Long Term Culture of Cells Derived from Mouse Blastocysts

The development of mouse blastocysts in primary culture has been followed for up to two months. The trophectoderm layer of the blastocyst gives rise to a monolayer of trophoblast cells; cells resembling both ectoplacental cone cells and primary giant cells are observed. The former can transform to giant cells, presumably secondary trophoblast, after several days in culture. Giant trophoblast cells are evident in the culture for much longer than the normal gestation period. Under the culture conditions described, the proportion of blastocysts showing substantial inner cell mass (ICM) proliferation in vitro is higher than that noted in previous studies. The ICM clumps develop into either egg cylinder-like structures, or, more commonly, into spherical, fluid-filled vesicles. The vesicles, which resemble yolk sac morphologically and biochemically [10, 11], continue to enlarge in size during several weeks of culture. The vesicles are attached to the underlying trophoblast monolayers by a stalk. Cells appear to migrate from this stalk out along the culture dish. The result after two to four weeks of culture is the appearance of a mixed monolayer containing a variety of different cell types. Secondary cultures of blastocyst cells have been continuously maintained in vitro for more than one year. Four lines of cells, all developing from the same pool of blastocysts, have been monitored for morphological, growth and biochemical properties, as well as chromosome number. Each line contained two or more morphologically distinct cell types, clearly indicated by cloning studies after eight months of culture. Doubling times and saturation densities among the four lines differed, as did biochemical properties. Although none of the cell lines resembled trophoblast biochemically after 7.5 months in culture, one line, MB4, possessed a number of biochemical properties in common with midgestation yolk sac. After a further five months of culture, some enzymes in the four lines were relatively unchanged; in other cases, notably with alkaline phosphatase, a sharp drop in enzyme activity was observed. One cell line, MB2, and specifically one of the cell types in this line, produced a yellow-orange pigment with a spectrum resembling that of a heme protein. After 7.5 months of culture, two of the four lines, MB21 and MB31, contained large numbers of cells with a diploid number of chromosomes. However, by 12.5 months in culture, the large majority of metaphases in all four cell lines possessed a hypotetraploid chromosome number. In a number of studies carried out to date, none of the cell lines generated tumors when injected into syngeneic hosts.     

Protein secretion by the mouse trophoblast during attachment and outgrowth in vitro

Protein secretion from mouse blastocysts undergoing attachment and trophoblast outgrowth in vitro was assessed. When Day 5 blastocysts were cultured in serum-containing medium, secretion of several 'attachment-associated' proteins (PAS) was initiated within 24 h, coincident with attachment and outgrowth. Those proteins characteristic of the pre-attachment blastocyst disappeared or made-up only a small portion of the secretions once attachment began. The major secreted protein from attached embryos, PA1, is a 35,000-45,000 Mr acidic glycoprotein with multiple isoelectric forms. PA2, a group of basic 40,000 Mr proteins and PA3 a group of 72,000 Mr proteins were also produced during outgrowth. PAS were secreted during outgrowth on fibronectin-coated plastic in serum-free medium, but not by blastocysts held in a non-attachment state during culture in serum-free medium on uncoated plastic. In pre-attachment blastocysts, secreted proteins were produced by trophoblast vesicles, but not by isolated inner cell masses. Both trophoblast vesicles growing out in vitro and surgically isolated trophoblast from spreading blastocysts had secreted protein patterns qualitatively similar to those of intact blastocyst outgrowths. The results indicate that development of trophoblast protein secretion continues through the period of outgrowth and giant cell transformation. These changes are apparently dependent on attachment of the blastocyst to a suitable substrate, but not dependent on any other serum influence.     

Interaction of extravillous trophoblast galectin-1 and mucin(s)—Is there a functional relevance?

ABSTRACT

In the course of embryo implantation extensive interaction of the trophoblast with uterine tissue is crucial for adequate trophoblast invasion. This interaction is highly controlled, and it has been pointed out that a specific glycocode and changes in glycosylation may be important for successful implantation and maintenance of pregnancy. Both uterine and trophoblast cells have been shown to express cell surface glycoconjugates and sugar binding proteins, such as mucins (MUC) and galectins (gals). An increasing number of studies have investigated potential candidates interacting in this process. However, knowledge about the biochemical nature of the interactions and their importance for trophoblast cell function, and, consequently, for pregnancy outcome are still lacking. This review is aimed at deliberating the possibility that mucins, as heavily glycosylated proteins, might be among the functionally relevant galectin ligands in human trophoblast, based on both published data and our original research.     

Signal transduction and biological function of placenta growth factor in primary human trophoblast

Placenta growth factor (PlGF), a member of the vascular endothelial growth factor family of angiogenic factors, is prominently expressed by trophoblast. In addition to its role as a paracrine angiogenic factor within the placenta and endometrium, presence of its receptor, Flt-1, on trophoblast suggests that PlGF also may have an autocrine role(s) in regulating trophoblast function. To elucidate its role in trophoblast, we examined the signal transduction and functional responses of primary human trophoblast to PlGF. Exogenous PlGF induced specific activation of the stress-activated protein kinase (SAPK) pathways, c-Jun-N terminal kinase (JNK) and p38 kinase, in primary term trophoblast with little to no induction of the extracellular signal regulated kinase (ERK-1 and -2) pathways. In contrast, PlGF induced significant ERK-1 and -2 activity in human umbilical vein endothelial cells but did not induce JNK or p38 activity. PlGF-induced activation of the SAPK signaling pathways protected trophoblast from growth factor withdrawal-induced apoptosis, but it did not protect trophoblast from apoptosis induced by the pro-inflammatory cytokines, interferon gamma and tumor necrosis factor alpha. These results provide the first direct evidence of a biochemical and functional role for PlGF/Flt-1 in normal trophoblast and suggest that aberrant PlGF expression during pregnancy may impact upon trophoblast function as well as vascularity within the placental bed.     

Implantation and invasiveness of mouse blastocysts on uterine monolayers

We have utilized monolayers of mouse uterine cells as a substratum for blastocyst implantation in vitro. In this system, blastocysts attach, and trophoblast cells initially grow out in an invasive manner, displacing the cells of the monolayer. A similar phenomenon is observed when any of a number of established cell types constitute the monolayer. After 5 to 6 days in culture, trophoblast cells lose their invasive capacity, although viability is unaffected. These events appear to be analogous to those taking place during implantation in utero or in ectopic sites. After treatment with dextran-norit, progesterone and estrogen can no longer be detected in fetal calf serum by a sensitive radioimmunoassay. Nevertheless, blastocysts can implant normally in medium supplemented with treated serum, indicating the likelihood that, unlike implantation in vivo, progesterone and estrogen are not required for the analogous event in vitro.     

Biochemical,biophysical, and genetic changes of porcine trophoblast‐derived stem‐like cells during differentiation as evaluated using Raman microspectroscopy,Atomic force microscopy,and quantitative polymerase chain reaction

Porcine trophoblast‐derived stem‐like cells grown into serum medium start to differentiate and become senescent within 30 days. However, trophoblast‐derived cells, cultured in vitro in a defined and non‐serum medium, have the regenerative properties, such as indefinite passage and foreign DNA receptivity, similar to stem cells. To evaluate the biochemical, biophysical, and genetic changes of the terminal differentiation of trophoblast derived cells, Raman microspectroscopy, atomic force microscopy, and qPCR were applied. It was found that Raman spectral intensities of characteristic peaks, cell morphology, and Young's modulus can be used to distinguish differentiated and undifferentiated trophoblast cells. In addition, 17 cytoskeleton and extracellular matrix‐related genes were significantly impacted by medium type (non‐serum versus serum). Our findings suggest that Raman microspectroscopy and atomic force microscopy—both considered as label‐free, non‐invasive techniques—can be applied to distinguish differentiated trophoblast cells, and cellular biochemical information and biophysical properties can be indicative of cellular differences during cell differentiation. In addition, most of cytoskeleton‐related genes exhibit similar pattern to that of Young's modulus during trophoblast cell differentiation, indicating the potential connection between cytoskeleton‐related genes and cellular stiffness. genesis 53:749–761, 2015. © 2015 Wiley Periodicals, Inc.     

Retinoic acid promotes differentiation of trophoblast stem cells to a giant cell fate.

Trophoblast stem cell (TS cell) lines have the ability to differentiate into trophoblast subtypes in vitro and contribute to the formation of placenta in chimeras. In order to investigate the possible role of retinoic acid (RA) in placentation, we analyzed the effects of exogenous RA on TS cells in vitro and the developing ectoplacental cone in vivo. TS cells expressed all subtypes of the retinoid receptor family, with the exception of RARbeta, whose expression was stimulated in response to RA. TS cells treated with RA were compromised in their ability to proliferate and exhibited properties of differentiation into trophoblast giant cells. During TS cell differentiation into trophoblast subtypes induced by withdrawal of FGF4, RA treatment further illustrated its role in the specification of cell fate by the promotion of differentiation into giant cells and the suppression of spongiotrophoblast formation. Moreover, administration of RA during pregnancy resulted in the overabundance of giant cells at the expense of spongiotrophoblast cells. RA hereby acts as an extracellular signal whose potential function can be linked to specification events mediating trophoblast cell fate. Taken together with the spatial patterns of giant-cell formation and RA synthesis in vivo, these findings implicate a function for RA in giant-cell formation during placentation.     

The human villous cytotrophoblast: Interactions with extracellular matrix proteins, endocrine function, and cytoplasmic differentiation in the absence of syncytium formation

Human syncytiotrophoblasts are derived from villous cytotrophoblasts by cell fusion. Coincident with this morphologic transformation, trophoblasts acquire specific endocrine functions, including elaboration of chorionic gonadotropin (hCG). We wondered if syncytia formation was a prerequisite for biochemical differentiation or simply was one part of the differentiation program. By growing purified human cytotrophoblasts under serum-free conditions and manipulating the culture surface, we were able to disassociate morphologic from biochemical differentiation. We have shown previously (Endocrinology 1986, 118:1567) that human cytotrophoblasts grown in the presence of fetal calf serum flatten out, aggregate, and form functional syncytiotrophoblasts in vitro over 24-96 hr. Here we demonstrate that when grown in the absence of serum, the cells do not undergo these morphologic changes, but remain as individual spherical cells. If the culture surface was precoated with fibronectin or a variety of collagens, but not albumin, the cells regained their ability to flatten, aggregate, and form syncytia. Attachment to and syncytia formation on fibronectin was blocked by the addition of the R-G-D-S-containing peptide, Gly-Arg-Gly-Asp-Ser-Pro. Attachment to and syncytia formation on type I collagen was blocked by anti-human fibronectin F(ab')2 fragments, while association with type IV collagen was not affected by this antibody, suggesting that fibronectin mediates trophoblast association with type I collagen, but not type IV. Although syncytia formation did not occur when cytotrophoblasts were cultured under serum-free conditions in the absence of ECM proteins, biochemical differentiation was not affected. These cells secreted hCG at the same rate under serum-free conditions whether they were plated on plastic only--which prevented syncytia formation--or fibronectin, laminin or, type IV collagen--which allowed syncytia formation to occur. Furthermore, cytoplasmic differentiation in the absence of syncytia formation was confirmed by performing transmission electron microscopy on cytotrophoblasts grown under serum-free conditions in the presence of 8-bromo-cAMP. We conclude that syncytia formation is not a prerequisite for biochemical differentiation, but simply part of the trophoblast differentiation program.     

Decidual natural-killer-cell interaction with trophoblast: cytolysis or cytokine production?

At the implantation site, the uterine mucosa (decidua) is infiltrated by large numbers of natural killer (NK) cells. These NK cells are in close contact with the invading fetal trophoblast and we have proposed that they might be the effector cells that control the implantation of the allogeneic placenta. Recent characterization of NK cell receptors and their HLA class I ligands has suggested potential mechanisms by which NK cells might interact with trophoblast. However, what happens as a result of this interaction is not clear. The traditional method for investigating NK cell function in vitro is the protection from lysis of target cells by expression of HLA class I antigens. This might not be an accurate reflection of what happens in vivo. Another function of NK cells is the production of cytokines on contact with target cells. This could be an important outcome of the interaction between decidual NK cells and trophoblast. Decidual NK cells are known to produce a variety of cytokines; trophoblast cells express receptors for many of these cytokines, indicating that they can potentially respond. In this way, decidual NK cells have a significant influence on trophoblast behaviour during implantation.     

Carbohydrate changes in pre- and peri-implantation mouse embryos as detected by a monoclonal antibody

We have examined the tissue and embryonic distribution of an antigen on a large polysaccharide that is recognized by a monoclonal antibody, IIC3, prepared against F9 teratocarcinoma cells. By immunofluorescence the antigen is first detected on compacted morulae and early blastocysts. It is strongly expressed on the primary endoderm and trophoblast of expanded blastocysts, but then disappears from the trophoblast of attached blastocysts in vitro. The binding of the antibody is completely inhibited by D-galactose and N-acetylgalactosamine. Fluoresceinated lectins were used to study further the changes in cell surface carbohydrates on trophoblast during implantation. Ricinus I, specific for terminal galactose, binds to preimplantation stages but does not bind to the trophoblast of the attached blastocyst. On the other hand, wheat germ agglutinin, specific for N-acetylglucosamine and sialic acid, binds to all preimplantation embryos and also to attached blastocysts (embryo proper and trophoblast). Neuraminidase treatment of blastocyst outgrowths enhances binding of both IIC3 and Ricinus I to the trophoblast; conversely, the binding of wheat germ agglutinin is decreased by this treatment. The results obtained in this study show changes of cell surface carbohydrates during early mouse development and suggest that sialic acid may be masking molecules on the surface of the trophoblast at the time of implantation.     

Participation of the mouse implanting trophoblast in nitric oxide production during pregnancy

While considerable progress has been made in elucidating nitric oxide (NO) regulatory mechanisms in the later stages of gestation, much less is known about its synthesis and role during embryo implantation. Thus, to evaluate the participation of the trophoblast in the production of NO during this phase, this study focused on NADPH-diaphorase activity and the distribution of NO synthase isoforms (NOS) using immunohistochemistry in pre- and postimplantation mouse embryos in situ and in vitro, as well as on NO production itself, measured as total nitrite, in trophoblast culture supernatants (Griess reaction). No NADPH-diaphorase activity was found in preimplanting embryos except after culturing for at least 48 h, when a few trophoblastic giant cells were positive. Conversely, postimplantation trophoblast cells either lodged into the implantation chamber (in situ) or after culturing (in vitro) showed intense NADPH-diaphorase activity. Also in the postimplantation trophoblast, the endothelial and inducible NOS (eNOS and iNOS) isoforms were immunodetected, under both in situ and in vitro conditions, although in different patterns. Extracts of ectoplacental cone also revealed bands of 135 and 130 kDa on SDS-PAGE that reacted with anti-eNOS and anti-iNOS, respectively, on Western blot. Analysis of the culture supernatant demonstrated that the nitrite concentration was 1) proportional to the number of cultured trophoblast cells, 2) almost completely abolished in the presence of N(omega)-nitro-L-arginine methyl ester, and 3) increased 2-fold in cultures stimulated with gamma-interferon. These results strongly suggest the production of NO from constitutive and inducible isoforms of NOS by the implanting mouse trophoblast. They also emphasize the possibility of the participation of these cells in vasodilatation and angiogenesis, and in cytotoxic mechanisms involved in the intense phagocytosis of injured maternal cells, which occur during the implantation process.     

Cell reproduction and genome multiplication in the proliferative and invasive trophoblast cell populations of mammalian placenta

Spatiotemporal "time-table" of ways of cell reproduction (mitosis, restitutional mitosis, endomitosis, endoreduplication) of trophoblast cell populations is described. The populations of mitotically active trophoblast cells (diploid and low-polyploid) are located mostly out of contact with maternal tissues. In rodent placenta they mainly switch from mitotic cycle to polyploidizing (restitutional) mitoses and reach 4c-8c. Thereafter they switch to endoreduplication and reach 16c-64c. Following a series of endoreduplication cycles a part of this cell population sets apart and penetrates deeply into the decidualized endometrium and myometrium, their capabilities for replication being lost progressively (in rodent--256c-1024c). The invasive trophoblast cells that reach 256c-1024c via endoreduplication simultaneously form a barrier between semiallogenic fetal and maternal tissues. Arrest of mitoses and complete repression of DNA replication after a series of endoreduplication cycles makes hardly probable the renewal of mitotic activity in the deeply invading tertiary giant trophoblast cells, thereby preventing the possibility of their ectopic expanding in the maternal tissues during the normal pregnancy.     

Heparin/heparan sulfate/CD44-v3 enhances cell migration in term placenta-derived immortalized human trophoblast cells

The function of CD44-v3 and heparin/heparan sulfate (HS) signaling was investigated during trophoblast cell migration to identify their role in the renewal of syncytial layer damage caused by increased hemodynamic turbulence in the intervillous space and maintenance of syncytial integrity in pre-eclampsia. We evaluated the effect of heparin/HS/CD44-v3-mediated processes during scratch wound closure in monolayer immortalized human trophoblast cells derived from term placenta (TCL-1 cells). Western blot analysis showed that these cultured human trophoblast cells express the epidermal growth factor receptor and CD44-v3 but do not express syndecan 4. An in vitro scratch wound healing assay showed enhanced migration of trophoblast cells in a dose-dependent manner in the presence of heparin compared with controls when cultured under serum-free conditions. Conversely, an anti-CD44 function-blocking antibody and CD44 siRNA suppressed the migration of trophoblast cells in the presence of heparin in a similar scratch assay. Furthermore, both heparin treatment and in vitro scratch wounding induced the phosphorylation of p21-activated kinase 1 (PAK1), whereas the anti-CD44-v3 antibody suppressed the heparin-induced phosphorylation of PAK1 in trophoblast cells. These results indicate that heparin/HS/CD44-v3-mediated signaling, in the absence of growth factor networks, enhances the direct repair of the damaged trophoblast layer through the migration of trophoblast cells. This renewed cell coverage may lead to the maintenance of syncytiotrophoblast cell function and an associated reduction in pathogenic soluble factors derived from the damaged trophoblast cells.     

Coordinated regulation of human trophoblast invasiveness by macrophages and interleukin 10

Trophoblast invasion and modification of the spiral arterioles are essential for the establishment of adequate uteroplacental blood flow during pregnancy. However, such vascular remodeling is deficient in preeclampsia. This disease is also associated with increased maternal levels of circulating proinflammatory cytokines such as tumor necrosis factor (TNF) and reduced levels of immunoregulatory cytokines such as interleukin 10 (IL10). We have previously shown that activated macrophages inhibit trophoblast invasiveness in vitro. The present study demonstrates that IL10 interferes with the invasion-inhibitory effect that activated macrophages exert on trophoblast cells. Co-culture experiments revealed that human lipopolysaccharide (LPS)-activated macrophages inhibited the ability of immortalized HTR-8/SVneo human trophoblast cells to invade through reconstituted extracellular matrix. This effect of activated macrophages on trophoblast invasiveness was paralleled by decreased expression of urokinase plasminogen activator receptor (PLAUR) on the surface of trophoblast cells, and by increased secretion of plasminogen activator inhibitor type 1 (SERPINE1). Exposure of LPS-treated macrophages to IL10 prior to co-culture prevented their ability to inhibit trophoblast invasion, PLAUR expression, and to stimulate SERPINE1 secretion. Interleukin 10 prevented macrophage activation by LPS as determined by the lack of secretion of TNF in the culture medium, and a neutralizing TNF antibody completely blocked the effect of macrophages on trophoblast invasion. These results indicate that decreased circulating levels of IL10 associated with preeclampsia may contribute to inadequate trophoblast invasion and remodeling of the uterine spiral arterioles.     

Trophoblast giant cell release of placental lactogens: temporal and regional characteristics

Placental lactogen (PL) production by rat trophoblast giant cells was studied using in vitro methods. The influence of trophoblast giant cell location within the conceptus and day of trophoblast giant cell isolation on the type of PL released in vitro were investigated. The effect of trophoblast giant cell location on the amount of PL, progesterone, and testosterone released in vitro was also evaluated. Trophoblast giant cells release two types of PLs in vitro; a high-molecular-weight lactogen, PL-1, and a low-molecular-weight lactogen, PL-2. The type of PL released by trophoblast giant cells was not influenced by their location within the conceptus at the time of dissection. Location did influence the amount of hormone produced by trophoblast giant cells. Mural trophoblast giant cells were more active in the production of PL, progesterone, and testosterone. The type of PL released by trophoblast giant cells is highly dependent upon the day of gestation the cells are removed for study. Trophoblast giant cells isolated on Day 10 of gestation release predominantly PL-1, while those cells isolated 24 hr later (Day 11 of gestation) release predominantly PL-2. The switch from PL-1 to PL-2 production that occurs in vivo does not occur under the in vitro conditions employed in this report.     

Developmental changes in the ploidy of mouse implanting trophoblast cells in vitro

Shortly after the onset of implantation, polar mouse trophoblast cells proliferate and give rise to the ectoplacental cone, constituted by two distinct cell populations: undifferentiated, diploid cells and giant cells. Giant cells characteristically exhibit exaggerated dimensions and polyploid nuclei. In this study, we employ ectoplacental cones as a dynamic source of trophoblast giant cells to analyze cell proliferation, cell death, and ploidy under in vitro conditions. Our results show that DNA synthesis and the increase in the cell number are relevant only during the first 24 h of culture. Subsequently, DNA synthesis still occurs, mainly in the giant cell compartment, while the number of cells gradually decreases. Cell death by injury and apoptosis was also observed in the non-giant cell compartment of the ectoplacental cone. These findings suggest that the first 24 h of culture are crucial to the mitotic activity of the ectoplacental cone cells that gradually ceases, favoring the endoreduplication process. The DNA synthesis index during the subsequent experimental intervals emphasizes accumulation of DNA for the polyploidization. There was clear correlation between DNA content and nuclear dimension. The ploidy values for the trophoblast giant cells varied from 2C up to 368C in the giant cells, but were not as expressive as those known from in vivo conditions, probably due to the absence of regulatory factors specific to the embryonic-maternal interface. In situ hybridization and histochemistry for the nucleolus-organizing region showed that trophoblast nuclei have only two marker signals, indicative of a typical polytenic process. This present study elucidates important aspects of trophoblast behavior and provides new information on trophoblast physiology in vivo and in vitro.