Implantation in the rhesus monkey: initial penetration of endometrium

Temporally resolved single cell transcriptomics in a human model of amniogenesis

Amniogenesis is triggered in a collection of pluripotent epiblast cells as the human embryo implants. To gain insights into the critical but poorly understood transcriptional machinery governing amnion fate determination, we examined the evolving transcriptome of a developing human pluripotent stem cell-derived amnion model at the single cell level. This analysis revealed several continuous amniotic fate progressing states with state-specific markers, which include a previously unrecognized CLDN10+ amnion progenitor state. Strikingly, we found that expression of CLDN10 is restricted to the amnion-epiblast boundary region in the human post-implantation amniotic sac model as well as in a peri-gastrula cynomolgus macaque embryo, bolstering the growing notion that, at this stage, the amnion-epiblast boundary is a site of active amniogenesis. Bioinformatic analysis of published primate peri-gastrula single cell sequencing data further confirmed that CLDN10 is expressed in cells progressing to amnion. Additionally, our loss of function analysis shows that CLDN10 promotes amniotic but suppresses primordial germ cell-like fate. Overall, this study presents a comprehensive amniogenic single cell transcriptomic resource and identifies a previously unrecognized CLDN10+ amnion progenitor population at the amnion-epiblast boundary of the primate peri-gastrula.

Temporally resolved early bone morphogenetic protein-driven transcriptional cascade during human amnion specification

Amniogenesis, a process critical for continuation of healthy pregnancy, is triggered in a collection of pluripotent epiblast cells as the human embryo implants. Previous studies have established that bone morphogenetic protein (BMP) signaling is a major driver of this lineage specifying process, but the downstream BMP-dependent transcriptional networks that lead to successful amniogenesis remain to be identified. This is, in part, due to the current lack of a robust and reproducible model system that enables mechanistic investigations exclusively into amniogenesis. Here, we developed an improved model of early amnion specification, using a human pluripotent stem cell-based platform in which the activation of BMP signaling is controlled and synchronous. Uniform amniogenesis is seen within 48 hr after BMP activation, and the resulting cells share transcriptomic characteristics with amnion cells of a gastrulating human embryo. Using detailed time-course transcriptomic analyses, we established a previously uncharacterized BMP-dependent amniotic transcriptional cascade, and identified markers that represent five distinct stages of amnion fate specification; the expression of selected markers was validated in early post-implantation macaque embryos. Moreover, a cohort of factors that could potentially control specific stages of amniogenesis was identified, including the transcription factor TFAP2A. Functionally, we determined that, once amniogenesis is triggered by the BMP pathway, TFAP2A controls the progression of amniogenesis. This work presents a temporally resolved transcriptomic resource for several previously uncharacterized amniogenesis states and demonstrates a critical intermediate role for TFAP2A during amnion fate specification.

Temporally resolved early BMP-driven transcriptional cascade during human amnion specification

Amniogenesis, a process critical for continuation of healthy pregnancy, is triggered in a collection of pluripotent epiblast cells as the human embryo implants. Previous studies have established that BMP signaling is a major driver of this lineage specifying process, but the downstream BMP-dependent transcriptional networks that lead to successful amniogenesis remain to be identified. This is, in part, due to the current lack of a robust and reproducible model system that enables mechanistic investigations exclusively into amniogenesis. Here, we developed an improved model of early amnion specification, using a human pluripotent stem cell-based platform in which the activation of BMP signaling is controlled and synchronous. Uniform amniogenesis is seen within 48 hours after BMP activation, and the resulting cells share transcriptomic characteristics with amnion cells of a gastrulating human embryo. Using detailed time-course transcriptomic analyses, we established a previously uncharacterized BMP-dependent amniotic transcriptional cascade, and identified markers that represent five distinct stages of amnion fate specification; the expression of selected markers was validated in early post-implantation macaque embryos. Moreover, a cohort of factors that could potentially control specific stages of amniogenesis was identified, including the transcription factor TFAP2A. Functionally, we determined that, once amniogenesis is triggered by the BMP pathway, TFAP2A controls the progression of amniogenesis. This work presents a temporally resolved transcriptomic resource for several previously uncharacterized amniogenesis states and demonstrates a critical intermediate role for TFAP2A during amnion fate specification.

An atlas of rabbit development as a model for single-cell comparative genomics

Traditionally, the mouse has been the favoured vertebrate model for biomedical research, due to its experimental and genetic tractability. However, non-rodent embryological studies highlight that many aspects of early mouse development, such as its egg-cylinder gastrulation and method of implantation, diverge from other mammals, thus complicating inferences about human development. Like the human embryo, rabbits develop as a flat-bilaminar disc. Here we constructed a morphological and molecular atlas of rabbit development. We report transcriptional and chromatin accessibility profiles for over 180,000 single cells and high-resolution histology sections from embryos spanning gastrulation, implantation, amniogenesis and early organogenesis. Using a neighbourhood comparison pipeline, we compare the transcriptional landscape of rabbit and mouse at the scale of the entire organism. We characterize the gene regulatory programmes underlying trophoblast differentiation and identify signalling interactions involving the yolk sac mesothelium during haematopoiesis. We demonstrate how the combination of both rabbit and mouse atlases can be leveraged to extract new biological insights from sparse macaque and human data. The datasets and computational pipelines reported here set a framework for a broader cross-species approach to decipher early mammalian development, and are readily adaptable to deploy single-cell comparative genomics more broadly across biomedical research.

Evolutionary divergence of embryo implantation in primates

Implantation of the conceptus into the uterus is absolutely essential for successful embryo development. In humans, our understanding of this process has remained rudimentary owing to the inaccessibility of early implantation stages. Non-human primates recapitulate many aspects of human embryo development and provide crucial insights into trophoblast development, uterine receptivity and embryo invasion. Moreover, primate species exhibit a variety of implantation strategies and differ in embryo invasion depths. This review examines conservation and divergence of the key processes required for embryo implantation in different primates and in comparison with the canonical rodent model. We discuss trophectoderm compartmentalization, endometrial remodelling and embryo adhesion and invasion. Finally, we propose that studying the mechanism controlling invasion depth between different primate species may provide new insights and treatment strategies for placentation disorders in humans. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Early human trophoblast development: from morphology to function

Human pregnancy depends on the proper development of the embryo prior to implantation and the implantation of the embryo into the uterine wall. During the pre-implantation phase, formation of the morula is followed by internalization of blastomeres that differentiate into the pluripotent inner cell mass lineage, while the cells on the surface undergo polarization and differentiate into the trophectoderm of the blastocyst. The trophectoderm mediates apposition and adhesion of the blastocyst to the uterine epithelium. These processes lead to a stable contact between embryonic and maternal tissues, resulting in the formation of a new organ, the placenta. During implantation, the trophectoderm cells start to differentiate and form the basis for multiple specialized trophoblast subpopulations, all of which fulfilling specific key functions in placentation. They either differentiate into polar cells serving typical epithelial functions, or into apolar invasive cells that adapt the uterine wall to progressing pregnancy. The composition of these trophoblast subpopulations is crucial for human placenta development and alterations are suggested to result in placenta-associated pregnancy pathologies. This review article focuses on what is known about very early processes in human reproduction and emphasizes on morphological and functional aspects of early trophoblast differentiation and subpopulations.

Trophectoderm differentiation to invasive syncytiotrophoblast is promoted by endometrial epithelial cells during human embryo implantation

STUDY QUESTION

How does the human embryo breach the endometrial epithelium at implantation?

SUMMARY ANSWER

Embryo attachment to the endometrial epithelium promotes the formation of multinuclear syncytiotrophoblast from trophectoderm, which goes on to breach the epithelial layer.

WHAT IS KNOWN ALREADY

A significant proportion of natural conceptions and assisted reproduction treatments fail due to unsuccessful implantation. The trophectoderm lineage of the embryo attaches to the endometrial epithelium before breaching this barrier to implant into the endometrium. Trophectoderm-derived syncytiotrophoblast has been observed in recent in vitro cultures of peri-implantation embryos, and historical histology has shown invasive syncytiotrophoblast in embryos that have invaded beyond the epithelium, but the cell type mediating invasion of the epithelial layer at implantation is unknown.

STUDY DESIGN, SIZE, DURATION

Fresh and frozen human blastocyst-stage embryos (n = 46) or human trophoblast stem cell (TSC) spheroids were co-cultured with confluent monolayers of the Ishikawa endometrial epithelial cell line to model the epithelial phase of implantation in vitro. Systems biology approaches with published transcriptomic datasets were used to model the epithelial phase of implantation in silico.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Human embryos surplus to treatment requirements were consented for research. Day 6 blastocysts were co-cultured with Ishikawa cell layers until Day 8, and human TSC spheroids modelling blastocyst trophectoderm were co-cultured with Ishikawa cell layers for 48 h. Embryo and TSC morphology was assessed by immunofluorescence microscopy, and TSC differentiation by real-time quantitative PCR (RT-qPCR) and ELISA. Single-cell human blastocyst transcriptomes, and bulk transcriptomes of TSC and primary human endometrial epithelium were used to model the trophectoderm-epithelium interaction in silico. Hypernetworks, pathway analysis, random forest machine learning and RNA velocity were employed to identify gene networks associated with implantation.

MAIN RESULTS AND THE ROLE OF CHANCE

The majority of embryos co-cultured with Ishikawa cell layers from Day 6 to 8 breached the epithelial layer (37/46), and syncytiotrophoblast was seen in all of these. Syncytiotrophoblast was observed at the embryo-epithelium interface before breaching, and syncytiotrophoblast mediated all pioneering breaching events observed (7/7 events). Multiple independent syncytiotrophoblast regions were seen in 26/46 embryos, suggesting derivation from different regions of trophectoderm. Human TSC spheroids co-cultured with Ishikawa layers also exhibited syncytiotrophoblast formation upon invasion into the epithelium. RT-qPCR comparison of TSC spheroids in isolated culture and co-culture demonstrated epithelium-induced upregulation of syncytiotrophoblast genes CGB (P = 0.03) and SDC1 (P = 0.008), and ELISA revealed the induction of hCGβ secretion (P = 0.03). Secretory-phase primary endometrial epithelium surface transcriptomes were used to identify trophectoderm surface binding partners to model the embryo-epithelium interface. Hypernetwork analysis established a group of 25 epithelium-interacting trophectoderm genes that were highly connected to the rest of the trophectoderm transcriptome, and epithelium-coupled gene networks in cells of the polar region of the trophectoderm exhibited greater connectivity (P < 0.001) and more organized connections (P < 0.0001) than those in the mural region. Pathway analysis revealed a striking similarity with syncytiotrophoblast differentiation, as 4/6 most highly activated pathways upon TSC-syncytiotrophoblast differentiation (false discovery rate (FDR < 0.026)) were represented in the most enriched pathways of epithelium-coupled gene networks in both polar and mural trophectoderm (FDR < 0.001). Random forest machine learning also showed that 80% of the endometrial epithelium-interacting trophectoderm genes identified in the hypernetwork could be quantified as classifiers of TSC-syncytiotrophoblast differentiation. This multi-model approach suggests that invasive syncytiotrophoblast formation from both polar and mural trophectoderm is promoted by attachment to the endometrial epithelium to enable embryonic invasion.

LARGE SCALE DATA

No omics datasets were generated in this study, and those used from previously published studies are cited.

LIMITATIONS, REASONS FOR CAUTION

In vitro and in silico models may not recapitulate the dynamic embryo-endometrial interactions that occur in vivo. The influence of other cellular compartments in the endometrium, including decidual stromal cells and leukocytes, was not represented in these models.

WIDER IMPLICATIONS OF THE FINDINGS

Understanding the mechanism of human embryo breaching of the epithelium and the gene networks involved is crucial to improve implantation success rates after assisted reproduction. Moreover, early trophoblast lineages arising at the epithelial phase of implantation form the blueprint for the placenta and thus underpin foetal growth trajectories, pregnancy health and offspring health.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by grants from Wellbeing of Women, Diabetes UK, the NIHR Local Comprehensive Research Network and Manchester Clinical Research Facility, and the Department of Health Scientist Practitioner Training Scheme. None of the authors has any conflict of interest to declare.

Uterine cellular changes during mammalian pregnancy and the evolution of placentation

There are many different forms of nutrient provision in viviparous (live bearing) species. The formation of a placenta is one method where the placenta functions to transfer nutrients from mother to fetus (placentotrophy), transfer waste from the fetus to the mother and respiratory gas exchange. Despite having the same overarching function, there are different types of placentation within placentotrophic vertebrates, and many morphological changes occur in the uterus during pregnancy to facilitate formation of the placenta. These changes are regulated in complex ways but are controlled by similar hormonal mechanisms across species. This review describes current knowledge of the morphological and molecular changes to the uterine epithelium preceding implantation among mammals. Our aim is to identify the commonalities and constraints of these cellular changes to understand the evolution of placentation in mammals and propose directions for future research. We compare and discuss the complex modifications to the ultrastructure of uterine epithelial cells and show that there are similarities in the changes to the cytoskeleton and gross morphology of the uterine epithelial cells, especially of the apical and lateral plasma membrane of the cells during the formation of a placenta in all eutherians and marsupials studied to date. We conclude that further research is needed to understand the evolution of placentation among viviparous mammals, particularly concerning the level of placental invasiveness, hormonal control and genetic underpinnings of pregnancy in marsupial taxa.

Mammalian primordial germ cell specification

The peri-implantation window of mammalian development is the crucial window for primordial germ cell (PGC) specification. Whereas pre-implantation dynamics are relatively conserved between species, the implantation window marks a stage of developmental divergence between key model organisms, and thus potential variance in the cell and molecular mechanisms for PGC specification. In humans, PGC specification is very difficult to study in vivo To address this, the combined use of human and nonhuman primate embryos, and stem cell-based embryo models are essential for determining the origin of PGCs, as are comparative analyses to the equivalent stages of mouse development. Understanding the origin of PGCs in the peri-implantation embryo is crucial not only for accurate modeling of this essential process using stem cells, but also in determining the role of global epigenetic reprogramming upon which sex-specific differentiation into gametes relies.

MUC20 expression marks the receptive phase of the human endometrium

RESEARCH QUESTION

How does mucin MUC20 expression change during the menstrual cycle in different cell types of human endometrium?

DESIGN

Study involved examination of MUC20 expression in two previously published RNA-seq datasets in whole endometrial tissue (n = 10), sorted endometrial epithelial (n = 44) or stromal (n = 42) cell samples. RNA-Seq results were validated by quantitative reverse transcription polymerase chain reaction (qRT-PCR) in whole tissue (n = 10), sorted epithelial (n = 17) and stromal (n = 17) cell samples. MUC20 protein localization and expression were analysed in human endometrium by immunohistochemical analysis of intact endometrial tissue (n = 6) and also Western blot of cultured stromal and epithelial cells (n = 2).

RESULTS

MUC20 is differentially expressed in the endometrium between the pre-receptive and receptive phases. We show that MUC20 is predominantly expressed by epithelial cells of the receptive endometrium, both at the mRNA (RNA-Seq, P = 0.005; qRT-PCR, P = 0.039) and protein levels (Western blot; immunohistochemistry, P = 0.029).

CONCLUSION

Our results indicate MUC20 as a novel marker of mid-secretory endometrial biology. We propose a model of MUC20 function in the hepatocyte growth factor (HGF)-activated mesenchymal-epithelial transition (MET) receptor signalling specifically in the receptive phase. Further investigations should reveal the precise function of MUC20 in human endometrium and the possible connection between MUC20 and HGF-activated MET receptor signalling. MUC20 could potentially be included in the list of endometrial receptivity markers after further clinical validation.

A historical review of blastocyst implantation research

Research development on blastocyst implantation was reviewed in three sections: primate implantation, ungulate farm animal implantation, and the general process of blastocyst implantation in small rodents. Future research directions of this area are suggested.

The role of invasive trophoblast in implantation and placentation of primates

We here review the evolution of invasive placentation in primates towards the deep penetration of the endometrium and its arteries in hominoids. The strepsirrhine primates (lemurs and lorises) have non-invasive, epitheliochorial placentation, although this is thought to be derived from a more invasive type. In haplorhine primates, there is differentiation of trophoblast at the blastocyst stage into syncytial and cellular trophoblast. Implantation involves syncytiotrophoblast that first removes the uterine epithelium then consolidates at the basal lamina before continuing into the stroma. In later stages of pregnancy, especially in Old World monkeys and apes, cytotrophoblast plays a greater role in the invasive process. Columns of trophoblast cells advance to the base of the implantation site where they spread out to form a cytotrophoblastic shell. In addition, cytotrophoblasts advance into the lumen of the spiral arteries. They are responsible for remodelling these vessels to form wide, low-resistance conduits. In human and great apes, there is additional invasion of the endometrium and its vessels by trophoblasts originating from the base of the anchoring villi. Deep trophoblast invasion that extends remodelling of the spiral arteries to segments in the inner myometrium evolved in the common ancestor of gorilla, chimp and human.

Implantation and Establishment of Pregnancy in Human and Nonhuman Primates

Implantation and the establishment of pregnancy are critical for the propagation of the species, but yet remain the limiting steps in human and primate reproduction. Successful implantation requires a competent blastocyst and a receptive endometrium during a specific window of time during the menstrual cycle to initiate the bilateral communication required for the establishment of a successful pregnancy. This chapter provides an overview of these processes and discusses the molecular mechanisms associated with implantation of the blastocyst and decidualization of the uterus in primates.

Quantifying the biomechanics of conception: L-selectin-mediated blastocyst implantation mechanics with engineered "trophospheres"

An estimated 12% of women in the United States suffer from some form of infertility. In vitro fertilization (IVF) is the most common treatment for infertility encompassing over 99% of all assisted reproductive technologies. However, IVF has a low success rate. Live birth rates using IVF can range from 40% in women younger than 35 years to 4% in women older than 42 years. Costs for a successful IVF outcome can be upward of $61,000. The low success rate of IVF has been attributed to the inability of the blastocyst to implant to the uterus. Blastocyst implantation is initiated by L-selectin expressing cells, trophoblasts, binding to L-selectin ligands, primarily sialyl Lewis X (sLeX), on the uterine surface endometrium. Legal and ethical considerations have limited the research on human subjects and tissues, whereas animal models are costly or do not properly mimic human implantation biochemistry. In this work, we describe a cellular model system for quantifying L-selectin adhesion mechanics. L-selectin expression was confirmed in Jeg-3, JAR, and BeWo cell lines, with only Jeg-3 cells exhibiting surface expression. Jeg-3 cells were cultured into three-dimensional spheres, termed "trophospheres," as a mimic to human blastocysts. Detachment assays using a custom-built parallel plate flow chamber show that trophospheres detach from sLeX functionalized slides with 2.75 × 10(-3) dyn of force and 7.5 × 10(-5) dyn-cm of torque. This work marks the first time a three-dimensional cell model has been utilized for quantifying L-selectin binding mechanics related to blastocyst implantation.

Iron transportation across the placenta

According to the classification of placental types among animals, the transfer of iron through the placenta can occur via: absorption connected to transferin through the outer surface of the trophoblast in direct contact with circulating maternal blood; absorption of the erythrocytes by the chorionic epithelium in direct contact with accumulation of blood extravased from haemotophagous areas; absorption by the chorionic epithelium in direct contact with iron enriched secretions from the endometrial glands and absorption by extravasations of the blood in the maternal-fetal surface and the subsequent phagocytosis of the erythrocytes by trophoblast cells described in bovine, small ruminants, canine and feline. The function of erythrophagocytosis observed after the extravasation of blood in the maternal-fetal interface is undefined in several species. Possibly, the iron is transferred to the fetus through the trophoblastic erythrophagocytosis in the hemophogous area of the placenta and also in the endometrial glands. In this literature survey, new methods of studies regarding placental transfer involving iron and other nutrients necessary for survival and maintenance of embryonic fetus to birth are proposed.

Cytoskeletal remodelling proteins identified in fetal-maternal interface in pregnant women and rhesus monkeys

The uterus undergoes dramatic remodelling in preparation for embryo implantation and pregnancy establishment. A receptive uterus is pivotal for embryo attachment, implantation and the eventual formation of a hemochorial placenta. We have previously identified by proteomics that tropomyosin alpha-4 chain (TPM4), protein disulfide isomerase A1 (PDIA1) and src substrate cortactin 8 (SRC8) were up regulated in the decidualized stromal cells during the late secretory phase of the menstrual cycle in women. These three proteins are associated with cytoskeletal remodelling. This study determined the localization of these three cytoskeletal proteins in the fetal-maternal interface including the decidual cells in the 1st trimester of pregnancy in women and rhesus monkeys. Immunohistochemical analysis revealed that TPM4, PDIA1 and SRC8 were all expressed by the decidual cells and the wall of the spiral arterioles in pregnant women. Similar expression pattern were also found in the rhesus monkey. In addition, TPM4, PDIA and SRC8 were also localized to the trophoblast cells, further highlighting the importance of these cytoskeletal remodelling proteins in early pregnancy.

Expression of endometrial protein kinase a during early pregnancy in bonnet monkeys (Macaca radiata)

Embryo-induced signaling pathways are considered to be important for initiation and sustenance of pregnancy. However many of these pathways remain to be deciphered in primates. In the present study, differential display RT-PCR was used to identify genes or gene fragments that are differentially expressed in endometrium of bonnet monkeys (Macaca radiata) on Day 6 of pregnancy. Of several fragments found to be differentially expressed, a fragment of 567 base pair (named GG1) was characterized in detail. GG1 was highly represented in endometrium of pregnant animals compared with that of nonpregnant animals. Sequencing analysis revealed homology of this fragment to exons 7, 8, 9, and 10 and surprisingly to intron 6 of cAMP-dependent protein kinase A (PKA) regulatory type I alpha (tissue-specific extinguisher 1) (PRKAR1A). The increased expression of this fragment in gestational endometrium was confirmed by quantitative PCR studies. Two transcripts of 3.0 kilobase (kb) and 1.5 kb were detected in Northern blot probed with labeled GG1. Protein expressions of alpha regulatory (PRKAR1A) and alpha catalytic (PRKCA) subunits of PKA were also higher in gestational endometrium compared with that in nongestational endometrium. Further in vitro studies using human endometrial explants demonstrated regulation of PRKAR1A (or GG1) and prostaglandin-endoperoxide synthase 2 or cyclooxygenase 2 (PTGS2) by estradiol. This is the first study to date on the differential expression of PKA in primate endometrium during early pregnancy and its in vitro regulation by estradiol.

Trophoblast invasion

The establishment of a pregnancy depends upon mechanisms of forming close contacts between embryonic/fetal and maternal tissues. The starting point is the attachment of the blastocyst to the uterine wall, and this critical step is followed by a series of different morphogenetic events leading to placentation. These processes depend on the invasive properties of extra-embryonic trophectoderm-derived cells that show their highest expression in species with haemochorial placentation, i.e. Insectivores, rodents, and primates including the human. In this review we will concentrate upon the human with occasional reference to other species.

Expression of VEGF-receptor system in conceptus during peri-implantation period and endometrial and luteal expression of soluble VEGFR-1 in the pig

In view of the importance of vascular events observed during gestation, it was hypothesized that the VEGF-receptor system plays a critical role during early pregnancy and maternal recognition of pregnancy in pigs. This hypothesis was tested by examining the expression of the VEGF-receptor system in the porcine conceptus. Additionally, the endometrium, corpus luteum (CL) and embryos were studied for the expression of soluble VEGF receptor 1 (sVEGFR-1), the strong endogenous antagonist of VEGF. The expression patterns show that VEGF164 mRNA levels increase gradually in line with conceptus development, whereas VEGF120 and VEGFR-2 remain unchanged during the peri-implantation period. Interestingly, elevated VEGFR-1 expression was observed in conceptuses on days 15-16 of gestation (P<0.05). Comparison of the endometrial sVEGFR-1 mRNA expression revealed up-regulation on days 12 and 15-16 of pregnancy (P<0.01 and P<0.05, respectively). Furthermore, increased sVEGFR-1 levels were observed on day 12 of the estrous cycle in the CL (P<0.05). Concluding, it seems that conceptus-derived VEGF164 plays crucial role in peri-implantation vascular events in pigs. These results support a potential role of VEGFR-1 in the proper growth and development of porcine conceptus during pregnancy. Moreover, expression patterns of sVEGFR-1 in the endometrium of pregnant pigs suggest that it may participate in vascular remodeling important for successful implantation. Finally, luteal sVEGFR-1 may be involved in the maintenance of CL function whenever pregnancy occurs in pigs.

Assessment of VEGF-receptor system expression in the porcine endometrial stromal cells in response to insulin-like growth factor-I, relaxin, oxytocin and prostaglandin E2

Several factors participate in regulation of growth and development as well as angiogenesis of the uterus during pregnancy, and hence little is known about the role of hormonal regulation of vascular endothelial growth factor (VEGF)-receptor system expression. This study has examined the effect of insulin-like growth factor-I (IGF-I), relaxin (RLX), oxytocin (OT) and prostaglandin (PG) E(2), on VEGF secretion and VEGF-receptor system mRNA expression in the porcine endometrial stromal cells. IGF-I and RLX were identified as the most effective inducers of VEGF secretion and mRNA expression. Although PGE(2) stimulated VEGF secretion and VEGF164 mRNA expression, OT inhibited both secretion and mRNA expression of VEGF. When tested for VEGF receptors (R), all factors failed to affect their mRNA expression. Media conditioned by stromal cells collected after IGF-I and RLX treatment significantly increased endothelial cell proliferation and this effect was blocked by soluble VEGFR-1. These data suggest that during early pregnancy IGF-I, RLX and PGE(2) can affect VEGF expression in the endometrium and therefore may support uterine and embryo development, implantation and pregnancy.

Endometrial modifications during early pregnancy in bonnet monkeys (Macaca radiata)

The present study was undertaken to investigate endometrial modifications that occur before embryo invasion in bonnet monkeys (Macaca radiata). These changes were analysed in luminal epithelium, glandular epithelium and stroma of endometrial functionalis on Day 6 post ovulation from pregnant and non-pregnant animals (n = 4 each) by transmission electron microscopy. Distinct features (i.e. loss of columnar shape by epithelial cells, changes in mitochondrial size and diffused apicolateral gap junctions) were observed in the luminal and glandular epithelium in pregnant animals. Stromal compaction was also observed in pregnant animals. Further, immunogold localisation studies demonstrated significantly higher expression (P < 0.05) of oestrogen receptor alpha, an oestrogen-regulated gene, in the glandular epithelium and stroma of the endometrium in pregnant animals compared with non-pregnant animals. Expression of two other genes known to be regulated by oestradiol, namely beta-actin and cyclo-oxygenase-1, were also significantly higher (P < 0.05) in the endometria of pregnant animals. These studies demonstrate marked changes in the endometrium before embryo invasion in bonnet monkeys. These studies also indicate altered oestrogenic activity in the uterine milieu before embryo invasion.

Implantation in the macaque: expansion of the implantation site during the first week of implantation

Data accumulated over several years of investigating implantation in macaque monkeys have been used to estimate the rate of expansion of the initial implantation site, the increase in volume of the site, and the rate of arterial invasion by cytotrophoblast columns. In addition the expansion of the secondary implantation site has also been estimated. The primary implantation site expands from an average diameter of 0.268 mm on day 10 to 4.93 mm on day 16-17. It expands in thickness from 0.064 mm on day 10 to 0.96 mm on day 16-17, and in volume from 0.0036 mm(3) on day 10 to 18.34 mm(3) on day 16-17. During this period of rapid expansion in extent and volume of the implantation site, trophoblast invades the endometrium, forms a lacunar stage, and initiates villus formation; consequently these very considerable changes in structure occur when the implantation is still very small yet growing rapidly. The secondary site expands from 0.23 mm in diameter on day 12 to 2.8mm on day 17. The rate of penetration of cytotrophoblast into endometrial arteries diminishes from 0.602 mm per day on day 12 to 0.171 mm per day on day 16, using the straight-line method of estimation. This diminution in rate is consistent with the hypothesis that cytotrophoblast cells generated at the anchoring villi migrate over cytotrophoblast cells that have attached to the endothelium of the endometrial arteries in advancing the intraluminal columns of trophoblast. It is hoped that the summaries provided will be useful to investigators using macaque monkeys to analyze aspects of implantation in primates.

Expression of prostasin and protease nexin-1 in rhesus monkey (Macaca mulatta) endometrium and placenta during early pregnancy

Serine proteases have been documented to play key roles in uterine matrix turnover and trophoblastic invasion during implantation. Roles of prostasin serine protease in these processes, however, are currently unclear. The present study was first conducted to investigate the colocalization of prostasin and its cognate serpin, protease nexin-1 (PN-1), in rhesus monkey endometrium and placenta on days 12, 18, and 26 of pregnancy by using in situ hybridization (ISH) and immunohistochemistry. With ISH, expression of prostasin mRNA was intensely localized in the glandular epithelium on days 12 and 18 and in the placental villi, trophoblastic column, trophoblastic shell, and fetal-maternal border on days 18 and 26. With the progress of pregnancy, expression level in the glandular epithelium was significantly decreased, and the accumulation in the placental compartments was further increased. In addition, the stroma and arterioles exhibited modest levels of prostasin signals. However, expression level of PN-1 in these compartments on adjacent sections in the three stages of early pregnancy was weak or below the level of detection. Prostasin protein expression in the endometrium was found to be consistent with the distribution patterns revealed in the ISH experiments. It may be suggested from these results that prostasin is involved in endometrial epithelial morphology establishment, tissue remodeling, and trophoblastic invasion during early pregnancy. The cognate serpin PN-1 was not coordinately expressed along with prostasin, creating a tissue environment favorable for proteolytic activities of prostasin during early pregnancy events.

Expression of tissue inhibitor of metalloproteinase-4 (TIMP-4) in endometrium and placenta of rhesus monkey (Macaca mulatta) during early pregnancy

The tissue inhibitors of metalloproteinases (TIMPs) are secreted as important regulators of the matrix metalloproteinases (MMPs). TIMP-4 is the most recently characterized member of the TIMPs family. In the present study, we examined the expression and localization of the TIMP-4 transcript and protein in endometrium and placenta of rhesus monkey (Macaca mulatta) on days 12, 18 and 26 of pregnancy using RT-PCR, in situ hybridization, and immunohistochemistry. The fragment of TIMP-4 gene from rhesus monkey uterine samples shared 95% identity with the corresponding region of human homologue. On day 12 of pregnancy, TIMP-4 mRNA was mainly expressed in the glandular and luminal epithelium. On days 18 and 26 of pregnancy, the expression of TIMP-4 mRNA tended to decline in glandular epithelium and there were strong staining in the placental villi. Furthermore, TIMP-4 mRNA was very faint or undetectable in the stromal cells, endothelial cells of arterioles and myometrium at any stages of pregnancy. The results of immunohistochemical analysis were similar to that of its mRNA. These findings indicate that TIMP-4 might play an important role in glandular secretion, endometrial tissue remodeling and invasion of the trophoblast cells by regulating MMPs in a localized manner in the uteri of rhesus monkey during early pregnancy.

Priming and remodelling of human placental bed spiral arteries during pregnancy--a review

It is now well known that in order to establish human hemochorial placentation and to provide a progressive increase in blood supply to the growing fetus, the uterine spiral arteries must undergo considerable alterations. This physiological modification is thought to be brought about by the interaction of invasive cytotrophoblast with the spiral artery vessel wall. Despite intensive research our understanding of the mechanisms that control human trophoblast invasion in normal, let alone abnormal pregnancy, are still poorly understood. This is partly due to difficulties in obtaining "true" placental bed biopsies and most investigators have relied on in vitro models of trophoblast invasion. Clearly interpretation of such studies must be tempered with a degree of caution. This review outlines why the placental bed is important, how we can sample and study it, what morphology actually occurs in the placental bed spiral arteries during pregnancy and then briefly summarise the findings on the placental bed in pre-eclampsia.

Uterine receptivity and the plasma membrane transformation

This review begins with a brief commentary on the diversity of placentation mechanisms, and then goes on to examine the extensive alterations which occur in the plasma membrane of uterine epithelial cells during early pregnancy across species. Ultrastructural, biochemical and more general morphological data reveal that strikingly common phenomena occur in this plasma membrane during early pregnancy despite the diversity of placental types--from epitheliochorial to hemochorial, which ultimately form in different species. To encapsulate the concept that common morphological and molecular alterations occur across species, that they are found basolaterally as well as apically, and that moreover they are an ongoing process during much of early pregnancy, not just an event at the time attachment, the term 'plasma membrane transformation' is suggested which also emphasises that alterations in this plasma membrane during early pregnancy are key to uterine receptivity.

Compartmental distinctions in uterine Muc-1 expression during early pregnancy in cynomolgous macaque (Macaca fascicularis) and baboon (Papio anubis)

BACKGROUND

Loss of the transmembrane mucin, Muc-1, is a molecular correlate of the acquisition of uterine receptivity to embryo adhesion in most species examined. In macaques, two distinct adhesion events occur at opposite sides of the uterus. Attachment to the secondary site is delayed relative to the primary site. The aim was to determine if Muc-1 is removed at secondary sites prior to trophoblast attachment.

METHODS

We examined Muc-1 expression in the uteri of cynomolgus macaque and baboon during early implantation by immunocytochemistry.

RESULTS

Luminal epithelia were devoid of Muc-1 at all stages examined at both primary and secondary adhesion sites. Loss of Muc-1 in luminal epithelia was found to be maternally determined, accompanied membrane transformation in both macaque and baboon, and at secondary implantation sites, preceded trophoblast attachment. In contrast, glandular epithelia in pregnant macaques exhibited a temporal and compartmentalized gradient of Muc-1 loss confined to the implantation sites. Glandular epithelia in the pregnant baboon uterus were uniformly negative for Muc-1.

CONCLUSIONS

Restriction of the Muc-1 loss in glandular epithelia to conceptual cycles may reflect the fundamental distinctions among epithelia of the various uterine compartments and the differential modulation of Muc-1 that occurs within these compartments in conceptual and non-conceptual cycles.

Spontaneous epithelial plaques in the uterus of a non-pregnant cynomolgus monkey (Macaca fascicularis)

Two epithelial plaques were observed in the uterus of a 4 years 8 month old non-pregnant cynomolgus monkey. Normally, epithelial plaques occur near the implantation site of primates during early pregnancy but can also be induced by various procedures. Both plaques consisted of clusters and nest of cells with large nuclei and a faintly basophilic cytoplasm. Marked cellular pleomorphism was seen, including cells with giant nuclei and binucleated cells. Further histological features were vacuolation, mitosis and PAS-positive granules in several cells and polymorphonuclear infiltration in the periphery of the plaque. This is the second description of a spontaneous epithelial plaque in a primate and the first in a cynomolgus monkey.

Animal models of implantation

Implantation is an intricately timed event necessary in the process of viviparous birth that allows mammals to nourish and protect their young during early development. Human implantation begins when the blastocyst both assumes a fixed position in the uterus and establishes a more intimate relationship with the endometrium. Due to the impracticalities of studying implantation in humans, animal models are necessary to decipher the molecular and mechanical events of this process. This review will discuss the differences in implantation between different animal models and describe how these differences can be utilized to investigate discrete implantation stages. In addition, factors that have been shown to be involved in implantation in the human and other various animal models including growth factors, cytokines, modulators of cell adhesion, and developmental factors will be discussed, and examples from each will be given.

The human cytotrophoblastic cell, a mononuclear chameleon

The human placenta represents an abundant; easily accessible and unlimited study material (at birth a human placenta provides about 500 g of trophoblast). Cytotrophoblastic cells (CTB) are one constituent of the human placenta and represent epithelial cells with fascinating properties: They are able to fuse to form syncytia, can behave like immotile polarized epithelial cells, can phenocopy stromal fibroblasts or endothelial cells or undergo a mesenchymal-like transformation that converts them into non proliferative and highly invasive cells. Like a chameleon, CTB are thus able to adapt to their immediate environment by phenocopying their neighbor cells. This review describes the different routes that CTB follow during their differentiation pathways, the regulation of these at the molecular level, it gives also an overview of the pathologies associated with faulty pathways and describes the usual phenotypic markers used to identify the different CTB subsets. This review is intended to stimulate investigators not acquainted with the field of placental biology to use CTB as a model to study important biological functions in vitro, such as cell fusion, cell invasion and cell transformation.

Morphological events in the primate endometrium in the presence of a preimplantation embryo, detected by the serum preimplantation factor bioassay

BACKGROUND

Hormonal modulation of the endometrium towards receptivity is well established; however, the role of embryonic stimuli in modulation of the endometrium prior to implantation, especially in primates, is unknown. The aim of the present study was to evaluate the endometrial histology when the embryo was present in its vicinity prior to implantation.

METHODS

Preimplantation factor (PIF) bioassay was used as a tool to detect the presence of an embryo in the uterine lumen of mated bonnet monkeys (Macaca radiata) (n=9). The control group comprised seven non-mated animals. The specificity of the PIF bioassay for the presence of an embryo was tested by studies in pregnant humans and monkeys. The effects of embryonic stimuli on the endometrial morphology were analysed by routine haematoxylin-eosin staining. The expressions of CD34, an endothelial cell marker, alpha-smooth muscle actin (alpha-SMA), a marker for blood vessel maturation, and prolactin, a marker of endometrial decidualization, were studied by immunohistochemistry.

RESULTS

That PIF is embryo specific was established by its presence in sera of pregnant humans, monkeys and also in embryo culture media. Six mated bonnet monkeys were found to be PIF positive. Morphologically, the endometria from these PIF-positive animals showed the presence of the pre-epithelial plaque reaction, increased angiogenesis and stromal compaction. The significantly increased number of CD34- and alpha-SMA-positive blood vessels (P<0.05) in the endometria of PIF-positive animals indicated increased angiogenesis in response to embryonic stimuli. The endometrial expression of immunoreactive prolactin was also significantly increased (P<0.05) in the PIF-positive animals, indicating decidualization.

CONCLUSIONS

Using PIF as a marker to detect early pregnancy in bonnet monkeys, we have shown that the embryo induces a pre-epithelial plaque type of reaction, increased angiogenesis and decidual reaction in the endometrium prior to implantation.

Reproductive adaptations to a large-brained fetus open a vulnerability to anovulation similar to polycystic ovary syndrome

During the ovarian or menstrual cycle, prior to ovulation, many female primates exhibit a relatively prolonged follicular phase and terminate the postovulatory luteal phase with menstrual bleeding. The prolonged follicular phase is a trait that distinguishes primate from nonprimate species. It enables extended estrogen-induced proliferation and growth of the uterine endometrium prior to progesterone-induced maturation during the luteal phase to accommodate a potential pregnancy with a rapidly invading placenta. Progressive development of both an extended duration of estrogen-induced, preimplantation endometrial proliferation and a rapidly invading placenta across the Primate order may well have been necessary to accommodate differentiation and growth of an increasingly large fetal brain. Prolongation of the follicular phase in primates has also led to the isolation of the final stages of follicle selection (growth deviation of the dominant follicle from its contemporaries) solely within the follicular phase and thus outside the protection of luteal phase progesterone inhibition of pituitary luteinizing hormone (LH) secretion. Such primate reproductive characteristics put the latter stages of ovarian follicle selection at risk of exposure to excessive pituitary secretion of LH. Excessive secretion of LH during follicle selection could result not only in impaired follicle development, excessive ovarian androgen secretion, and ovulation failure, but also in excessive estrogenic stimulation of the uterine endometrium without intervening menstrual periods. Such reproductive abnormalities are all found in a single, prevalent infertility syndrome afflicting women in their reproductive years: polycystic ovary syndrome (PCOS). We propose that successful female reproductive adaptations to accommodate the growth demands of large-brained primate fetuses have facilitated a particular vulnerability of higher primates to hypergonadotropic disruption of ovulatory function, as found in PCOS.

Human physiological adaptation to pregnancy: inter- and intraspecific perspectives

Reproductive success requires successful maternal physiological adaptation to pregnancy. An interspecific perspective reveals that the human species has modified features of our haplorhine heritage affecting the uteroplacental circulation. We speculate that such modifications - including early implantation and deep, widespread invasion of fetal (trophoblast cells) into and resultant remodeling of maternal uterine vessels - are responses to or compensation for the biomechanical constraints imposed by bipedalism which, in turn, render our species susceptible to the pregnancy complication of preeclampsia. Preeclampsia is characterized by incomplete remodeling of maternal uterine vessels as the result of shallow trophoblast invasion, which in turn reduces uteroplacental blood flow and frequently leads to intrauterine growth restriction (IUGR). Using an intraspecific perspective, we consider the fitness-related consequences of variation in uteroplacental blood flow during high-altitude pregnancy. Although birth weights are reduced at high altitudes in Bolivia, multigenerational Andean residents are relatively protected from altitude-associated IUGR. Our preliminary data suggest that Andean women have greater uteroplacental oxygen delivery than European high-altitude residents due to more complete growth and remodeling of maternal uterine vessels. Identification of the physiological and genetic mechanisms involved in such inter- and intraspecific variations in pregnancy physiology will likely be useful for understanding human evolution and contemporary challenges to successful reproduction.

Modification of uterine vasculature during pregnancy in macaques

Embryonic development in macaques includes extensive modification of the uterine vasculature by fetal trophoblast cells. Soon after the onset of blastocyst attachment to the endometrium, syncytial trophoblast cells intrude between endometrial epithelial cells, resulting in focal epithelium loss. Trophoblast cells continue to move into the endometrial stroma and encounter superficial uterine capillaries. These capillaries are penetrated by trophoblast, which permits maternal blood to leave the maternal circulation and enter lacunae formed within the mass of trophoblast cells. Cytotrophoblast cells enter the uterine vessels and attach to the endothelium via cell adhesion molecules prior to migration into confluent spiral arterioles, against the flow of blood. As intra-arterial cytotrophoblast cells migrate, they displace adjacent endothelium, produce matrix metalloproteinases, traverse the tunica intima, and reside in the tunica media as intramural trophoblast. Intramural trophoblast cells disrupt the tunica media and become surrounded by an extensive extracellular matrix. In areas proximal to the placenta, the entire circumferences of spiral arteries are modified in this way. In the same arteries, distal to the placenta and farther "upstream," trophoblast-mediated changes to the arterial wall are less extensive. Uterine veins are modified by trophoblast only in the area immediately next to the trophoblast shell, with no trophoblast migration. The functional consequence of this trophoblast activity may be to ensure an adequate flow of maternal blood to the placenta, thus enhancing the survival of the fetus.

Development of the utero-placental circulation: the role of carbon monoxide and nitric oxide in trophoblast invasion and spiral artery transformation

It is now well known that in order to establish human hemochorial placentation and to provide a progressive increase in blood supply to the growing fetus, the uterine spiral arteries must undergo considerable alterations. This physiological modification is thought to be brought about by the interaction of invasive cytotrophoblast with the spiral artery vessel wall. Despite intensive research our understanding of the mechanisms that control human trophoblast invasion in normal, let alone abnormal pregnancy, are sill poorly understood. This is partly due to difficulties in obtaining "true" placental bed biopsies and most investigators have relied on in vitro models of trophoblast invasion. This article describes the morphological changes that occur within the placental bed throughout human pregnancy along with a review of the various studies which have attempted to sample the placental bed. Thereafter, follows a review of the evidence that invasive trophoblast can release the vasoactive agents nitric oxide and/or carbon monoxide which, in turn, could contribute to early physiological changes in spiral arteries prior to destruction of the smooth muscle within the vessel wall. Current evidence supports the idea that trophoblast-derived carbon monoxide may contribute to spiral artery modification. In contrast there is no evidence for a similar role by nitric oxide.

The human placental bed revisited

Pre-eclampsia, fetal growth restriction and spontaneous miscarriage have all been linked to abnormalities in trophoblast invasion into the placental bed. Despite intensive research our understanding of the mechanisms that control human trophoblast invasion in normal, let alone abnormal pregnancy, are still poorly understood. This is partly due to difficulties in obtaining "true" placental bed biopsies and most investigators have relied on in vitro models to try and understand the processes which control trophoblast invasion. The purpose of this article is to review the various studies which have attempted to sample the placental bed along with their success rate in doing so and secondly to review our current understanding of the morphological changes which occur in the placental bed in normal pregnancy, pre-eclampsia and spontaneous miscarriage.

Transforming growth factor beta expression in human placenta and placental bed during early pregnancy

Normal human pregnancy depends on physiological transformation of spiral arteries. Pre-eclampsia and fetal growth restriction are associated with impaired trophoblast invasion and spiral artery transformation. Recent data obtained from studies on placenta suggest that temporal changes in expression of TGF-beta3 play a key role in trophoblast invasion and that over-expression of TGF-beta3 in pre-eclampsia is responsible for inadequate trophoblast invasion. There are, however, no studies of specific TGF-betas in the placental bed throughout pregnancy although this is where the invasive trophoblast and spiral arteries are located. In this study we have used immunohistochemistry, Western blot analysis, ELISA and RT-PCR to examine the expression of TGF-beta1, TGF-beta2 and TGF-beta3 in placental bed biopsies and placentas from 7--19 weeks' gestation. The results show that TGF-beta1, 2 and 3 are expressed in the placenta throughout this time but the striking temporal changes in TGF-beta3 expression previously reported were not observed. Extravillous trophoblast within the placental bed expressed TGF-beta2 but not TGF-beta1 or TGF-beta3 while extracellular TGF-beta1 and cytoplasmic TGF-beta2 were detected in decidua. These data suggest that TGF-beta1 and TGF-beta2 but not TGF-beta3 may play a role in trophoblast invasion.

Ultrastructure of epithelial plaque formation and stromal cell transformation by post-ovulatory chorionic gonadotrophin treatment in the baboon (Papio anubis)

BACKGROUND

To understand factors controlling endometrial responses to pregnancy, we have established a model using the baboon and examined the effects of infused human chorionic gonadotrophin (HCG) on the preparation of the luminal epithelium and stromal cell differentiation for the establishment of pregnancy.

METHODS

The ultrastructure of endometrium from normal day 10 post-ovulation animals, cycling females treated with either HCG or FSH (control), and a day 15 pregnant animal has been compared.

RESULTS

In the control endometrium, the luminal epithelium was smooth and regular, with underlying spindle shaped stromal cells. In pregnancy, the luminal epithelium underwent a plaque reaction, while stromal cells enlarged and developed filament-rich cell processes. Infusion of HCG produced changes similar to those seen in pregnancy, with generalized plaque formation and stromal decidualization, while in the animal treated with FSH there was no response.

CONCLUSIONS

This study indicates that infusion of HCG into the uterus can duplicate many of the responses of the endometrium to pregnancy, although in this case the plaque reaction involved the whole of the luminal epithelium, rather than only the implantation site as in pregnancy.

Characteristics of the uterine luminal surface epithelium at preovulatory and preimplantation stages in the marmoset monkey

Light and electron microscopy was used to examine the apical luminal epithelial surface of the uterus at preovulatory and preimplantation stages in the marmoset monkey. Luminal surface charge, detected by cationic ferritin staining, progressively decreased from preovulation to day 11 of pregnancy. The smooth, regular apical plasma membrane at preovulatory stages was in contrast to the convoluted, irregular surface observed during early pregnancy, especially at 1 day before blastocyst implantation. Profiles of microvilli were also altered, becoming thicker and more irregular during early pregnancy. Within the epithelial cell body, cyclic morphologic changes were seen, largely in association with secretory organelles. Giant phagocytic bodies were prominent at all stages examined, although their composition and intensity of staining varied throughout the cycle. Weak to moderate estrogen alpha and progesterone receptor immunostaining of the luminal epithelium was found during preovulatory and early pregnancy stages. This study describes complex cyclic changes in the morphology and biochemical make-up of the uterine luminal epithelial surface in a New World monkey in preparation for blastocyst attachment.

Expression of matrix metalloproteinase-2, -9, -14, and tissue inhibitors of metalloproteinase-1, -2, -3 in the endometrium and placenta of rhesus monkey (Macaca mulatta) during early pregnancy

The extracellular matrix proteolytic machinery has long been recognized as one of the most important mechanisms for regulating trophoblast invasion. Matrix metalloproteinases (MMPs) are a group of proteases involved in this process, and their activities are regulated by tissue inhibitors of MMPs (TIMPs). In this study, we collected rhesus monkey uteri on Days 12, 18, and 26 of pregnancy and examined the mRNA expression of MMP-2, -9, -14, and TIMP-1, -2, -3, as well as the activities of MMP-2 and -9 by using in situ hybridization and gelatin zymography, respectively. The results showed that MMP-2 and -9 were expressed earlier than MMP-14 and TIMPs in the pregnant endometrium. MMP-14 and TIMP-2 mRNAs appeared in perivascular decidual cells earlier than MMP-2 mRNA. On Day 26 of pregnancy, placental villi expressed little MMP-2, -14, and TIMP transcripts but abundant MMP-9 mRNA. Furthermore, MMP-2, -9, -14, and TIMP-1, -2, -3 were highly expressed on the fetal-maternal border but were absent in the myometrium. TIMP-3 mRNA in the endometrium was specifically localized to some cells lining the outer membrane of several groups of arterioles. Combined with the results obtained by gelatin zymography, we found that active MMP-2 existed in the endometrium throughout these three phases, while MMP-9 showed considerable activities only on Days 18 and 26 of pregnancy. The data suggest key roles for MMP-2 and -9 in invasion of trophoblast cells into the endometrium and the development of the placenta and might indicate that these processes are regulated by MMP-14 and TIMP-1, -2, and -3.

Effect of vaginally administered fumagillin on the morphology of implantation stage endometrium in the rhesus monkey

Intravaginal administration of an anti-angiogenic agent, fumagillin, during blastocyst implantation, inhibits pregnancy establishment in a dose-related manner in the rhesus monkey. In the present study, mated female rhesus monkeys were vaginally inserted with tampons containing vehicle (group 1; n = 5) and test agent (fumagillin, 4 mg/animal; group 2; n = 6) on cycle day 20, and endometrial tissue samples were collected on cycle day 24 from all monkeys and processed for morphometric and ultrastructural analysis. Concentrations of estradiol-17beta, progesterone and chorionic gonadotrophin in peripheral circulation were determined. From serum profiles of hormones, two monkeys in group 1, and one animal in group 2 appeared pregnant. Endometrial morphology revealed histologic evidence of pregnancy in three of six fumagillin-treated animals, while other three fumagillin-treated animals showed degenerative changes in glands and venules along with marked extravasation. It is possible that the function of corpus luteum was affected by fumagillin treatment resulting in inadequate progesterone production (p <0.05), and consequent inadequate endometrial secretory preparation and receptivity, as revealed from decline in apical movement of vacuoles (p <0.05) and increase (p <0.05) in extravasation of red cells and leukocytes.

Plasminogen activators and inhibitors are transcribed during early macaque implantation

Plasminogen activators and inhibitors may be important early in primate implantation but evidence for this is sparse in non-human primates. We define the expression of urokinase type plasminogen activator (uPA), tissue-type plasminogen activator (tPA), plasminogen activator inhibitor type 1 (PAI-1) and type 2 (PAI-2), the receptor for uPA (uPAR) and fibrin/fibrinogen in monkey implantation sites. In situ hybridization and immuno-histochemical localization of rhesus monkey implantation sites (day 15-16 postovulation) indicate: (1) uPA mRNA is localized to placental trophoblast, epithelial plaque and endometrial stroma. (2) tPA mRNA is mainly expressed in glandular cells of endometrium. (3) PAI-1 expression is linked to a specific population of trophoblasts that confront maternal cells, adding support to our view that it has a regulatory role in trophoblast invasion. (4) Localization of tPA antigen confirms that uterine glands are the major source of tPA and that it is also closely associated with fibrin(ogen) suggesting its possible function during implantation is fibrinolysis. (5) Unlike uPA mRNA, however, the distribution of uPA protein and its cell surface receptor uPAR suggests that it mediates trophoblast invasion and plays a significant role in angiogenesis. (6) PAI-2, the inhibitor associated with pregnancy in humans, was found in unidentified cells located specifically along the maternofetal junction. This localization adjacent to areas of cell death at the maternofetal junction implies that it may have a role as a protective curtain with anti-apoptotic function. In conclusion our results suggest that gene expression of PAs and PAIs in early implantation sites are tissue-specific, location-sensitive and function-related.

The plasma membrane transformation facilitates pregnancy in both reptiles and mammals

Mechanisms of placentation are very diverse in mammals and range from types in which the uterine epithelium is breached by the implanting blastocyst to those where the epithelium remains intact. Despite these differences in mechanisms, the initial response of the plasma membrane of uterine epithelial cells is remarkably similar across mammalian species which has led to the term 'plasma membrane transformation' to encapsulate the concept of a common beginning to implantation. Membrane phenomena similar to those of mammals have now been observed in some viviparous lizards at the ultrastructural level during early pregnancy, and we propose extending the concept of 'plasma membrane transformation' to lizards with live birth.

Implantation in the marmoset monkey: expansion of the early implantation site

This study was initiated to examine the early stages of trophoblast adhesion and invasion during implantation in the marmoset. Seven implantation sites were found in the uteri of four marmosets taken between days 13 and 15 of gestation. Three implantation sites in two uteri were examined in detail by electron microscopy. Between days 13 and 15, the marmoset implantation site expanded peripherally by adding areas where syncytial trophoblast penetrated between uterine luminal epithelial cells. Such penetrating masses often bridged openings of endometrial glands, shared junctional complexes with the uterine epithelial cells between which they are infiltrating, and subsequently reached the residual basal lamina of the uterine luminal epithelium. Centripetal to the peripheral region was an intermediate region in which syncytial trophoblast overlay individual clusters of epithelial cells and rested along the basal lamina. In this region there was some evidence of fusion of syncytial trophoblast with uterine epithelial cells. In the central region of the implantation site near the inner cell mass and amnion the trophoblast formed elaborate lamellipodia in relation to the basal lamina. In one of the three specimens examined with electron microscopy there were two foci where trophoblast penetrated through the basal lamina. It was also in the central region that trophoblast penetrated farthest into the uterine glands. The gland cells closest to trophoblast were less closely associated and lost their columnar shape, forming large round cells similar to the epithelial plaque cells of other primates. Where two blastocysts implanted on the same side of the uterus a conjoint membrane was formed which in regions consisted solely of syncytial trophoblast with two basal surfaces and two basal laminas. The prolonged period of time when the implantation site expands within the plane of the uterine epithelium (trophoblastic plate stage) and the peripheral to central sequence in extent of development make this primate a particularly useful animal for studies of trophoblast adhesion to and penetration of the uterine luminal epithelium.

Comparative placental structure

The primary function of all placentas is to act as an interface between the mother and fetus that allows, and even promotes, appropriate metabolic exchanges. This function is accomplished by bringing maternal and fetal blood into close apposition while maintaining separation of the maternal and fetal circulatory systems. Despite the common physiological functions shared by placentas, however, examination of placental morphology from different animal groups reveals a remarkable diversity of species-specific structural organization.The separation of fetal and maternal blood is always maintained by an elaboration of extraembryonic fetal tissues that cover fetal blood vessels. In some species the outermost layer of this fetal tissue, the trophoblast, is in direct contact with maternal blood. In many other species uterine tissues also contribute to the selective barrier separating the two blood systems. In addition to morphological variation among placentas of different animal groups, placentas undergo substantial structural modifications during pregnancy in a single species. In some animals different types of placentas function successively, or concurrently during a single pregnancy.As a result of these myriad details of placental structure, effective evaluation of fetal-maternal transfer of drugs must consider not only the components of the interhemal barrier of the fully developed placenta characteristic for each species, but also the placental structures functioning at each gestational stage of the fetus.

Immunohistochemical localization of receptors for progesterone and oestradiol-17 beta in the implantation site of the rhesus monkey

The aim of the present study was to examine the cellular basis of the involvement of oestradiol and progesterone in blastocyst implantation in the primate. To this end, the cellular distribution of receptors for oestradiol (ER) and progesterone (PR) in fetal trophoblast cells and in endometrial compartments of timed lacunar (pre-villous) and villous stages of placentation in primary implantation sites collected on days 13-22 of gestation were investigated in rhesus monkeys. Both in pre-villous stage tissue and in villous stage tissue, cytotrophoblast cells and syncytiotrophoblast cells and other trophoblast derived cells were PR positive, while they were generally ER negative. Maternal endometrial cells were ER negative, while epithelial cells, stromal cells and vascular endothelial cells in maternal endometrium showed heterogeneous staining patterns for PR depending on their relative location; these patterns, however, correlated well with glandular hyperplasia and differentiation, stromal-decidual transformation and vascular response seen during blastocyst implantation.