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Regular Articles |
From the Academic Department of Obstetrics and
Gynaecology,*
University College London, London; and the
Department of Anatomy,
University of
Cambridge, Cambridge, United Kingdom
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Introduction |
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Attempts to address the question using Doppler ultrasound techniques have obtained conflicting results,8-10 and it is clear that the debate will not be resolved while discussion focuses on issues such as the relative sensitivities of ultrasound equipment or the precise localization of recordings from within the placenta. We have therefore adopted a physiological approach. One of the major implications of the new theory is that the oxygen tension within the early feto-placental unit will be considerably lower than previously anticipated, rising with the onset of maternal blood flow. We tested this prediction by measuring the oxygen concentration within the placental intervillous space with a highly accurate and sensitive multiparameter probe at different gestational ages.
Aerobic metabolism is inextricably associated with the generation of
reactive oxygen species, and the rate of their formation is
proportional to the prevailing oxygen tension.11
These
species are potentially extremely hazardous and so a complex system of
antioxidant defenses has evolved to meet this challenge (Figure 1)
. To gain further evidence of the
oxygen concentration at the cellular level we therefore assayed the
mRNA concentration and activity of the principal antioxidant enzymes
catalase, glutathione peroxidase (GPX), and copper/zinc and manganese
superoxide dismutases (SOD) within placental tissues at different ages.
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Materials and Methods |
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Measurement of intrauterine gases was performed in 30 healthy women undergoing surgical termination of pregnancy under general anesthesia for psychosocial reasons at 7 to 16 weeks of gestation. All women gave their written informed consent to participate in this study which had been approved by the University College London Hospitals Committee on the Ethics of Human Research.
Oxygen Measurements
The Paratrend 7 monitoring system with automated sensor
calibration (Diametrics Medical, St. Paul, MN) contains a
sterile electrochemical sensor (Clark electrode) for measuring oxygen
concentrations (Figure 2)
. It is
calibrated with three precision gases supplied by the manufacturer, and
has a range of 0 to 120 mmHg and 95% confidence intervals of ±1 mmHg.
The 0 to 90% response time is <150 seconds.
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. Maternal peripheral-arterial blood saturation was evaluated continuously by pulse oximetry with a probe applied to a finger.
Collection and Processing of Experimental Tissue
After the probe measurements, placental tissue was obtained under ultrasound guidance using a chorionic villous sampling procedure to minimize tissue damage and contamination. Villous tissue was immediately washed in ice-cold saline and dissected away from associated blood clots, membranes, and maternal tissues. Samples were either snap-frozen in liquid N2 and stored at -80°C, fixed immediately for 2 hours at room temperature in 4% formaldehyde in 0.1 mol/L Pipes buffer (pH 7.0), or fixed in 2% glutaraldehyde in 0.1 mol/L Pipes buffer.
Anti-Oxidant Enzyme Activity Assays
Frozen villous tissue was thawed on ice and 100 mg wet weight dissected away. Tissue was homogenized in 50 mmol/L sodium phosphate buffer (pH 7.4), 1 mmol/L phenylmethylsulfonyl fluoride, 0.05% Triton X-100, and a soluble fraction component was prepared by centrifugation at 20,000 x g for 30 minutes at 4°C. The supernatant was removed and the pelleted material washed using the phosphate buffer. After further centrifugation the two supernatants were combined, mixed, and assayed immediately. Protein concentrations were determined using the Lowry method and all concentrations adjusted to 2 mg/ml. All assays were repeated 12 times and the mean recorded. Assays of antioxidant activities in the pelleted material indicated that a maximum of 5% of total activities was retained in this fraction.
Total SOD Activity
The final assay mixture comprised the following: 65 mmol/L sodium
phosphate (pH 7.8), 25 µmol/L nitroblue tetrazolium, 100 µmol/L
hypoxanthine, 0.02 U/ml xanthine oxidase, and 1, 2, 5, or 10 µg of
homogenate protein. The reaction was allowed to equilibrate at 37°C
for 30 seconds and then absorbance change at 570 nm was monitored for 2
minutes. Relative activities are expressed as 
OD/minutes/mg protein.
Catalase Activity
The final assay mixture contained 50 mmol/L sodium phosphate (pH
7.0), 12 mmol/L hydrogen peroxide and 2, 5, or 10 µg of homogenate
protein. The disappearance of hydrogen peroxide was followed
spectrophotometrically at 230 nm at 25°C. Rates are expressed as
OD/minutes/mg protein. The addition of catalase inhibitor
3-amino-1,2,4-triazole, pre-incubated for 2 minutes with homogenate at
25°C, resulted in rates which barely registered on the
spectrophotometric output.
GPX Activity
Cellular activity was assayed using a kit
(Calbiochem-Novabiochem). In brief, the final reaction mixture
comprised the following assay buffer (pH 7.6) 0.2 mmol/L NADPH, 1
mmol/L glutathione, 0.4 U/ml glutathione reductase, 0.22 mmol/L
tert-butyl hydroperoxide and 5, 10, or 20 µg homogenate
protein. After a pre-incubation of 30 seconds, utilization of NADPH at
37°C was followed spectrophotometrically (340 nm) for 2 minutes, and
rates are expressed as OD/minutes/mg protein. Two types of control
blanks were performed using water instead of homogenate or water
instead of tert-butyl hydroperoxide.
Reduced Glutathione Assay
Concentrations were determined using a kit (Calbiochem-Novabiochem, Nottingham, UK). Placental tissue was minced in ice-cold 5% metaphosphoric acid, and homogenized using a tight-fitting Teflon pestle. The homogenate was centrifuged at 3,000 x g for 10 minutes at 4°C. The final assay mixture contained: assay buffer (200 mmol/L potassium phosphate (pH 7.8) containing 0.2 mmol/L diethylene triamine penta-acetic acid, 0.025% Lubrol), 0.6 mmol/L chromogenic reagent, 1.5% w/v sodium hydroxide, and 5, 10, or 20 µl of homogenate supernatant. The reaction mixture was incubated at 25°C for 10 minutes in the dark and the absorbance then measured at 400 nm. The assay was made quantitative using a standard curve with reduced glutathione concentrations between 5 and 100 µmol/L. All calculations for glutathione concentration were performed after subtraction of readings for buffer controls.
Northern Blots
Total RNA was prepared from 11 samples ranging in gestational age from 7 to 14 weeks using Trizol reagent (Gibco BRL, Paisley, UK) according to the manufacturer’s protocol. Samples of RNA (15 µg) were electrophoresed on a 0.8% formaldehyde/agarose gel and transferred onto positively charged nylon membrane (Roche Molecular Biochemicals, Lewes, UK). The cDNA of the antioxidant enzymes catalase, GPX, Cu/ZnSOD, and MnSOD were subcloned from PCR Blunt (Invitrogen, Groningen, The Netherlands) into pGEM-4Z vector (Pharmacia Biotech, Uppsala, Sweden). Blots were hybridized with the 32P-labeled cDNA probes separately. The plasmids were prepared with a Plasmid Isolation Kit (Qiagen, Crawley, UK). An 18S ribosomal RNA probe was used to normalize loading. The membranes, which were washed to high stringency, were put into a phosphor-imaging cassette with a storage screen. The image was detected using the Storm Imaging System (Molecular Dynamics, Uppsala, Sweden).
Immunohistochemical Staining of Inducible Hsp 70 and Nitrotyrosines
Tissue fixed in paraformaldehyde was embedded in paraffin wax and sectioned at 5 µm. Sections were dewaxed in xylene, rehydrated, treated with 0.1% H202 for 30 minutes, rinsed in phosphate-buffered saline (PBS), blocked for 20 minutes in 1.5% goat serum/PBS, and incubated for 2 hours at room temperature with the primary antibodies; rabbit anti-nitrotyrosine (2 µg/ml; Chemicon, Temecula, CA), and rabbit polyclonal anti-Hsp 70 inducible form (1 µg/ml; StressGen, York, UK). Further processing for colorimetric detection was according to the instructions for the Vectastain ABC kit (Vector Laboratories, Burlingame, CA). Negative controls were performed alongside substituting nonimmune rabbit serum for primary antibody against Hsp 70i. For nitrotyrosines the primary antibody was pre-incubated with 10 mmol/L 3-nitro-L-tyrosine overnight.
For quantification of Hsp 70i expression adjacent sections were
incubated with a fluorescein isothiocyanate-labeled secondary
anti-rabbit antibody. After blinding for gestational age they were
viewed with a Leica true confocal scanner-Windows NT confocal
microscope using a x40 objective, a 488-nm excitation wavelength and a
530/30 band-pass filter for emitted light. The pinhole setting was 1
Airey-disk equivalent. Laser power, the acousto-optic threshold filter
setting, and detection photomultiplier tube gain remained constant
throughout. Under these conditions the thickness of the confocal
optical section is 
0.5 µm. This was positioned within the
thickness of the 5-µm physical section, thus eliminating the effects
of variation in tissue section thickness. To avoid bleaching, five
randomly selected fields of view per section were rapidly saved for
later analysis. Quantitative assessment was achieved using the Leica
true confocal scanner-Windows NT quantification software to assess mean
fluorescent intensity measurements along randomly positioned lines
projected through the longest axis of the syncytiotrophoblast or the
stromal core. The mean of these values was then recorded for each
placenta.
Electron Microscopy
Samples were secondary fixed in 1% osmium tetroxide in Pipes buffer for 1 hour at room temperature and embedded in Araldite epoxy resin. Ultra-thin sections (150 nm) were counterstained with uranyl acetate followed by lead citrate, and viewed using a Philips CM100 microscope (FEI/Philips, Eindhoven, The Netherlands). For each placenta at least 20 mitochondria were selected at random by viewing areas of syncytiotrophoblast immediately adjacent to grid bars, and the volume fraction of the intracristal space was estimated by point counting.12
Statistics
The data were analyzed with a statistics package (Statview, SAS Institute, Cary, NC), and results were considered statistically significant at P < 0.05. Relationships between variables were first tested by correlation analysis, and if significant, were explored further using bivariate scatterplots and the LOWESS (locally weighted scatterplot smoother) technique.
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Results |
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The tip of the probe could be identified at all times using ultrasound, allowing for accurate positioning of the sensors. No acute complication was associated with the use of the Paratrend probe in the placenta or decidua. In particular, insertion of the needle and sensor did not appear to damage the tissues as shown by an absence of bleeding and/or any echographic signs of hematoma. The probe however proved to be delicate, and was damaged on four occasions when entering the placenta and on a further eight when moving from the placenta into the decidua.
Once inserted into the area of interest the measurements stabilized rapidly. The mean coefficient of variation (SD/mean x 100) of the measurements during the 5 to 8 minutes of monitoring was 3.6% for the placenta, and 3.3% for the decidua. For technical reasons relating to the size of the probe, and the size and delicacy of the placental tissues, it was not possible to obtain reliable measurements before 8 weeks of gestation.
Maternal arterial blood oxygen saturation monitored by the finger-tip clip remained constant during the procedure, with values ranging between 96 and 99% (mean, 97.5%; SD ± 0.7), confirming that the patients were well ventilated throughout.
Probe Measurements
There was a highly significant positive correlation between the
mean PO2 recorded within the intervillous space
and the gestational age (n = 26,
r = 0.80, P < 0.001). However, a
bivariate scattergram illustrated that the data fell into two sets, an
early set including weeks 7 to 11 and a later set including weeks 12 to
15. If each data set was analyzed separately no correlation existed
(n = 14, r = 0.46,
P > 0.05 and n = 12, r
= 0.02, P > 0.05 for the early and late values,
respectively). In view of this finding a sigmoid curve was fitted to
the data using the LOWESS technique with a tension of 60 (Figure 3)
. This analysis indicated that the
oxygen tension within the placenta rises steeply during weeks 10 to 12
of pregnancy.
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. This pattern confirms that more maternal blood enters the placenta
from the endometrium as pregnancy advances rather than there being
significant changes in maternal oxygenation. Antioxidant Defenses
GPX activity within the placental tissues showed a strong positive
correlation with gestational age (Table 1)
, with activity increasing slowly until
the end of the tenth week and faster thereafter (Figure 4a)
. A similar pattern was observed in
the concentration of reduced glutathione although these data showed
considerable scatter at early ages (Table 1
and Figure 4b
). Catalase
activity also correlated with gestational age (Table 1)
, but in this
case activity rose steadily and then plateaued at 12 weeks (Figure 4c)
. Although total SOD activity demonstrated a trend toward increased
activity with age, this failed to reach statistical significance (Table 1
and Figure 4d
).
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. However, although the concentration of reduced glutathione
seemed to increase with rising oxygen tensions this trend failed to
reach statistical significance. mRNA Concentrations
A representative blot is shown in Figure 5
. For statistical analysis the samples
were divided into two groups according to gestational age; before and
after 10 weeks, respectively. Mean gestational age in the first group
was 56 days (n = 5, SD = 9.3) and in the
second group was 92.7 days (n = 6, SD =
6.7). Densitometry readings relative to 18S ribosomal RNA revealed that
the mRNA concentrations for catalase, Cu/ZnSOD and GPX all increased
with gestational age (t = 2.51,
P = 0.033; t = 2.60, P
= 0.029 and t = 2.21, P = 0.05,
respectively). Two bands were observed for MnSOD at 1 kb and 4 kb. No
change was observed in the former, and although it appeared that there
was an increase in the latter the difference failed to reach
statistical significance (t = 1.56,
P = 0.15).
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Expression of Hsp 70i was extremely low in the earliest 6- to
7-week tissue (Figure 6a)
, but showed a
marked increase in 8- to 9-week villi with particularly strong
immunolabeling in the syncytiotrophoblast and endothelial cells (Figure 6b)
. Thereafter expression seemed to decline in tissues of older
gestational age and became uniform between the different cell types
(Figure 6c)
. All negative controls demonstrated minimal background
staining (Figure 6d)
. Two runs of the quantification procedure were
performed because of the physical limitations imposed by immunolabeling
and viewing a large number of sections under the same conditions. Each
run contained a complete range of ages, although the second
concentrated on samples from 7 to 10 weeks. The data for each run are
therefore internally consistent, but the two runs are not strictly
comparable because of possible variations in labeling conditions or
microscope laser power. The two data sets are therefore plotted as
scattergrams (Figure 7)
. In each case low
levels of expression were observed in the trophoblast of the earliest
samples. However, there was a sharp peak of fluorescence intensity in
samples from 8 to 9 weeks of gestational age, with later samples
showing intermediate levels of expression. Values for the stroma were
consistently lower than those for the trophoblast layer, but followed a
similar pattern.
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A generally similar pattern of immunolabeling was observed for
nitrotyrosine residues as for Hsp 70i, with a marked contrast between
gestational ages. Residues were virtually undetectable in the earliest
6- to 7-week tissues, were increased throughout the 8- to 9-week villi,
but reduced again at 10 to 12 weeks (Figure 6, e–g)
. Controls using
the pre-incubated primary antibody displayed minimal staining (Figure 6h)
.
Mitochondrial Morphology
In the 6-week tissue the mitochondrial profiles were smoothly oval
or circular in shape, and the cristae were clearly visible as a series
of classically arranged interdigitating folds. The matrix was
moderately electron-dense (Figure 8a)
. At
9 to 10 weeks the appearances were very different. The profiles
appeared smaller in area, and more irregular in outline. Fewer cristae
were visible, and often these were in the form of a single circle
within the matrix. The intracristal space within the cristae was
substantially dilated (Figure 8b)
. By 12 to 14 weeks cristae could be
clearly defined again although they were more irregularly arranged than
in the early specimens (Figure 8c)
. Quantification confirmed that the
percentage volume of the organelles occupied by intracristal space
showed an initial rise coinciding with the Hsp 70 expression, and a
subsequent decline as gestational age increased (Figure 8d)
. A
second order polynomial regression showed these changes to be
statistically significant (F = 3.94,
P < 0.05).
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Discussion |
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Our data are consistent with classical morphological descriptions of early placental development. These state that during implantation the trophoblast invades into capillaries and veins within the superficial endometrium, and that maternal erythrocytes are present in the precursors of the intervillous space. This represents a capillary circulation only, however, and direct connections between the spiral arteries and the intervillous space cannot be observed before the ninth week of gestation.17,18 Before that stage the distal segments of the arteries are occluded by aggregates of cytotrophoblast cells derived from the developing cytotrophoblastic shell and the villous cell columns.4,19 Arterial communication with the enlarging intervillous space is therefore restricted to a network of narrow intercellular spaces, ensuring that any flow is sluggish or restricted to a plasma filtrate. Both these phenomena could account for the low oxygen tension measured within the placenta on account of the small quantity of oxygen they deliver. The loosening of some of the arterial plugs at around 10 weeks allows maternal blood to enter the intervillous space more freely, accounting for the change in flow patterns detected with Doppler ultrasound at the end of the first trimester.7,9 As a result the total quantity of oxygen delivered to the placenta will rise, leading to an increase in the PO2 within the intervillous space.
Although it is clear the steepest rise in intervillous
PO2 occurs at around the tenth week it is not
possible to identify precisely when the rise begins. Before 8 weeks the
placenta proved too small to accommodate the probe so earlier data are
not available. It is likely however that the onset of the circulation
is a gradual event, and that considerable individual variation occurs
between placentas. Aerobic metabolism is inevitably associated with the
generation of free radical species, principally through the transfer of
single electrons to molecular oxygen during their passage along the
mitochondrial electron-transport chain. This occurs at the level of
either NADH dehydrogenase or coenzyme Q, and results in the formation
of the superoxide anion,
O2·-. The rate of formation
is proportional to the prevailing oxygen concentration,11
and a series of enzymatic antioxidant defenses have evolved to quench
this and other resultant radicals. The manganese form of SOD located
within the mitochondrial matrix readily dismutates
O2·- to hydrogen peroxide,
H2O2 (Figure 1)
. Although
not a radical, H2O2 can
react with free ferrous iron to yield the highly reactive hydroxyl
radical, HO·. It is therefore important that
H2O2 is maintained at
physiological concentrations, and the enzymes GPX and catalase detoxify
it to oxygen and water. GPX is located both within the mitochondrial
matrix and the cytoplasm, whereas catalase is restricted primarily to
peroxisomes. GPX uses reduced glutathione as a substrate, and so the
intracellular pool of this tripeptide represents an important redox
buffer. Indeed, it has been estimated that the pool contains >90% of
all cellular reducing equivalents.20
Superoxide anions may
also be formed within the endoplasmic reticulum, and here the Cu/Zn
form of SOD is the first enzyme in the detoxification pathway (Figure 1)
. It is therefore essential that the correct balance of activities of
the antioxidant enzymes is maintained to ensure global cell
protection.21
Our data confirm that the expression and
activities of the principal antioxidant enzymes increase with
gestational age, and that this is probably in response to the changes
in maternal blood flow. Attempts were made to separate the activities
of Cu/Zn and MnSOD using sodium cyanide to block the cytoplasmic
enzyme, but as highly inconsistent results were obtained this proved
not to be possible. Previous work using a native polyacrylamide gel
electrophoresis activity gel indicated an increase in activity of both
enzymes with gestational age,12
and this is supported by
the mRNA data reported here. Two splice variants were detected for
MnSOD, and in human cell lines it has been demonstrated that the 4-kb
variant is the more rapidly responsive of the two.22
This
variant showed the greater differences with gestational age in our
samples, although the differences failed to reach statistical
significance. Our results are therefore in general agreement with the
findings of Takehara et al23
who reported an increase in
placental total SOD and catalase activity from 5 weeks of gestation to
term. It is pertinent that from their scattergrams it appears that most
of the increase occurred toward the end of the first trimester.
If free radical generation exceeds the capacity of the antioxidant defenses then oxidative stress results. In this situation indiscriminate damage to proteins, lipids, and DNA can occur, leading to a loss of cell function. Recent work has confirmed that the protein deformations caused by oxidative stress induce the expression of Hsps in a similar manner to hyperthermia.13 Oxidative modification of protein thiol groups causes thermal stable proteins to become destabilized and to undergo thermal denaturation at physiological temperatures. The resultant molten-globule intermediates cause trimerization of the transcription factor Hsf-1, which confers DNA binding activity, so leading to gene activation. Hsps act as molecular chaperones to sequestrate damaged proteins, giving them an opportunity to refold or directing them to proteolytic pathways. Their expression represents an adaptive response aimed at preventing the aggregation of denatured proteins within the cytosol, and overexpression of both the constitutive and inducible forms of Hsp 70 confers increased resistance to oxidative stress in cell lines.24 The peak of Hsp 70 expression we observed within the syncytiotrophoblast at 8 to 9 weeks therefore provides a sensitive marker of oxidative stress occurring in vivo.
This conclusion is further supported by the concurrent detection of
elevated concentrations of nitrotyrosine residues. Although
O2·-, is rapidly dismutated
by SOD, it also reacts avidly with nitric oxide to form peroxynitrite,
a potent oxidant capable of initiating lipid peroxidation and nitrating
tyrosine residues on a variety of proteins. This can lead to
inactivation of enzyme activity, as for example has been demonstrated
for MnSOD.25
Because peroxynitrite has a relatively long
half-life (1 second) and is freely diffusible through biological
membranes its potential targets are widespread.
The syncytiotrophoblast is particularly vulnerable to oxidative stress for two reasons. Firstly, because of its location on the villous surface this tissue will be the first to experience the increase in intervillous PO2, and so will be the principal beneficiary in terms of aerobic respiration. Secondly, we have previously shown that the syncytiotrophoblast contains much lower concentrations of the antioxidant enzymes than other villous tissues during early gestation.12,26,27 Low levels of MnSOD will place the mitochondria at particular risk of O2·--mediated damage. It has been known for many years that the ultrastructural morphology of mitochondria changes reversibly according to their metabolic activity.28 The orthodox appearances of interdigitating cristae seen during periods of low oxygen consumption reflect a low respiration rate. By contrast, high respiration rates are associated with a reduction in volume, condensation of the matrix, and an increase in the intracristal space. On this basis, our findings are consistent with a general switch from anaerobic to aerobic metabolism within the syncytiotrophoblast at the end of the first trimester. The boundary between normal metabolic changes and damage resultant on oxidative stress is not clear however, and they may represent different stages along a gradual spectrum. For example, the cytotoxic action of tumor necrosis factor is mediated through increased free radical generation within mitochondria, and is associated with disruption of their cristae and loss of enzyme function.29 The circular arrangements of cristae we observed at 9 to 10 weeks may represent early stages in the formation of the onion-skin whorls induced by tumor necrosis factor, indicating a degree of oxidative stress. We have previously observed qualitatively similar changes in mitochondrial morphology in villi maintained under 21% oxygen, where they are accompanied by loss of enzyme activity and of mitochondrial membrane potential as evidenced by rhodamine-123 fluorescence.12,30 It is likely therefore that mitochondrial function is impaired during onset of the maternal circulation, although clearly the situation is recoverable as our samples represent snapshots of what would otherwise have been normal on-going pregnancies. We suggest this may be a period of rapid mitochondrial turnover, with the formation of new mitochondria equipped with elevated concentrations of antioxidant defenses to cope with the increasing oxygen tension.
Overall therefore, it would seem that the onset of the full maternal
arterial circulation to the placenta is associated with a transient
period of placental oxidative stress in normal pregnancies (Figure 9)
. We hypothesize that this is because
of a temporary imbalance between increased generation of free radical
species as a result of the rapidly rising oxygen tension, and
adaptations in the antioxidant defenses to quench them. This burst of
stress may serve an important physiological function in normal
placental development by triggering differentiation
pathways.31,32
For example, it may switch cytotrophoblast
cells from a proliferative to an invasive phenotype, so stimulating the
migration of extravillous trophoblast into the endometrium where they
play a key role in the conversion of the spiral
arteries.33-35
Failure of this process is associated with
pre-eclampsia, a common complication of pregnancy associated with poor
perfusion of the placenta and chronic oxidative
stress.36,37
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.12,38
Oxidation of thiol groups on proteins
incorporated in the inner mitochondrial membrane is thought to promote
the mitochondrial permeability transition through opening of the BCL-2
regulated mega-channel. Loss of ionic gradients and the release of
small proteins such as cytochrome c into the cytoplasm may
result in activation of the caspase cascade and hence
apoptosis.39
Enzymes located in the matrix and on the
inner membrane, including those of the respiratory chain, will also be
vulnerable through direct oxidation, but in addition the peroxidation
of the membrane lipids can significantly impair their
function.25,40,41
If sufficiently severe these changes
lead to depletion of adenosine triphosphate stores, and may
cause cells to undergo either necrosis or apoptosis. Because the
syncytiotrophoblast is responsible for all placental hormone synthesis
and active transport,42
pregnancy failure would rapidly
ensue. The magnitude of the stress will depend on the rate of increase
in PO2 within the intervillous space, and on the
state of the placental antioxidant defenses (Figure 9)
. In cases of
inevitable ongoing miscarriage and of missed abortion there is
premature and excessive entry of maternal blood into the intervillous
space.9,43
Histological examination of the decidua in such
cases reveals that trophoblast invasion is less extensive than normal,
and that plugging of the apical portions of the spiral arteries by
endovascular aggregations of cytotrophoblast cells is
incomplete.44
Hence, the onset of the maternal circulation
is likely to have been precipitate. Placental tissues are most
susceptible to lipoperoxidation at 7 weeks of gestational
age,23
and this will be further exacerbated if antioxidant
defenses are depleted. It has been suggested that women with low
concentrations of serum selenium, the transition metal embedded within
GPX, experience a higher rate of early pregnancy failure, although this
was not confirmed by a recent case-controlled study.45,46
It has also been reported that mice deficient in glucose-6-phosphate
dehydrogenase, an enzyme involved in maintaining the available pool of
reduced glutathione, show a higher rate of intrauterine deaths and
fetal resorptions than controls.47
There may be a human
corollary to this phenomenon, for the incidence of G6PD-deficient -/y
males and -/- females born among all G6PD-deficient groups in Europe
is >50% lower than expected.48
These data suggest a
lower survival rate than expected for the affected embryos. Oxidative
stress of the syncytiotrophoblast may therefore act as the final common
pathway for a number of diverse conditions associated with early
pregnancy failure.
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Acknowledgements |
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Footnotes |
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Supported by a grant from the Medical Research Council (G9701485), and University College London Hospital Special Trustees, London, United Kingdom.
Accepted for publication September 11, 2000.
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 J. M. Catov, L. M. Bodnar, R. B. Ness, N. Markovic, and J. M. Roberts Association of Periconceptional Multivitamin Use and Risk of Preterm or Small-for-Gestational-Age Births Am. J. Epidemiol., August 1, 2007; 166(3): 296 - 303. [Abstract] [Full Text] [PDF]
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N. Soleymanlou, A. Jurisicova, Y. Wu, M. Chijiiwa, J. E. Ray, J. Detmar, T. Todros, S. Zamudio, M. Post, and I. Caniggia Hypoxic Switch in Mitochondrial Myeloid Cell Leukemia Factor-1/Mtd Apoptotic Rheostat Contributes to Human Trophoblast Cell Death in Preeclampsia Am. J. Pathol., August 1, 2007; 171(2): 496 - 506. [Abstract] [Full Text] [PDF]
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 S. J. Germain, G. P. Sacks, S. R. Soorana, I. L. Sargent, and C. W. Redman Systemic Inflammatory Priming in Normal Pregnancy and Preeclampsia: The Role of Circulating Syncytiotrophoblast Microparticles J. Immunol., May 1, 2007; 178(9): 5949 - 5956. [Abstract] [Full Text] [PDF]
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M. Al-Asmakh, H. Race, S. Tan, and M.H.F. Sullivan The effects of oxygen concentration on in vitro output of prostaglandin E2 and interleukin-6 from human fetal membranes Mol. Hum. Reprod., March 1, 2007; 13(3): 197 - 201*. [Abstract] [Full Text] [PDF]
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 S. J. Renaud, S. K. Macdonald-Goodfellow, and C. H. Graham Coordinated Regulation of Human Trophoblast Invasiveness by Macrophages and Interleukin 10 Biol Reprod, March 1, 2007; 76(3): 448 - 454. [Abstract] [Full Text] [PDF]
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