Localization of fibronectin, laminin-entactin, and entactin in Reichert's membrane by immunoelectron microscopy

Transcriptional network governing extraembryonic endoderm cell fate choice

The mechanism by which transcription factor (TF) network instructs cell-type-specific transcriptional programs to drive primitive endoderm (PrE) progenitors to commit to parietal endoderm (PE) versus visceral endoderm (VE) cell fates remains poorly understood. To address the question, we analyzed the single-cell transcriptional signatures defining PrE, PE, and VE cell states during the onset of the PE-VE lineage bifurcation. By coupling with the epigenomic comparison of active enhancers unique to PE and VE cells, we identified GATA6, SOX17, and FOXA2 as central regulators for the lineage divergence. Transcriptomic analysis of cXEN cells, an in vitro model for PE cells, after the acute depletion of GATA6 or SOX17 demonstrated that these factors induce Mycn, imparting the self-renewal properties of PE cells. Concurrently, they suppress the VE gene program, including key genes like Hnf4a and Ttr, among others. We proceeded with RNA-seq analysis on cXEN cells with FOXA2 knockout, in conjunction with GATA6 or SOX17 depletion. We found FOXA2 acts as a potent suppressor of Mycn while simultaneously activating the VE gene program. The antagonistic gene regulatory activities of GATA6/SOX17 and FOXA2 in promoting alternative cell fates, and their physical co-bindings at the enhancers provide molecular insights to the plasticity of the PrE lineage. Finally, we show that the external cue, BMP signaling, promotes the VE cell fate by activation of VE TFs and repression of PE TFs including GATA6 and SOX17. These data reveal a putative core gene regulatory module that underpins PE and VE cell fate choice.

Developmental and mechanical roles of Reichert's membrane in mouse embryos

Embryonic development and growth in placental mammals proceeds in utero with the support of exchanges of gases, nutrients and waste products between maternal tissues and offspring. Murine embryos are surrounded by several extraembryonic membranes, parietal and visceral yolk sacs, and amnion in the uterus. Notably, the parietal yolk sac is the most outer membrane, consists of three layers, trophoblasts and parietal endoderm (PaE) cells, and is separated by a thick basal lamina termed Reichert's membrane (RM). RM is composed of extracellular matrix (ECM) initially formed as the basement membrane of the trophectoderm of pre-implanted embryos and followed by the heavy deposition of ECM mainly produced in PaE cells of post-implanted embryos. In addition to the physiological roles of RM, such as gas and nutrient exchange, it also plays a crucial role in cushioning and dispersing intrauterine pressures exerted on embryos for normal egg-cylinder morphogenesis. Mechanistically, such intrauterine pressures generated by uterine smooth muscle contractions appear to be involved in the elongation of the egg-cylinder shape, along with primary axis formation, as an important biomechanical element in utero. This review focuses on our current views of the roles of RM in properly buffering intrauterine mechanical forces for mouse egg-cylinder morphogenesis. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Expression and function of the ID1 gene during transforming growth factor-β1-induced differentiation of human embryonic stem cells to endothelial cells

ID1 can mediate transforming growth factor-β (TGF-β)/activin receptor-like kinase-1 (ALK1)-induced (and Smad-dependent) migration in endothelial cells (ECs). However, the role that ID1 plays during differentiation of human embryonic stem cells (hESCs) into ECs induced by TGF-β1 remains unclear. In this study, a hESC differentiation model that recapitulates the developmental steps of vasculogenesis during the early stages of embryonic development was used to explore this question. We found that TGF-β1 increases endothelial cell differentiation and inhibits endothelial tube formation. Furthermore, at an early stage of differentiation, TGF-β1 may induce in vitro differentiation of hESCs into ECs by inhibiting expression of ID1, while at a later stage of differentiation, TGF-β1 may stimulate the proliferation and migration of ECs via the ALK1/Smad1/5/ID1 pathway. Downregulation of ID1 by gene silencing can lead to acceleration of TGF-β1-induced hESC differentiation into ECs and inhibition of proliferation and migration of ECs. This study may reveal some mechanisms of in vivo vasculogenesis in the early stages of embryonic development.

Establishment of endoderm progenitors by SOX transcription factor expression in human embryonic stem cells

In this study, we explore endoderm cell fate regulation through the expression of lineage-determining transcription factors. We demonstrate that stable endoderm progenitors can be established from human ES cells by constitutive expression of SOX7 or SOX17, producing extraembryonic endoderm and definitive endoderm progenitors, respectively. In teratoma assays and growth factor-mediated differentiation, SOX7 cells appear restricted to the extraembryonic endoderm, and SOX17 cells demonstrate a mesendodermal phenotype in teratomas and the ability to undergo endoderm maturation in vitro in the absence of cytokine-mediated endoderm induction. These endoderm progenitor cells maintain a stable phenotype through many passages in culture, thereby providing new tools to explore the pathways of endoderm differentiation.

A simple enzymatic method for parietal yolk sac removal in early postimplantation mouse embryos

Crucial aspects of axial development in mice occur at early postimplantation stages from the time of implantation to the appearance of the primitive streak. However, this period of development is notoriously refractory to experimental approaches due to the small size of the conceptus and to the presence of the parietal yolk sac, a protective tripartite membrane that surrounds the developing egg cylinder. Here, we describe a method that combines enzymatic digestion and mechanical manipulation to remove the parietal yolk sac of conceptuses at stages between 5.5 and 6.5 days post coitum. This method, which is compatible with whole-mount in situ hybridization and immunostaining techniques, offers a significant improvement over conventional dissection techniques, and it will greatly facilitate research in early mammalian development.

Imprinted X-inactivation in extra-embryonic endoderm cell lines from mouse blastocysts

The extra-embryonic endoderm lineage plays a major role in the nutritive support of the embryo and is required for several inductive events, such as anterior patterning and blood island formation. Blastocyst-derived embryonic stem (ES) and trophoblast stem (TS) cell lines provide good models with which to study the development of the epiblast and trophoblast lineages,respectively. We describe the derivation and characterization of cell lines that are representative of the third lineage of the blastocyst –extra-embryonic endoderm. Extra-embryonic endoderm (XEN) cell lines can be reproducibly derived from mouse blastocysts and passaged without any evidence of senescence. XEN cells express markers typical of extra-embryonic endoderm derivatives, but not those of the epiblast or trophoblast. Chimeras generated by injection of XEN cells into blastocysts showed exclusive contribution to extra-embryonic endoderm cell types. We used female XEN cells to investigate the mechanism of X chromosome inactivation in this lineage. We observed paternally imprinted X-inactivation, consistent with observations in vivo. Based on gene expression analysis, chimera studies and imprinted X-inactivation, XEN cell lines are representative of extra-embryonic endoderm and provide a new cell culture model of an early mammalian lineage.

Parietal endoderm cell line from a rat blastocyst

We have derived a cell line, RE1, from a pre-implantation rat blastocyst, resembling morphologically the L2 cell line from a parietal yolk sac carcinoma of the rat, as well as parietal endoderm cell lines of the mouse. The sub-cellular organization and epithelial characteristics of RE1 cells are described. The cells express cytokeratins of simple epithelia, and vimentin; and demonstrate synthesis of proteins of the extracellular matrix, such as laminin and collagen IV. Extensive Reichert's-like basement membrane is formed by RE1 cells when grown in suspension as aggregates. Cells have a microvillous surface morphology and abundant, rough endoplasmic reticulum which is swollen with apparent secretory material. These morphological and cytochemical features are characteristic of parietal endoderm cells in vivo, and the RE1 cell line is deduced to be rat parietal endoderm. In addition, RE1 cells were examined for expression of stage-specific embryonic antigens: cells reacted with antibody against SSEA-1/TEC-1 and EMA-1, constituting the first observation of parietal endoderm cells expressing the respective epitopes. RE1-cell monolayers did not generate transepithelial resistances or potential differences in vitro, consistent with their formation of leaky epithelia. Our observations on RE1-cell morphology and ultrastructure are consistent with the occurrence of epithelial-mesenchyme transitions in culture.

Identification of two distinct functions for TGF-beta in early mouse development

In this study the function of transforming growth factor-beta (TGF-beta) in preimplantation mouse embryos was examined. By RT-PCR, mRNA for the signalling type I (T beta R-I) and type II (T beta R-II) receptors for TGF-beta was shown to be present in two distinct time windows: in fertilized oocytes and at the blastocyst stage. The function of TGF-beta at these times was analysed in two ways. Firstly, the TGF-beta signalling pathway was blocked by injecting a DNA construct encoding a truncated T beta R-II, that acts as a dominant-negative receptor, in fertilized oocytes, and the effect on development was determined. Secondly, inner cell masses isolated at the blastocyst stage were cultured in vitro with and without TGF-beta under conditions that favour the outgrowth of parietal endoderm. The results show that TGF-beta signalling mediated by maternally expressed receptors is important for development of preimplantation embryos beyond the two-cell stage, and suggest a regulatory role for TGF-beta in the outgrowth of parietal endoderm.

Cooperative interactions between extracellular matrix, integrins and parathyroid hormone-related peptide regulate parietal endoderm differentiation in mouse embryos

The outgrowth of parietal endoderm (PE) cells from precursor endodermal cells is one of the first differentiation events that occur in mouse embryos. We have analyzed the molecular determinants of this process by placing isolated inner cell masses (ICMs) on defined extracellular matrix substrata in microdrop cultures. Differentiation and outgrowth of PE required a fibronectin substratum. Laminin supported the adhesion and outgrowth of visceral endoderm (VE) and actively suppressed the differentiation of PE in mixtures of fibronectin and laminin. Collagen type IV, gelatin, vitronectin or entactin supported little or no endodermal outgrowth. Trophectoderm (TE) cells have been implied to be important in PE induction in vivo. We found that recombination of ICMs in culture with TE cells, or with medium conditioned by TE cells, greatly increased the differentiation of PE. TE cells stimulated PE outgrowth on substrata other than fibronectin. One cytokine secreted by trophoblast and endodermal cells, parathyroid hormone-related peptide (PTHrP), was critical for outgrowth on any substratum. A function-perturbing antibody to PTHrP reduced the number of PE cells, whereas the addition of PTHrP increased that number. Furthermore, addition of PTHrP changed the substratum requirements for outgrowth, making laminin, vitronectin and low concentrations of fibronectin permissive for PE outgrowth. Immunostaining with anti-integrin antibodies showed that fully differentiated PE cells outgrowing on fibronectin expressed alpha 5, alpha 6 and alpha v beta 3 integrins. However, analysis of outgrowths in the presence of function-perturbing antibodies to alpha 5, alpha 6 and alpha v beta 3 integrins showed that these integrins directed PE outgrowth only on fibronectin, laminin and vitronectin substrata, respectively. We have shown that there is a cooperative interplay of extracellular matrix, integrins and PTHrP that modulates PE outgrowth.

Basement membrane components secreted by mouse yolk sac carcinoma cell lines

Three new cell lines (NE, ME, LRD) were cloned from mouse-embryo-derived teratocarcinomas and characterized on the basis of developmental, ultrastructural, and cytochemical criteria as nullipotent embryonal carcinoma (EC), pure parietal yolk sac (PYS) carcinoma and mixed parieto-visceral yolk sac carcinoma respectively. Cell lines NE and ME were composed of a monomorphous cell population; however, the morphology of ME was growth-medium-dependent. LRD was composed of a heterogeneous cell population and formed embryoid bodies. NE secreted soluble laminin, osteonectin, entactin and fibronectin but did not form visible pericellular matrix. ME formed pericellular matrix which was composed of laminin and entactin, but did not contain fibronectin. The LRD cells formed pericellular matrix which was composed of laminin, entactin and fibronectin. Whereas laminin from ME and LRD reacted with polyclonal antibodies and a monoclonal antibody to parietal yolk sac laminin, the laminin from NE cells was unreactive with the monoclonal antibody. Osteonectin was found in the supernatant of LRD and ME, but could not be demonstrated immunohistochemically in the extracellular matrix. We conclude that some extracellular matrix components, such as laminin and fibronectin, are produced not only by yolk sac carcinoma cells but by nullipotent EC as well, although the latter do not assemble them into extracellular matrix. Laminin produced by EC is immunochemically different from laminin secreted by yolk sac carcinoma. The extracellular matrix produced by mixed parieto-visceral yolk sac carcinoma is different from the matrix laid down by the pure PYS in that the latter does not contain fibronectin. The lack of osteonectin in the extracellular matrix of yolk sac carcinoma cells indicates that not all polypeptides secreted by these cell lines are incorporated into the extracellular matrix. The new cell lines described in this paper differ with regard to their capacity to form extracellular matrix and secrete its various components. Hence they could be used for further studies of basement membrane assembly in vitro.

Entactin: structure and function

Entactin is an integral and ubiquitous component of the basement membrane. The amino acid sequences of the mouse and human molecules have been determined and exhibit 85% sequence identity. The molecule is organized into three structural domains, an N-terminal globule (I) is linked to a smaller C-terminal globule (III) by a rigid stalk (II) largely consisting of cysteine-rich EGF-like homology repeats and a cysteine-rich thyroglobulin homology repeat. The molecule binds calcium ions and supports cell adhesion. However, its major function may be the assembly of the basement membrane. The carboxyl globule binds tightly to one of the short arms of laminin at the inner rodlike segment. This same region is also believed to be responsible for the attachment of entactin to type IV collagen at approximately 80 nm from its carboxyl noncollagenous end. Entactin therefore could serve as a bridge between the two most abundant molecules in the basement membrane. Supporting evidence for this role has been obtained from transfection of human choriocarcinoma, JAR, cells with the entactin gene. JAR cells synthesize laminin and type IV collagen but not entactin. Transfection of entactin into the cells stimulated incorporation of laminin and type IV collagen along with entactin into the extracellular matrix and into structures resembling focal contacts. The calcium-binding activity of entactin may play a role in the matrix assembly process. The protease sensitivity of entactin suggests that it may be a target for proteolytic activity during tissue remodeling, metastasis, and other events requiring the turnover of the basement membrane.

Distribution of laminin in the murine pituitary

The distribution and cellular localization of the glycoprotein laminin were investigated by light and electron microscopic immunocytochemistry in the adult murine pituitary gland. Immunoblots confirmed that laminin was the only protein in the pituitary gland of the adult male mouse to react with antilaminin serum. Laminin immunoreactivity was demonstrated at the light microscopic level simultaneously with that of beta-follicle stimulating hormone (beta-FSH) and beta-luteinizing hormone (beta-LH). In addition to its distribution is basal laminae, laminin immunoreactivity was coincidently expressed in gonadotrophs with the immunoreactivities of beta-FSH and beta-LH. Electron microscopic immunocytochemistry was employed on aldehyde-fixed sections embedded in L.R. White. Sites of binding of primary antisera to laminin were identified with affinity-purified secondary antisera directly coupled to 20 nm particles of colloidal gold. Three antisera recognizing laminin were compared and found to result in an identical pattern of immunoreactivity. Laminin was found extracellularly only in formed basal laminae in all three lobes of the pituitary and was not found in extracellular matrices of connective tissue. Laminin immunoreactivity was also found intracellularly in gonadotrophs but in none of the other endocrine or non-endocrine cells of the anterior lobe. Within gonadotrophs, only secretory granules were labeled. The majority, but not all, secretory granules were labeled in each of the gonadotrophs examined, and the proportion of granules labeled with laminin could not be increased by doubling the concentration of anti-laminin serum. Laminin immunoreactivity segregated with the subset of secretory granules containing beta-FSH. In contrast, laminin immunoreactivity was absent in the smaller subset of secretory granules that contain serotonin.(ABSTRACT TRUNCATED AT 250 WORDS)

Interferon-induced glomerular basement membrane and endothelial cell lesions in mice. An immunogold ultrastructural study of basement membrane components

Newborn Swiss mice were injected daily for the first week of life with mouse interferon alpha/beta. This treatment resulted in a delay in the maturation of the kidney and the development of glomerular abnormalities. The width of the glomerular basement membrane (GBM) was increased up to tenfold and was characterized by a marked thickening of the endothelial aspect of the GBM. The endothelial cells lining the capillary loops were also abnormal with many dilated regions of the rough endoplasmic reticulum that contained amorphous electron-opaque material. Immunogold studies showed that type IV collagen and laminin/entactin were distributed throughout the thickened GBM, and also within the dilated rough endoplasmic reticulum of the endothelial cells. These results show that the interferon-induced lesion within the glomerulus is associated with an accumulation of normal GBM components and that endothelial cells are involved in this pathologic process.

Hepatocytes may produce laminin in fibrotic liver and in primary culture

Previous studies have shown that laminin is present in basement membranes in normal liver but failed to identify cellular sources. We have investigated the extracellular and intracellular distribution of laminin in normal rat and human liver, in fibrotic human liver and in primary hepatocyte cultures from both species by light and electron microscopy using the indirect immunoperoxidase technique. In normal liver from both species, antibodies to laminin strongly stained basement membranes and formed discontinuous discrete deposits in the wall of the sinusoid. Vascular endothelial and bile duct cells as well as fat-storing cells and sinusoidal endothelial cells strongly stained for this glycoprotein while hepatocytes were negative. In fibrotic human liver, increased amounts of extracellular laminin were usually found. Continuous deposition in the space of Disse was observed in some cases. In addition to fat-storing cells and endothelial cells, hepatocytes were also sometimes positive. Normal rat and human hepatocytes synthesize and secrete laminin in conventional culture, but it remains soluble in the medium. By contrast, in coculture with another rat liver cell type, laminin accumulated around hepatocyte cords. These observations suggest that fat-storing cells and endothelial cells are the major sites of production of laminin in normal liver. However, when their environment is altered (e.g., liver injury, culture), adult hepatocytes are able to synthesize detectable amounts of laminin.

Distribution of laminin and fibronectin along peripheral trigeminal axon pathways in the developing chick

The trigeminal region of the chick was studied with indirect immunofluorescence in order to determine whether extracellular matrix components might be distributed in such a way as to guide trigeminal axons to their peripheral targets in the mandibular arch. Tissue sections from stages 13-15 and 21/22 were immunolabeled indirectly with affinity-purified antibodies raised against fibronectin and laminin, two extracellular matrix glycoproteins that support axon growth in vitro. Fibronectin was distributed ubiquitously throughout the head mesenchyme prior to and during initial axon growth from the brainstem (stages 13-15). Shortly after trigeminal axons reached their target tissues (stage 21/22), fibronectin immunolabeling was distributed throughout the head mesenchyme, but was present only at low levels in the trigeminal ganglion and motor nerve. Laminin immunolabeling was distributed in the lateral head mesenchyme at stage 13 as small specks and patches. At stage 14, when the motor axons first exit from the brainstem, short, linear arrays of laminin immunostaining were present from the basement membrane of the neural tube to the core of the mandibular arch, and many were parallel to the direction of axon growth. By stage 21/22 the trigeminal ganglion and motor root showed intense antilaminin immunofluorescence as did the central core of the mandibular arch. These studies suggest that the distribution of fibronectin within the head mesenchyme cannot give directional information to the growing trigeminal axons because of its homogeneous distribution. However, the initial distribution of laminin during the earliest stages of axon outgrowth may provide an extracellular matrix pathway that permits trigeminal axons to reach their targets.

Basement membrane matrix in vitro: focal binding of exogenous fibronectin to the matrix of teratocarcinoma-derived endodermal cells

Interaction of exogenous fibronectin with the basement membrane-like PYS-2 cell matrix, lacking fibronectin and hyaluronic acid but containing heparan sulfate proteoglycan, was studied in vitro. Both human plasma fibronectin and fibronectin in fetal calf serum bound to PYS-2 matrix; also, fragments of fibronectin containing heparin-binding domains but lacking the collagen-binding domain bound to the matrix. In immunoelectron microscopy the bound fibronectin was found as 20-40 nm globules or patches. Distribution of fibronectin differed from that of laminin and correlated best with that of heparan sulfate proteoglycan. The results suggest that the binding of fibronectin to basement membrane matrices is not due to random adherence but involves specific interactions with other components.

The extracellular matrix is an integrated unit: ultrastructural localization of collagen types I, III, IV, V, VI, fibronectin, and laminin in human term placenta

The human term placenta is used extensively as a source of extracellular matrix components. To elucidate the tissue distribution and interrelationships of seven of these components, monospecific antibodies directed against collagen types I, III, IV, V, VI, fibronectin, and laminin were reacted with human term placenta and studied by light and electron immunohistochemistry. Type I collagen was the basic structural unit of human term placenta, present as 30-35 nm, cross-banded fibers, often in the form of large fiber bundles. Type III collagen was present as thin 10-15 nm, beaded fibers often forming a meshwork which encased type I collagen fibers. Types V and VI collagen were present as 6-10 nm filaments, often closely associated with types I and III collagen. Type VI collagen also coated collagen fibers of all diameters, enhancing their periodicity, providing a staining pattern often similar to that observed with anti-fibronectin antibodies. Fibronectin was present in both maternal and fetal plasma and throughout the stroma of the chorionic villus, as both free filaments and coating collagen fibers. Basement membranes contained laminin and type IV collagen, but no fibronectin. In summary, the non-basement membrane proteins studied often codistributed with type I collagen, between and apparently attached to fibers, suggesting that they may act as binding proteins, linking type I fibers and bundles, to themselves and to other structures.

Basement membrane proteins, interstitial collagens, and fibronectin in neurofibroma

The distribution and nature of extracellular matrix proteins in neurofibroma tissue was studied by indirect immunofluorescence, immunoelectron microscopy, immunoblotting, and rotary shadowing. The most striking feature was an extensive network of basement membranes localized mainly around Schwann cells and small blood vessels. The major components, collagen IV, laminin, and nidogen, were mainly deposited in the lamina densa. Some laminin and nidogen could be extracted with 0.5 M NaCl and were shown by electrophoresis to have the characteristic chain and fragment patterns described previously for these proteins isolated from the mouse Engelbreth-Holm-Swarm (EHS) sarcoma. Fragments of collagen IV and collagen VI were solubilized by limited proteolytic digestion and identified after rotary shadowing. The more remote interstitial regions of the tumor contained cross-striated collagen fibrils which were composed of collagen III (diameter, 20-30 nm) or collagen I (diameter, 40-50 nm). Collagen fibrils thicker than 80 nm were not found. The interstitial regions also contained collagen VI as a fine filamentous network near cells and between collagen fibrils. Deposits of fibronectin were rather small and showed a scattered distribution. The data indicate that Schwann cells contribute considerably to matrix production in neurofibroma which may therefore be a suitable model for studying basement membranes of neuroectodermal origin.

Reichert's membrane as a model for studying the biosynthesis and assembly of basement membrane components

The major components of Reichert's membrane (laminin, type IV procollagen, entactin and heparan sulphate proteoglycan) are all synthesized by the parietal endoderm cells of the mouse embryo. Fibronectin is found mainly on the trophoblast side of Reichert's membrane and does not appear to be a major structural component. Parietal endoderm cells are thought to differentiate and migrate as individual cells from the margins of the epithelial visceral endoderm layer. They may interact with the type IV collagen in Reichert's membrane via a surface-associated protein of Mr = 47000 known as 'colligin'. Parietal endoderm cells are a rich source of mRNAs for basement membrane components and have been used to prepare a cDNA library in the expression vectors pUC8 and pUC9 from which cDNAs for type IV collagen and laminin B chains have been isolated.

Distribution of entactin in the basement membrane of the rat mammary gland. Evidence for a non-epithelial origin

Entactin, a sulfated glycoprotein with a molecular weight (MW) of about 150 kD, is present in vascular basement membranes and in the interstitial connective tissue of the mammary glands of virgin rats. It does not appear to be present in the basement membrane surrounding the mammary ductal system. However, in lactating mammary glands entactin is also present in the basement membrane region surrounding the secretory alveoli. Ultrastructural localisation of entactin reveals that it is present on the basal surface of epithelial cells, with patchy staining in the lamina lucida and lamina densa. Entactin also appears to be associated with interstitial collagen fibres. Mammary fibroblastic cells in culture are able to produce entactin, whereas mammary epithelial and myoepithelial cells, which synthesise the basement membrane proteins laminin and type IV collagen, fail to synthesise entactin.