Current topic: structural responses of the primate endometrium to implantation

Marmoset and human trophoblast stem cells differ in signaling requirements and recapitulate divergent modes of trophoblast invasion

Early human trophoblast development has remained elusive due to the inaccessibility of the early conceptus. Non-human primate models recapitulate many features of human development and allow access to early postimplantation stages. Here, we tracked the pre- to postimplantation transition of the trophoblast lineage in superficially implanting marmoset embryos in vivo. We differentiated marmoset naive pluripotent stem cells into trophoblast stem cells (TSCs), which exhibited trophoblast-specific transcriptome, methylome, differentiation potential, and long-term self-renewal. Notably, human TSC culture conditions failed to support marmoset TSC derivation, instead inducing an extraembryonic mesoderm-like fate in marmoset cells. We show that combined MEK, TGF-β/NODAL, and histone deacetylase inhibition stabilizes a periimplantation trophoblast-like identity in marmoset TSCs. By contrast, these conditions differentiated human TSCs toward extravillous trophoblasts. Our work presents a paradigm to harness the evolutionary divergence in implantation strategies to elucidate human trophoblast development and invasion.

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'.

Stromal cells of the endometrium and decidua: in search of a name and an identity

Human endometrial and decidual stromal cells are the same cells in different environments (non-pregnancy and pregnancy, respectively). Although some authors consider decidual stromal cells to arise solely from the differentiation of endometrial stromal cells, this is a debatable issue given that decidualization processes do not end with the formation of the decidua, as shown by the presence of stromal cells from both the endometrium and decidua in both undifferentiated (non-decidualized) and decidualized states. Furthermore, recent functional and transcriptomic results have shown that there are differences in the decidualization process of endometrial and decidual stromal cells, with the latter having a greater decidualization capacity than the former. These differences suggest that in the terminology and study of their characteristics, endometrial and decidual stromal cells should be clearly distinguished, as should their undifferentiated or decidualized status. There is, however, considerable confusion in the designation and identification of uterine stromal cells. This confusion may impede a judicious understanding of the functional processes in normal and pathological situations. In the present article we analyse the different terms used in the literature for different types of uterine stromal cells, and propose that a combination of differentiation status (undifferentiated, decidualized) and localization (endometrium, decidua) criteria should be used to arrive at a set of accurate, unambiguous terms. The cell identity of uterine stromal cells is also a debatable issue: phenotypic, functional and transcriptomic studies in recent decades have related these cells to different established cells. We discuss the relevance of these associations in normal and pathological situations.

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.

Immunomorphological changes in the rhesus monkey endometrium and decidua during the menstrual cycle and early pregnancy

PROBLEM

Throughout the reproductive cycle and into early pregnancy, the normal endometrium undergoes changes in a range of leukocytes, epithelia, stromal fibroblasts, and vascular structures caused by intersecting effects of hormone balance and embryo implantation. The direct investigation in humans of reproductive tract responses during normal and physiologically altered cycles is not practical or feasible. METHOD AND STUDY: The aim of this study was to define immunological and morphological changes through immunohistological and morphometric evaluation of the endometrium throughout the menstrual cycle and the decidua during early gestation in the rhesus monkey, a tractable experimental animal model.

RESULTS

A zone-dependent method for the immunohistological description of the rhesus uterine mucosa was established and showed that leukocyte infiltration, stromal cell decidualization, glandular and vascular responses were zone- and cell type-dependent, and changed throughout the cycle and early pregnancy. Morphological heterogeneity of uterine natural killer cells in the cycling endometrium and gestational decidua were consistent with the recent characterization of phenotypic subsets.

CONCLUSIONS

These data establish a morphological platform upon which to further study the regulation of endometrial responses to the hormonal mileau of pregnancy, the control of local leukocyte populations, and the responses to threatened pregnancy, infection, and inflammation.

Spontaneous lesions in the reproductive tract and mammary gland of female non-human primates

Because of their close phylogenic relationship with humans, the use of non-human primates (NHP) as experimental subjects has a long history in biomedical research. Although research topics have shifted focus and species used have changed, NHP remain vital as models in basic and applied research. While there is a wealth of information available on the spontaneous lesions of NHP, most of this information is fragmented, dated, or narrow in focus, often limited to single case reports. This review attempts to integrate this information to illustrate and enumerate the spectrum of spontaneous pathology of the reproductive tract and mammary gland of NHP. Although not the focus of this review, steroid-related changes are inextricably linked to these tissues, and brief consideration is given to this subject as well.

Uterine natural killer cells: a specialized differentiation regulated by ovarian hormones

In adult females of many species, a transient population of natural killer (NK) cells appears in cycles within the uterine endometrium (lining). Appearance of these lymphocytes coincides with specific phases of the ovarian hormone cycle and/or early pregnancy. Studies in rodents, women, and pigs dominate the literature and suggest the uterine (u)NK cells are an activated subset sharing many but not all features with circulating or lymphoid organ-residing NK cells. During successful murine pregnancy, uNK cells appear to regulate initiation of structural changes in the feed arterial systems that support maternal endometrial tissue at sites of implantation and subsequent placental development. These changes, which reverse after pregnancy, create a higher volume arterial bed with flaccid vessels unresponsive to vasoactive compounds. These unique pregnancy-associated arterial changes elevate the volume of low-pressure, nutrient-rich, maternal arterial blood available to conceptuses. Regulation of the differentiation, activation, and functions of uNK cells is only partially known, and there is lively debate regarding whether and how uNK cells participate in infertility or spontaneous abortion. This review highlights the biology of uNK cells during successful pregnancy.

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.

Inhibiting Uterine PC6 Blocks Embryo Implantation: An Obligatory Role for a Proprotein Convertase in Fertility1

Successful embryo implantation involves complex interactions between the embryo and the uterus and is critical in establishing pregnancy. Proprotein convertase (PC) 6 (PC6) is one of the PC endoproteases regulating protein function through posttranslational activation of precursor proteins, including growth and differentiation factors. Here we show that PC6 protein is induced in the uterine stromal cells specifically at the site of embryo attachment during early pregnancy in mice. In vivo blocking of uterine production of PC6 protein using morpholino antisense oligonucleotides in mice resulted in total inhibition of implantation, revealing a vital role for PC6 in modulating the uterus for embryo implantation. Studies in primates (rhesus monkey and human) showed a dramatic upregulation of endometrial PC6 during the phase of uterine receptivity and at implantation, particularly during a critical uterine cell differentiation process termed decidualization. Thus, the current studies have demonstrated that PC6 is an essential molecule in modulating uterine function to support the establishment of embryo implantation. Interestingly, PC6 is one of the PCs identified to be important in processing the coat protein of HIV; inhibition of PCs has been suggested to be an effective approach to reduce HIV transmission. We therefore propose the novel concept that PC6 could be a potential nonhormonal target in the female reproductive tract for dual protection for women, both in preventing pregnancy and reducing HIV infection.

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.

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.

Chorionic gonadotropin and uterine dialogue in the primate

Implantation is a complex spatio-temporal interaction between the growing embryo and the mother, where both players need to be highly synchronized to be able to establish an effective communication to ensure a successful pregnancy. Using our in vivo baboon model we have shown that Chorionic Gonadotropin (CG), as the major trophoblast derived signal, not only rescues the corpus luteum but also modulates the uterine environment in preparation for implantation. This response is characterized by an alteration in both the morphological and biochemical activity in the three major cell types: luminal and glandular epithelium and stromal fibroblasts. Furthermore, CG and factors from the ovary have a synergistic effect on the receptive endometrium. Novel local effects of CG which influence the immune system to permit the survival of the fetal allograft and prevent endometrial cell death are also discussed in this review. An alternate extracellular signal-regulated kinase (ERK) activation pathway observed in epithelial endometrial cells and the possibility of differential expression of the CG/LH-R isoforms during gestation, open many questions regarding the mechanism of action of CG and its signal transduction pathway within the primate endometrium.

Uterine receptivity and implantation: the regulation and action of insulin-like growth factor binding protein-1 (IGFBP-1), HOXA10 and forkhead transcription factor-1 (FOXO-1) in the baboon endometrium

In primates, the phase of the menstrual cycle when the uterus becomes receptive is initially dependent on estrogen and progesterone. Further morphological and biochemical changes are induced as a result of biochemical signals between the embryo and the maternal endometrium. Blastocyst implantation in the baboon usually occurs between 8 and 10 days post ovulation and is similar to that described for the rhesus macaque. In the baboon, when chorionic gonadotropin is infused in a manner that mimics blastocyst transit, this has physiological effects on the three major cell types in the uterine endometrium. The luminal epithelium undergoes endoreplication and distinct epithelial plaques are evident. The glandular epithelium responds by inducing transcriptional and post-translational modifications in the major secretory product, glycodelin. The stromal fibroblasts initiate their differentiation process into a decidual phenotype and are characterized by the expression of actin filaments. Decidualization, is the major change that occurs in the primate endometrium after conception. During this process the fibroblast-like stromal cells change morphologically into polygonal cells and express specific decidual proteins. Studies in the baboon demonstrated that insulin-like growth factor binding protein-1 (IGFBP-1) gene expression is a conceptus-mediated response. Subsequent studies in vitro established that IGFBP-1 is transcriptionally regulated by FOXO1 and HOXA10 which together upregulate the IGFBP-1 promoter activity. A baboon endometriosis model was utilized to determine if the changes observed during uterine receptivity in normally cycling animals were compromised. The data suggests that in animals with disease, markers of uterine receptivity are not appropriately expressed in the eutopic endometrium. It is possible that these differences influence the fertility of the animals with disease and the baboon could be used as a primate model to study the causes of infertility as a result of endometriosis.

Implantation: Embryonic Signals and the Modulation of the Uterine Environment—A Review

Interaction between maternal endometrium and embryo during implantation is mediated through the multiple molecules and signalling cascades, which are still not fully understood. This complex sequence of events results in the transformation of stromal cells into decidual cells (decidualization). The conceptus is essential to regulate these changes in baboon stromal fibroblasts, possibly through production of cytokines by the implanting embryo. The role of interleukin-1 (IL-1) system during implantation and its contribution to decidualization is discussed in this review. Decidualized endometrial stromal cells are thought to contribute to establishment of a successful pregnancy by expressing a number of gene products. One of the major products of decidual cells is insulin-like growth factor binding protein-1 (IGFBP-1). The IGFBP-1 serves as a modulator of insulin-like growth factor (IGF) protein action in the interactions between the decidua and invading trophoblast. The delineation of the mechanisms involved in cooperative action of transcription factors FKHR and HOXA10 leading to IGFBP-1 expression, as well as interrelationship between IL-1 and IGF signalling cascades will contribute to the understanding of events leading to successful implantation.

Dynamic Changes in Primate Endometrial Leukocyte Populations: Differential Distribution of Macrophages and Natural Killer Cells at the Rhesus Monkey Implantation Site and in Early Pregnancy

The distribution of uterine leukocytes during the periimplantation period cannot be readily evaluated in human pregnancy. Using immunohistochemistry we examined the distribution of macrophages, natural killer (NK) cells, and T cells in the non-pregnant endometrium and in the decidua at early stages of implantation and pregnancy in the rhesus monkey. CD68+ macrophages, CD56+ lymphocytes and CD3+ T cells were present in the proliferative and secretory endometrium. The number of macrophages and CD56+ lymphocytes dramatically increased at implantation (day 14-15 of pregnancy) and continued to be high in early pregnancy decidua. Macrophages were conspicuously more numerous in proximity to implantation site (decidua basalis) as compared to sites peripheral to the developing placenta (decidua parietalis), and were found in close association with cytotrophoblasts adjacent to the decidua, as well as around arteries invaded by extravillous cytotrophoblasts. In contrast to macrophages, CD56+ lymphocytes were more evenly distributed throughout the decidua. Few CD3+ T cells were seen in pregnancy, being scattered in the endometrial stroma with occasional aggregate formation. The distribution of uterine leukocytes vis-à-vis trophoblasts at the rhesus monkey implantation site and in early pregnancy suggests different roles for macrophages and uterine NK cells in the response to trophoblast invasion.

Heme oxygenases in pregnancy II: HO-2 is downregulated in human pathologic pregnancies

PROBLEM

We previously reported a diminished expression of the heme-degrading enzymes heme oxygenases (HO)-1 and HO-2 in decidua and placenta from mice undergoing Th1-mediated abortion, strongly indicating the protective effect of HO in murine pregnancy maintenance. Here we investigated whether the expression of HO-1 and HO-2 is also reduced at the feto-maternal interface of pathologic human pregnancies.

METHOD OF STUDY

Immunohistochemistry was used to detect HOs expression in placental and decidual first-trimester tissue from patients with: spontaneous abortion (n = 14), choriocarcinoma (n = 14), hydatidiform mole (H-mole) (n = 12), compared with normally progressing pregnancies (n = 15). Further, we investigated early third-trimester decidual and placental tissue from patients with pre-eclampsia (n = 13) compared with fetal growth retardation (n = 14) as age-matched controls.

RESULTS

In first trimester tissue, we observed a significant reduction of HO-2 expression in invasive trophoblast cells, endothelial cells, and syncytiotrophoblasts in samples from patients with spontaneous abortion compared with normal pregnancy. H-mole samples showed a diminished expression of HO-2 in invasive trophoblast cells and endothelial cells in comparison with NP, whereas choriocarcinoma samples showed no significant differences compared with the control. In third trimester tissue, HO-2 was also reduced in syncytiotrophoblasts and invasive trophoblast cells from pre-eclampsia compared with samples from fetal growth retardation. HO-1 expression was diminished in all pathologies investigated; however, the differences did not reach levels of significance.

CONCLUSIONS

Our data indicate that HOs play a crucial role in pregnancy and low expression of HO-2, as observed in pathologic pregnancies, may lead to enhanced levels of free heme at the feto-maternal interface, with subsequent upregulation of adhesion molecules, allowing enhanced inflammatory cells migration to the feto-maternal interface.

Confocal laser scanning microscope study of cytokeratin immunofluorescence differences between villous and extravillous trophoblast: Cytokeratin downregulation in pre-eclampsia

Pre-eclampsia is a disease characterized by failures in interstitial implantation. One product of the implantation process is the basal plate; a structure whose complexity makes it hard to fully appreciate the pathological changes in significant diseases of pregnancy. This article describes our use of CLSM immunofluorescence to examine the cytokeratin composition of the cells of trophoblastic origin in the term placental basal plate. Large differences in the content of the structural polymeric protein were compared using analysis of digital images. We show that greater pancytokeratin immunofluorescence is observed in extravillous cytotrophoblast cells as compared with villous trophoblast. There is a >30-fold difference in the mean area percent of the most intensely immunofluorescent pixels in the tissue containing these cells. This is a very high, statistically significant difference as defined by the Wilcoxon Signed Ranks Test Asym. Sig. (two-tailed): P < 0.001. The most invasive population of cells of the trophoblast lineage (the extravillous trophoblast) exhibits a significant reduction in cytokeratin immunofluorescence when comparisons of healthy and pre-eclamptic pregnancies are made. This ratio was 2.4:1. It was tested using the Mann-Whitney U-test. From healthy to pre-eclamptic the reduction was from mean rank 83.42((healthy)) to 51.13((pre-eclamptic)). The difference was very highly statistically significant (n = 53 + 75 = 128; U = 984.500; Z = -4.852; P < 0.001). There was also less cytokeratin-related immunofluorescence in villous trophoblast when healthy villi were compared with pre-eclamptic villi. The observed alterations in trophoblastic cytoskeletal components are expected to damage the anchorage and motility of cells. The extravillous trophoblast is known to be necessary for implantation. This leads to a cellular hypothesis of the failure of implantation resulting in reduced depth of uterine invasion and failure to adapt the spiral arterioles for low-pressure perfusion of the intervillus space, two well-known features of pre-eclampsia. The reduction in cytokeratin-related immunofluorescence in the villus trophoblast seen on comparing healthy term placentae with those from pre-eclamptics implies that the trophoblast is a weaker epithelial layer in the hypertensive pregnancy. This could account for the rise in deported trophoblast associated with pre-eclampsia. Deported trophoblast has been invoked as the systemic messenger that leads to generalized maternal hypertension seen in this condition.

Evidence of a limited contribution of feto-maternal interactions to trophoblast differentiation along the invasive pathway

Trophoblast differentiation is a key event in human placental development. During extravillous trophoblast (EVT) differentiation, stem cells from the anchoring villi detach from their basement membrane and proliferate to form aggregates called trophoblast cell columns (TCCs). They subsequently invade the decidua and differentiate into interstitial and endovascular trophoblasts. The influence of the decidua on EVT differentiation is controversial. We therefore compared the pattern of trophoblast differentiation marker expression in viable intrauterine and tubal pregnancies, as decidual cell markers (prolactin [PRL] and insulin-like growth factor binding Protein-1 [IGFBP1]) were only expressed in endometrial implantation sites. Extravillous trophoblast differentiation in anchoring villi from uterine and ectopic pregnancies exhibited a comparable phenotypical switch: alpha6 integrin subunit, E-cadherin, EGF receptor, Ki 67 and connexin 40 were localized in the proximal part of the TCC, while alpha5beta1 and alpha1 integrins, c-erb B2, hPL and HLA-G were expressed by invasive cytotrophoblasts. The cyclin-dependent kinase inhibitors p16 and p57 were mainly detected in invasive cytotrophoblasts some distance from the columns. However, the TCC was markedly longer in tubal pregnancy than in intrauterine pregnancy. These findings suggest that the decidua is not necessary to trigger EVT invasion, but that it is likely to limit the extent of the TCC and to accelerate the onset of EVT migration.

Distribution of Immunocompetent Cells in Decidua of Controlled and Uncontrolled (Choriocarcinoma/Hydatidiform mole) Trophoblast Invasion

PROBLEM

Pregnancy has been considered as a model of successfully controlled tissue invasion where trophoblast cells infiltrate the maternal decidua without being rejected or without destroying the tissue. In choriocarcinoma (CC) and hydatidiform mole (HM), a dysregulation of invasive (malignant/benign) trophoblast cells is present. Immunocompetent cells (IC) are known to be involved in rejection pathways of malignant cells and can also be identified in early pregnancy decidua. The aim of the present study was to identify the phenotype of IC in decidua of women with normal pregnancy (NP), CC and HM.

METHODS

Immunocompetent cells were detected by immunohistochemistry in decidual tissue from first trimester NP (n = 10), CC (n = 12) and HM (n = 11) using antibodies against CD8+, CD3+, CD56+, CD68+ cell surface markers and mast cell tryptase (MCT). A scaled eye piece was used for cell counting to obtain semiquantitative results. Statistical analysis was performed using Wilcoxon rank/Mann-Whitney tests.

RESULTS

We observed a significantly increased number of lymphocytes positive for CD8, CD3 and MCT positive granulocytes in CC and HM compared with the samples from NP (all P < or = 0.001). Lymphocytes positive for natural killer (NK) cell marker CD56 were significantly decreased in CC and HM versus NP (P < or = 0.001). The number of CD68 positive cells (macrophages) were not significantly different among the tissue pools.

CONCLUSION

The increase of CD8/CD3 T cells and mast cells in CC and HM and the decrease of CD56 cells, compared with NP, suggests the necessity of a balance between T and NK cells in controlling trophoblast invasion.

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.

Endometrial function: cell specific changes in the uterine environment

The uterus undergoes dynamic changes during the cycle and these events are largely driven by ovarian steroids. However, in the presence of an embryo, an additional series of changes that are not otherwise observed predominate. The ability of the embryo to modulate the uterine environment is restricted to a specific time of the cycle which is termed the 'window of receptivity'. Changes that occur within this window of receptivity and immediately following implantation can be divided into three distinct phases. The first phase, regulated by estrogen and progesterone, is characterized primarily by changes in both the luminal and glandular epithelial cells in preparation for blastocyst apposition and attachment. If the action of progesterone is antagonized, these changes are inhibited and the uterus is maintained in a pre-receptive state. The second phase is in the further modulation of these steroids induced changes by embryonic signals. In the primate, infusion of chorionic gonadotropin in a manner that mimics blastocyst transit, results in the endoreplication and plaque formation in the luminal epithelium. The glandular epithelium responds by increasing transcriptional and post-translational modifications of secretory proteins and the stromal fibroblasts initiate their differentiation process into a decidual phenotype. The final phase is associated with trophoblast invasion and remodeling of the endometrial stromal compartment. The most dramatic effect is on the stromal fibroblasts, which in response to embryonic stimuli, differentiate into decidual cells, the major cell type of the gestational endometrium. Thus, during the window of receptivity, signals from the embryo can dramatically alter the morphological and functional characteristics of the uterine endometrium. We suggest that these changes are critical to ensure prolonged maintenance of endometrial function during gestation and facilitate 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.

The role of chorionic gonadotropin (CG) in blastocyst implantation

Implantation is a complex spatio-temporal interaction between the genotypically different embryo and the mother. Success of this event requires the synchronization of development and effective biochemical communications from both sides. Chorionic gonadotropin (CG), which is a major embryonic signal in the primate, is a glycoprotein hormone synthesized and secreted by the trophoblast. Various isoforms exist in plasma, urine, and blastocyst culture medium, a result of posttranslational modifications. The exponential secretion of CG and its long circulatory half-life extends the life span of corpus luteum to maintain the supply of progesterone during the first 6-8 weeks of pregnancy. To study the direct effects of CG in the uterus, we used the baboon (Papio anubis) as a non-human primate model. In vivo stimulation with CG during the window of uterine receptivity results in further morphologic and biochemical modifications of the receptive endometrium. These are characterized by the plaque reaction in the luminal epithelium, an increase in glycodelin expression and secretion by the glandular epithelium, and the differentiation of subepithelial stromal fibroblasts characterized by expression of the alpha smooth muscle actin (alpha SMA). Pretreatment with progesterone receptor antagonist (PRa) completely or partially inhibits these effects. The signal transduction pathway activated by CG in primate endometrial epithelial cells involves the protein kinase A (PKA)-independent phosphorylation of extracellular signal regulated kinase (ERK 1/2). This alternate signal transduction pathway may prevent CG Receptor (R) downregulation at the implantation site and enhance epithelial cell proliferation and differentiation. Thus, our results suggest that CG plays an important role in implantation in addition to its luteotrophic role.

Early Implantation Events in the Baboon (Papio anubis) with Special Reference to the Establishment of Anchoring Villi

The development of the baboon anchoring villus has been studied from day 14 to day 48 of gestation, using light and electron microscopy. At day 14, cords of trophoblast could be seen streaming into the endometrium, invading maternal vessels and forming blood-filled lacunae; by 20 days gestation some of these had differentiated into distinctive anchoring villi, with an outer covering of syncytiotrophoblast and inner cytotrophoblast cells which differed from those of floating villi in that a subpopulation detached from the syncytium to form an interconnecting network of cells within the centre of the villus. Subsequent migration of cytotrophoblast into the endometrium formed the cytotrophoblastic shell while fibrillin-like extracellular matrix biosynthesis within the body of the villus provided a firm mechanical support. At the trophoblast-decidual interface, a zone of necrosis and phagocytosis initially developed, which became less extensive with time, so that by 40 days a stable interface was evident with only residual pockets of necrosis. During this period, there was differentiation of decidual cells which by 28 days developed characteristic pedunculated cell processes, and later became surrounded by a basal lamina. The factors that control detachment of cytotrophoblast from the syncytium and the biosynthesis of the specialized, fibrillar extracellular matrix, features that are not apparent in other placental villi, require further investigation, possibly by carefully controlled in vitro experimentation.

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.

Assessment of hormonally active agents in the reproductive tract of female nonhuman primates

Using the ovariectomized macaque model of postmenopausal women's health, we investigated the effects of long-term treatments (5 weeks-3 years) with estradiol, conjugated equine estrogens (CEE), esterified estrogens, progestins such as medroxyprogesterone acetate (MPA) and nomegestrol acetate, CEE + MPA, tamoxifen, soybean phytoestrogens (SPEs), a variety of putative selective estrogen receptor modulators (SERMs), and androgens. Agents tested were selected on the basis of beneficial effects on arteries and/or bone. Doses were scaled on a caloric or serum-concentration basis to approximate human clinical doses. We evaluated endometrial and mammary gland histopathology and morphometry and used immunohistochemistry to evaluate cell proliferation and expression of estrogen receptor alpha and progesterone receptor (PR). Both estradiol and CEE induced endometrial hyperplasia. MPA antagonized epithelial proliferation induced by CEE in endometrium and induced pseudodecidual stromal hyperplasia in some animals. Tamoxifen induced endometrial polyps, cystic hyperplasia, stromal fibrosis, and PR expression but not Ki-67 expression. SPEs were not estrogenic at dietary doses and antagonized estrogen-induced proliferation in the endometrium and breast. Nandrolone induced mucometra and an adenomyosis-like change. The potential SERM 17 alpha dihydroequilenin did not have uterotrophic or mammotrophic effects. In general, experimental findings in macaques have been predictive of outcomes in human clinical trials of the same agents.

Trophinin expression in the mouse uterus coincides with implantation and is hormonally regulated but not induced by implanting blastocysts

Trophinin mediates apical cell adhesion between two human cell lines, trophoblastic teratocarcinoma and endometrial adenocarcinoma. In humans, trophinin is specifically expressed in cells involved in implantation and early placentation. The present study was undertaken to establish trophinin expression by the mouse uterus. In the pregnant mouse uterus, trophinin transcripts are expressed during the time which coincides with the timing of blastocyst implantation. Trophinin is also expressed in the nonpregnant mouse uterus at estrus stage. Uteri from ovariectomized mice did not express trophinin, whereas strong expression was induced by estrogen but not by progesterone. Trophinin transcripts and protein were found in the pseudopregnant mouse uterus. No differences were detected in trophinin expression by the uteri in the pregnant, pseudopregnant, and pseudopregnant received blastocysts. In delayed implantation model, trophinin proteins were found in both luminal and glandular epithelium, whereas dormant blastocysts were negative for trophinin. Upon activation with estrogen, however, no significant changes were detected either in the blastocyst or in the uterus. These results indicate that ovarian hormones regulate trophinin expression by the mouse uterus, and that an implanting blastocyst has no effect on trophinin expression in the surrounding endometrial luminal epithelial cells.

Endometrial receptivity and the window of implantation

Implantation is a highly co-ordinated event that involves both embryonic and endometrial participation. The endometrium expresses a sophisticated repertoire of proteins during the menstrual cycle many of which help to define a period of receptivity collectively known as the 'window of implantation'. Many of these factors, which are temporally aligned with this window, are now seen as chemical messengers that are recognized by the embryo and facilitate embryonic growth and differentiation. The use of such proteins as biomarkers has also advanced our understanding of the implantation process and may identify women with implantation failure and infertility. While the study of endometrial receptivity is still evolving, the field is growing rapidly and will probably enhance our ability to diagnose and treat couples with infertility, especially in the arena of assisted reproductive technologies (ART).

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.

Localization of ubiquitin and ubiquitin cross-reactive protein in human and baboon endometrium and decidua during the menstrual cycle and early pregnancy

We have examined the distribution of ubiquitin and the related ubiquitin cross-reactive protein (UCRP) in paraffin-embedded sections of human and baboon endometrium and decidua by immunoperoxidase or immunofluorescence cytochemistry with antibodies raised against ubiquitin, UCRP, CD45, and insulin-like growth factor-binding protein-1. Anti-ubiquitin immunoreactivity was present in the nonpregnant endometrium, particularly in the glandular epithelial cells, and up-regulated in endometrial stromal cells as they decidualized at the beginning of pregnancy. Anti-UCRP immunoreactivity was absent from nonpregnant tissue but accumulated to high levels in decidual cells during pregnancy. Western blotting indicated that immunoreactivity was primarily due to the presence of ubiquitin and UCRP conjugated to other proteins, and that although levels of ubiquitin-protein conjugates do not change substantially during pregnancy, decidualization is accompanied by the appearance of conjugates of UCRP. Baboon uterine tissues demonstrated a similar distribution of the two proteins, which indicates that the baboon may be a useful model for study of the role of the ubiquitin system and UCRP in the establishment of pregnancy in humans.

Modulation of the baboon (Papio anubis) uterine endometrium by chorionic gonadotrophin during the period of uterine receptivity

This study was undertaken to determine the modulation of uterine function by chorionic gonadotrophin (CG) in a nonhuman primate. Infusion of recombinant human CG (hCG) between days 6 and 10 post ovulation initiated the endoreplication of the uterine surface epithelium to form distinct epithelial plaques. These plaque cells stained intensely for cytokeratin and the proliferating cell nuclear antigen. The stromal fibroblasts below the epithelial plaques stained positively for alpha-smooth muscle actin (alphaSMA). Expression of alphaSMA is associated with the initiation of decidualization in the baboon endometrium. Synthesis of the glandular secretory protein glycodelin, as assessed by Western blot analysis, was markedly up-regulated by hCG, and this increase was confirmed by immunocytochemistry, Northern blot analysis, and reverse transcriptase-PCR. To determine whether hCG directly modulated these uterine responses, we treated ovariectomized baboons sequentially with estradiol and progesterone to mimic the hormonal profile of the normal menstrual cycle. Infusion of hCG into the oviduct of steroid-hormone-treated ovariectomized baboons induced the expression of alphaSMA in the stromal cells and glycodelin in the glandular epithelium. The epithelial plaque reaction, however, was not readily evident. These studies demonstrate a physiological effect of CG on the uterine endometrium in vivo and suggest that the primate blastocyst signal, like the blastocyst signals of other species, modulates the uterine environment prior to implantation.

Expression of G proteins in human placentas from pregnancies complicated by gestational hypertension

Preeclampsia (gestational hypertension) is accompanied by decreased hPL and increased hCG levels in maternal serum. The expression of these peptides as well as the endocrine mechanisms responsible for their regulation in preeclampsia are unknown. We have demonstrated that regulatory GTP-binding proteins (G proteins) are implicated in the modulation of hPL production by placentas from normal pregnancies. In order to extend our knowledge on placental endocrinology, we analyzed in this study the expression of hPL and beta-hCG mRNAs as well as placental G protein alpha-subunits in pregnancies complicated by gestational hypertension. Western and Northern blot analyses were respectively performed on membrane protein and total mRNA preparations from human placentas of preeclamptic (n = 7) and normal pregnancies (n = 4). The levels of hPL and beta-hCG mRNAs were respectively 108% and 105% of those from normal placentas, suggesting that the altered circulating levels of hPL and beta-hCG are not related to dysfunctional mRNA expression of these peptides. The autoradiographs for G proteins and their mRNAs showed no difference in G protein expression between preeclamptic and normal tissues. Specifically, G alpha i2, G alpha i3, G alpha o, G alpha s, and G alpha q/11 levels reached 87%, 81%, 91%, 99%, and 103% respectively of those from normal placentas. In parallel with the protein levels, their mRNAs expression were respectively 93%, 89%, 113%, 104%, and 94% of normal values for G alpha i2, G alpha i3, G alpha o, G alpha s, and G alpha q/11. These results suggest that neither a change in hPL and beta-hCG expression nor a change in signal transduction machinery is implicated in the altered circulating levels of hPL and beta-hCG in preeclampsia.

Identification and characterization of a fibroblast marker: FSP1

We performed subtractive and differential hybridization for transcript comparison between murine fibroblasts and isogenic epithelium, and observed only a few novel intracellular genes which were relatively specific for fibroblasts. One such gene encodes a filament-associated, calcium-binding protein, fibroblast-specific protein 1 (FSP1). The promoter/enhancer region driving this gene is active in fibroblasts but not in epithelium, mesangial cells or embryonic endoderm. During development, FSP1 is first detected by in situ hybridization after day 8.5 as a postgastrulation event, and is associated with cells of mesenchymal origin or of fibroblastic phenotype. Polyclonal antiserum raised to recombinant FSP1 protein stained the cytoplasm of fibroblasts, but not epithelium. Only occasional cells stain with specific anti-FSP1 antibodies in normal parenchymal tissue. However, in kidneys fibrosing from persistent inflammation, many fibroblasts could be identified in interstitial sites of collagen deposition and also in tubular epithelium adjacent to the inflammatory process. This pattern of anti-FSP1 staining during tissue fibrosis suggests, as a hypothesis, that fibroblasts in some cases arise, as needed, from the local conversion of epithelium. Consistent with this notion that FSP1 may be involved in the transition from epithelium to fibroblasts are experiments in which the in vitro overexpression of FSP1 cDNA in tubular epithelium is accompanied by conversion to a mesenchymal phenotype, as characterized by a more stellate and elongated fibroblast-like appearance, a reduction in cytokeratin, and new expression of vimentin. Similarly, tubular epithelium submerged in type I collagen gels exhibited the conversion to a fibroblast phenotype which includes de novo expression of FSP1 and vimentin. Use of the FSP1 marker, therefore, should further facilitate both the in vivo studies of fibrogenesis and the mapping of cell fate among fibroblasts.

Expression of adhesion molecules by endovascular trophoblast and decidual endothelial cells: implications for vascular invasion during implantation

During the process of implantation, maternal spiral arteries within the decidua are invaded by trophoblast cells that adhere to and migrate along the luminal surface of the vascular endothelial cells. This phenomenon resembles the events that occur during the migration of neutrophils into an acute inflammatory site, therefore it is possible that similar mechanisms are involved. Indeed, previous observations have shown that endovascular trophoblast expresses the blood group-related antigen sialyl Le(x). In this study, we show, by immunohistology, the expression of both E- and P-selectin by vascular endothelial cells only in the decidua basalis and not in decidua parietalis. In contrast, ICAM-1 is expressed by all vascular endothelium throughout the decidua. Expression of VCAM-1 is variable at the implantation site, and is not expressed by vascular endothelial cells in decidua parietalis. Interestingly, we demonstrate the strong expression of a polysialylated form of NCAM by endovascular trophoblast. Our data suggests that vascular invasion by trophoblast is regulated by the expression of appropriate adhesion molecules which permit interaction between endovascular trophoblast and decidual endothelial cells.

Structural interactions of trophoblast and uterus during hemochorial placenta formation

It is argued that the successful placental arrangements are those that decrease the exposure of the trophoblast to the efferent side of the cellular immune response. Examples are taken from three different groups with hemochorial placentation, to show that their placental morphology places most of the trophoblast in contact with maternal blood, not maternal connective tissue. In addition, maternal cells of the junctional area are modified either before contact with trophoblast (rat) or just after trophoblast invasion (primate), or the region of contact is limited as in the armadillo, in which maternal blood sinuses are expanded to form intervillous spaces.

Granulated metrial gland cells: a natural killer cell subset of the pregnant murine uterus

The metrial gland develops in the uterus of many rodent species during normal pregnancy. It is a maternally-derived tissue that contains stromal and vascular elements plus a population of large cells, striking in their light microscopic appearance due to the presence of numerous cytoplasmic granules. These cells, which have become known in mice and rats as granulated metrial gland (GMG) cells, are derived from bone marrow precursors and recent work suggests they are a subset of lymphocytes belonging to the natural killer (NK) cell lineage. The functions of GMG cells during normal gestation have not been clearly defined. In vitro, GMG cells have been shown to produce cytokines and their cytokine profile is altered upon addition of medium containing the T cell growth factor interleukin-2 (IL-2). GMG cell granules contain the cytolytic protein perforin but GMG cells have a very limited capacity to kill in vitro unless they have been stimulated by IL-2 or interferon-gamma. Histological study of GMG cells has suggested they preferentially associate with fetal trophoblast. Since trophoblast appears resistant to immune lysis, except by IL-2-activated effector lymphocytes, and because resorbing murine embryos become infiltrated by lytic cells of the NK cell lineage, it is important to establish whether GMG cells are activated by pregnancy-associated events to play a major lytic role in vivo.