Communication compartments in the gastrulating mouse embryo

We characterized the pattern of gap junctional communication in the 7.5-d mouse embryo (at the primitive streak or gastrulation stage). First we examined the pattern of dye coupling by injecting the fluorescent tracers, Lucifer Yellow or carboxyfluorescein, and monitoring the extent of dye spread. These studies revealed that cells within all three germ layers are well coupled, as the injected dye usually spread rapidly from the site of impalement into the neighboring cells. The dye spread, however, appeared to be restricted at specific regions of the embryo. Further thick section histological analysis revealed little or no dye transfer between germ layers, indicating that each is a separate communication compartment. The pattern of dye movement within the embryonic ectoderm and mesoderm further suggested that cells in each of these germ layers may be subdivided into smaller communication compartments, the most striking of which are a number of "box-like" domains. Such compartments, unlike the restrictions observed between germ layers, are consistently only partially restrictive. In light of these results, we further monitored ionic coupling to determine if some coupling might nevertheless persist between germ layers. For these studies, Lucifer Yellow was coinjected while ionic coupling was monitored. The injected Lucifer Yellow facilitated the identification of the impalement sites, both in the live specimen and in thick sections in the subsequent histological analysis. By using this approach, all three germ layers were shown to be ionically coupled, indicating that gap junctional communication is maintained across the otherwise dye-uncoupled "germ layer compartments." Thus our results demonstrate that partially restrictive communication compartments are associated with the delamination of germ layers in the gastrulating mouse embryo. The spatial distribution of these compartments are consistent with a possible role in the underlying development.     

Gap junctional communication in the post-implantation mouse embryo.

We studied the extent of cell-to-cell communication via junctional channels in in vitro-implanted mouse blastocysts by monitoring ionic coupling and the spread of two injected low molecular weight dyes, fluorescein and Lucifer yellow. In the early attached embryos, both trophoblasts and cells of the inner cell mass (ICM) were ionically coupled to one another. Dye injections in either trophoblasts or ICM cells resulted in spread to the entire embryo. As older and more developed embryos were examined, the spread of injected dye was progressively more limited. In the most developed embryos examined, dye injected into a cell in the ICM region resulted in spread throughout the ICM but not into the surrounding trophoblast cells, while dye injected into a trophoblast cell did not spread to any other cell in the embryo. Simultaneous monitoring of ionic coupling and dye injections in embryos of intermediate stages in this transition revealed that the trophoblast and ICM cells were ionically coupled, even across the apparent boundary where no dye was observed to pass. In the latest stage embryos examined in which no injected dye was observed to move out of the ICM, ionic coupling was still observed between the cells of the ICM and the trophoblasts. Furthermore, in the more developed embryos, dye injected into the ICM region frequently was not transferred to all the cells of the ICM, thus suggesting a further compartmentalization of due spread within the ICM. Our observations that ionic coupling is more extensive than the detectable spread of injected dyes may perhaps reflect a reduced number of junctional channels. With fewer channels less dye would pass between cells, so that, together with continuous quenching, the transfer of injected dye would not be detectable. This partial segregation of cell-to-cell communication as indicated by the limited dye spread may parallel specific differentiation processes, in particular that of giant trophoblast, embryonic ectoderm and extraembryonic endoderm differentiation.     

Communication compartments in the ectoderm of embryos of Patella vulgata

Summary Patterns of gap junctional communication in the ectoderm of embryos of Patella vulgata have been studied by intracellular injection of the fluorescent dye Lucifer Yellow, and by analysis of its subsequent spread to adjacent cells (dye-coupling). We found that dye-coupling became progressively restricted to different domains of the ectoderm, forming communication compartments. These communication compartments are characterized by their high coupling abilities within the compartment, and reduction of coupling across their boundaries. During development, the pretrochal (anterior) ectoderm becomes subdivided into two communication compartments, the apical organ and the anlage of the head ectoderm. The posttrochal (posterior) ectoderm becomes subdivided into different communication compartments in two successive phases. Firstly, in the 15-h embryo the dorsal and ventral domains of the ectoderm form separate communication compartments. A dorso-ventral communication boundary restricts the passage of dye between the two domains. Secondly, in the 24-h embryo dye-coupling becomes further compartmentalized in both the dorsal and ventral domains. These compartments correspond to the anlagen of different ectodermal structures. In order to study whether any level of coupling persists between the ectodermal compartments we injected currents through a microelectrode inserted into one cell of one compartment and monitored its spread by means of a second microelectrode inserted into one cell of another compartment (electrical coupling). Despite the absence of dye-coupling, electrical coupling between the ectodermal dye-coupling compartments was detected, which suggests that some level of communication is maintained between compartments. Our results demonstrate that within the ectoderm layer of Patella vulgata the transfer of dyes becomes progressively restricted to communication compartments and, concomitantly with the specification of the different ectodermal anlagen, these compartments become subdivided into smaller communication compartments.     

Expression of the EMILIN-1 gene during mouse development.

Expression of EMILIN-1, the first member of a newly discovered family of extracellular matrix genes, has been investigated during mouse development. EMILIN-1 mRNA is detectable in morula and blastocyst by RT-PCR. First expression of the gene is found by in situ hybridization in ectoplacental cone in embryos of 6.5 days and in extraembryonic visceral endoderm at 7.5 days. The allantois is also labeled. Staining of ectoplacental cone-derived secondary trophoblast giant cells and spongiotrophoblast is strong up to 11.5 days and then declines. In the embryo, high levels of mRNA are initially expressed in blood vessels, perineural mesenchyme and somites at 8.5 days. Later on, intense labeling is identified in the mesenchymal component of organs anlage (i.e. lung and liver) and different mesenchymal condensations (i.e. limb bud and branchial arches). At late gestation staining is widely distributed in interstitial connective tissue and smooth muscle cell-rich tissues. The data suggest that EMILIN-1 may have a function in placenta formation and initial organogenesis and a later role in interstitial connective tissue.     

Teratoid Tumors Derived from Mouse Embryos Grown in an Immunologically Privileged Site

Mouse early embryos and embryo fragments were transplanted into an immunologically privileged site, consisting of a glass cylinder previously implanted under the skin of adult mice in order to test their tumor producing potential, in allogeneic adult recipients. The highest yield of tumors was obtained upon transplantation of 6 1/2 day old embryos in toto. i.e., including the embryonic and extraembryonic areas. Histological examination showed teratomas composed of differentiated tissues derived from the three germ layers containing isolated foci of undifferentiated cells and nodules of trophoblast giant cells. Areas exhibiting the histological appearance of yolk sac carcinoma were also observed. Transplantation of the whole 6 1/2 day old egg cylinder, including the ectoplacental cone, and the isolated embryonic area produced a lower incidence of teratomas with a reduced variety of differentiated tissues. No yolk sac carcinoma was found in these grafts. The ectoplacental cone of 6 1/2 day embryos produced no tumors. Grafts of genital ridges from 12 1/2 day embryos gave rise to teratomas with well differentiated tissues of embryonic and extraembryonic origin. Areas ressembling yolk sac carcinoma were also observed. The life span of trophoblastic giant cells within the glass cylinder was significantly longer than in other experimental systems.     

纤粘连蛋白对小鼠胚胎体外发育和体外着床的作用

应用小鼠胚泡和外胎盘锥体外培养的方法,研究了纤粘连蛋白对小鼠胚泡发育及胚泡或外胎盘锥粘附和扩展的影响。结果显示,纤粘连蛋白对小鼠胚泡发育有一定的促进作用;对胚泡及外胎盘锥的粘附和胚泡初生滋养层细胞及外胎盘锥次生滋养层细胞扩展均有显著促进作用。纤粘连蛋白分子活性位点的合成肽段精－苷－天冬－丝氨酸可有效抑制纤粘连蛋白对胚泡或外胎盘锥发育、粘附和扩展的促进作用。结果表明,纤粘连蛋白在小鼠胚胎发育和着床过     

Expression of syndecan, a putative low affinity fibroblast growth factor receptor, in the early mouse embryo.

Syndecan is an integral membrane proteoglycan that binds cells to several interstitial extracellular matrix components and binds to basic fibroblast-growth factor (bFGF) thus promoting bFGF association with its high-affinity receptor. We find that syndecan expression undergoes striking spatial and temporal changes during the period from the early cleavage through the late gastrula stages in the mouse embryo. Syndecan is detected initially at the 4-cell stage. Between the 4-cell and late morula stages, syndecan is present intracellularly and on the external surfaces of the blastomeres but is absent from regions of cell-cell contact. At the blastocyst stage, syndecan is first detected at cell-cell boundaries throughout the embryo and then, at the time of endoderm segregation, becomes restricted to the first site of matrix accumulation within the embryo, the interface between the primitive ectoderm and primitive endoderm. During gastrulation, syndecan is distributed uniformly on the basolateral cell surfaces of the embryonic ectoderm and definitive embryonic endoderm, but is expressed with an anteroposterior asymmetry on the surface of embryonic mesoderm cells, suggesting that it contributes to the process of mesoderm specification. In the extraembryonic region, syndecan is not detectable on most cells of the central core of the ectoplacental cone, but is strongly expressed by cells undergoing trophoblast giant cell differentiation and remains prominent on differentiated giant cells, suggesting a role in placental development. Immunoprecipitation studies indicate that the size of the syndecan core protein, although larger than that found in adult tissues (75 versus 69 x 10(3) Mr), does not change during peri-implantation development. The size distribution of the intact proteoglycan does change, however, indicating developmental alterations in its glycosaminoglycan composition. These results indicate potential roles for syndecan in epithelial organization of the embryonic ectoderm, in differential axial patterning of the embryonic mesoderm and in trophoblast giant cell function.     

Dynamic expression of manganese superoxide dismutase during mouse embryonic organogenesis

The balance between reactive oxygen species production and antioxidant defense enzymes in embryos is necessary for normal embryogenesis. To determine the dynamic expression profile of manganese superoxide dismutase (MnSOD) in embryos, which is an essential antioxidant enzyme in embryonic organogenesis, the expression level and distribution of MnSOD mRNA and protein were investigated in mouse embryos, as well as extraembryonic tissues on embryonic days (EDs) 7.5-18.5. MnSOD mRNA levels were remarkably high in extraembryonic tissues rather than in embryos during these periods. MnSOD protein levels were also higher in extraembryonic tissues than in embryos until ED 16.5, but the opposite trend was found after ED 17.5. MnSOD mRNA was observed in the chorion, allantois, amnion, ectoderm, ectoplacental cone and neural fold at ED 7.5 and in the neural fold, gut, ectoplacental cone, outer extraembryonic membranes and primitive heart at ED 8.5. After removing the extraembryonic tissues, the prominent expression of MnSOD mRNA in embryos was seen in the sensory organs, central nervous system and limbs on EDs 9.5-12.5 and in the ganglia, spinal cord, sensory organ epithelia, lung, blood cells and vessels, intestinal and skin epithelia, hepatocytes and thymus on EDs 13.5-18.5. Strong MnSOD immunoreactivity was observed in the choroid plexus, ganglia, myocardium, blood vessels, heapatocytes, pancreatic acinus, osteogenic tissues, brown adipose tissue, thymus and skin. These findings suggest that MnSOD is mainly produced from extraembryonic tissues and then may be utilized to protect the embryos against endogenous or exogenous oxidative stress during embryogenesis.     

Localization of Brachyury (T) in embryonic and extraembryonic tissues during mouse gastrulation

T-box gene family members have important roles during murine embryogenesis, gastrulation, and organogenesis. Although relatively little is known about how T-box genes are regulated, published gene expression studies have revealed dynamic and specific patterns in both embryonic and extraembryonic tissues of the mouse conceptus. Mutant alleles of the T-box gene Brachyury (T) have identified roles in formation of mesoderm and its derivatives, such as somites and the allantois. However, given the cell autonomous nature of T gene activity and conflicting results of gene expression studies, it has been difficult to attribute a primary function to T in normal allantoic development. We report localization of T protein by sectional immunohistochemistry in both embryonic and extraembryonic tissues during mouse gastrulation, emphasizing T localization within the allantois. T was detected in all previously reported sites within the conceptus, including the primitive streak and its derivatives, nascent embryonic mesoderm, the node and notochord, as well as notochord-associated endoderm and posterior neurectoderm. In addition, we have clarified T within the allantois, where it was first detected in the proximal midline of the late allantoic bud (approximately 7.5 days postcoitum, dpc) and persisted within an expanded midline domain until 6-somite pairs (s; approximately 8.5 dpc). Lastly, we have discovered several novel T sites, including the developing heart, visceral endoderm, extraembryonic ectoderm, and its derivative, chorionic ectoderm. Together, these data provide a unified picture of T in the mammalian conceptus, and demonstrate T's presence in unrelated cell types and tissues in highly dynamic spatiotemporal patterns in both embryonic and extraembryonic tissues.     

The endoderm of the mouse embryo arises by dynamic widespread intercalation of embryonic and extraembryonic lineages

The cell movements underlying the morphogenesis of the embryonic endoderm, the tissue that will give rise to the respiratory and digestive tracts, are complex and not well understood. Using live imaging combined with genetic labeling, we investigated the cell behaviors and fate of the visceral endoderm during gut endoderm formation in the mouse gastrula. Contrary to the prevailing view, our data reveal no mass displacement of visceral endoderm to extraembryonic regions concomitant with the emergence of epiblast-derived definitive endoderm. Instead, we observed dispersal of the visceral endoderm epithelium and extensive mixing between cells of visceral endoderm and epiblast origin. Visceral endoderm cells remained associated with the epiblast and were incorporated into the early gut tube. Our findings suggest that the segregation of extraembryonic and embryonic tissues within the mammalian embryo is not as strict as believed and that a lineage previously defined as exclusively extraembryonic contributes cells to the embryo.     

Yolk sac carcinoma derived from the rat epiblast as a renal isograft

We report the novel observation that a biphasic, parieto-visceral (PYS/VYS) yolk sac carcinoma can develop from the isolated epiblast of the pre-primitive streak rat embryo in a prolonged cultivation in vivo as a renal isograft. Late 7-day rat egg cylinders were dissected free of the ectoplacental cone and the Reichert's membrane. The middle segment of the cylinder, in which the embryonic and the extraembryonic cell layers partly overlap, were also removed. From the rest of the cylinder the 4 cell layers were isolated and transplanted separately under the kidney capsule of isogenic adult males. After 4 weeks the hypoblast was resorbed, the extraembryonic ectoderm gave rise to hemorrhagic cysts and trophoblastic giant cells, the extraembryonic (visceral yolk sac) endoderm formed benign cystic PYS/VYS tumors, and the epiblast developed into a benign teratoma. After prolonged (7-30 weeks) development of these teratomas as isografts, a malignant yolk sac carcinoma (YSC) developed in 45% of them. It destroyed the teratoma and the recipient's kidney, metastasized to peritoneum and other sites, and caused abundant ascites containing clustered tumor cells. The primary tumor was retransplantable subcutaneously as well as intraperitoneally, and displayed the characteristics of the mixed or biphasic PVYS carcinoma, with a progressive loss of the VYS component with time. Several data are apparently in favor of its origin by transdifferentiation rather than from undifferentiated cells.     

Glucose transporter expression in rat embryo and uterus during decidualization, implantation, and early postimplantation.

Efficient transfer of glucose from the mother to the embryonic compartment is crucial to sustain the survival and normal development of the embryo in utero, because the embryo's production of this primary substrate for oxidative metabolism is minimal. In the present study, the temporal sequence of expression of the sodium-independent facilitative glucose transporter isoforms GLUTs 1, 3, 4, and 5 was investigated in the developing rat uteroembryonic unit between conception and Gestational Day 8 using immunohistochemistry. The GLUTs 1, 3, and 4 were expressed in the embryonic tissues after the start of implantation, being colocalized in the parietal endoderm, visceral endoderm, primary ectoderm, extraembryonic ectoderm, and the ectoplacental cone. In the uterus, a faint GLUT1 labeling emerged, but not until Gestational Day 3, in the luminal epithelium, endometrial stroma, and decidual cells. The intensity of GLUT1 staining increased in the latter population with progressing decidualization. Endometrial glands and myometrial smooth muscle cells stained neither for GLUT1 nor for GLUT3 until postimplantation. During all developmental stages examined, GLUT4 was visualized throughout the pregnant rat uterus, as was GLUT3 (with the above-mentioned exceptions). The density of GLUT5 was generally less than the sensitivity of the immunohistochemical detection method in all tissues investigated. In conclusion, the data point to a significant expression of the high-affinity glucose transporters GLUTs 1, 3, and 4 in the rat uteroembryonic unit, providing supportive evidence for an important role of facilitative glucose diffusion during peri-implantation development.     

Polarity of the mouse embryo is anticipated before implantation

In most species, the polarity of an embryo underlies the future body plan and is determined from that of the zygote. However, mammals are thought to be an exception to this; in the mouse, polarity is generally thought to develop significantly later, only after implantation. It has not been possible, however, to relate the polarity of the preimplantation mouse embryo to that of the later conceptus due to the lack of markers that endure long enough to follow lineages through implantation. To test whether early developmental events could provide cues that predict the axes of the postimplantation embryo, we have used the strategy of injecting mRNA encoding an enduring marker to trace the progeny of inner cell mass cells into the postimplantation visceral endoderm. This tissue, although it has an extraembryonic fate, plays a role in axis determination in adjacent embryonic tissue. We found that visceral endoderm cells that originated near the polar body (a marker of the blastocyst axis of symmetry) generally became distal as the egg cylinder formed, while those that originated opposite the polar body tended to become proximal. It follows that, in normal development, bilateral symmetry of the mouse blastocyst anticipates the polarity of the later conceptus. Moreover, our results show that transformation of the blastocyst axis of symmetry into the axes of the postimplantation conceptus involves asymmetric visceral endoderm cell movement. Therefore, even if the definitive axes of the mouse embryo become irreversibly established only after implantation, this polarity can be traced back to events before implantation.