Blastocyst implantation in the Chinese hamster (Cricetulus griseus)

The trophoblast giant cells of cricetid rodents

Giant cells are a prominent feature of placentation in cricetid rodents. Once thought to be maternal in origin, they are now known to be trophoblast giant cells (TGCs). The large size of cricetid TGCs and their nuclei reflects a high degree of polyploidy. While some TGCs are found at fixed locations, others migrate throughout the placenta and deep into the uterus where they sometimes survive postpartum. Herein, we review the distribution of TGCs in the placenta of cricetids, including our own data from the New World subfamily Sigmodontinae, and attempt a comparison between the TGCs of cricetid and murid rodents. In both families, parietal TGCs are found in the parietal yolk sac and as a layer between the junctional zone and decidua. In cricetids alone, large numbers of TGCs, likely from the same lineage, accumulate at the edge of the placental disk. Common to murids and cricetids is a haemotrichorial placental barrier where the maternal-facing layer consists of cytotrophoblasts characterized as sinusoidal TGCs. The maternal channels of the labyrinth are supplied by trophoblast-lined canals. Whereas in the mouse these are lined largely by canal TGCs, in cricetids canal TGCs are interspersed with syncytiotrophoblast. Transformation of the uterine spiral arteries occurs in both murids and cricetids and spiral artery TGCs line segments of the arteries that have lost their endothelium and smooth muscle. Since polyploidization of TGCs can amplify selective genomic regions required for specific functions, we argue that the TGCs of cricetids deserve further study and suggest avenues for future research.

Endometrial glycogen metabolism during early pregnancy in mice

Glucose is critical during early pregnancy. The uterus can store glucose as glycogen but uterine glycogen metabolism is poorly understood. This study analyzed glycogen storage and localization of glycogen metabolizing enzymes from proestrus until implantation in the murine uterus. Quantification of diastase-labile periodic acid-Schiff (PAS) staining showed glycogen in the glandular epithelium decreased 71.4% at 1.5 days postcoitum (DPC) and 62.13% at DPC 3.5 compared to proestrus. In the luminal epithelium, glycogen was the highest at proestrus, decreased 46.2% at DPC 1.5 and 63.2% at DPC 3.5. Immunostaining showed that before implantation, glycogen metabolizing enzymes were primarily localized to the glandular and luminal epithelium. Stromal glycogen was low from proestrus to DPC 3.5. However, at the DPC 5.5 implantation sites, stromal glycogen levels increased sevenfold. Similarly, artificial decidualization resulted in a fivefold increase in glycogen levels. In both models, decidualization increased expression of glycogen synthase as determine by immunohistochemistry and western blot. In conclusion, glycogen levels decreased in the uterine epithelium before implantation, indicating that it could be used to support preimplantation embryos. Decidualization resulted in a dramatic increase in stromal glycogen levels, suggesting it may have an important, but yet undefined, role in pregnancy.

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.

Two pathways of progesterone action in the human endometrium: implications for implantation and contraception

The endometrium is the site of implantation and pregnancy. Preparation for this important biological event relies primarily on progesterone, which takes the estrogen-primed endometrium toward a state of receptivity. As a steroid target tissue, the endometrium is also prone to abnormal growth sometimes leading to the development of hyperplasia or cancer. It is the balance between estrogen and progesterone that maintains the endometrium in a state of health and provides the synchronous timing necessary for a successful implantation to occur. In our efforts to understand the role of progesterone in the endometrium we have focused on the use of specific protein biomarkers. Based on examination of a cell adhesion molecule, the alphavbeta3 integrin, and its ligand, osteopontin, we have come to conclude that progesterone action can be direct or indirect. Progesterone acting on the stromal compartment provides paracrine mediators that influence epithelial gene expression. Conversely, acting directly, progesterone may primarily stimulate gene expression of the endometrial epithelium. The complexity of the system is extended since progesterone itself works through two different receptor isoforms. Regulated differential expression of PR-A versus PR-B also appears to fine tune the effect of progesterone on specific genes. Progesterone may also inhibit specific genes that undergo cyclic variation during the menstrual cycle. Together, using in vitro models we have shown that progesterone dynamically regulates gene expression in the endometrium and that imbalances between estrogen and progesterone may have far reaching consequences on normal cycle fecundity and on the balance between health and disease in this hormone-target tissue.

Early maternal changes contributing to the formation of the chorioallantoic and yolk sac placentas in rat: a morphological study

In order to determine the maternal changes contributing to the formation of the chorioallantoic and yolk-sac placentas, rat gestation sites were examined by light and electron microscopy on days 7 through 10 of pregnancy. On day 7, the implantation chamber showed different compartments and contained the blastocyst in the antimesometrial chamber. The epithelial lining of the implantation chamber disappeared at the antimesometrial chamber, transformed into disintegrated cells in the mesometrial chamber, and showed signs of the programmed cell death in the decidual crypt. On day 8, the mesometrial chamber lumen contained red blood cells and it was continuous with subepithelial sinusoids. The endothelial cells lining the mesometrial sinusoids also showed some characteristics of the sprouting type angiogenesis such as hypertrophy and cell proliferation. While the yolk-sac placental circulation was more obvious with participation of the giant trophoblasts at the antimesometrial pole of the conceptus on day 9, the antimesometrial cells showed autophagic degeneration after the formation of the chorioallantoic placenta on day 10. The contribution of the regional cell death and angiogenesis to form both of the two placentas are discussed.

Temporal and morphologic characteristics of pinopod expression across the secretory phase of the endometrial cycle in normally cycling women with proven fertility

OBJECTIVE

To assess the temporal and morphologic characteristics of pinopod expression on the surface of endometrium across the secretory phase, in LH-timed endometrial samples in normal, healthy women.

DESIGN

Prospective, randomized study.

SETTING

Academic teaching hospital.

PATIENT(S)

Sixty-eight healthy volunteers with proven fertility.

INTERVENTION(S)

Urinary LH-timed endometrial and blood sampling was performed on each subject on a randomly selected day of the secretory phase.

MAIN OUTCOME MEASURE(S)

Histologic dating, assessment of pinopods using scanning electron microscopy, and comparison with serum P levels.

RESULT(S)

Eighty-six endometrial tissue samples obtained from 68 subjects were evaluated under scanning electron microscopy. Pinopods were first observed on luteal day 5, corresponding with the onset of the midluteal phase increase in serum P levels. Pinopods persisted for the entire duration of the secretory phase, but their morphology changed as the cycle advanced.

CONCLUSION(S)

The present findings demonstrate that pinopods are a characteristic feature of the mid to late secretory phase endometrial epithelium, exhibit cycle-dependent changes in morphology, and are most prominent during the putative implantation interval.

Understanding the apical surface markers of uterine receptivity: pinopods-or uterodomes?

The plasma membrane of uterine epithelial cells is very sensitive to ovarian hormones and protrusions of the apical portion of this membrane have been used as indicators of endocrine status and preparation for implantation in the human uterus in particular. Protrusions of the apical plasma membrane were first identified in rats and mice where their established pinocytotic function gave rise to the name 'pinopod'. In humans and many other animals however, little evidence of the functional nature of such protrusions is available but what is available suggests that human 'pinopods' (useful though they are as indicators of endocrine status) might be more similar morphologically to other, larger, membrane protrusions, or apical domes, which have been shown not to be pinocytotic. Hence, I propose that these latter protrusions, including those in the human uterus, should be referred to by a term which does not imply a particular function and have settled on the name 'uterodome'.

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.

Loss of laminin and type IV collagen in uterine luminal epithelial basement membranes during blastocyst implantation in the mouse

BACKGROUND

Removal of the uterine luminal epithelium and its basement membrane is necessary for successful implantation of invasive blastocysts. Few reports, however, have specifically addressed the penetration and loss of the uterine luminal epithelial basement membrane (UEBM). We investigated the loss of UEBM by examining the distribution of laminin and type IV collagen.

METHODS

Blastocyst implantation sites were collected from mice on days 5, 6, and 7 of pregnancy. Paraffin sections were prepared from these tissues and processed with standard immunoperoxidase techniques to reveal the distribution of laminin and type IV collagen.

RESULTS

On day 5 of pregnancy blastocysts were adherent to the uterine epithelium. The epithelium and UEBM were complete and uninterrupted. On day 6 the juxtaembryonic uterine epithelium was lost and focal discontinuities were seen along the UEBM. By 1200 hr the UEBM had receded to the region near the ectoplacental cone, but staining was reduced for both antigens over the entire region surrounded by decidual cells. This decreased staining of the UEBM occurred in areas not yet occupied by trophoblast cells. On day 7 the UEBM was lost over the entire embryonic half of the uterine lining, corresponding to the distribution of decidual cells.

CONCLUSIONS

Progressive loss of the UEBM occurred in a consistent spatiotemporal pattern following the onset of blastocyst implantation. Diminished immunoreactivity of laminin and type IV collagen in the UEBM was closely correlated with the area occupied by decidualized endometrial stroma and occurred in areas not yet in contact with trophoblast cells. We conclude that decidual cells are instrumental in the removal of UEBM during early pregnancy.

Implantation and decidualization in rodents

This article reviews the main events of embryo-implantation and decidualization in rodents. In common laboratory rodents the embryo attaches to the uterine epithelial lining, usually on days 4 to 6 of pregnancy. A progressive degree of proximity between trophoblast and epithelium occurs until the epithelial cells undergo apoptosis and detach from the basement membrane. During the attachment stage, the spindle-shaped connective tissue cells that underlie the epithelium next to the embryos transform into polyhedral and closely packed decidual cells. Following the epithelial detachment and the breaching of the basement membrane the embryo is thus in direct contact with decidual cells. These cells accumulate organelles associated with synthesis of macro-molecules, intermediate filaments, and eventually lipid droplets and glycogen. Another remarkable feature of decidual cells is the establishment of gap and adherens intercellular junctions. Differentiation of fibroblasts into decidual cells advances antimesometrially and mesometrially, creating in the endometrium several regions of cells with different morphology. The whole phenomenon of decidualization which is normally triggered by the embryo can be artificially induced in pseudo-pregnant or hormonally-prepared animals with the use of diverse stimuli. The uterine epithelium is probably responsible for the transduction of the initial stimulus. Prostaglandins have been shown to be important in the induction of decidualization. More recently other substances such as leukotrienes, platelet-activating factor (PAF), and transforming growth factor (TGF) have been thought to play a role in induction. Much evidence points to prostaglandin production by the decidual cells. New proteins such as a luteotropic factor, desmin, and other molecules were shown to be produced after rat stromal cells undergo decidual transformation. The extracellular matrix of the mouse decidua contains very thick collagen fibrils. Mouse decidual cells are also very active in phagocytosing the thick fibrils, contributing to the remodeling and involution of the decidua that accompanies embryonic growth. Radioautographic data indicates that mouse decidual cells produce and secrete collagen and sulfated proteoglycans.

Penetration of the uterine epithelial basement membrane during blastocyst implantation in the mouse

For many species, blastocyst implantation is associated with a reduction in the number of cellular and extracellular matrix layers which separate the trophoblast from maternal vasculature. Following loss of uterine epithelial cells along the distal mural trophoblast, the mouse blastocyst encounters the residual epithelial basement membrane. This sheet of extracellular matrix must be breached and later removed prior to trophoblast invasion of the uterine stroma and formation of the placenta. The interactions between the trophoblast, luminal epithelial basement membrane, and decidual cells during the time when embryonic and uterine stromal cells first achieve contact were examined in this study. Distal mural trophoblast of activated delay blastocysts was in contact with the residual luminal epithelial basement membrane 36 hr after estrogen administration. This portion of the basement membrane contained areas in which the usual linear appearance was changed to an irregular, tortuous profile. The lamina densa frequently appeared flocculent and diffuse. Cytoplasmic processes from trophoblast and decidual cells simultaneously perforated the basement membrane at multiple discrete loci. With further development the basement membrane was lost, leaving trophoblast and decidual cells in close contact over large areas. In normally implanting blastocysts a similar stage of embryonic development, as described above, was attained by 0400 hr on day 6 of pregnancy. Regions of convoluted epithelial basement membrane were also seen in these implantation sites. However, only decidual cell processes were seen penetrating the residual basement membrane. These processes extended to the fetal side of the basement membrane and separated that matrix from overlying trophoblast. They contained organelles and formed rudimentary intercellular junctions with the trophoblast.(ABSTRACT TRUNCATED AT 250 WORDS)