimens, while villous cytotrophoblast, mesenchymal cells of the villous cores and extravillous cytotrophoblast of cell columns and cell islands were all negative for bcl-2.

Differences regarding patient and specimen specific characteristics between ruptured and non-ruptured tubal ectopic pregnancies, such as gestational weeks, %PA, S and I, were found to be statistically significant as shown by the Wilcoxon two-sample test (Tables I and IIGoGo). Statistically significantly higher (P = 0.0009) immunostaining for bcl-2 was found in ruptured tubal ectopic pregnancies (Figure 1bGo).


View this table:
[in this window]
[in a new window]
 
Table I. Mean values ± SD of patient specific characteristics, such as age, gestational weeks, size of the adnexal mass in cm measured by transvaginal sonography, and human chorionic gonadotrophin (HCG) are compared between non-ruptured and ruptured ectopic pregnancies. There were no statistically significant differences
 

View this table:
[in this window]
[in a new window]
 
Table II. Mean values ± SD of specimen specific characteristics, such as percentage of positive stained area (%PA), hue (0–360° on the standard colour wheel), saturation (in pixels), and intensity (in pixels) are compared between non-ruptured and ruptured ectopic pregnancies
 
For correlation analyses between bcl-2 immunostaining and patient characteristics, Pearson correlation coefficients revealed a correlation coefficient of r = 0.44 (P = 0.008) for gestational weeks and %PA in the whole patient collective. In ruptured ectopic pregnancies, a correlation coefficient of r = 0.76 (P = 0.003) for gestational weeks and ß-HCG was revealed. In non-ruptured ectopic pregnancies, a statistically significant correlation for gestational weeks and %PA of r = 0.51 (P = 0.03) was revealed.

Using the method of stepwise logistic regression, the variables percentage of bcl-2 immunostained area (%PA) (ß = 0.203, P = 0.0036, odds ratio=1.23) and S (ß = –0.458, P = 0.02, odds ratio = 0.63) entered the model. The intercept of the stepwise logistic regression model was ß = 3.229. For a probability threshold of 0.5 (<0.5 = non-ruptured ectopic pregnancy, >0.5 = ruptured ectopic pregnancy), this logistic regression model yields a sensitivity and specificity of 94.4%. It should be mentioned that the estimation of sensitivity and specificity will be positively biased in the learning sample. Moreover, on account of small sample sizes, the confidence interval for sensitivity and specificity were calculated (72.7%; 99.9%).


    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
The main functions of human placental villi syncytiotrophoblast are to act as an immunological barrier, to regulate maternal–fetal exchange of molecules, and to provide hormone production for 9 months (Sakuragi et al., 1994Go; Lea et al., 1997Go). The expression of bcl-2 in the syncytiotrophoblast seems to reflect a fundamental mechanism by which this important epithelial component of the placenta is preserved (Sakuragi et al., 1994Go). It was stated (Kim et al., 1995Go) that the decrease in bcl-2 expression with increasing gestational age of the placenta was due to the fact that bcl-2 acted as a proliferation and differentiation-related marker of trophoblast, a function which was no longer necessary in the term placenta. In contrast to these results, bcl-2 was also reported to be involved in the preservation of the syncytiotrophoblast until delivery without any decrease in immunostaining (Marzioni et al., 1998Go). Furthermore, it was suggested that bcl-2 positivity of the syncytiotrophoblast proved the relationship between bcl-2 expression and cell proliferation (Marzioni et al., 1998Go). In the current study, a statistically significant correlation (P = 0.008) was found between gestational weeks and percentage of positive bcl-2 immunostained area in the whole patient collective.

While other trophoblast-associated apoptotic markers, such as Fas and Fas ligand, have been described in co-occurrence with bcl-2 in term placenta, an apoptotic process mediated by Fas ligand has been suggested to play a role in placental invasion during early trophoblast implantation which underscores similarities between the trophoblast and neoplastic cells (Uckan et al., 1997Go). It was suggested that the invasiveness of ectopic trophoblast towards and through the muscularis zone of the tubal wall, consequently leading to tubal rupture, might be due to unlimited cell survival and high proliferative activity reflected by elevated bcl-2 expression in the syncytiotrophoblast overlying the cytotrophoblastic layer. There have also been efforts to identify parameters reflecting proliferative cell activity (Klein et al., 1995aGo; Klein et al., 1995bGo; Kiss et al., 1997Go) or biological activity (Kemp et al., 1996Go; Kemp et al., 1997Go) of ectopic trophoblast. Particularly in cases of asymptomatic ectopic pregnancies, the identification of parameters reflecting the biological activity of ectopic trophoblast could enable selection of those patients who require surgical intervention rather than medical treatment. The results of this study are in agreement with others (Klein et al., 1995aGo,bGo) in which it was postulated that the degree to which trophoblast proliferation has progressed yields a better basis for therapeutic decisions, as evidence of invasive growth might indicate a decision against medical treatment.

While bcl-2 protein expression has been shown in first trimester chorionic villi of intrauterine pregnancies (Sakuragi et al., 1994Go), in the current study, bcl-2 was immunolocalized in first trimester ectopic pregnancies. In agreement with other investigators (Lea et al., 1997Go), the greater intensity of syncytiotrophoblast immunostaining in the group of ruptured ectopic pregnancies in the current patient group might reflect different local environments, e.g. tumour necrosis factor alpha (TNF-{alpha}), which might also affect bcl-2 expression. As the inhibitory effect of bcl-2 on apoptosis may occur through proapoptotic proteins, elevated expression of bax, too, for example, could counter the protective effect of bcl-2. Enhanced bcl-2 expression in the syncytiotrophoblast overlying subtrophoblastic fibrin deposits was suggested to be necessary for the preservation of the placenta during gestation and for reparative processes of the trophoblast (Marzioni et al., 1998Go). It was already emphasized that subtrophoblastic intravillous fibrinoid was the result of immunological processes altering the syncytiotrophoblastic barrier, thus increasing bcl-2 expression in the overlying syncytium (Kaufmann et al., 1996Go).

As the preliminary data presented here demonstrated statistically significantly elevated bcl-2 immunostaining in ruptured human tubal ectopic pregnancies, further studies are being conducted at our department to reveal whether the localization of other members of the bcl-2 family is involved in tubal rupture. This suggested that the extensive bcl-2 expression reflecting unlimited cell survival of ectopic pregnancies might play a pivotal role in the development of tubal rupture. On the basis of immunohistochemical findings, the establishment of a circulating marker for tubal rupture is the goal of further investigations.


    Acknowledgements
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
The authors are deeply grateful to Professor H.Koelbl, Halle/Saale, Germany, for his support since 1996. They wish to thank Professor H.Plenk from the Bone and Biomaterials Research Laboratory for critically reading the manuscript, Dr S.Slobodinsky for many fruitful discussions, Professor G.Breitenecker for kindly providing them with the histological slides and E.Bock, J.Stani, and G.Dudek for expert technical assistance. This study was supported by the Medizinisch-Wissenschaftlichen Fonds des Bürgermeisters der Bundeshauptstadt Wien as well as by the Ludwig Boltzmann Institute for Gynaecological Oncology & Reproductive Medicine.


    Notes
 
4 To whom correspondence should be addressed. E-mail: elisabeth.kucera{at}akh-wien.ac.at Back


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
Cotter, T.G., Lennon, S.V., Glynn, J.G. et al. (1990) Cell death via apoptosis and its relationship to growth, development and differentiation of both tumour and normal cells. Anticancer Res., 10, 1153–1159.[ISI][Medline]

Hengartner, M.O. (1997) Apoptosis and the shape of death. Dev. Genet., 21, 245–248.[ISI][Medline]

Hockenbery, D., Nunez, G., Milliman, C. et al. (1990) Bcl-2 is an inner mitochondrial membrane protein that blocks programmed cell death. Nature, 348, 334–336.[ISI][Medline]

Huppertz, B., Frank, H.G. and Kaufmann, P. (1999) The apoptosis cascade – morphological and immunohistochemical methods for its visualisation. Anat. Embryol. Berl., 200, 1–18.[ISI][Medline]

Job, S.N., Fernandez, H., Bouyer, J. et al. (1999) Ruptured tubal ectopic pregnancy: risk factors and reproductive outcome: results of a population-based study in France. Am. J. Obstet. Gynecol., 180, 938–944.[ISI][Medline]

Kaufmann, P., Huppertz, B. and Frank, H.G. (1996) The fibrinoids of the human placenta: origin, composition and functional relevance. Anat. Anz., 178, 485–501.[Medline]

Kemp, B., Funk, A. and Rath, W. (1996) Doppler sonographic criteria for viability in ectopic pregnancy in correlation with histology. Int. J. Gynaecol. Obstet., 54, 179–181.[ISI][Medline]

Kemp, B., Funk, A., Hauptmann, S. et al. (1997) Doppler sonographic criteria for viability in symptomless ectopic pregnancies [letter]. Lancet, 349, 1220–1221.[ISI][Medline]

Kim, C.J., Choe, Y.J., Yoon, B.H., et al. (1995) Patterns of bcl-2 expression in placenta. Pathol. Res. Pract., 191, 1239–1244.[ISI][Medline]

Kiss, H., Klein, M., Egarter, C. et al. (1997) Proliferative cell activity in correlation to human chorionic gonadotrophin release of trophoblast tissue of tubal pregnancy. Hum. Reprod., 12, 383–386.[Abstract]

Klein, M., Graf, A., Kiss, H. et al. (1995a) Impact of trophoblast penetration through the basal membrane on the efficacy of drug therapy in tubal pregnancies. Hum. Reprod., 10, 439–441.[Abstract]

Klein, M., Graf, A.H., Hutter, W. et al. (1995b) Proliferative activity in ectopic trophoblastic tissue. Hum. Reprod., 10, 2441–2444.[Abstract]

Kokawa, K., Shikone, T. and Nakano, R. (1998a) Apoptosis in human chorionic villi and decidua during normal embryonic development and spontaneous abortion in the first trimester. Placenta, 19, 21–26.[ISI][Medline]

Kokawa, K., Shikone, T. and Nakano, R. (1998b) Apoptosis in human chorionic villi and decidua in normal and ectopic pregnancy. Mol. Hum. Reprod., 4, 87–91.[Abstract]

Lea, R.G., al Sharekh, N., Tulppala, M. et al. (1997) The immunolocalisation of bcl-2 at the maternal-fetal interface in healthy and failing pregnancies. Hum. Reprod., 12, 153–158.[ISI][Medline]

Lea, R.G., Riley, S.C., Antipatis, C. et al. (1999) Cytokines and the regulation of apoptosis in reproductive tissues: a review. Am. J. Reprod. Immunol., 42, 100–109.[ISI][Medline]

Marx, L., Arck, P., Kapp, M. et al. (1999) Leukocyte populations, hormone receptors and apoptosis in eutopic and ectopic first trimester human pregnancies. Hum. Reprod., 14, 1111–1117.[Abstract/Free Full Text]

Marzioni, D., Muhlhauser, J., Crescimanno, C. et al. (1998) BCL-2 expression in the human placenta and its correlation with fibrin deposits. Hum. Reprod., 13, 1717–1722.[Abstract]

Mol, B.W., Hajenius, P.J., Engelsbel, S. et al. (1999) Can noninvasive diagnostic tools predict tubal rupture or active bleeding in patients with tubal pregnancy? Fertil. Steril., 71, 167–173.[ISI][Medline]

Sakuragi, N., Matsuo, H., Coukos, G. et al. (1994) Differentiation-dependent expression of the BCL-2 proto-oncogene in the human trophoblast lineage. J. Soc. Gynecol. Invest., 1, 164–172.[ISI][Medline]

Thompson, E.B. (1994) Apoptosis and steroid hormones. Mol. Endocrinol., 8, 665–673.[ISI][Medline]

Toki, T., Horiuchi, A., Ichikawa, N. et al. (1999) Inverse relationship between apoptosis and Bcl-2 expression in syncytiotrophoblast and fibrin-type fibrinoid in early gestation. Mol. Hum. Reprod., 5, 246–251.[Abstract/Free Full Text]

Uckan, D., Steele, A., Cherry, Wang, B.Y. et al. (1997) Trophoblasts express Fas ligand: a proposed mechanism for immune privilege in placenta and maternal invasion. Mol. Hum. Reprod., 3, 655–662.[Abstract]

Wyllie, A.H. (1994) Apoptosis. Death gets a brake. Nature, 369, 272–273.[ISI][Medline]

Wyllie, A.H., Kerr, J.F. and Currie, A.R. (1980) Cell death: the significance of apoptosis. Int. Rev. Cytol., 68, 251–306.[Medline]

Submitted on October 30, 2000; accepted on February 26, 2001.





This Article
Abstract
FREE Full Text (PDF )
Alert me when this article is cited
Alert me if a correction is posted
Services
Email this article to a friend
Similar articles in this journal
Similar articles in ISI Web of Science
Similar articles in PubMed
Alert me to new issues of the journal
Add to My Personal Archive
Download to citation manager
Search for citing articles in:
ISI Web of Science (3)
Request Permissions
Google Scholar
Articles by Kucera, E.
Articles by Sliutz, G.
PubMed
PubMed Citation
Articles by Kucera, E.
Articles by Sliutz, G.