BIOLOGY OF REPRODUCTION (2014) 91(4):98, 1–8
Published online before print 3 September 2014.
DOI 10.1095/biolreprod.114.122846
Minireview
The Human Endometrium as a Sensor of Embryo Quality1
Nick S. Macklon2,3
and Jan J. Brosens4
3
Academic Unit of Human Development and Health, University of Southampton, and BRC in Nutrition, University
Hospital Southampton NHS Foundation Trust, Southampton, United Kingdom
4
Division of Reproductive Health, Warwick Medical School, Clinical Sciences Research Laboratories, University
Hospital, Coventry, United Kingdom
ABSTRACT
Human reproduction is characterized by a high degree of
embryo wastage, which is largely ascribed to a high prevalence
of embryo aneuploidy. It is proposed that maternal strategies
have evolved that prevent inappropriate investment in invasive,
but poorly viable embryos. Key to this is the emerging concept of
the endometrium as biosensor, first identified in human in vitro
embryo/decidualized stromal cell coculture systems and recently
confirmed in an in vivo mouse model. In this review, the
growing supporting experimental evidence for the biosensor
component of decidualized endometrium is outlined, and recent
insights into the nature of the embryo-derived signal detected by
the endometrium and the biological processes by which this
signal is thought to be converted into a go or no-go endometrial
response are described. Finally, the clinical implications of this
new paradigm of the choosy uterus are addressed.
decidualization, embryo, endometrium, implantation, selection
INTRODUCTION
Compared with other mammalian species, human reproductive
efficiency is not impressive. The probability of achieving a
pregnancy within one menstrual cycle, defined as the monthly
fecundity rate (MFR), is 20%–30%. In contrast, the MFR may
be as high as 80% in baboons and 90% in rabbits [1–3]. Those
concerned about the impact of the rising population on the
Earth’s resources may find some reassurance in this observation,
but for couples suffering from the distress of reproductive
failure and their clinicians it is an exercising source of
frustration. Progress in assisted reproductive techniques has led
to the majority of couples undergoing in vitro fertilization
(IVF) treatment obtaining embryos for transfer, but only
around half implant, and up to half of these are lost soon after
[4]. However, this high rate of peri-implantation loss is not
solely a feature of assisted conception treatment. The first
indications of the high peri-implantation attrition that characterizes
human reproduction were revealed by Hertig and
colleagues 1950s exploration of 210 postovulation hysterectomy
specimens where only 34 yielded fertilized ova, good, bad,
and indifferent [5]. The development of sensitive urinary
human chorionic gonadotropin (hCG) assays led to the
landmark study of Wilcox et al. [6] in which serial urinary
samples collected over 6 mo or until a positive pregnancy test
were collected from 221 women trying to conceive. Urinary
hCG measurements revealed that implantation occurred in 19%
of cycles, but more than a third were then lost, a majority
before any clinical sign of conception had presented. When
considered in conjunction with subsequent studies, it has been
estimated that around 50% of human conceptions fail to
progress to an ongoing pregnancy [7]. A greater understanding
of the etiology and teleology of peri-implantation loss is key to
improving reproductive outcomes, but human studies are
marred by ethical and technical challenges. However, a
combination of informative animal models and novel in vitro
systems is beginning to throw new light on what has previously
been considered the implantation ‘‘black box’’.
THE NEED TO BE DISCERNING
The exceptional rate of early pregnancy loss that characterizes
human reproduction is now thought to derive from two key
features of human embryos: their intrinsic invasiveness and
their high prevalence of chromosomal abnormalities. Genomewide
screening of individual blastomeres taken from highquality
cleavage-stage embryos IVF has shown that around
70% have complex chromosomal abnormalities, mostly arising
from mitotic rather than meiotic nondisjunction [8–10]. This
rate of aneuploidy, estimated to be at least 10 times greater than
that observed in other mammalian species, appears to decrease
during development to the blastocyst stage. Fluorescent in situ
hybridization analysis of 10 chromosomes in individual
blastomeres from viable human IVF embryos has shown that
the prevalence of aneuploidy decreases from 83% on Day 4 of
development to 42% on Day 8 [11]. Consistent with these
findings, microarray comparative genomic hybridization analysis
of 5 to 10 trophectoderm cells from 1046 blastocysts
identified a total of 1113 distinct aneuploidies in 608 (58%)
embryos [10], emphasizing that peri-implantation human
embryos are intrinsically chromosomally diverse and predominantly
mosaic.
1
N.S.M. was supported by the NIHR BRC in Nutrition, Southampton,
UK, and by Zon Mw, The Netherlands. J.J.B. was supported the
Biomedical Research Unit in Reproductive Health, a joint initiative
between the University Hospitals Coventry and Warwickshire NHS
Trust and Warwick Medical School.
2
Correspondence: Nick S. Macklon, University Department of Obstetrics
and Gynaecology, Princess Anne Hospital, Coxford Road, Southampton
SO16 5YA, United Kingdom. E-mail: n.s.macklon@soton.ac.uk.
Received: 26 June 2014.
First decision: 21 July 2014.
Accepted: 6 August 2014.
Ó 2014 by the Society for the Study of Reproduction, Inc.
eISSN: 1529-7268 http://www.biolreprod.org
ISSN: 0006-3363
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Such observations raise a number of pressing questions.
What is the biological significance of this high prevalence of
aneuploidy and mosaicism, and what is the evolutionary
advantage conferred by embryo aneuploidy? Remarkably little
attention is given to these questions, perhaps because the
prevailing thinking in assisted reproductive technology circles
remains that the optimal embryo for transfer must be uniformly
diploid.
An obvious teleological explanation for the genetic diversity
of human embryos is that it renders implantation events
intrinsically adaptable to a changing ecology, which arguably
is essential for procreation and thus survival of a species that
exhibits relatively few implantation events over their lifespan.
In this context, emerging data are challenging the view that
aneuploidy is uniformly detrimental for cells and organisms.
For example, a variety of stress signals can induce chromosomal
instability in budding yeast, but, importantly, the
resultant aneuploidy then drives rapid phenotypic evolution
and confers stress resistance [12, 13].
Considering that the cytogenetic anomalies in human
embryos resemble closely those found in cancer cells [9], it
is also not inconceivable that a degree of genomic instability in
embryos may confer an implantation advantage or even
compensate for loss of key nidation genes [14]. If so, it
follows that the perfectly diploid human blastocyst may have
maximal developmental potential but perhaps lack intrinsic
implantation competence, not unlike mouse embryos. These
conjectures clearly require further testing. However, it is
incontrovertible that the rather peculiar human embryo imposes
an important reproductive challenge: how to facilitate implantation
while simultaneously safeguarding the mother against
prolonged investment in potentially developmentally abnormal
embryos [15]. In recent years, experimental evidence has
emerged indicating that spontaneous decidualization of the
endometrium coupled to cyclic menstruation and regeneration
represents an ingenious strategy to meet this challenge.
THE SELECTIVE DECIDUA
Endometrial decidualization, a process imperative for
pregnancy in all species with invading embryos, is characterized
by the transformation of stromal fibroblasts into secretory
decidual cells [16]. In contrast to most species, the human
endometrium decidualizes in response to endocrine rather than
embryonic cues. Hence, decidualization is a feature of the
midluteal phase in all ovulatory cycles, whether they lead to
conception or not. The term decidua refers in part to the
intrinsic ability of differentiated stromal cells to self-destruct
and slough off in response to falling progesterone levels, which
in turn accounts for cyclic menstruation in women [17–19].
Duplication of an ancestral b-luteinizing hormone gene
enabled human embryos to avoid guaranteed maternal
destruction by ensuring continuous ovarian progesterone
production in the early stages of pregnancy through the
production of beta hCG [20]. Hence, decidual cells are a priori
programmed to select against embryos that are perceived to
lack fitness because of insufficient hCG production. Similarly,
the transition from histiotrophic nutrition of the early conceptus
to active maternal perfusion of the placenta toward the end of
the first trimester of pregnancy causes dramatic changes in
local oxygen tension and production of free radicals [21, 22].
Arguably, this transition could be viewed as imposing a stress
test on the feto-maternal interface, thus increasing the
likelihood that a failing pregnancy is disposed of in a timely
manner. From both an evolutionary and clinical perspective,
the later the pregnancy is lost in gestation, the greater the harm
to maternal fitness and health.
While the mechanisms outlined above may account for
clinical miscarriages, they do not explain the high attrition rate
during the peri-implantation period. They also do not explain
why complex aneuploidies and large-scale complex chromosomal
errors that are prevalent in preimplantation embryos
have rarely been reported in miscarriage tissues [10]. Recently,
additional sophisticated and dynamic functions have been
uncovered that position the decidua as the key determinant of
successful implantation and as the active component of the
maternal strategy to cope discerningly with genomically
unstable blastocysts.
The first evidence supporting the concept of active human
embryo selection at implantation emerged from coculture
studies, consisting of single hatched human blastocysts
cultured for 3 days on a layer of primary human endometrial
stromal cells that had been decidualized with a cyclic AMP
analog and medroxyprogesterone acetate [23]. While it might
be considered unlikely that 50 000 decidualizing cells would
produce a measureable response to a single human embryo,
profound changes in the production of a number of key
cytokines and growth factors were observed. Moreover, the
nature of this response was opposite to what had been
expected. The pervasive embryo-centric paradigm predicted
that developmentally competent embryos would signal their
viability to the decidualized stromal cells, which would
respond by up-regulating production of proimplantation
modulators, such as interleukin-1 beta (IL-1b), heparin-binding
EGF-like growth factor (HB-EGF), or leukemia inhibitory
factor (LIF). However, in comparison to control cultures, highgrade
embryos had little impact on the supernatant concentrations
of 14 factors measured by multiplex immunoassay. By
contrast, embryos that showed morphological signs of
developmental impairment elicited a strong response in
decidualizing cells, characterized by selective inhibition of
IL-1b, -6, -10, -17, and -18, as well as eotaxin (CCL11) and
HB-EGF secretion (Fig. 1). This putative biosensor function
resides in the decidual phenotype because human embryos did
not trigger a response when cocultured with undifferentiated
endometrial stromal cells [23].
The mechanism underlying this novel concept of human
embryo selectivity was unclear and difficult to study directly.
To address these challenges, a study was designed in which
decidualizing cells were exposed for 12 h to pooled
conditioned medium from human embryos deemed insufficient
for uterine transfer and high-quality embryos that resulted in an
on-going pregnancy after single embryo transfer, thus providing
unequivocal proof of their developmental competence. A
third pool of unconditioned embryo culture media was applied
as a control. Genomewide expression profiling uncovered only
15 decidual genes responsive to soluble signals from competent
human embryos. In contrast, and consistent with the previously
observed cytokine response [23], 449 maternal genes were
deregulated in response to medium conditioned by poor-quality
embryos. Gene ontology studies showed half of these genes to
be associated with the broad biological process of transport,
translation, and cell cycle regulation [14].
This concept of the decidualized endometrium responding
more profoundly to the developmentally incompetent than the
competent embryo is consistent with previous studies asserting
that the healthy embryo, needing only to invest energy in
growth and development, is metabolically quiet in comparison
to less viable embryos that must additionally engage in repair
and apoptosis [24–26]. This concept introduces the possibility
that an embryo may be so quiet that it fails to trigger any
MACKLON AND BROSENS
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supportive maternal response, a hypothesis that would be
consistent with the conjecture that some degree of aneuploidy
may confer certain advantages to the implanting embryo. The
converse notion to that of the high-quality, quiet embryo is that
of the struggling, metabolically noisy embryo producing a
source of putative signals to which the decidualized stroma
responds by deregulating a repertoire of genes implicated in
implantation.
In response to signals present in conditioned media derived
from impaired human embryos, the most down-regulated gene
in the array analysis of exposed decidualized stromal cells was
HSPA8, which encodes a ubiquitously and constitutively
expressed member of the heat shock protein 70 family of
molecular chaperones involved in protein assembly and folding
[27]. HSPA8 is a multifaceted molecular chaperone that can
represent up to 1% of total cellular protein content. It is
involved in clatherin-mediated endocytosis, assembly of
multiprotein complexes, transport of nascent polypeptides,
and regulation of protein folding. It is also a major regulator of
chaperone-mediated autophagy [28, 29]. Knockdown of
HSPA8 in decidualizing cells compromised the secretion of
two sensitive differentiation markers, prolactin and insulin-like
growth factor binding protein 1, and induced endoplasmic
reticulum (ER) stress [14]. In keeping with the role of HSPA8
as a pivotal sensor molecule for embryonic signals, transfection
of a luciferase reporter that becomes activated under ER stress
conditions confirmed that soluble signals from developmentally
impaired human embryos induce a proteotoxic stress
response in decidualizing cells.
These in vitro studies provided evidence for a putative
mechanism by which the decidualized stromal cells sense
developmentally compromised embryos. However, in order to
elucidate whether this mechanism could be identified in vivo,
the uteri of mice were flushed with naive or conditioned human
embryo culture medium. Analysis of the uterine transcriptome
confirmed the findings of the in vitro studies, with around six
times as many genes showing altered expression in response to
media conditioned by impaired versus competent human
embryos [14]. Intriguingly, signals emanating from competent
human embryos triggered a very specific transcriptional
response in the mouse uterus, characterized by the induction
of multiple metabolic genes. Moreover, in excess of 30% of
FIG. 1. A) Primary human endometrial stromal cells were first decidualized for 5 days and then cocultured with human embryos (magnification 340) or
not (control, magnification 3100). Over the 72-h coculture period, 30 embryos arrested (arrested) whereas 11 continued to develop normally
(developing). B) Analysis of the culture supernatants revealed that the presence of an arresting embryo inhibited the secretion of a number of factors. Data
for HB-EGF and Il-1b are shown (median and interquartile range: box ¼ 25%–75%, whiskers ¼ 10%–90%) The letters above the box plots indicate
significant differences between groups (P , 0.01). Adapted from Teklenburg et al. [23] with permission.
EMBRYO SELECTION AT IMPLANTATION
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regulated genes coded for known implantation factors,
including COX-2, cytochrome p450 26a1, and osteopontin.
Hence, the endometrial response to embryonic signals in this
model was not limited to negative selection but also
demonstrated a component consistent with the established
paradigm of a competent embryo evoking a supportive
intrauterine environment [14, 30–32]. A dual-phase response
of the endometrium can therefore be proposed consisting of
recognition and selection. The ability of the luminal epithelium
to recognize a high-quality embryo and modulate the decidual
response it encounters on breaching the epithelium would aid
subsequent nurturing and development in the postimplantation
phase. This supportive implantation response is intrinsically
lacking in the presence of a developmentally impaired embryo.
However, chromosomal instability, invasiveness, and the
inherent unpredictability of low-quality embryos could mean
that many will breach the luminal epithelium but are then
selected against and disposed of as a consequence of the
ensuing decidual stress response.
EMBRYO SELECTION: THE ROLE OF DECIDUAL MI-
GRATION
Once the embryo has successfully breached the luminal
epithelium, it is thought to continue its journey into the
endometrial stroma as the active party, invading through what
has been considered a mechanically passive decidual matrix.
However, recent studies indicate that active decidual cell
migration and encapsulation of the conceptus are integral steps
in the implantation process. By placing human or mouse
blastocysts on a monolayer of decidualized stromal cells,
Grewal et al. [33] observed that stromal cell motility is
obligatory for blastocyst implantation and trophoblast outgrowth.
Moreover, time-lapse recordings revealed that the
stromal cells moved around the embryo to accommodate its
expansion [33, 34]. Decidualizing stromal cells have been
shown to provide a more favorable matrix for trophoblast
expansion than do undifferentiated cells [35], indicating that
the choreography of stromal cell movement actively assists
expansion. Further work showing trophoblast spheroids to
expand to a greater extent when cultured on a monolayer of
decidualized versus undifferentiated stromal cells provided
more evidence supporting directed cell migration as a key
element of the decidual phenotype [36].
Further confirmation for this comes from recent imaging
studies, which revealed that decidualizing endometrial stromal
cells are programmed to migrate toward implantation-competent
blastocysts [37]. This chemotactic and invasive migration
of endometrial stromal cells is triggered by signals emanating
from the trophoblast, especially platelet-derived growth factorAA
(PDGF-AA) [36, 38–40]. Transcriptome analysis of
trophectoderm cells from human blastocysts and matched
endometrial biopsies confirmed that secretion of embryoderived
PDGF-AA occurs in tandem with increased expression
of its receptor, PDGF-Ra, in endometrial cells [41]. Other local
growth factors that may serve to fine-tune directed and
nondirected migration of decidual cells at the implantation site
include PDGF-BB and HB-EGF [39]. The active participation
of the decidualized stroma in the encapsulation of the embryo
illustrated by these studies raises the question as to whether
invasion with its implications of offensive incursion is the
appropriate term to describe this phase of human implantation.
A more appropriate term that better reflects this closely
choreographed pas de deux may be embedding.
A modified wound assay, consisting of a monolayer of
decidualizing cells with a single human embryo placed in the
migratory zone, has been used to further test the biosensor
hypothesis. Weimar et al. [37] observed that the intrinsic
propensity of decidual cells to migrate toward a conceptus was
confined to high-quality human embryos and entirely inhibited
in the presence of chromosomally abnormal tripronuclear
embryos (Fig. 2). Taken together, these data indicate that
decidual cells have the capacity to actively hinder invasion and
outgrowth of abnormal human embryos that have breached the
luminal epithelium.
EMBRYO-DERIVED COMPETENCE SIGNALS
How human embryos signal their developmental competence
remains an unresolved question that if answered could
have considerable clinical implications for embryo selection
and modulation of endometrial receptivity. Recently, evidence
has emerged from both human and murine studies that
implicates embryo-derived serine-proteases as candidate signals.
Elegant studies in the mouse uterus have demonstrated
that trypsin produced by the implanting blastocyst cleaves and
activates epithelial Naþ
channels present on the apical border
of luminal epithelial cells (14, 42]. This leads in turn to an
inward current, membrane depolarization, and Ca2þ
entry into
epithelial cells though voltage-gated L-type Ca2þ
channels.
The resultant Ca2þ
transients then help promote phosphorylation
and activation of transcription factor CREB, which in turn
up-regulates COX-2 and leads to PGE2 release [42].
Conditioned medium from human embryos also trigger Ca2þ
oscillations in human endometrial epithelial cells (EECs).
Importantly, the Ca2þ
transients induced in EECs by
developmentally competent embryos (DCEs) are very shortlived,
lasting approximately 5 min. In contrast, low-quality
human embryos trigger prolonged and disorganized Ca2þ
oscillations in EECs, leading to a uterine stress response (Fig.
3) [14]. The precise nature and origins of embryo-derived
serine proteases are not clear. Furthermore, whether or not
protease activity in embryo culture medium could lead to
FIG. 2. The migration zone after adding a high-quality, low-quality, or no
embryo. The migratory response of decidualized human endometrial
stromal cells (H-EnSCs) from normally fertile (A–C) and RM women (D–F)
was analyzed in the absence of a human embryo (A and D), in presence of
a high-quality embryo (B and E), or a low-quality embryo (C and F). Phase
contrast pictures were taken 18 h after creating the migration zone. The
dotted line represents the front of the migration zone directly after its
creation. As a reference for the position of the embryo, the bottom of the
plate was marked. The arrows indicate the position of the embryo. All the
pictures were taken with 253 magnification. (Reprinted from Weimar et
al. [37]) with permission.
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improved selection of embryos for transfer warrants further
investigation.
A REPRODUCTIVE TRADE-OFF: RECEPTIVITY VERSUS
SELECTIVITY
A further test of the validity of the selective biosensor
function of decidualized endometrium is to consider the likely
clinical manifestations caused by decidual selection. A
predictable consequence of a reduced ability to recognize
embryonic signals would be increased frequency of implantation
of impaired embryos that embark on a developmental
trajectory destined to fail as a clinical miscarriage. Conversely,
failure to respond to signals of high-quality embryos will result
in a suboptimal environment for subsequent development and
placenta formation, thus also increasing the risk of miscarriage.
If this supposition is correct, persistently impaired endometrial
selectivity is predicted to result in recurrent early
pregnancy loss in conjunction with paradoxical superfertility.
This hypothesis was supported by a study on the time-topregnancy
(TTP) intervals in 560 women presenting with a
history of three of more consecutive miscarriages. Women with
recurrent miscarriage (RM) were shown to be highly fecund,
with TTP further decreasing in those with a history of five or
more miscarriages. The observed incidence of achieving three
or more pregnancies within 6 mo was much higher than would
be predicted given a normal MFR of 20%. Indeed 40% of
women with recurrent pregnancy loss could be considered
superfertile when defined by a mean TTP of 3 mo or less. On
the other hand, prolonged endometrial receptivity rather than
impaired selectivity would also shorten the interpregnancy
interval as well as the miscarriage rate by promoting out-ofphase
implantation. This conjecture is also supported indirectly
by clinical observations. For example, in the aforementioned
study of Wilcox and colleagues [43], 84% of conceptions were
detected between 8 to 10 days after ovulation. Among the 102
pregnancies identified on Day 9 after ovulation, 13% ended in
an early pregnancy loss. This proportion rose to 26% with
implantation on Day 10, to 52% on Day 11, and to 82% on Day
12 and beyond. Taken together, these clinical observations
FIG. 3. Positive and negative mechanisms contribute to active selection of human embryos at implantation. A) Developmentally competent human
embryos secrete evolutionarily conserved serine proteases that activate epithelial Naþ channel (ENaC) expressed on luminal epithelial cells (box),
triggering calcium signaling and, ultimately, induction of factors involved in implantation and postimplantation embryo development, such as
cyclooxygenase-2 (COX-2). In this model, the luminal surface epithelium amplifies and relays embryonic signals to the underlying stroma to optimize
postimplantation development. B) By contrast, the luminal epithelium may not be much of a barrier for developmentally compromised embryos that are
characterized by excessive protease production. However, expansion of the endoplasmic reticulum (ER) in underlying decidualizing cells renders them
particularly sensitive to aberrant embryonic signals, leading to proteotoxic stress, active withdrawal of key decidual factors, tissue breakdown, and,
ultimately, elimination of an unwanted conceptus.
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indicate that unrestrained endometrial receptivity and lack of
embryo selection both contribute to subsequent pregnancy
failure. This concept has been further supported by in vitro
wound healing assays in which the suppression of stromal cell
migration observed in the presence of a developmentally
incompetent embryo did not occur when the decidual cells had
been obtained from women with RM (Fig. 2) [37].
Emerging evidence indicates that an aberrant decidual
response both prolongs receptivity phenotype of the endometrium
and impairs the ability of the endometrium to engage in
embryo quality control. Decidualization is often depicted as an
all or nothing static phenotype, defined by the induction of
marker genes, such as PRL or IGFBP1. This perception is
misguided because the expression of several decidual gene
networks change profoundly as the differentiation process
unfolds. For example, a recent PCR array analysis of 84
inflammatory mediators identified 70 up-regulated cytokines,
interleukins, and their receptors in cells decidualized for 2 days
compared to undifferentiated control cells [44]. By 8 days of
differentiation, only 12 transcripts remained elevated while 34
other mRNAs were now expressed below the level of that seen
in undifferentiated cells. This suggests that decidual transformation
is at least a biphasic process, characterized initially by
an acute-phase inflammatory response, which is followed by a
profound anti-inflammatory response [44]. Arguably, the antiinflammatory
phenotype coincides with the marked expansion
of the decidual ER, up-regulation of various molecular
chaperones, including HSPA8, and thus the ability of the cell
to sense and respond to embryonic signals in a tailored fashion.
The biological relevance of the transient inflammatory
decidual initiation response was examined by flushing mouse
uteri with conditioned medium of undifferentiated and
decidualizing primary human endometrial stromal cell cultures
prior to embryo transfer. Strikingly, strong expression of the
receptivity genes and effective embryo implantation were only
observed if mice were exposed to the inflammatory secretome
that marks the early stages of the decidual process. Compared
to control cultures, this initial proinflammatory decidual
response was both prolonged and disordered in primary
cultures established from RM patients. Furthermore, when
flushed through the mouse uterus, secreted factors from
decidualizing RM cultures not only prolonged the window of
receptivity but also increased the incidence of pathological
implantation sites, characterized by focal bleeding, immune
cell infiltration, and fetal demise [44]. Additional studies have
shown that differentiating human endometrial stromal cells
from RM patients are characterized by lower induction of
decidual marker genes, increased vulnerability to oxidative
apoptosis, aberrant responses to hCG, and failure to discriminate
between high- and low-quality human embryos in cell
migration assays [37, 45, 46].
Taken together, these observations indicate that the
transition of decidual cells from the proinflammatory initiation
phase to a fully secretory anti-inflammatory phenotype
balances the receptivity versus selectivity traits of the human
endometrium. Thus, an excessive decidual response will curtail
the window of receptivity and increase the disposal efficacy of
embryos, thereby reducing the incidence of miscarriages but
also increasing the likelihood of conception delay or recurrent
implantation failure after IVF. Conversely, a disordered
decidual response will increase the likelihood of both
pregnancy as well as miscarriage by facilitating out-of-phase
implantation [47] (Fig. 4). Importantly, spontaneous decidualization
is inextricably linked to menstruation and cyclic
regeneration; a process that involves recruitment of extrauterine
stem cells and activation of local progenitor cells. Thus, the
quality of the decidual process, that is, the balance of the
receptive versus selective traits, will vary in response to the
cumulative impact of menstruations and other reproductive
events, such as miscarriage and parturition.
CONCLUSIONS
From a clinical perspective, RM and implantation failure
have remained frustratingly devoid of effective therapeutic
strategies. However, the emerging evidence that the decidualizing
endometrium is an active participant and key determinant
of successful implantation promises to open up new therapeutic
avenues. Moreover, the recognition that endometrial function
can be considered in terms of both the epithelial feature of
receptivity or ability to recognize and respond supportively to a
high-quality embryo and the decidual function of selectivity
offers a new paradigm for the investigation and management of
early reproductive failure.
Similar to its previously proposed role in the bovine species
[47], the decidualized human endometrium is emerging as an
active gatekeeper to implantation in the human. This novel
model requires further validation but addresses a number of
imperatives characteristic of human reproduction. The remarkable
prevalence of aneuploidy in human embryos demands
maternal investment in effective means of preventing invasive
but chromosomally chaotic embryos from establishing a
clinical pregnancy destined to fail. Menstruation, a process
triggered by spontaneous decidualization of the endometrium
in an embryo-independent manner and almost unique to
humans among mammalian species, may have emerged as a
strategy for early detection and active rejection of developmentally
abnormal embryos that have breached the luminal
epithelium. At the same time, cyclic regeneration of the
FIG. 4. Balancing endometrial receptivity versus selectivity. Human
preimplantation embryos are intrinsically diverse. To be reproductively
successful, the maternal endometrium must be receptive as well as
selective, meaning acquiring the ability to mount a secretory response that
is tailored to an individual embryo. The purpose of a tailored maternal
response is either to support further development of high-quality embryos
or to trigger early disposal of an unwanted conceptus. A prolonged or
unopposed receptive phenotype will lead to miscarriage and all but the
most developmentally competent embryos. Conversely, premature or
excessive decidualization will increase the barrier function of the
endometrium and reduce the likelihood of pregnancy. Notably, as
maternal age advances, the incidence of embryonic aneuploidy increases,
thus requiring a greater need for endometrial selection in order to avoid
recurrent pregnancy failure.
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endometrium provides a mechanism to continuously rebalance
the receptivity and selectivity traits of the endometrium, thus
increasing the likelihood of reproductive success.
It is likely that the response of the luminal epithelium serves
to transduce and amplify signals coming from competent
embryos in a way that renders the underlying decidual layer
more receptive to invasion. Conversely, in the presence of a
poor-quality embryo, evidence from recent studies suggests
that the network of supportive decidual processes is not
activated. Instead, decidual cells mount a stress response,
leading to withdrawal of key implantation factors and active
withdrawal of maternal support for a developmentally
compromised embryo. An economic and evolutionarily
conserved signaling system by which the embryo indicates
its competence may be represented by embryonic serine
proteases [14].
Taken together, there is accumulating experimental and
clinical evidence to support a bimodal and biphasic endometrial
response to the implanting embryo characterized by
recognition and then selective elements. The clinical implications
of this are potentially far reaching. To date, clinical
interventions aimed at treating RM and implantation failure
have been characterized by their similarity and disappointing
efficacy. Further understanding of these dual processes may
reveal that recurrent implantation failure and RM are caused by
unbalancing of these functional endometrial traits; the former
reflecting a net overselective phenotype, and the latter an
inadequately discerning endometrium. A key challenge now is
to unravel the molecular processes that control the timely
transition from a proinflammatory, receptive decidual phenotype
to a fully secretory and selective decidual phenotype.
Once fully elucidated, new and effective approaches to
modulate implantation await.
REFERENCES
1. Chard T. Frequency of implantation and early pregnancy loss in natural
cycles. Baillieres Clin Obstet Gynaecol 1991; 5:179–189.
2. Foote RH, Carney EW. Factors limiting reproductive efficiency in selected
laboratory animals. Ann NY Acad Sci 1988; 541:683–696.
3. Stevens VC. Some reproductive studies in the baboon. Hum Reprod
Update 1997; 3:533–540.
4. Boomsma CM, Kavelaars A, Eijkemans MJ, Lentjes EG, Fauser BC,
Heijnen CJ, Macklon NS. Endometrial secretion analysis identifies a
cytokine profile predictive of pregnancy in IVF. Hum Reprod 2009; 24:
1427–1435.
5. Hertig AT, Rock J, Adams EX, Menkin MC. Thirty-four fertilized human
ova, good, bad and indifferent, recovered from 210 women of known
fertility; a study of biologic wastage in early human pregnancy. Pediatrics
1959; 23:202–211.
6. Wilcox, AJ Weinberg, CR, O’Connor, JF, Baird, DD, Schlatterer, JP,
Canfield, RE, Armstrong, EG, Nisula. BC. Incidence of early loss of
pregnancy. N Engl J Med 1988; 319:189–194.
7. Macklon NS, Geraedts JP, Fauser BC. Conception to ongoing pregnancy:
the ‘black box’ of early pregnancy loss. Hum Reprod Update 2002; 8:
333–343.
8. Mertzanidou A, Wilton L, Cheng J, Spits C, Vanneste E, Moreau Y,
Vermeesch JR, Sermon K. Microarray analysis reveals abnormal
chromosomal complements in over 70% of 14 normally developing
human embryos. Hum Reprod 2013; 28:256–264.
9. Vanneste E, Voet T, Le Caignec C, Ampe M, Konings P, Melotte C,
Debrock S, Amyere M, Vikkula M, Schuit F, Fryns JP, Verbeke G, et al.
Chromosome instability is common in human cleavage-stage embryos.
Nat Med 2009; 15:577–583.
10. Fragouli E, Alfarawati S, Spath K, Jaroudi S, Sarasa J, Enciso M, Wells D.
The origin and impact of embryonic aneuploidy. Hum Genet 2013; 132:
1001–1013.
11. Santos MA, Teklenburg G, Macklon NS, Van Opstal D, Schuring-Blom
GH, Krijtenburg PJ, de Vreeden-Elbertse J, Fauser BC, Baart EB. The fate
of the mosaic embryo: chromosomal constitution and development of Day
4, 5 and 8 human embryos. Hum Reprod 2010; 25:1916–1926.
12. Chen G, Bradford WD, Seidel CW, Li R. Hsp90 stress potentiates rapid
cellular adaptation through induction of aneuploidy. Nature 2012; 29:482:
246–250.
13. Oromendia AB, Dodgson SE, Amon A. Aneuploidy causes proteotoxic
stress in yeast. Genes Dev 2012; 26:2696–2708.
14. Brosens JJ, Salker MS, Teklenburg G, Nautiyal J, Salter S, Lucas ES, Steel
JH, Christian M, Chan YW, Boomsma CM, Moore JD, Hartshorne GM, et
al. Uterine selection of human embryos at implantation. Sci Rep 2014; 6:
3894.
15. Haig D. Genetic conflicts in human pregnancy. Q Rev Biol 1993; 68:
495–532.
16. Gellersen B, Brosens IA, Brosens JJ. Decidualization of the human
endometrium: mechanisms, functions, and clinical perspectives. Semin
Reprod Med 2007; 25:445–453.
17. Brosens JJ, Gellersen B. Death or survival—progesterone-dependent cell
fate decisions in the human endometrial stroma. J Mol Endocrinol 2006;
36:389–398.
18. Finn CA. Menstruation: a nonadaptive consequence of uterine evolution.
Q Rev Biol 1998; 73:163–173.
19. Emera D, Romero R, Wagner G. The evolution of menstruation: a new
model for genetic assimilation: explaining molecular origins of maternal
responses to fetal invasiveness. Bioessays 2012; 34:26–35.
20. Talmadge K, Boorstein WR, Fiddes JC. The human genome contains
seven genes for the beta-subunit of chorionic gonadotropin but only one
gene for the beta-subunit of luteinizing hormone. DNA 1983; 2:281–289.
21. Jauniaux E, Watson AL, Hempstock J, Bao YP, Skepper JN, Burton GJ.
Onset of maternal arterial blood flow and placental oxidative stress. A
possible factor in human early pregnancy failure. Am J Pathol 2000; 157:
2111–2122.
22. Hempstock J, Jauniaux E, Greenwold N, Burton GJ. The contribution of
placental oxidative stress to early pregnancy failure. Hum Pathol 2003; 34:
1265–1275.
23. Teklenburg G, Salker M, Molokhia M, Lavery S, Trew G, Aojanepong T,
Mardon HJ, Lokugamage AU, Rai R, Landles C, Roelen BA, Quenby S, et
al. Natural selection of human embryos: decidualizing endometrial stromal
cells serve as sensors of embryo quality upon implantation. PLoS One
2010; 5:e10258.
24. Leese HJ. Quiet please, do not disturb: a hypothesis of embryo metabolism
and viability. Bioessays 2002; 24:845–849.
25. Brison DR, Houghton FD, Falconer D, Roberts SA, Hawkhead J,
Humpherson PG, Lieberman BA, Leese HJ. Identification of viable
embryos in IVF by non-invasive measurement of amino acid turnover.
Hum Reprod 2004; 19:2319–2324.
26. Sturmey RG, Hawkhead JA, Barker EA, Leese HJ. DNA damage and
metabolic activity in the preimplantation embryo. Hum Reprod 2009; 24:
81–91.
27. Hartl FU, Hayer-Hartl M. Converging concepts of protein folding in vitro
and in vivo. Nat Struct Mol Biol 2009; 16:574–581.
28. Agarraberes FA, Dice JF. A molecular chaperone complex at the
lysosomal membrane is required for protein translocation. J Cell Sci
2001; 114:2491–2499.
29. Stricher F, Macri C, Ruff M, Muller S. HSPA8/HSC70 chaperone protein:
structure, function, and chemical targeting. Autophagy 2013; 9:
1937–1954.
30. Das SK, Wang XN, Paria BC, Damm D, Abraham JA, Klagsbrun M,
Andres GK, Dey SK. Heparin-binding EGF-like growth factor gene is
induced in the mouse uterus temporally by the blastocyst solely at the site
of its apposition: a possible ligand for interaction with blastocyst EGFreceptor
in implantation. Development 1994; 120:1071–1083.
31. Dey SK, Lim H, Das SK, Reese BC, Paria T, Daikoku T, Wang H.
Molecular clues to implantation. Endocr Rev 2004; 25:341–373.
32. Singh H, Nardo L, Kimber SJ, Aplin JD. Early stages of implantation as
revealed by an in vitro model. Reproduction 2010; 139:905–914.
33. Grewal S, Carver JG, Ridley AJ, Mardon HJ. Implantation of the human
embryo requires Rac1-dependent endometrial stromal cell migration. Proc
Natl Acad Sci U S A 2008. 21:105(42):16189–161094.
34. Grewal S, Carver JG, Ridley AJ, Mardon HJ. Human endometrial stromal
cell rho GTPases have opposing roles in regulating focal adhesion
turnover and embryo invasion in vitro. Biol Reprod 2010; 83:75–82.
35. Gonzalez M, Neufeld J, Reimann K, Wittmann S, Samalecos A, Wolf A,
Bamberger AM, Gellersen B. Expansion of human trophoblastic spheroids
is promoted by decidualized endometrial stromal cells and enhanced by
heparin-binding epidermal growth factor-like growth factor and interleukin-1
b. Mol Hum Reprod 2011; 17:421–433.
36. Gellersen B, Wolf A, Kruse M, Schwenke M, Bamberger AM. Human
endometrial stromal cell-trophoblast interactions: mutual stimulation of
EMBRYO SELECTION AT IMPLANTATION
7 Article 98
Downloadedfromwww.biolreprod.org.Downloadedfromhttps://academic.oup.com/biolreprod/article/91/4/98,1-8/2434351byMasarykovaUniverzitauseron16April2024
chemotactic migration and promigratory roles of cell surface molecules
CD82 and CEACAM1. Biol Reprod 2013; 88:80.
37. Weimar CH, Kavelaars A, Brosens JJ, Gellersen B, de Vreeden-Elbertse
JM, Heijnen CJ, Macklon NS. Endometrial stromal cells of women with
recurrent miscarriage fail to discriminate between high- and low-quality
human embryos. PLoS One 2012; 7:e41424.
38. Gellersen B, Reimann K, Samalecos A, Aupers S, Bamberger AM.
Invasiveness of human endometrial stromal cells is promoted by
decidualization and by trophoblast-derived signals. Hum Reprod 2010;
25:862–873.
39. Schwenke M, Kno¨fler M, Velicky P, Weimar CH, Kruse M, Samalecos A,
Wolf A, Macklon NS, Bamberger AM, Gellersen B. Control of human
endometrial stromal cell motility by PDGF-BB, HB-EGF and trophoblastsecreted
factors. PLoS One 2013; 8:e54336.
40. Weimar CH, Macklon NS, Post Uiterweer ED, Brosens JJ, Gellersen B.
The motile and invasive capacity of human endometrial stromal cells:
implications for normal and impaired reproductive function. Hum Reprod
Update 2013; 19:542–557.
41. Haouzi D, Dechaud H, Assou S, Monzo C, de Vos J, Hamamah S.
Transcriptome analysis reveals dialogues between human trophectoderm
and endometrial cells during the implantation period. Hum Reprod 2011;
26:1440–1449.
42. Ruan YC, Guo JH, Liu X, Zhang R, Tsang LL, Dong JD, Chen H, Yu
MK, Jiang X, Zhang XH, Fok KL, Chung YW, et al. Activation of the
epithelial Naþ channel triggers prostaglandin E2
release and production
required for embryo implantation. Nat Med 2012; 18:1112–1117.
43. Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the
conceptus and loss of pregnancy. N Engl J Med 1999; 340:1796–1799.
44. Salker MS, Nautiyal J, Steel JH, Webster Z, Sucurovic S, Nicou M, Singh
Y, Lucas ES, Murakami K, Chan YW, James S, Abdallah Y, et al.
Disordered IL-33/ST2 activation in decidualizing stromal cells prolongs
uterine receptivity in women with recurrent pregnancy loss. PLoS One
2012; 7:e52252.
45. Salker MS, Christian M, Steel JH, Nautiyal J, Lavery S, Trew G, Webster
Z, Al-Sabbagh M, Puchchakayala G, Fo¨ller M, Landles C, Sharkey AM, et
al. Deregulation of the serum- and glucocorticoid-inducible kinase SGK1
in the endometrium causes reproductive failure. Nat Med 2011; 7(11):
1509–1513.
46. Salker M, Teklenburg G, Molokhia M, Lavery S, Trew G, Aojanepong T,
Mardon HJ, Lokugamage AU, Rai R, Landles C, Roelen BA, Quenby S, et
al. Natural selection of human embryos: impaired decidualization of
endometrium disables embryo-maternal interactions and causes recurrent
pregnancy loss. PLoS One 2010; 5:e10287.
47. Quenby S, Vince G, Farquharson R, Aplin J. Recurrent miscarriage: a
defect in nature’s quality control? Hum Reprod 2002; 17:1959–1963.
48. Mansouri-Attia N, Sandra O, Aubert J, Degrelle S, Everts RE, GiraudDelville
C, Heyman Y, Galio L, Hue I, Yang X, Tian XC, Lewin HA, et
al. Endometrium as an early sensor of in vitro embryo manipulation
technologies. Proc Natl Acad Sci U S A 2009; 106:5687–5692.
MACKLON AND BROSENS
8 Article 98
Downloadedfromwww.biolreprod.org.Downloadedfromhttps://academic.oup.com/biolreprod/article/91/4/98,1-8/2434351byMasarykovaUniverzitauseron16April2024