Revie
Phy
imp
of Health, Department of Health and Human Services, Bethesda, MD 20892, USA
a r t i c l e i n f o
3.1. Estrogenic derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
. . . . . . . . . . . . . 00
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
0098-2997/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.
⇑ Corresponding authors. Addresses: State Key Laboratory of Reproductive Biology, Institute of Zoology, Chinese Academy of Sciences, 1 Beichen West
Road, Chaoyang District, Beijing 100101, PR China. Tel.: +86 10 64807868; fax: +86 10 64807099 (H. Wang), C.S. Mott Center for Human Growth and
Development, Wayne State University School of Medicine, 275 East Hancock Street, Detroit, MI 48201-1405, USA. Fax: +1 313 577 8554 (D.R. Armant).
E-mail addresses: hbwang@ioz.ac.cn (H. Wang), D.Armant@wayne.edu (D.R. Armant).
1 These authors contributed equally to this work.
Molecular Aspects of Medicine xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Molecular Aspects of Medicine
journal homepage: www.elsevier .com/locate /mam
http://dx.doi.org/10.1016/j.mam.2012.12.011
4.1. Steroid hormones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
4.2. Cytokines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
3.2. Cannabinoid signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Wnt signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. Embryo-derived signals for implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4. Uterine receptivity: unique status of uterine differentiation conducive for embryo implantation. . . . . . . . . . . .
Please cite this article in press as: Zhang, S., et al. Physiological and molecular determinants of embryo implantation. Molecular As
Medicine (2013), http://dx.doi.org/10.1016/j.mam.2012.12.011
. . . . 00
. . . . 00
. . . . 00
underlying mechanisms governing embryo implantation should generate new strategies
to rectify implantation failure and improve pregnancy rates in women.
� 2012 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
2. Maternal hormonal environment required for embryo implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
3. Embryonic preparation for implantation necessitates ‘‘blastocyst activation’’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Article history:
Available online xxxx
Keywords:
Blastocyst activation
Uterine receptivity
Blastocyst attachment
Embryo implantation
Decidualization
a b s t r a c t
Embryo implantation involves the intimate interaction between an implantation-compe-
tent blastocyst and a receptive uterus, which occurs in a limited time period known as
the window of implantation. Emerging evidence shows that defects originating during
embryo implantation induce ripple effects with adverse consequences on later gestation
events, highlighting the significance of this event for pregnancy success. Although a mul-
titude of cellular events and molecular pathways involved in embryo–uterine crosstalk
during implantation have been identified through gene expression studies and genetically
engineered mouse models, a comprehensive understanding of the nature of embryo
implantation is still missing. This review focuses on recent progress with particular atten-
tion to physiological and molecular determinants of blastocyst activation, uterine receptiv-
ity, blastocyst attachment and uterine decidualization. A better understanding of
Shuang Zhang a,b,1, Haiyan Lin a,1, Shuangbo Kong a,b,1, Shumin Wang a, Hongmei Wang a,
Haibin Wang a,⇑, D. Randall Armant c,d,⇑
a State Key Laboratory of Reproductive Biology, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, PR China
bGraduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
cWayne State University School of Medicine, Detroit, MI 48201-1405, USA
d Program in Reproductive and Adult Endocrinology, Eunice Kennedy Shriver National Institute for Child Health and Human Development, National Institutes
siological and molecular determinants of embryo
lantation
w
pects of
Suc
and a
event
bryo,
accom
t
spons
survei
2 S. Zhang et al. /Molecular Aspects of Medicine xxx (2013) xxx–xxx
Please
Medic
es to the semi-allogenic embryo. However, it remains largely unclear how the blastocyst escapes maternal immune
llance at the time of implantation. With the emergence of advanced technologies, a global analysis of gene and protein
o support embryo survival. It is also thought that the decidua functions as a barrier against maternal immunological re-
’’ (Ma et al., 2003; Paria et al., 1993; Rogers and Murphy, 1989; Yoshinaga, 1980). In response to the implanting em-
the surrounding uterine stroma undergoes cellular transformation, a process known as decidualization, to
modate embryonic growth and invasion (Lim and Wang, 2010). Locally induced decidua provides a positive feedback
bryo–uterine interactions during early pregnancy remains to be explored in depth.
The crosstalk between the blastocyst and the uterus can only occur during a brief period, namely the ‘‘window of implan-
tation
ntation (Dey et al., 2004). However, the hierarchical landscape of the molecular signaling pathways that govern em-
2002; Curtis Hewitt et al., 2002). Molecular and genetic evidence indicates that ovarian hormones together with locally pro-
duced signaling molecules, including cytokines, growth factors, homeobox transcription factors, lipid mediators and mor-
phogen genes, function through autocrine, paracrine and juxtacrine interactions to specify the complex process of
impla
ntation.
cessful implantation requires synchronization between the acquisition of implantation competency by the blastocyst
receptive state in the uterine endometrium (Dey et al., 2004; Tranguch et al., 2005b; Wang and Dey, 2006). These two
s are precisely regulated by maternal hormones, in particular, ovarian estrogen and progesterone (Conneely et al.,
Early pregnancy loss, occurring during the periimplantation period before pregnancy is recognized clinically, is a rela-
tively common phenomenon in humans (Cockburn and Rossant, 2010; Norwitz et al., 2001). For example, even in natural
conception, the maximum chance of successful pregnancy occurring in a given menstrual cycle is limited to about 30% (Zina-
man et al., 1996). Only 50–60% of all conceptions advance beyond 20 weeks of gestation (Norwitz et al., 2001). Among the
pregnancies that are lost, implantation failure is the major cause, reaching approximate 75% (Wilcox et al., 1988). Further-
more, 1 out of 7 couples worldwide are suffering from infertility (Forti and Krausz, 1998). Despite significant developments
in in vitro fertilization and embryo transfer (IVF–ET) technology that have overcome many underlying causes of infertility,
pregnancy success rates remain relatively low, mainly due to implantation failure (Miller et al., 2012; Norwitz et al., 2001;
Wilcox et al., 1993). Therefore, it is imperative to address this global issue by investigating the mysteries of embryo
impla
4.3. Homeobox transcription factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
4.4. Developmental genes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5. Cell–cell interactions: the nature of embryo implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5.1. Uterine luminal closure for blastocyst apposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5.2. The trophectoderm–uterine epithelium interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5.3. The epithelial–stromal interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5.3.1. Uterine epithelial responsiveness to estrogen signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5.3.2. Stromal responsiveness to progesterone signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5.4. Uterine glands for embryo implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5.4.1. Uterine adenogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5.4.2. Glandular–luminal epithelial interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5.4.3. Glandular–stromal interaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
6. Molecular basis of decidualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
6.1. Steroid hormones: central players in decidualization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
6.2. Epithelial signals controlling stromal decidualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
6.3. Cell-cycle regulators during decidualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
6.4. Molecular and cellular aspects of immune tolerance during decidualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
6.4.1. Genes regulating immune tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
6.4.2. Immune cells during decidualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
7. Emerging concept: the quality of implantation determines the quality of ongoing pregnancy . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
8. Implications for human fertility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
9. Perspectives and closing remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
1. Introduction
In mammals, a new life begins with the union of an egg with a sperm, a process known as fertilization (Wassarman,
1999). Following fertilization, the zygote undergoes several rounds of divisions and morphogenesis to form the blastocyst,
an embryonic stage with two distinct cell lineages: the outer specialized trophectodermal epithelium and the inner cell mass
(Cockburn and Rossant, 2010; Wang and Dey, 2006). The blastocyst participates in the first physical and physiological inter-
action with the maternal endometrium to initiate implantation (Red-Horse et al., 2004; Wang and Dey, 2006). A bidirectional
crosstalk is essential for normal implantation thus the success of pregnancy, since perturbations will generate adverse out-
comes for subsequent development, including decidualization and placentation, with potential loss of the pregnancy (Chen
et al., 2011; Song et al., 2002; Wilcox et al., 1999; Ye et al., 2005).
cite this article in press as: Zhang, S., et al. Physiological and molecular determinants of embryo implantation. Molecular Aspects of
ine (2013), http://dx.doi.org/10.1016/j.mam.2012.12.011
expression in the implanting embryo and uterus has been undertaken in several studies to unravel the molecular networks
that control implantation in mice, as well as in humans (Hamatani et al., 2004b; Haouzi et al., 2011; Hu et al., 2008; Kao et al.,
2002; Reese et al., 2001; Riesewijk et al., 2003; Yoon et al., 2004; Yoshioka et al., 2000). However, due to experimental dif-
ficulties and ethical restrictions, our understanding of human implantation still relies predominantly on animal models, par-
ticularly the mouse. Gene-knockout mouse models provide valuable information that has been used to construct a tentative
molecular basis of implantation. Since embryo implantation is a dynamic developmental process that integrates many sig-
naling molecules into a precisely orchestrated program, it is important to understand the hierarchical landscape of the path-
ways governing these processes to generate new strategies to correct implantation failure and improve pregnancy rates in
women. This review will examine our understanding of signaling cascades that regulate embryo implantation and decidu-
alization derived from gene expression studies and genetically engineered mouse models.
2. Maternal hormonal environment required for embryo implantation
In the majority of eutherian mammals, implantation occurs in a fixed interval of time after ovulation when the corpus
luteum is fully formed (Finn and Martin, 1974). In humans, this is during the luteal phase of the menstrual cycle, while
in rodents, it is in the diestrous phase of the estrous cycle. It has been well established that estrogen and progesterone
are principal hormones in this process. According to their dynamic fluctuating levels, the reproductive cycle is divided into
three stages (Finn and Martin, 1974; Wang and Dey, 2006). The first stage is the proestrous or follicular phase in women
during which estrogen levels are very high (Michael, 1976; Yoshinaga et al., 1969). The second stage is a period when the
levels of both hormones are low immediately after ovulation. Finally, the luteal stage is when both progesterone and estro-
gen are secreted from the corpus luteum. Embryo implantation occurs at the luteal phase. For example, at this stage in mice,
the level of progesterone is gradually increased, owing to an enhanced secretion from newly formed corpora luteum, accom-
panied by a preimplantation surge of estrogen on day 4 of pregnancy (day 1 = day of vaginal plug), while embryo implan-
tation takes place at the midnight of day 4 (McCormack and Greenwald, 1974; Wang and Dey, 2006) (Fig. 1A). Based on
S. Zhang et al. /Molecular Aspects of Medicine xxx (2013) xxx–xxx 3
Fig. 1. Hormonal control of embryo implantation in mice. (A) Steroid hormone patterns are illustrated during indicated days of the estrous cycle, uterine
receptivity and early pregnancy. Estrogen secretion (red curve) is high at ovulation after the luteinizing hormone surge. Soon afterwards, progesterone (blue
curve) increases beginning in the late afternoon of proestrus. If mating is successful, the newly formed corpora luteum, stimulated by mating behavior, will
secrete progesterone from day 3 onward. On day 4, a small surge of estrogen cooperates with progesterone to induce uterine receptivity. Blastocyst
implantation occurs at midnight of day 4. After implantation, progesterone is required for decidualization, placentation and completion of pregnancy. (B)
Diagrams depicting cross-sections of the preimplantation uterus (day 1, day 4) and implantation sites (day 5, day 8). On day 1, the luminal epithelium of the
non-receptive uterus is highly branched. On day 4, the uterus is receptive with the opposing luminal epithelium that closes around an implanting
blastocyst. On day 5, the mural trophectoderm of the blastocyst attaches to the antimesometrial luminal epithelium. The stromal cells underlying the
invading embryo then proliferate and differentiate to form an avascular primary decidual zone (PDZ) on the afternoon of day 5. Stroma cells next to the PDZ
continue proliferation and differentiation to form a well-vascularized secondary decidual zone (SDZ) by day 8. AM, antimesometrial side; Bl, blastocyst; Em,
embryo; E2, estradiol-17b; GE, glandular epithelium; LE, luminal epithelium; M, mesometrial side; P4, progesterone; S, stroma. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Zhang, S., et al. Physiological and molecular determinants of embryo implantation. Molecular Aspects of
Medicine (2013), http://dx.doi.org/10.1016/j.mam.2012.12.011
of a small dose of estrogen (Krehbiel, 1941). Later studies provide direct evidence for the role of luteal estrogen in normal
4 S. Zhang et al. /Molecular Aspects of Medicine xxx (2013) xxx–xxx
implantation (Cochrane and Meyer, 1957; Whitten, 1958), showing that the timing of ovariectomy before or after luteal
phase estrogen is critical for the induction of delayed implantation. For example, the blastocyst implants normally when
ovariectomy is performed after preimplantation ovarian estrogen secretion, whereas if ovariectomy takes place before estro-
gen secretion, the embryo does not implant and the uterus enters into a condition of delayed implantation. With progester-
one supplementation, the blastocyst remains quiescent, but it can be induced to implant by exogenous estrogen (Paria et al.,
1993). These findings clearly indicate that preimplantation estrogen secretion is crucial for blastocyst implantation into a
progesterone-primed receptive uterus. Since preovulatory estrogen is secreted in most species, it is thought that proestrous
estrogen optimizes the subsequent response just before implantation (Barkley et al., 1979).
Notably, the requirement for ovarian estrogen in implantation is species-specific. In species such as guinea pig, rhesus
monkey, rabbit and golden hamster, progesterone alone is adequate for implantation (Harper et al., 1969; Heap and
Deanesly, 1967; Heap et al., 1981; Kwun and Emmens, 1974; Psychoyos, 1973, 1986). However, participation of estrogen
in implantation of these species may not be completely excluded. A hypothesis has been proposed that blastocysts in these
species could synthesize and secrete estrogen locally to initiate implantation (Dey et al., 2004; Wang and Dey, 2006). In
agreement with this, aromatase, an enzyme for estrogen synthesis, is detected in the blastocyst of hamster and rabbit, while
such an aromatase is absent in mice (Dickmann et al., 1975; Hoversland et al., 1982a; Reese et al., 2008; Sengupta et al.,
1983; Sholl et al., 1983). It remains unclear wh