Biology of Reproduction 63, 1575B-1579 (2000)
© 2000 Society for the Study of Reproduction, Inc.
Human Fetal Membranes: Their Preterm Premature Rupture1
Gillian D. Bryant-Greenwood2,a, and
Lynnae K. Millara
a Pacific Biomedical Research Center, University of Hawaii, Honolulu, Hawaii 96822
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ABSTRACT
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At the 1999 annual meeting of the Society for the Study of Reproduction there were three speakers in the minisymposium entitled ``I've got to get out of here: fetal-maternal interactions involved in parturition''. The primary focus was on research progress in understanding the mechanisms involved in human parturition. Although the title of the symposium emphasized the need to ``get out'', there was considerable emphasis on understanding the problem of ``getting out too early'' or preterm birth. While preterm birth is unusual in most species, it is of major clinical importance in the human. The data presented by one of the speakers is reviewed here with a focus on preterm labor and preterm premature rupture of the fetal membranes as mechanisms involved in the diverse pathology of preterm birth.
decidua, parturition, placenta, relaxin, syncytiotrophoblast
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INTRODUCTION
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Human parturition is a complex physiological process, in which the synchronization of uterine contractions, cervical dilatation, and fetal membrane rupture is achieved through systemic and autocrine/paracrine signaling events that are beginning to be elucidated. Spontaneous preterm birth is an unusual, even unknown problem, in most species other than the human. The reasons for this are unclear but may be due to our upright posture, imposing a greater mechanical challenge on the tissues than in other species. The predominantly localized mechanisms in the human that appear to impose the time limit on gestation may make human parturition different from that of most other species. We need to understand the basic systemic and the local events that govern the timing of normal birth and how these events are interrelated to synchronize the tissue changes. The more pressing clinical problem is that of preterm birth that occurs in about 11% of all pregnancies [1]. It has become clear in recent years that in order to understand the causes of preterm birth, it has to be studied as a pathological process, as the signaling events may differ between term and preterm birth. Preterm birth is defined as a pregnancy ending before 37 wk gestation and is a leading cause of perinatal and neonatal mortality. The preterm birth rate has continued to rise in the United States, in spite of the emphasis on making prenatal care available to more women. Preterm birth disproportionately affects the minority populations in this country. It may result in long-term health problems for the surviving infants and often, significant psychosocial difficulties for the family unit. Preterm birth can occur secondary to preterm labor, preterm premature rupture of the fetal membranes, an incompetent cervix, or secondary to intentional delivery for significant fetal or maternal complications like fetal growth restriction or pre-eclampsia.
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PRETERM LABOR AND PRETERM PREMATURE RUPTURE OF THE FETAL MEMBRANES
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Fifty percent of all preterm births are due to premature labor, defined as regular uterine contractions with progressive cervical dilatation [1]. Preterm birth can occur secondary to a number of diverse pathologies, some of which are shown in Figure 1. Chronic inflammation as well as acute infection, occurring locally within the uterus, can cause the production of autocrine/paracrine hormones, as well as the classical cytokines, that then activate the myometrium causing preterm labor. This is a reasonable biological response, because the mother and fetus may have a greater chance of survival with the infected tissues expulsed. Understanding the involvement of infection has been a major focus of attention in the field, particularly with respect to chorioamnionitis and its affect on cytokine release [2]. Maternal placental vasculopathy has been shown to be a separate risk factor for preterm birth, distinct from infection [3]. A group of growth disorders of the fetal-placental unit are likely to be linked to a group of mechanical disorders, because in the event of inadequate growth, there is increased mechanical stress imposed upon the tissues. There is unfortunately little known about the coordination of the growth of the fetus, placenta, and the fetal membranes and that of the uterus that contains them. The effects of maternal stress upon the autocrine/paracrine system controlling labor, is clearly of importance to preterm birth [4]. This may be linked to the proposed "placental clock" of normal pregnancy, in which placental corticotropin-releasing hormone levels appear to be a marker of gestational length [5]. Thus in Figure 1, the principal factors likely to affect the autocrine/paracrine balance of hormones and cytokines to initiate preterm labor are shown. The same substances can also act via an alternative pathway, increasing the available active matrix metalloproteinases (MMPs) via alteration of the MMP/tissue inhibitor of metalloproteinase (TIMP) balance. The consequences of this would be increased matrix degradation in the fetal membranes and cervix. This can lead to decidual activation and increased production of cytokines and prostaglandins, leading to myometrial activation. Alternatively, after membrane rupture, ascending infection is a rapid process that also terminates in uterine contraction. Therefore, this is often described as the common terminal pathway to parturition, because these are the required terminal pathways for either preterm or term birth.
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FIG. 1. Diagram of the diverse pathologies that can potentially cause preterm labor, preterm premature rupture of the fetal membranes, or cervical ripening
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The rupture of the fetal membranes before the onset of regular uterine contraction at term takes place in some 8–10% of pregnancies [6]. This can result in increased rates of maternal and fetal infection. However, membranes rupture prematurely before 37 wk gestation in approximately 1% of all pregnancies, and this is associated with 30–40% of preterm deliveries [6]. There is undoubtedly a need for identifying markers for the etiologies of preterm labor and preterm premature rupture of the membranes, shown in Figure 1, that could be used to predict patients at risk for preterm delivery. The current methods of predicting women at risk are too insensitive and nonspecific to be of real clinical value [7]. It is likely that there are markers that could be of clinical use, because it is becoming increasingly evident that many of the events leading to preterm birth are more likely to be chronic than acute. There is an urgent need therefore to apply the latest methodologies to identify these markers.
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DECIDUAL/PLACENTAL RELAXINS, A FOCUSED STUDY
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Relaxin is classically a systemic hormone produced by the corpus luteum of pregnancy with a role in the remodelling of the reproductive tissues to both accommodate the pregnancy and to prepare for parturition. It was the need to find a source of human relaxin that led to the identification of the decidual cell and placental syncytiotrophoblast as sources of the hormone [8, 9]. We then proposed that by analogy with the follicle and the role of relaxin as an intrafollicular modulator of follicle wall structure prior to ovulation, that decidual and placental relaxins could act locally to alter the structure of the fetal membranes before their rupture [10]. We noted that both the follicular and amniotic compartments are initially closed and not in equilibrium with the circulatory system. The events of ovulation and parturition are both controlled systemically and locally, are nonreversible, and take place within selective barriers. Ovulation alters the function of the supporting follicle cells, while parturition involves the loss of the entire placental-fetal membrane system, thereby eliminating any need for neutralization or reversal of the terminal events.
Our first studies used dispersed human amnion and chorion cells and the addition of porcine relaxin, the only relaxin then available [10]. It was shown by classical binding studies that a specific relaxin receptor was present on these cells and that secretion of both collagenase and plasminogen activator were increased after a 32-h exposure to relaxin in vitro. We had the temerity to suggest that the human fetal membranes were novel target tissues for relaxin and that the endogenous hormone in vivo might cause a similar release of collagenolytic enzymes leading to the weakening and rupture of the fetal membranes [9]. This proposal was put into context by the simultaneous study of eight other local and systemic candidate hormones for causing increased or decreased secretion of collagenolytic enzymes. From this we acknowledged that relaxin was but one of many potential hormones capable of modulating the structure of the fetal membrane extracellular matrix in this manner [10].
It took some 14 yr of work on human relaxin to be able to repeat these studies with the homologous hormone [11–13]. Meanwhile, the MMP field had expanded from a single collagenase to over 20 members of this important enzyme family [14]. The availability of several cDNA probes to some key collagenases allowed us to show that their expression does indeed alter over the peripartal period, at a time of rapid autocrine/paracrine hormone changes [15].
Recent work has extended these observations to show that human relaxin added to fetal membrane explants in vitro can cause a dose-dependent increase in the expression of specific genes, proteins, and activities of interstitial collagenase (MMP-1), stromelysin-1 (MMP-3), and gelatinase B (MMP-9), but not gelatinase A (MMP-2) or the inhibitor TIMP-1 [12, 13]. Local relaxin could therefore cause activation of a specific enzyme cascade that can result in the degradation of a broad spectrum of extracellular matrix components. These data complement work from other laboratories that have shown that human relaxin in vitro causes a 30% decrease in the tensile strength of the fetal membrane [16]. The mechanism by which relaxin causes this is currently unknown. The expression of the two human relaxin genes was then quantitated in the decidua and placenta of patients in a number of clinical situations. Any infected tissues were rigorously eliminated from the studies in order to uncover a relaxin pathway to MMP production distinct from that of the cytokine pathways to MMP production. In addition, quantitative Northern analyses with a complementary RNA probe to both human relaxins was used to confirm quantitative in situ hybridization data, obtained with a series of oligoprobes [17]. These studies showed that there was significantly more total relaxin expressed in patients with preterm premature rupture of the fetal membranes compared to patients with preterm labor or preterm Cesarean section for maternal medical complications [17]. A similar group of tissues collected at term failed to show any significant differences in relaxin expression, which suggests that relaxin is involved in the pathology of preterm premature rupture of the fetal membranes and not in their normal rupture at term. We carried out a study to show whether this overexpression of relaxin was indeed independent of chorioamnionitis. Based upon the histological detection of neutrophil invasion of the tissues, the classical cytokines, interleukin IL-1, IL-6, and IL-8, were indeed increased with severe infection; relaxin expression, however, was not [18]. On the other hand, relaxin expression was increased in the decidua of the noninfected preterm prematurely ruptured patients, confirming our previous data.
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DECIDUAL AND PLACENTAL RELAXINS IN A BROADER CONTEXT
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We now need to put this upregulation of relaxin gene expression into a pathway of events taking place within the fetal membranes in order to understand what is happening in this tissue just prior to its rupture. In order to gain such insight, we have collected large numbers of prematurely ruptured fetal membranes and very carefully selected a few from healthy women with minimal potential confounding variables. We have matched these with control membranes from patients having a preterm Cesarean section for a medical complication of pregnancy. None of the patients experienced labor. The mRNAs from these pairs of tissues have been used for cDNA expression arrays, containing over 600 cytokines/receptors and cell interaction genes. Fortunately, the relaxin H2 gene was included in these arrays. The data show that relaxin expression is indeed upregulated in those tissues that prematurely rupture, by approximately the same order of magnitude as determined from quantitative in situ hybridization and Northern analysis in our previous studies. However, some 39 other genes were also upregulated and 15 were downregulated in the preterm prematurely ruptured membranes. The challenge remains to place relaxin into a pathway or pathways derived from these data.
In parallel studies, an analysis of the 5' and 3' untranslated regions of the two human relaxin genes has been undertaken in order to seek structural features that could act as gene-specific transcription regulators [19, 20]. It was particularly interesting to find that the 5' ends of the relaxin genes could be divided into a proximal highly homologous segment and a distal nonhomologous region. Within the proximal region were several putative regulatory elements common to both genes, suggesting a similar regulatory mechanism. A distinct gene-specific regulation may also exist for the individual relaxin genes because cis elements specific to each gene were identified; moreover, the divergence at the distal region of their 5' upstream sequences may provide the structural features that act as gene-specific transcription regulators [19]. With this information at hand, we have recently shown that the addition of progesterone to fetal membrane explants and to a choriocarcinoma cell line (JAR cells) that express both relaxin genes causes the downregulation of the H2 relaxin gene but has no effect on the relaxin H1 gene (to be published). This is in agreement with our 5' upstream analysis that showed a glucocorticoid response element in the relaxin H2 and none in the H1 sequence [19]. Thus, by a combination of exploratory work using the latest available technology, we have identified progesterone as a potential modulator of decidual relaxin production.
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ROLE OF DISTENTION IN FETAL MEMBRANE ACCOMMODATION AND RUPTURE
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The human fetal membranes are composed of specialized cells and extracellular matrix [21]. Little is known about membrane growth through gestation or how it is controlled and coordinated with the growth of the fetus and placenta. It has been shown that there is rapid mitotic division of the amniotic epithelium in early pregnancy, but that this decreases by the 6th mo and reaches low values thereafter [22]. The amniotic cavity expands most rapidly in the last trimester, and the epithelial cells must replicate or hypertrophy at a rate sufficient to maintain this continuous epithelium. Concurrent studies on the mitotic and apoptotic indices throughout gestation are in progress and should clarify how these are coordinated to allow sufficient net growth to accommodate the growing uterine contents. Extracellular matrix remodeling is also necessary to allow for accommodation; this is a complex process involving the concurrent degradation and synthesis of many highly complex proteins [23]. A net loss of extracellular matrix occurs from the fetal membranes during the last weeks of gestation [24], at the same time collagen synthesis continues until term, although at a lower level than in early pregnancy [25]. We have shown that decidual and placental relaxins are produced constitutively [17] and are capable of stimulating the production of the MMPs [12, 13]. Interleukin-8 is a constitutively produced cytokine also capable of increasing the production of the MMPs [26]. The excessive local production of relaxin or IL-8 could cause a greater than normal loss of the extracellular matrix, weaken the tissue, and predispose it to rupture.
We and others have proposed that the increased mechanical distention of the fetal membranes over this period of rapid fetal and placental growth is important. The stretching of this tissue is likely to cause the increased expression of specific genes that would influence the structure of the extracellular matrix [27]. The degree of fetal membrane distention in vivo was measured by assessing the intrauterine surface area in patients of different gestational ages by using calipers placed horizontally at 5-cm intervals on the maternal abdomen between the symphysis pubis and the fundus of the uterus [28]. These authors determined the ratios of the surface area of the membrane in vivo to the surface area of the expelled sac in vitro at preterm and term, showing an approximate twofold distention by term. The membrane when expelled at term is therefore only about half the size that it is in vivo. However, the methodology used lacked precision, and we have recently used ultrasonographically generated images and three-dimensional models of the uterine cavity, together with surface area measurements in vitro, to gain a more accurate measurement of the distention of the fetal membranes in vivo [27]. Our measurements show that the membranes are stretched by 70% at term, showing that the measurements made with calipers directly on the maternal abdomen overestimated the uterine surface area. The measured change between preterm and term was however 30% in both studies; the fetal membranes are indeed under a state of increasing tension as gestation advances.
During the course of measuring the surface areas of many fetal membranes after their expulsion, in addition to their differences in size, we noted marked differences in elasticity. We have shown that there is a very fine elastic component in the fetal membrane that is well cross-linked and likely to contribute significantly to the elastic recoil properties of the tissue [29]. We showed that the expression of the tropoelastin gene is greater at preterm than at term, coinciding with the measured increase in elasticity of the tissue over this period [28].
The unidirectional stretching of the fetal membranes has been shown to induce an acute increase in IL-8 expression [26, 30]. Physical forces are known to influence cell growth, differentiation, apoptosis and movement [31] and must affect multiple genes in the tissue. Because IL-8 is also a classical cytokine that responds to bacterial infection as well as distention, we initially used a sterile system of amniotic epithelial cells (WISH cells), grown on a silicone elastomer sheet, coated with placental extracellular matrix to mimic their in vivo substrate. This sheet of cells was placed into a specially designed multidirectional distention device to distend the cells to approximately the amount measured in vivo at term. At the end of a 4-h distention period, we used the RNA isolated from cells placed into the device but left undistended and subtracted this from the RNA isolated from the distended cells by the method of suppression subtractive hybridization [32]. There was a 9-fold increase in expression of IL-8 mRNA on distention and a 2.8-fold increase in a novel cytokine, pre-B cell enhancing factor (PBEF) [33]. When full-thickness fetal membranes from preterm and term patients before labor were similarly distended for 4 h, quantitative Northern analysis of IL-8 and PBEF showed significant upregulation in the expression of these two genes, by 3.5-fold and 2.7-fold, respectively. However, fetal membranes from patients obtained at term that were distended in the multidirectional stretching device showed less upregulation in the expression of these two genes, suggesting that the term membranes had already undergone extensive distention-induced changes throughout gestation and had passed the time of their maximum sensitivity to distention. Similar results at preterm and term were obtained when we cloned an additional three genes by suppression subtractive hybridization after distending the full-thickness fetal membranes [34]. These genes were the interferon-stimulated gene encoding a 54-kDa protein, the gene for huntingtin-interacting protein 2, and an expressed sequence tag that was recently deposited into the database and is the gene for the human interleukin enhancing binding factor 2. The functions of these novel genes in the fetal membranes are unknown. We sought changes in the expression of the distention-inducible genes in vivo and determined that four of the five genes were significantly upregulated by normal labor and delivery. It is not clear if these genes were upregulated secondary to the repetitive forces applied to the membrane during labor or due to other factors.
The accommodation of the fetal membranes to the rapid fetal and placental growth taking place in the last trimester of gestation is a complex process involving cell division and hypertrophy, extracellular matrix remodeling, apoptosis, as well as the ability to stretch and recoil (elasticity). A problem with any one of these could potentially be negated to some extent by compensation from one of the other components. There is a need to understand more fully the role of each and how they interact to achieve full accommodation of the tissue during gestation, if we are to understand the problem of the preterm rupture of the fetal membranes.
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ACKNOWLEDGMENTS
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We thank Mrs. S. Yamamoto and the past and present members of this laboratory for their dedication and contributions to this work.
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FOOTNOTES
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First decision: 10 May 2000.
1 This work was supported by NIH grants HD06633 and HD24314 and grants to the University of Hawaii and Kapiolani Medial Center under the Research Centers in Minority Institutions Program of the NCRR (RR1A1-03061 and RR-11091). 
2 Correspondence: G.D. Bryant-Greenwood, Department of Cell and Molecular Biology, University of Hawaii, 1960 East-West Road, Honolulu, HI 96822. FAX: 808 956 9481; gbg{at}pbrc.hawaii.edu
Accepted: June 26, 2000.
Received: April 20, 2000.
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REFERENCES
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-
Goldenberg RL. Prevention of premature birth. New Engl J Med 1998; 339:313–320.[Free Full Text]
-
Gomez R, Ghezzi F, Romero R, Monuz H, Tolosa J, Rojas I. Premature labor and intraamniotic infection. Clin Perinatol 1995; 22:281–342.[Medline]
-
Arias F, Rodriquez L, Rayne SC, Kraus FT. Maternal placental vasculopathy and infection: two distinct subgroups among patients with preterm labor and preterm ruptured membranes. Am J Obstet Gynecol 1993; 168:589–591.
-
Hobel CJ, Dunkel-Schetter C, Roesch S. Maternal stress as a signal to the fetus. Prenatal Neonatal Med 1998; 3:116–120.
-
McLean M, Bisits A, Davies J, Woods R, Lowry P, Smith R. A placental clock controlling the length of human pregnancy. Nat Med 1995; 1:460–463.[CrossRef][Medline]
-
Parry S, Strauss JF. Mechanisms of disease—premature rupture of the fetal membranes. New Engl J Med 1998; 338:663–670.[Free Full Text]
-
Lockwood CJ, Kuczynski E. Markers of risk for preterm delivery. J Perinat Med 1999; 27:5–20.[CrossRef][Medline]
-
Yamamoto SY, Kwok S, Greenwood FC, Bryant-Greenwood GD. Relaxin purification from human placental basal plates. J Clin Endocrinol Metab 1981; 51:601–604.
-
Bryant-Greenwood GD. Relaxin as a new hormone. Endocr Rev 1982; 3:62–90.[Medline]
-
Koay ESC, Too CKL, Greenwood FC, Bryant-Greenwood GD. Relaxin stimulates collagenase and plasminogen activator secretion by dispersed human amnion and chorion cells in vitro. J Clin Endocrinol Metab 1983; 56:1332–1334.[Abstract]
-
Garibay-Tupas JL, Maaskant RA, Greenwood FC, Bryant-Greenwood GD. Characteristics of the binding of 32P-labelled human relaxins to the human fetal membranes. J Endocrinol 1995; 145:441–448.[Abstract]
-
Qin X, Garibay-Tupas J, Chua PK, Cachola L, Bryant-Greenwood GD. An autocrine/paracrine role of human decidual relaxin. I. Interstitial collagenase (MMP-1) and tissue plasminogen activator. Biol Reprod 1997; 56:800–811.[Abstract]
-
Qin X, Chua PK, Ohira RH, Bryant-Greenwood GD. An autocrine/paracrine role of human decidual relaxin. II. Stromelysin-1 (MMP-3) and tissue inhibitor of matrix metalloproteinase-1 (TIMP-1). Biol Reprod 1997; 56:812–820.[Abstract]
-
Nagase H, Woessner JF. Matrix metalloproteinases. J Biol Chem 1999; 274:21491–21494.[Free Full Text]
-
Bryant-Greenwood GD, Yamamoto SY. Control of peripartal collagenolysis in the human chorio-decidua. Am J Obstet Gynecol 1995; 172:63–70.[CrossRef][Medline]
-
Petersen LK, Helmig R, Oxlund H, Vogel I, Uldbjerg N. Relaxin induced weakening of human fetal membranes in vitro. Eur J Obstet Gynecol Reprod Biol 1994; 57:123–128.[CrossRef][Medline]
-
Bogic LV, Yamamoto SY, Millar LK, Bryant-Greenwood GD. Developmental regulation of the human relaxin genes in the decidua and placenta: overexpression in the preterm premature rupture of the fetal membranes. Biol Reprod 1997; 57:908–920.[Abstract]
-
Millar LK, Boesche M, Yamamoto S, Killeen J, DeBuque L, Chen R, Bryant-Greenwood G. A relaxin-mediated pathway to preterm premature rupture of the fetal membranes that is independent of infection. Am J Obstet Gynecol 1998; 179:126–134.[CrossRef][Medline]
-
Garibay-Tupas JL, Csiszar K, Fox M, Povey S, Bryant-Greenwood GD. Analysis of the 5'-untranslated regions of the human relaxin H1 and H2 genes and their chromosomal localization on chromosome 9p24.1 by radiating hybrid and breakpoint mapping. J Mol Endocrinol 1999; 23:355–365.[Abstract]
-
Garibay-Tupas JL, Bao S, Kim MT, Tashima LS, Bryant-Greenwood GD. Isolation and analysis of the 3' untranslated regions of the human relaxin H1 and H2 genes. J Mol Endocrinol 2000; 24:241–252.[Abstract]
-
Bryant-Greenwood GD. The extracellular matrix of the human fetal membranes: structure and function. Placenta 1998; 19:1–11.[Medline]
-
Schmidt W. The amniotic fluid compartment: the fetal habitat. Adv Anat Embryol Cell Biol 1992; 127:1–100.[Medline]
-
Hulboy DL, Rudolph LA, Matrisian LM. Matrix metalloproteinases as mediators of reproductive function. Mol Hum Reprod 1997; 3:27–45.[Abstract/Free Full Text]
-
Skinner SJM, Campos GA, Liggins GC. Collagen content of human amniotic membranes: effect of gestation length and premature rupture. Obstet Gynecol 1981; 57:487–489.[Abstract/Free Full Text]
-
Casey ML, MacDonald PC. Interstitial collagen synthesis and processing in human amnion–a property of the mesenchymal cells. Biol Reprod 1996; 55:1253–1260.[Abstract]
-
El Maradny E, Kanayama N, Halim A, Maehara K, Terao T. Stretching of fetal membranes increases the concentrations of interleukin-8 and collagenase activity. Am J Obstet Gynecol 1996; 174:843–849.[CrossRef][Medline]
-
Millar LK, Stollberg J, DeBuque L, Bryant-Greenwood GD. Fetal membrane distention: determination of the intrauterine surface area and distention of the fetal membranes preterm and at term. Am J Obstet Gynecol 2000; 182:128–134.[CrossRef][Medline]
-
Parry-Jones E, Priya S. A study of the elasticity and tension of the fetal membranes and the relation of the area of the gestational sac to the area of the uterine cavity. Br J Obstet Gynecol 1976; 83:205–212.[Medline]
-
Hieber AD, Corcino D, Motosue J, Sandberg LB, Roos PJ, Yu SY, Csiszar K, Kagan HM, Boyd CD, Bryant-Greenwood GD. The detection of elastin in the human fetal membranes: proposed molecular basis for elasticity. Placenta 1997; 18:301–312.[CrossRef][Medline]
-
Maehara K, Kanayama N, Maradny EE, Uezato T, Fujita M, Terao T. Mechanical stretching induces interleukin-8 gene expression in fetal membranes: a possible role for the initiation of human parturition. Eur J Obstet Gynecol Reprod Biol 1996; 70:191–196.[CrossRef][Medline]
-
Hopkin K. Turning on cells with a twist or a tug. J NIH Res 1996; 8:23–25.
-
Diatchenko L, Lau Y, Campbell A, Chenchik A, Moquadam F, Huang B. Suppression subtractive hybridization: a method for generating differentially regulated or tissue-specific cDNA probes and libraries. Proc Natl Acad Sci U S A 1996; 93:6025–6030.[Abstract/Free Full Text]
-
Nemeth E, Tashima LS, Yu Z, Bryant-Greenwood GD. Fetal membrane distention. I. Differentially expressed genes regulated by acute distention in amniotic epithelial (WISH) cells. Am J Obstet Gynecol 2000; 182:50–59.[CrossRef][Medline]
-
Nemeth E, Millar LK, Bryant-Greenwood GD. Fetal membrane distention. II. Differentially expressed genes regulated by acute distention in vitro. Am J Obstet Gynecol 2000; 182:60–67.[CrossRef][Medline]
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ABSTRACT
INTRODUCTION
