Human Reproduction vol.15 no.3 pp.578–583, 2000
Production of inhibin forms by the fetal membranes,
decidua, placenta and fetus at parturition
Simon C.Riley1,4, Rosemary Leask1, Claire Balfour1,
Janet E.Brennand2 and Nigel P.Groome3
1Department
of Obstetrics and Gynaecology, University of
Edinburgh, 37 Chalmers Street, Edinburgh, 2Department of
Obstetrics and Gynaecology, University of Glasgow, Glasgow and
3School of Biological and Molecular Sciences, Oxford Brookes
University, Oxford, UK
4To
whom correspondence should be addressed
Inhibins are regulators of paracrine and endocrine function
during pregnancy, but their intrauterine sites of secretion
are not well established. In amniotic fluid, inhibin A-,
inhibin B- and inhibin pro-αC-containing isoforms were
present in high concentrations, whereas in maternal serum,
inhibin A and pro-αC forms were present in high amounts,
with low concentrations of inhibin B. In fetal cord serum,
inhibin pro-αC was present in all samples, inhibin B was
detectable in male but not female fetuses, with no detectable
inhibin A in either sex. From cultured explants, both
inhibin A and B were secreted by chorion laeve, whereas
only inhibin A was secreted by placenta, with both tissues
secreting inhibin pro-αC. Only low concentrations of both
dimeric inhibins and pro-αC forms were secreted by decidua parietalis and amnion. The dual perfused placental
cotyledon secreted both inhibin A and pro-αC into maternal
perfusate, but only inhibin pro-αC into the fetal circulation
and less than to the maternal side. We conclude that
trophoblast is the predominant source of dimeric inhibins,
but with markedly different secretion depending on its
intrauterine location. There was a significant decrease in
inhibin A and pro-αC in amniotic fluid collected at term
active labour compared to elective Caesarean section (P <
0.001). This may reflect a local change in inhibin/activin
processing at labour, likely in chorion laeve trophoblast
cells, which may be important in the paracrine control of
the feto-maternal communication required to maintain
pregnancy and initiate labour.
Key words: fetal membranes/inhibin/labour/placenta/pregnancy/ trophoblast
Introduction
The inhibins are heterodimeric glycoprotein hormones of the
transforming growth factor-β (TGFβ) superfamily which were
initially described for their suppression of follicle stimulating
hormone (FSH) secretion (Qu and Thomas, 1995; Petraglia,
1997). They are made up of a common α-subunit and one
of two β-subunits, βA and βB. These give rise to two mature
578
32 kDa dimeric inhibins, inhibin A (α-βA) and inhibin B
(α-βB), while the activins are made of dimers of the β-subunits.
Other β-subunits have been recently reported but the formation
of novel inhibin forms and their roles are not well defined
(Fang et al., 1996). Towards the end of human pregnancy,
concentrations of both immunoreactive and bioactive inhibin
are present in increasing amounts in maternal serum
(Muttukrishna et al., 1995; Qu and Thomas, 1995; Petraglia,
1997; Fowler et al., 1998). All inhibin isoforms are present in
extracts of term human placenta, with the placenta as the
principal source of inhibin in the maternal circulation (de
Kretser et al., 1994). At term, the placenta, fetal membranes
and decidua express the mRNA and also the protein to the
α-subunit and both the βA- and βB-subunit forms as determined by in-situ mRNA analysis and immunohistochemistry
(Petraglia et al., 1991a, 1993). However, the likely sources
and secretion of the specific inhibin isoforms into the different
compartments of pregnancy, the amniotic fluid and fetal and
maternal circulation, and their role in the regulation of placental
and fetal membrane function are still not well defined
(Petraglia, 1997).
From in-vitro studies it appears that inhibins are involved
in the paracrine regulation of prostaglandin, human chorionic
gonadotrophin (HCG) and progesterone release (Petraglia
et al., 1989; Petraglia et al., 1993; Qu and Thomas 1995),
but how the different inhibin isoforms are involved in the
maintenance of pregnancy and subsequent initiation and
maintenance of labour is unclear. Measurement of inhibins
has proved difficult due to the lack of selectivity of the
α-subunit-directed immunoassays, which cannot measure the
bioactive dimeric isoforms and are confounded by the
presence of high concentrations of free α-subunit isoforms.
Furthermore, both immuno- and bioassays can be affected
by the presence of activins and the activin/inhibin binding
protein follistatin. Specific assays have been developed to
identify the two dimeric inhibins A and B (Groome et al.,
1994, 1996) and inhibin isoforms containing pro and αC
forms which are predominantly free α-subunit (Groome
et al., 1995; Robertson et al., 1997). In this study we have
measured the dimeric and α-subunit forms of inhibin in the
different compartments of pregnancy at term before and at
labour. We have identified using in-vitro techniques the
principal tissues that secrete inhibins within the uterus. Our
findings have revealed that inhibin production is tissuedependent with directional secretion, resulting in the different
bioactive isoforms secreted into different compartments in
late gestation. These findings are important in elucidating
the regulation and potential roles of the different inhibin
© European Society of Human Reproduction and Embryology
Intrauterine sites of inhibin secretion at parturition
isoforms for the paracrine and endocrine control of pregnancy
and parturition.
(iv) lactate concentrations remained within the normal low range (⬍2
mmol/l; Benediktsson et al., 1997) throughout the sampling period,
and (v) there were no significant morphological changes on subsequent
histological examination.
Materials and methods
Collection of fluid and tissue samples
Matched samples of amniotic fluid and maternal and cord serum were
obtained from women undergoing elective Caesarean section (n ⫽
15; 38–40 weeks; dated from last menstrual period) and after
spontaneous vaginal delivery (n ⫽ 15; 38–41 weeks; not associated
with an intrauterine infection by clinical assessment, although no
subsequent histology was performed) at term. Immediately after
delivery by elective Caesarean section, samples of fetal urine (n ⫽
8; only clear samples that were uncontaminated with blood were
retained) at the first micturition and lung fluid (expired immediately
after delivery; n ⫽ 4) were collected from the neonate. All samples
were centrifuged (1000 g) and stored at –20°C prior to immunoassay
for the different inhibin isoforms. For explant tissue culture, placentae
with attached fetal membranes were collected from women undergoing
elective Caesarean section at term (⬎37 weeks; not associated with
labour). Decidual tissue was dissected from the myometrial aspect of
the posterior uterine wall, from a site away from the placental bed,
after delivery of the placenta. For placental perfusion, placentae were
collected immediately after spontaneous vaginal delivery at term (n ⫽
6). Ethical approval for the collection of all fluid and tissue samples
was obtained from the Lothian Trust Ethical Committee with the
informed and written consent of patients.
Explant tissue culture
Explants of tissues were cultured as described previously by this
laboratory (Brennand et al., 1995). Discs of amnion (12 mm diameter;
wet weight 10–20 mg) and chorion laeve with adherent decidua
(9 mm diameter; 15–25 mg) were prepared using a cork borer, and
pieces of villous placental tissue (collected from the middle of a
central cotyledon; 20–30 mg) or decidua (20–30 mg) from the
maternal aspect by curettage were placed on absorbent capillary
matting. Tissues were maintained in culture medium (RPMI1640;
Life Technologies Ltd, Paisley, UK) supplemented with 10% fetal
calf serum (Life Technologies) plus antibiotics in a water saturated
air/5% CO2 atmosphere at 37°C. After 24 h, culture medium was
collected and stored at –20°C prior to immunoassay.
Dual perfusion of placental cotyledon
The in-vitro isolated dual perfused human placental cotyledon was
used, as described previously (Schneider et al., 1972) with minor
modifications (Benediktsson et al., 1997). Within 15 min of delivery,
a peripheral intact cotyledon with parallel chorionic artery and vein
and also the maternal lacunae were cannulated and perfused with
Krebs’ solution, which was supplemented with dextran (20 g/l; 74
kDa) in the fetal perfusate. To mimic partial pressures in vivo, the
solutions used to perfuse the maternal and fetal circulations were
gassed with 95% O2/5% CO2 and 95% N2/5% CO2, respectively. The
cotyledon was perfused for 40 min to flush out residual blood and
allow circulatory perfusion pressures to stabilize. Perfusates were not
recycled. Samples (10 min) were collected after this equilibration
period and stored at –20°C prior to assay. Viability and integrity of
each preparation was assessed by establishing (i) the input perfusion
rate in both fetal (6 ml/min) and maternal (10 ml/min) circuits
equalled outflows, (ii) there was adequate exchange of O2 from the
maternal to fetal circuits (step-up of pressure from ⬍40 to ⬎70
mmHg on fetal side), (iii) at the end of the experiment, the fetal
vasculature responded to a bolus of noradrenaline (20 mg; Sigma),
Immunoassays for inhibin isoforms
The inhibin assays used were two-site enzyme-linked immunosorbent
assay (ELISA). These assays have been previously described, characterized and their specificities and cross-reactivities detailed (Groome
et al., 1994, 1995, 1996) and were performed with minor modifications
(Riley et al., 1996; Wallace et al., 1997). Immunoassay plates (96well; Nunc Maxisort, Life Technologies Ltd) were coated passively
with specific mouse monoclonal capture antibodies raised to peptide
sequences of the inhibin βA (antibody E4) and βB (C5) subunits and
the pro- portion of the inhibin-α subunit (INPRO) which conferred
the assay specificity to inhibin A-, inhibin B- and pro- and αCcontaining inhibin immunoreactivity. Plates were dried using a dry
coating reagent and stored at 4°C (diluted 1:1 with H2O; Bionostics
Ltd, Wyboston, UK). For the inhibin A and B assays samples and
standards were diluted as appropriate with fetal calf serum, incubated
in sodium dodecyl sulphate (2% final volume) for 3 min at 100°C
which improves the signal and removes false positive results. Samples
were then pretreated with H2O2 (1% final volume) for 30 min at
23°C, which increases antibody reactivity by modification of the βsubunit epitopes (Knight and Muttukrishna, 1994), prior to adding
to the appropriate ELISA plate. In the assay for pro-αC inhibin
immunoreactivity all dilutions were performed in triton assay diluent
and samples were applied directly to the plate without pretreatment
(Groome et al., 1995).
The standard used for the inhibin A and inhibin B assay is a
partially immunopurified inhibin standard from human follicular fluid
calibrated against recombinant 32 kDa human inhibin A or inhibin
B (Genentech, CA, USA) with results expressed in terms of this
recombinant form. These assays have been validated for amniotic
fluid (Riley et al., 1996; Wallace et al., 1997) and plasma samples
(Groome et al., 1994; Robertson et al., 1997). For the assay for
inhibin forms containing pro- and αC immunoreactivity the capture
antibody (INPRO) was raised against a sequence of the pro-αC
subunit, with a highly purified preparation of pro-αC inhibin as
standard, as described previously (Groome et al., 1995). The pro-αC
assay predominantly measures the 36 and 29 kDa forms (Robertson
et al., 1997), although it may possess some cross-reactivity with
larger forms of dimeric inhibin containing the α-subunit pro sequence
as indicated by immunoblot (Groome et al., 1995). All assays utilized
the same detection antibody (R1), the Fab fraction raised against the
N-terminal region of the 20 kDa inhibin-α subunit which is conjugated
to alkaline phosphatase. This alkaline phosphatase activity was
detected in the inhibin A and pro-αC inhibin assays by addition of the
substrate p-nitrophenylphosphate (Kirkegaard and Perry Laboratories,
Gaithersburg, MA, USA), then measuring absorbancy at 405 nm
using a microplate reader (Molecular Devices Corp., Menlo Park,
CA, USA) with integrated software (Softmax; Molecular Devices),
or for inhibin B using an amplification kit (Life Technologies Ltd)
and measuring absorbancy at 490 nm. The assay sensitivity, and
intra- and inter-plate coefficients of variation were: for inhibin A, 7
pg/ml and 6% and 8%; for inhibin B, 15 pg/ml and 8% and 10%;
and for inhibin pro-αC, 3 pg/ml and 4% and 6%, respectively.
Data analysis
All sample sets had a normal distribution and statistical differences
were assessed using Student’s t-test. Differences were recognized as
significant when P ⬍ 0.05.
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S.C.Riley et al.
Table I. Concentrations of inhibin isoforms containing pro and αC in amniotic fluid and maternal serum
Amniotic fluid
Maternal serum
Inhibin A (pg/ml)
Inhibin B (pg/ml)
Pro-αC inhibin (pg/ml)
CS
SVD
CS
SVD
CS
SVD
1381 (180)
915 (109)
951* (106)
1216 (236)
1695 (484)
30 (4)
1333 (199)
35 (5)
3213 (345)
1085 (132)
1155** (194)
1211 (159)
Values are mean ⫾ SEM; n ⫽ 15 in each group.
Amniotic fluid and maternal serum were collected before the onset of labour at elective Caesarean section
(CS) and at delivery after spontaneous onset of labour (SVD).
*Significantly (P ⬍ 0.05) lower at delivery compared with prior to the onset of labour.
**Significantly (P ⬍ 0.001) lower at delivery compared with prior to the onset of labour.
Figure 1. Concentrations (pg/ml; n ⫽ 15) of inhibin A (round
symbols), inhibin B (square symbols) and inhibin isoforms
containing pro and αC (triangular symbols) in amniotic fluid
collected before the onset of labour at elective term Caesarean
section (cs; open symbols) and at spontaneous delivery at term
(svd; solid symbols). Significantly (*P ⬍ 0.05; **P ⬍ 0.001) lower
at delivery compared to elective Caesarean section prior to the
onset of labour.
Results
Amniotic fluid
Inhibin A, inhibin B and inhibin containing pro and αC
immunoreactivity were all present in a similar range of
concentrations in samples of amniotic fluid (Figure 1 and
Table I). There were significant decreases in forms containing
pro-αC inhibin (P ⬍ 0.001) and inhibin A (P ⬍ 0.05) present
at the time of delivery when compared to samples collected
at elective Caesarean section. There was no significant change
in inhibin B between these times.
Maternal serum
Inhibin A was present in maternal plasma in much higher
amounts than inhibin B (Table I). Inhibin containing pro and
αC isoforms was also present in high concentrations in
maternal serum. There were no significant differences in the
580
Figure 2. Concentrations (pg/ml) of inhibin A, inhibin B (square
symbols) and inhibin isoforms containing pro and αC (triangular
symbols) in umbilical cord blood collected at term (elective
Caesarean section and spontaneous delivery samples combined)
from male (open symbols; n ⫽ 17) and female fetuses (solid
symbols; n ⫽ 13). Note different scales for dimeric inhibins and
pro-αC-containing forms. nd represents none detected as below
sensitivity of assay.
concentrations of inhibin A or pro-αC inhibin between samples
collected before the onset of labour at elective Caesarean
section or after active labour and delivery. There were no
differences in immunoreactive pro-αC inhibin or in inhibin A
according to fetal sex.
Fetus: cord serum, fetal urine and expirated lung fluids
The concentrations of inhibin isoforms in cord serum are
shown in Figure 2. There were no differences between concentrations of inhibins in cord serum from fetuses of the same
sex collected before the onset of labour at elective Caesarean
section and after labour, so these groups were combined
(Figure 2). Immunoreactive pro-αC inhibin was present in
cord serum from both male (2689 ⫾ 276 pg/ml; mean ⫾
SEM; n ⫽ 17) and female (2674 ⫾ 379; n ⫽ 13) fetuses at
similar concentrations with no significant differences between
sexes. These concentrations in cord blood are similar to those
present in maternal serum. Inhibin B was only detectable in
cord serum from male fetuses (110 ⫾ 11 pg/ml; mean ⫾ SEM;
Intrauterine sites of inhibin secretion at parturition
Figure 3. Secretion (output expressed as pg/mg wet weight tissue/
24 h; mean ⫾ SEM; n ⫽ 6 different placentae) of inhibin A (open
bars), inhibin B (solid bars) and immunoreactive inhibin containing
pro and αC (hatched bars) by explants of amnion, chorion laeve
with adherent decidua parietalis, decidua parietalis (collected from
the myometrial aspect) and villous placenta obtained at elective
Caesarean section at term and maintained in culture for 24 h.
n ⫽ 17), but was undetectable in serum collected from females.
No dimeric inhibin A was detectable in cord serum from either
sex. In fetal urine (n ⫽ 8), no inhibin A, inhibin B or pro-αC
immunoreactivity were detectable. In fetal lung fluids (n ⫽
4), inhibin A, inhibin B and pro and αC immunoreactive
inhibin isoforms were present in similar amounts to those
found in the respective matched amniotic fluid sample (data
not shown). This is probably not surprising as this fluid is
made up principally of amniotic fluid inhaled during fetal
breathing movements.
Secretion of inhibins by explants of placenta, amnion, chorion
laeve and decidua parietalis
The placenta and chorion laeve were the tissue explants which
secreted the greatest amounts of both dimeric and pro-αC
subunit forms of inhibin into culture medium (Figure 3).
Placental explants secreted only inhibin A in large concentrations with barely detectable concentrations of inhibin B,
whereas explants of chorion laeve secreted inhibin A and also
inhibin B in high concentrations. Both placenta and chorion
laeve secreted large amounts of pro-αC inhibin forms. Decidua
parietalis and amnion secreted only low amounts of inhibin
A, inhibin B and pro-αC isoforms into culture medium.
Secretion of inhibins by the dual perfused placental cotyledon
In the maternal perfusate, dimeric inhibin A was present, with
no detectable dimeric inhibin B (Figure 4). In the perfusate
collected from the fetal circulation, no dimeric inhibin A or
inhibin B were detectable. Inhibin isoforms containing proαC immunoreactivity were secreted into both the maternal and
fetal output, with a significantly (P ⬍ 0.05) greater output in
the maternal compared to the fetal effluent.
Figure 4. Secretion (pg/min/cotyledon; mean ⫾ SEM; n ⫽ 6
different placentae) of inhibin A (A; open bars), inhibin B (B; solid
bars) and inhibin isoforms containing pro and αC (pro-αC; hatched
bars) into the maternal and fetal circulations of the in-vitro isolated
dual perfused placental cotyledon preparation. nd represents none
detected as below sensitivity of assay.
Discussion
This study has demonstrated that there are several different
sites of secretion of bioactive dimeric inhibin A and inhibin
B isoforms, and inhibin containing pro and αC isoforms, with
specific patterns of secretion in the amniotic fluid and maternal
and fetal circulations. Furthermore, inhibin A and inhibin pro
and αC decrease in amniotic fluid with the onset of labour.
Both dimeric inhibin forms are present in similar concentrations
in amniotic fluid whereas only inhibin A is the predominant
form in the maternal circulation. Trophoblast tissues are the
likely main source of dimeric inhibins. Placental trophoblast
secretes inhibin A, while trophoblast of the chorion layer of
the fetal membranes secretes inhibin A and inhibin B in similar
amounts, apparently unidirectionally into the amniotic fluid.
In the fetus, secretion of dimeric inhibin B into the fetal
circulation is sex specific with the testis as the likely source.
The source of inhibin pro-αC-containing forms in the fetal
circulation may be due, at least in part, to secretion by the
placenta. No inhibin A is present in the fetal circulation.
The predominant source of the inhibin A and inhibin B in
amniotic fluid is likely trophoblast of chorion laeve of the
fetal membranes. The main sources of amniotic fluid production
are via the fetal membranes and fetal urine, with some
contribution from the fetal lung (Gilbert and Brace, 1993).
Chorion explants secrete similar amounts of inhibin A and B
and this is the same pattern of secretion in amniotic fluid.
Although no attempt was made to separate adherent decidua
from the chorion explants, this presence of trophoblast cells
would seem to be the determinant of secretion of dimeric
inhibins. This is because decidua parietalis obtained from the
myometrial aspect, which contains little contamination by
trophoblast cells, only secreted low concentrations of inhibin
isoforms. In decidua parietalis, mRNA to the β-subunit isoforms is expressed and this probably results in mostly activin
secretion (Petraglia et al., 1990, 1997). There is no secretion
581
S.C.Riley et al.
of inhibins via the fetal urine. The fetal lung cannot be
discounted as a potential source, but lung fluids obtained at
first expiration after delivery contained similar concentrations
to those found in the amniotic fluid, as would be anticipated, as
it largely contains amniotic fluid. The source of immunoreactive
pro-αC inhibin isoforms is less easy to define and may be
secreted from several sites, including the amnion, chorion
laeve and decidua parietalis. The fetus may also be a site of
production of pro-αC inhibin, which is present in the fetal
circulation although the mechanism of secretion into amniotic
fluid is unclear but would probably be via the lung. These
results are consistent with findings in the first trimester (Riley
et al., 1996, 1998) where dimeric inhibins A and B and proαC isoforms and activin A are present in extra-embryonic
coelomic fluid, with pro-αC but no dimeric inhibin in amniotic
fluid. This indicates that the trophoblast layer is the principal
site of dimeric inhibin production at this stage. It is only when
the amnion fuses to the chorion laeve with the loss of the
extra-embryonic coelom, and the amniotic cavity becomes the
sole intrauterine compartment, that dimeric inhibins are present
in the amniotic fluid and increase with gestation (Wallace
et al., 1997). This unidirectional secretion of inhibin B secretion
into the amniotic fluid but not into maternal serum is similar
to that of prolactin, which is produced by decidua parietalis
but is only present in amniotic fluid (Rosenberg et al., 1980),
demonstrating that protein hormones can be directed into
amniotic fluid.
In the maternal circulation, high concentrations of inhibin
pro and αC and inhibin A are present, with low concentrations
of inhibin B. This is in the same range of concentrations as
reported previously (Fowler et al., 1998). The likely site of
secretion of inhibin A and inhibin pro-αC is the syncytiotrophoblast of the placenta. Protein and mRNA to α and βA subunits
are localized at this site (Petraglia et al., 1992; Petraglia, 1997)
and placental trophoblast cells secrete inhibin A (Keelan et al.,
1994; Qu and Thomas, 1995). This secretion of inhibin A by
placenta is unidirectional into maternal blood, whereas inhibin
pro-αC is secreted in both directions. The ovary may also
remain a source of circulating inhibins at term (Kettel et al.,
1991; Illingworth et al., 1996b), and also the adrenal gland
(Spencer et al., 1992; Munro et al., 1999). The chorion laeve
of the fetal membranes may contribute to the low concentrations
of inhibin B found in the maternal circulation at term.
In the fetal circulation dimeric inhibin B, which is the
important isoform in the male (Illingworth et al., 1996a), was
detected in cord blood from male but not female fetuses,
confirming our previous findings (Wallace et al., 1997). Immunoreactive inhibin containing pro-αC was present in similar
amounts in cord serum from both fetuses of both sexes,
although no inhibin A was detectable in cord serum from
fetuses of either sex. Our current findings are in agreement
with the reports using the non-selective assays for free αsubunit which demonstrated the presence of inhibins in cord
serum (Massa et al., 1992; Billiar et al., 1995; Rombauts
et al., 1996). Billiar et al. (1995) reported that there was no
dimeric inhibin present although their assay was not able to
detect inhibin B as detected in this and previous studies
(Wallace et al., 1997). In the fetus, only the steroidogenic
582
tissues of the testis and adrenal gland express the α-subunit
mRNA, whereas the mRNA to both the βA and βB subunits is
expressed in multiple organ systems (Rabinovici et al., 1991;
Jaffe et al., 1993; Tuuri et al., 1994), possibly reflecting activin
secretion. Therefore the fetal testis, not the adrenal gland, is
the likely source of dimeric inhibin B as it is specific to the
male fetus. Immunoreactive pro-αC inhibin detected in the
fetal circulation may be due, at least in part, to placental
secretion as shown by output into this circuit in the perfused
placental cotyledon. The fetal adrenal gland also expresses
mRNA to inhibin α (Munro et al., 1999) and may be a source
of pro-αC inhibin in cord serum, irrespective of fetal sex.
These studies provide evidence for a decrease in local αsubunit production within the fetal membranes at parturition,
with the reduction of both inhibin pro-αC and also inhibin A
in amniotic fluid at spontaneous delivery. The likely site for
this control is the trophoblast cells of chorion laeve, which
secrete both bioactive dimeric inhibin isoforms. The placenta
secretes only inhibin A in significant amounts into the maternal
circulation with no changes at labour. However, the role of
inhibins in the regulation of labour is not well defined. They
are probably diverse due to their secretion into both maternal
circulation with potential for an endocrine role, as well as
being local regulators at the feto-maternal interface. Inhibins
have a diverse range of functions which are important in the
paracrine signalling to initiate labour including control of
steroidogenesis (Petraglia et al., 1987) and peptide hormone
and prostaglandin secretion (Petraglia et al., 1993; Qu and
Thomas, 1995) and monocyte chemotaxis (Petraglia et al.,
1991b). The presence of the novel inhibin receptors (Hertan
et al., 1999) in the uterus remains to be defined, but inhibins
may modulate activin action at the level of activin receptors
(Martens et al., 1997) which are expressed by placenta, fetal
membranes and decidua (Petraglia et al., 1997). Local decreases
in inhibin A and α-subunit production altering the balance of
inhibins and activins may modulate paracrine mechanisms
involved in the timing of the onset and for maintaining the
cascade of stimuli required for progression to delivery.
Acknowledgements
The authors wish to thank Dr David C.Howe for collecting the
decidual samples for culture, Dr Rafn Benediktsson and Mr Alistair
Greystoke for help with the placental perfusion and Mr Tom McFetters
for help with the illustrations.
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Received on August 23, 1999; accepted on November 19, 1999
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