Distribution of maternal immunoglobulins in the mouse uterus and embryo in the days after implantation

Mechanisms of formation and functions of the early embryonic cavities

As the early mouse embryo develops, fundamental steps include the sequential formation of the first lumens in the murine conceptus. The first cavity established in the pre-implantation embryo is the blastocoel, followed by the emergence of the proamniotic cavity during the peri-implantation stages. The mouse embryo is a dynamic system which switches its modes of lumenogenesis before and after implantation. The blastocoel emerges in between the basolateral membranes, whereas the proamniotic cavity is formed on the apical interface. Defects in the sculpting of these luminal spaces are associated with developmental abnormalities and embryonic lethality. Here, we review the mechanisms by which these early embryonic cavities are formed and discuss the cavities in terms of their common and stage-specific principles of lumenogenesis and their functions.

Deciphering epiblast lumenogenesis reveals proamniotic cavity control of embryo growth and patterning

During the peri-implantation stages, the mouse embryo radically changes its appearance, transforming from a hollow-shaped blastocyst to an egg cylinder. At the same time, the epiblast gets reorganized from a simple ball of cells to a cup-shaped epithelial monolayer enclosing the proamniotic cavity. However, the cavity's function and mechanism of formation have so far been obscure. Through investigating the cavity formation, we found that in the epiblast, the process of lumenogenesis is driven by reorganization of intercellular adhesion, vectoral fluid transport, and mitotic paracellular water influx from the blastocoel into the emerging proamniotic cavity. By experimentally blocking lumenogenesis, we found that the proamniotic cavity functions as a hub for communication between the early lineages, enabling proper growth and patterning of the postimplantation embryo.

PRDM1/BLIMP1 is widely distributed to the nascent fetal-placental interface in the mouse gastrula

BACKGROUND

PRDM1 is a transcriptional repressor that contributes to primordial germ cell (PGC) development. During early gastrulation, epiblast-derived PRDM1 is thought to be restricted to a lineage-segregated germ line in the allantois. However, given recent findings that PGCs overlap an allantoic progenitor pool that contributes widely to the fetal-umbilical interface, posterior PRDM1 may also contribute to soma.

RESULTS

Within the posterior mouse gastrula (early streak, 12-s stages, embryonic days ∼6.75-9.0), PRDM1 localized to all tissues containing putative PGCs; however, PRDM1 was also found in all three primary germ layers, their derivatives, and two presumptive growth centers, the allantoic core domain and ventral ectodermal ridge. While PRDM1 and STELLA colocalized predominantly within the hindgut, where putative PGCs reside, other colocalizing cells were found in non-PGC sites. Additional PRDM1 and STELLA cells were found independent of each other throughout the posterior region, including the hindgut. The Prdm1-Cre-driven reporter supported PRDM1 localization in the majority of sites; however, some Prdm1 descendants were found in sites independent of PRDM1 protein, including allantoic mesothelium and hindgut endoderm.

CONCLUSIONS

Posterior PRDM1 contributes more broadly to the developing fetal-maternal connection than previously recognized, and PRDM1 and STELLA, while overlapping in putative PGCs, also co-localize in several other tissues. Developmental Dynamics 246:50-71, 2017. © 2016 Wiley Periodicals, Inc.

Dynamic Pattern of HOXB9 Protein Localization during Oocyte Maturation and Early Embryonic Development in Mammals

Background

We previously showed that the homeodomain transcription factor HOXB9 is expressed in mammalian oocytes and early embryos. However, a systematic and exhaustive study of the localization of the HOXB9 protein, and HOX proteins in general, during mammalian early embryonic development has so far never been performed.

Results

The distribution of HOXB9 proteins in oocytes and the early embryo was characterized by immunofluorescence from the immature oocyte stage to the peri-gastrulation period in both the mouse and the bovine. HOXB9 was detected at all studied stages with a dynamic expression pattern. Its distribution was well conserved between the two species until the blastocyst stage and was mainly nuclear. From that stage on, trophoblastic cells always showed a strong nuclear staining, while the inner cell mass and the derived cell lines showed important dynamic variations both in staining intensity and in intra-cellular localization. Indeed, HOXB9 appeared to be progressively downregulated in epiblast cells and only reappeared after gastrulation had well progressed. The protein was also detected in the primitive endoderm and its derivatives with a distinctive presence in apical vacuoles of mouse visceral endoderm cells.

Conclusions

Together, these results could suggest the existence of unsuspected functions for HOXB9 during early embryonic development in mammals.

Critical roles of type III phosphatidylinositol phosphate kinase in murine embryonic visceral endoderm and adult intestine

The metabolism of membrane phosphoinositides is critical for a variety of cellular processes. Phosphatidylinositol-3,5-bisphosphate [PtdIns(3,5)P(2)] controls multiple steps of the intracellular membrane trafficking system in both yeast and mammalian cells. However, other than in neuronal tissues, little is known about the physiological functions of PtdIns(3,5)P(2) in mammals. Here, we provide genetic evidence that type III phosphatidylinositol phosphate kinase (PIPKIII), which produces PtdIns(3,5)P(2), is essential for the functions of polarized epithelial cells. PIPKIII-null mouse embryos die by embryonic day 8.5 because of a failure of the visceral endoderm to supply the epiblast with maternal nutrients. Similarly, although intestine-specific PIPKIII-deficient mice are born, they fail to thrive and eventually die of malnutrition. At the mechanistic level, we show that PIPKIII regulates the trafficking of proteins to a cell's apical membrane domain. Importantly, mice with intestine-specific deletion of PIPKIII exhibit diarrhea and bloody stool, and their gut epithelial layers show inflammation and fibrosis, making our mutants an improved model for inflammatory bowel diseases. In summary, our data demonstrate that PIPKIII is required for the structural and functional integrity of two different types of polarized epithelial cells and suggest that PtdIns(3,5)P(2) metabolism is an unexpected and critical link between membrane trafficking in intestinal epithelial cells and the pathogenesis of inflammatory bowel disease.

Spatial restriction of bone morphogenetic protein signaling in mouse gastrula through the mVam2-dependent endocytic pathway

The embryonic body plan is established through positive and negative control of various signaling cascades. Late endosomes and lysosomes are thought to terminate signal transduction by compartmentalizing the signaling molecules; however, their roles in embryogenesis remain poorly understood. We showed here that the endocytic pathway participates in the developmental program by regulating the signaling activity. We modified the mouse Vam2 (mVam2) locus encoding a regulator of membrane trafficking. mVam2-deficient cells exhibited abnormally fragmented late endosomal compartments. The mutant cells could terminate signaling after the removal of the growth factors including TGF-β and EGF, except BMP-Smad1/Smad5 signaling. mVam2-deficient embryos exhibited ectopic activation of BMP signaling and disorganization of embryo patterning. We found that mVam2, which interacts with BMP type I receptor, is required for the spatiotemporal modulation of BMP signaling, via sequestration of the receptor complex in the late stages of the endocytic pathway.

Antagonism between Smad1 and Smad2 signaling determines the site of distal visceral endoderm formation in the mouse embryo

The anterior–posterior axis of the mouse embryo is established by formation of distal visceral endoderm (DVE) and its subsequent migration. The precise mechanism of DVE formation has remained unknown, however. Here we show that bone morphogenetic protein (BMP) signaling plays dual roles in DVE formation. BMP signaling is required at an early stage for differentiation of the primitive endoderm into the embryonic visceral endoderm (VE), whereas it inhibits DVE formation, restricting it to the distal region, at a later stage. A Smad2-activating factor such as Activin also contributes to DVE formation by generating a region of VE positive for the Smad2 signal and negative for Smad1 signal. DVE is thus formed at the distal end of the embryo, the only region of VE negative for the Smad1 signal and positive for Smad2 signal. An inverse relation between the level of phosphorylated Smad1 and that of phosphorylated Smad2 in VE suggests an involvement of antagonism between Smad1- and Smad2-mediated signaling.

Maternal immunological recognition mechanisms during pregnancy

One of the most intriguing shortcomings of modern immunology, especially transplantation immunology, is its failure to provide a satisfactory final explanation for the consistent non-rejection of immunogenetically alien conceptuses in utero, even in specifically preimmunized females. Certainly, there is no shortage of hypotheses. Over the past few years various observations have utterly refuted the simplistic notion that the much-mated but nulliparous or gravid females are immunologically unaware of the presence and activities of allogeneic cells transiently or chronically within their reproductive tracts. This evidence has engendered the belief that some kind(s) of active response(s) on the part of the female, following early recognitive events, plays an important, if not essential, role, in conjunction with adaptive hypoantigenicity of the trophoblast, not only in conferring selective benefits upon the conceptus from the time of implantation, but also in affording it protection from the possible hazards of a cellular immunity. Evidence is emerging that during mating, implantation, placentation, and gestation (1) various fetal and histocompabibility antigens are presented to the mother in a unique manner; (2) she does respond to these; and (3) her responses aid in the establishment and maintenance of a harmonious state of immunological coexistence with her fetus. There are reasonable grounds for believing that a complete understanding of the immunobiology of the maternal-fetal relationship may facilitate significant advances in both transplantation and tumour immunology.

Aquaporin water channel genes are differentially expressed and regulated by ovarian steroids during the periimplantation period in the mouse

The periimplantation period is marked by edematous changes in the uterus. In the mouse, increased uterine vascular permeability occurs in response to estrogen and certain vasoactive mediators, but the mechanisms that regulate fluid transport during implantation are not fully understood. Aquaporins (AQPs) are a family of membrane channel proteins that facilitate bulk water transport. To assess their role in implantation, we examined the expression of AQPs 0-9 in the mouse uterus on d 1-8 of pregnancy. Our results show distinct uterine expression patterns for AQP1, AQP4, and AQP5. AQP1 is localized to the inner circular myometrium throughout the periimplantation period. AQP4 is highly expressed in the luminal epithelium on d 1 of pregnancy but barely detectable at the time of implantation. AQP5 is expressed at low levels in the glandular epithelium during early pregnancy but is markedly increased on d 5. By immunohistochemistry, AQP5 is localized in the basolateral region of the uterine glands. Treatment of adult ovariectomized mice with replacement steroids demonstrates an estrogen-induced shift in AQP1 signals from the myometrium to the uterine stromal vasculature, suggesting a role in uterine fluid imbibition. In contrast, AQP5 is induced only in estrogen-treated, progesterone-primed uteri. We also observed expression of AQP8 in the inner-cell mass and AQP9 in the mural trophectoderm of the implanting blastocyst. Collectively, these results suggest that members of the AQP family are involved in embryo and uterine fluid homeostasis during implantation.

Fetal-maternal immune interaction: blocking antibody and survival of the fetus

In the late 1940s it became clear that the homograft reaction was essentially the result of an immune response. Subsequently, Medawar commented on the apparent paradox of the survival of the mammalian fetus in the face of such a potential (cell-mediated) immune response. In an outbred population the fetal-placental unit will be antigenically different to the mother by virtue of its complement of paternal genes and additionally there may be developmental or stage-specific gene products that are immunogenic. Many mechanisms have been proposed to account for the survival of the fetus in the face of a potential immune attack and, while many of these have been investigated in considerable detail, there has been no clear-cut indication that any one plays a predominant role. Either control of immune rejection of the fetus is exercised by an as yet undiscovered mechanism or, more probably, by a combination of some or all of the mechanisms that have been proposed by many workers over the last three decades. Potential controlling processes, which will be reviewed briefly, include: systemic and local modification of maternal responsiveness; altered expression of MHC antigens on extra-embryonic tissues; the placenta as a barrier; and blocking antibody responses. We discuss some of our recent studies in which we have started to look for potential blocking antibodies in a mouse model system. Cells secreting immunoglobulins M and G, characterized in hemolytic plaque assays, have been mapped to areas close to the midgestation mouse embryo, using an immunocryohistological technique. A scaled-down version of hybridoma technology has been used as an analytical probe of the specificity and isotype of immunoglobulin secreted by cells originating either from close to the embryo/fetus or from the para-aortic lymph nodes (PALN). So far monoclonal (IgG1) antibodies with specificity for embryonic cells have been derived together with some monoclonal immunoglobulins with as yet uncharacterized antibody specificity.

Immunology of pregnancy: towards a unifying hypothesis

The variable findings of hormonal-immunoregulation and the variable cellular and humoral immune responses in pregnancy have been considered in relationship to the physiological response. From such considerations it appears that the peripheral blood lymphocyte/leukocyte response in pregnancy is not important, but rather the local uterine immune response at implantation and throughout pregnancy. It is proposed, and evidence is presented, that a normal allogeneic immune response is initiated at the time of implantation of the blastocyst. This immune response regulates the invasive nature of the trophoblast and initiates the first stage of parturition. The initiation and maintenance of this immune response is based on an interplay between maternal and paternal HLA and trophoblast antigens. In the case of HLA-incompatible donor-recipient blastocyst transplants, a more pivotal role for immunoregulation by trophoblast antigens is proposed. This is because it is considered that the local uterine immune response suppresses the expression of allogeneic HLA. This concept is further developed in terms of haploid HLA suppression on maternal and fetal lymphocytes that cross the placenta. This is considered to allow the interaction of these lymphocytes with each other and explains maternal transfer of cell-mediated immunity.

Role of embryonic factors in implantation: recent developments

The embryonic factors influencing implantation have been studied extensively in laboratory and domestic animals, but not in primates, including humans. Species differences make extrapolation inadvisable. Embryonic factors affecting implantation include intrinsic features of the embryo, such as its genetic constitution, morphology and hatching. Abnormal genetic constitutions or unsuccessful transitions from maternal to embryonic transcription could account for many failures of early embryonic growth and implantation. Morphology per se does not greatly influence implantation, except when it reflects an abnormal genetic constitution, e.g. in severe fragmentation, although subtle effects may be detected as experimental techniques are refined. The initiation of differentiation and intraembryonic communication between cells and cell types has been studied in animal embryos. Signals must be exchanged between the embryo and the mother to ensure satisfactory implantation. These could include platelet activating factor, prostaglandins, histamine related factors, steroids, proteins, metabolic products and immune-active factors. No one factor seems to be totally responsible for alerting the mother to the presence of an embryo, and a concerted action of these and other agents is probably responsible. The process of implantation itself is poorly understood because of a lack of adequate experimental models. The expression of complementary proteins and the role of specific enzymes and markers of endometrial and embryonic competence are factors well worthy of further study. Knowledge about human implantation is increasing because of recent developments in assisted reproductive technology, and concepts arising from many years of research in animals should find clinical applications in understanding and controlling human reproduction.

Immunoglobulin-secreting cells in close proximity to the mid-gestation mouse embryo

A reverse haemolytic plaque assay has been used to detect cells secreting IgM and IgG from areas in close proximity to mid-gestation mouse embryos. These cells were obtained from the fetal-placental unit and from the decidual mass of mice on the 9th or 10th day of syngeneic pregnancy. Cryohistology using affinity prepared goat antibodies specific for mouse Ig isotypes, coupled to horseradish peroxidase by a second antibody, showed that free Ig and cells containing Ig, are located closely but externally to the extra-embryonic membranes and also in areas close to the developing placenta in the decidua basalis. We suggest that some of the identified Ig-forming cells may be producing antibodies specific for antigens on the extra-embryonic membranes and may thus be acting as blocking (enhancing) antibodies.

Progesterone regulation of secretory component (SC): uterine SC response in organ culture following in vivo hormone treatment

Sex steroid hormones are known to have profound effects on mucosal immunity. In the present study we evaluated the effects of progesterone on the uterine immune system by determining the changes in the levels of secretory component (SC) released from uterine tissues in culture following in vivo administration of progesterone to estradiol-stimulated ovariectomized rats. SC is a transport protein which moves IgA into external secretions such as intestinal and uterine secretions. SC release was determined by radioimmunoassay of the culture medium collected 24 h after introduction of uterine tissues into medium with or without cycloheximide. The net production of SC was reduced following progesterone administration. Reduction in SC levels followed a dose dependency and occurred irrespective of whether the progesterone was given before or during treatment with estradiol. These results support the hypothesis that progesterone plays a direct role in suppression of uterine SC production and release.

Localization of immunoglobulins in the mouse uterus, embryo, and placenta during the second half of pregnancy

Throughout the second half of pregnancy in mice there were many plasma cells containing immunoglobulins A (IgA) and G (IgG) in the uterine endometrium. There was intense staining of IgA in uterine glands at all stages, but little staining of IgG. The staining of both immunoglobulins (Igs) in the luminal epithelium was moderate to dark on day 11, slight on day 14, and increased from day 16 to term. From day 14 to term the endometrium exhibited folds or villi around each placenta. The cores of the villi contained many plasma cells of both isotypes, and the staining of extracellular Igs in the villous cores was darker than in nonvillous endometrium. Both Igs were detected in the uterine lumen, and in visceral and parietal yolk sac endoderm cells at all stages. Near term, the staining of Igs in the visceral yolk sac was darkest in the peripheral villous portion adjacent to the endometrial villi. From day 14 to term IgG was present in the visceral yolk sac mesenchyme and embryo, consistent with its transfer from the uterine lumen to the embryo via the vitelline circulation. In contrast, IgA was not detected in yolk sac mesenchyme until day 19, when only slight staining was observed, and IgA was never detected in the embryo. Most trophoblast giant cells contained both Igs on day 11. During the remainder of pregnancy, there was staining of both Igs in labyrinthine trophoblast and in a few giant cells adjacent to the parietal yolk sac on the placenta, but there was negligible staining in the spongiotrophoblast region. Our observations suggest that the local immune system in the mouse uterus may protect the embryo during the second half of pregnancy by secreting anti-microbial immunoglobulins A and G into the uterine lumen surrounding the visceral yolk sac, and may at the same time contribute to the transfer of maternal IgG to the embryo via the yolk sac and vitelline circulation.

Abnormal embryonic development induced by antibodies to rat visceral yolk-sac endoderm: isolation of the antigen and localization to microvillar membrane

An antigenic substance was isolated from rat visceral yolk-sac endoderm of the 18th-20th days of gestation by extraction with the nonionic detergent Nonidet P-40, Sephacryl S-300 gel filtration, and Ricinus communis agglutinin affinity chromatography. The rabbit antiserum directed against this antigenic substance when injected into pregnant rats during the period of organogenesis caused abnormal embryonic development, fetal growth retardation, and embryonic death. Ouchterlony gel diffusion analysis demonstrated that the antiserum formed one immunoprecipitin band against the crude detergent extract and a complete identity between the present visceral yolk-sac antigen and the renal glycoprotein antigen previously isolated (C. C. K. Leung, (1982) J. Exp. Med. 156, 372-384). The antigen eluted from the antibody affinity column appeared to consist of two major peptides of 60 and 30 kDa when analyzed by SDS-polyacrylamide gel electrophoresis. Indirect immunofluorescent and immunoperoxidase localization studies at the light microscopic level demonstrated that both rat renal proximal tubule and embryonic visceral yolk-sac endoderm at various gestational stages (including the organogenetic period) shared the same antigen. Indirect immunoperoxidase localization studies at the electron microscopic level demonstrated that the antigen was a part of (or associated with) the microvillar membrane and membrane invaginations at the base of the microvilli of the renal proximal tubule and visceral yolk-sac endoderm. In vivo immunoperoxidase localization studies demonstrated that the teratogenic antibodies localized within the large phagolysosomes and the apical vesicles of the visceral yolk-sac endoderm. It is postulated that visceral yolk-sac pathology was induced by the antibodies.

The secretory immune system in the uterus of the pregnant rat: production of secretory component by uterine tissues

Secretory component (SC) was measured in amniotic fluid, fetal serum, and maternal serum and compared with SC production during in vitro culture of uterine tissue segments from pregnant rats. The concentrations of SC in amniotic fluid did not change between days 14 and 20 of pregnancy. Similarly, there was no change in maternal or fetal serum during pregnancy, although, the levels of SC in sera were consistently higher than those in amniotic fluid. When uterine segments were incubated in vitro, release of SC was greater in the absence of cycloheximide than in the presence of cycloheximide at all stages of pregnancy. In contrast to SC values in amniotic fluid, however, SC production by uterine tissue changed markedly during pregnancy. SC levels were low during early pregnancy (day 7 post coitus) and increased to levels found in non-pregnant diestrous rats just prior to parturition (day 20). The findings suggest that the endocrine balance during pregnancy may play a central role in regulation of the uterus immune system. The pattern of SC release may reflect a need both to ensure protection of the fetus from the IgA immune system in early pregnancy and to prevent maternal infection during parturition by reactivation of this system.

Maternal immunoglobulins G, A and M in mouse uterus and embryo during the postimplantation period

The distribution of maternal IgG, IgA and IgM was studied by an immunoperoxidase technique on mouse uteri and embryo sections from 4.5 to 10 days of gestation. In the embryo, from day 5, IgG was found in the endoderm, and later in the trophoblast, distal and extraembryonic proximal endoderm, embryo cavity and early vitelline vessels. From day 6, IgA was found in the same areas but staining was less intense and was not seen in vitelline vessels. From day 8, small amounts of IgM were found in the same areas, but not in vitelline vessels. Between implantation sites, IgG and plasma cells containing IgA remained present in the stroma; most glands contained IgA, and the uterine lumen stained for IgA and IgG. IgG granules were found in the apical region of luminal epithelial cells in the implantation sites at 5 and 5.5 days; IgA granules were seen in the apical region of luminal epithelial cells between implantation sites from 4.5 to 6.5 days. These results suggest that IgA and some IgM are transmitted along with IgG to the early mouse embryo but do not reach the embryonic circulation, and that during the implantation period reabsorption of IgG and IgA from the uterine lumen into the epithelium differs, depending on the position of this epithelium in relation to the embryo. The persistent IgA and IgG secretion in the uterus after implantation suggests that during this period the mechanisms regulating immunoglobulin secretion may differ from those during the oestrous cycle.

Bone marrow origin of decidual cell precursors in the pseudopregnant mouse uterus

Decidual cells are considered to be the endproduct of a hormonally induced transformation of endometrial stromal cells of the uterus. However, the source of these precursors remains unknown. This study of evaluated the possibility of their bone marrow origin by an examination of the H-2 phenotype of decidual cells in pseudopregnant bone marrow chimeras. These chimeras were produced by repopulating lethally irradiated CBA/J female (H-2k) mice with bone marrow from (CBA/J x C57BL/6J) F1 female (H-2kb) mice. Pseudopregnancy was produced with a hormonal regimen followed by an oil-induced decidual stimulus. Chimerism was evaluated radioautographically by an identification of the donor-specific Kb phenotype on cells with an immunolabeling technique with monospecific anti-H-2 serum followed by radioiodinated protein A. The extent of chimerism as indicated by the degree of Kb labeling on decidual cells as well as macrophages contained within the decidual nodules was quantitatively compared with that seen on splenic lymphocytes. Fair to good chimerism, as reflected by labeling for the donor-specific marker (Kb), was seen on splenic lymphocytes and macrophages within the decidual nodules in 6 out of 11 animals. A similar level of chimerism was detected on decidual cells in all but one of these six, in which case this was low. One animal showed low chimerism in the spleen but good chimerism on the decidual cells. The remaining four mice were nonchimeric for all three cell types. These results indicate that decidual cells and macrophages appearing within the decidual nodules of pseudopregnant mice are ultimate descendants of bone marrow cells.

Immunological studies of mouse decidual cells. II. Studies of cells in artificially induced decidua

Cells from artificially induced decidual tissue (deciduoma) in the mouse were examined for Thy-1 surface antigen and receptors for the Fc portion of immunoglobulin G (FcR) and compared with cells of the normal decidua from 6 to day 9 of pregnancy. It was shown that (1) Thy-1 antigen is present on the same proportion of cells in decidua and deciduoma on day 6 and day 7, (2) FcR-bearing cells can be detected in similar numbers on day 6 and day 7 but this does not increase on day 8 in deciduoma as it does in decidua, and (3) progesterone treatment after induction of decidualization allowed further increase of FcR-bearing cells in deciduoma. These results present further evidence of the similarity between deciduoma and decidua in the mouse. They indicate that these two membrane markers are present in the early decidua, regardless of the presence of an embryo, and suggest that progesterone may play a part in the increase of FcR-bearing cells in the decidua during pregnancy.

Localisation of immunoglobulin (IgG) within the rat metrial gland

An immunohistological method was used to assess the IgG content of the rat metrial gland at different stages of pregnancy. The result apparently varied according to the type of fixative used. Saturated alcoholic mercuric chloride was found to produce the most consistent demonstration, with the IgG located in cells which also contained diastase-fast, PAS-positive granules. Using single cell suspensions prepared from metrial glands, significantly more cells were shown to contain cytoplasmic IgG at day 13, compared to day 14 of pregnancy. However, there were no other significant differences at other stages of pregnancy examined. Surface IgG was detected on a small proportion of the cells and the findings are discussed in relation to the hypothesis of a lymphocytic origin for the granulated metrial gland cells.

Placental permeability during early gestation in the hamster. Electron microscopic observations using horseradish peroxidase as a macromolecular tracer

The permeabilities of the parietal yolk sac placenta and the preplacental region of the hamster conceptus during early postimplantation (day 8) were compared by means of electron microscopy and a macromolecular protein tracer, horseradish peroxidase (HRP). HRP was administered by injection into the maternal venous system; samples of the two placental tissues were obtained for examination at intervals between 4 minutes and 1 hour later. The three layers of the parietal yolk sac wall (from outer to inner: capsular trophoblast, Reichert's membrance, parietal endoderm) appeared to provide little impediment to the passage of HRP from perivitelline maternal blood spaces to the yolk sac cavity. HRP passed through the outer trophoblast layer, both by way of intracellular fenestrae (60-200 nm diameter) and narrower intercellular channels, and completely permeated the meshwork of Reichert's membrane within minutes after injection. The inner parietal endoderm cell layer was widely discontinuous and clearly presented no barrier to HRP movement. HRP reaching the yolk sac cavity was avidly endocytosed by the viceral yolk sac epithelium. In contrast to the parietal yolk sac, the preplacental region of the conceptus was impermeable to HRP. Zonular occluding junctions located between contiguous cells of the chorionic ectoderm layer of the preplacenta were the obvious barrier to the HRP molecules. These results suggest that in this rodent species, during the early postimplantation period of gestation, the pareital yolk sac placenta potentially plays a more important role in the maternal-embryonic transfer of macromolecular substances than does the preplacenta.

Uptake of marker proteins by glycoprotein-containing cells of the pregnant rat uterus and placenta

A study was made of cells in the pregnant rat uterus and placenta known to contain glycoprotein inclusions to investigate their ability to endocytose marker proteins (fluorescein conjugated serum and horseradish peroxidase) injected into the maternal circulation. The visceral endoderm showed marked uptake of both proteins, though in later pregnancy this was restricted to the area of yolk sac adjacent to the chorio-allantoic placenta. The intracellular distribution of the endocytosed marker proteins resembled that of the glycoprotein inclusions. In the earlier stages some of the giant cell inclusions contained glycoprotein, some showed staining with the Dunn-Thompson technique for haemoglobin, and some showed peroxidase activity. There was endocytosis of both marker proteins by giant cells, and apparently this occurred independently of ingestion of red blood cells. Uptake by the giant cells persisted to a later stage in the area round the margin of the chorio-allantoic placenta than in the other giant cells. Endocytosis occurred in the labyrinthine trophoblast, and the glycoprotein inclusions found in this situation may represent material being transmitted or digested. The glycoprotein-containing granulated metrial gland cells showed no evidence of endocytotic activity, but there was uptake of both marker proteins by the associated stromal cells.

Freeze-fracture observations on the visceral yolk sac placenta of rats, mice and hamsters. With special reference to endodermal cell tight junctions

Freeze-fracture replicas of visceral yolk sac from rats, mice and hamsters in late stages of gestation were studied by electron microscopy. Special attention was directed toward determining the types of junctional specializations that exist between the columnar endoderm cells of this placental membrane. In all three species, well-developed, zonular tight (occluding) junctions were found on the contiguous lateral surfaces of the endoderm cells. The tight junctional network, located in an immediate subluminal position, was from 0.2--0.5 micrometers in depth and consisted at any point of 2--5, interconnecting, approximately 9 eta wide, strands (P-face) or shallow furrows (E-face). Patch-like aggregations of irregular intramembrane particles, characteristic of desmosomes (maculae adherentes), also were observed at scattered sites below the tight junctions. However, no evidence of gap (communicating) junctions was encountered. The endoderm cells of the rodent visceral yolk sac have been shown to play a central role in the selective transport of macromolecular substances from the maternal to the fetal system. Tight junctions may be vital to this endodermal cell function by preventing random paracellular fluxes of macromolecules.

Localization of H-2 antigens on mouse trophoblast cells

The presence of H-2 antigens of the paternal and maternal haplotypes on mouse trophoblast cells was examined at several stages of pregnancy by using a sensitive immunolabeling technique followed by quantitative radioautography. Results revealed the presence of H-2 antigens (determined by the K or D loci) of both parental haplotypes on the F1 trophoblast cells. At 14-16 d of gestation, the antigen density was equivalent to that on adult thymocytes and there was a further 50% increase on day 18. H-2 antigens of both parental haplotypes are also found to be expressed on 11-13 d trophoblast cells.

The occurrence of uterine stromal and intraepithelial monocytes and heterophils during normal late pregnancy in the rat

The steady decline in plasma progesterone level that occurs during the last week of pregnancy in the normal rat (Wiest, '70) provides good opportunity to study the effect of withdrawal of progesterone on uterine differentiation. Evidence is presented that tissue monocytes, heterophils, and eosinophils are regular components of the normal late gestational uterus and that their number increases as term approaches. Uterine monocytes and heterophils are located in the endometrial and myometrial stroma as well as within the basal intercellular compartment of the luminal epithelium. Stromal monocytes are distributed throughout the attenuated endometrium of late gestation, but are more common immediately beneath the luminal epithelium. In the myometrium, monocytes and heterophils occur, often as perivascular, clusters in the connective tissue septum that separates the two layers of smooth muscle. Eosinophils are present especially in the deep endometrial and myometrial stroma, and increase in number as plasma estrogen rises immediately before parturition. A small population of lymphocytes is regularly present. An important feature of the prepartum uterine stroma is the sparseness of macrophages. Near term, however, the beginnings of monocytic-macrophagic transformation are noticeable as the cell surface becomes more irregular and organelles associated with endocytic activity arise. The prepartum monocytes are positioned in the same histological sites that during the postpartum period of regression will be occupied by macrophages (Padykula and Campbell, '76). Since it is generally accepted that monocytes are precursors of macrophages, this spatial correlation raises the possibility that cellular preparations for regression commence before birth. The possible significance of prepartum monocytic infiltration is discussed in relation to the effect of changing plasma and uterine concentrations of progesterone on uterine collagenase activity. The steady increase in uterine leucocytes which occurs concomitantly with decreasing uterine binding capacity for progesterone supports the hypothesis by Siiteri et al. ('77) that progesterone in high local concentrations has an anti-inflammatory effect.

Immunological studies of mouse decidual cells. I. Membrane markers of decidual cells in the days after implantation

Mouse decidual cell suspensions from day 6 to day 8 of gestation were prepared by enzymatic treatment with collagenase and trypsin and tested for various membrane markers. (a) Besides H-2 antigens, Thy-1 antigens are present on about 50% of the cells; this may reflect the fibroblastic origin of decidual cells or be a marker expressed on some decidual cells possibly under hormonal control. (b) T or B lymphocytes, as defined by four Lyt antigens or surface immunoglobulins, are not present in significant amounts. (c) A substantial number of cells bearing receptors for the Fc portion of IgG (FcR) is detectable in the decidua, probably closely connected with trophoblast cells; these FcR-bearing cells may act in preventing excessive invasion of uterine tissue by trophoblast or could contribute to the protection of the embryo by interacting with maternal blocking antibodies and trophoblast. No receptors for for complement were detected, even after 16-20 h in culture after trypsin treatment.

The structure and differentiation of granulated metrial gland cells of the pregnant mouse uterus

A study was made with the light and electron microscopes of the granulated metrial gland cells of the decidua basalis of the pregnant mouse uterus, up to day 11 of pregnancy. The granulated metrial gland cells are large, up to 50 mu in diameter, mono- or binucleate and the glycogen rich cytoplasm typically contains many large glycoprotein granules which may be up to 5 mu in diameter. Morphological evidence is described in support of a lymphocyte-like cell being the precursor to the granulated metrial gland cell. This differentiation sequence is similar to that already proposed in the rat but differences between the ultrastructure of the mature metrial gland cells of rats and mice were noted.