Cell polarity and cell diversification during early mouse embryogenesis.

At the eight-cell stage of mouse development, the organization of blastomeres changes from radially symmetrical to polarized. This acquisition of cell polarity, followed by asymmetric divisions, leads to the formation of two phenotypically different cell types, which give rise to the first two cell lineages of the mouse blastocyst embryo, trophectoderm and the inner cell mass. Cell fate, controlled by positional information, is not irreversibly fixed during differentiation, providing the embryo with considerable developmental flexibility.     

Transparent things: Cell fates and cell movements during early embryogenesis of zebrafish

Development of an animal embryo involves the coordination of cell divisions, a variety of inductive interactions and extensive cellular rearrangements. One of the biggest challenges in developmental biology is to explain the relationships between these processes and the mechanisms that regulate them. Teleost embryos provide an ideal subject for the study of these issues. Their optical lucidity combined with modern techniques for the marking and observation of individual living cells allow high resolution investigations of specific morphogenetic movements and the construction of detailed fate maps. In this review we describe the patterns of cell divisions, cellular movements and other morphogenetic events during zebrafish early development and discuss how these events relate to the formation of restricted lineages.     

Spatial and temporal patterns of cell division during early Xenopus embryogenesis

We describe the spatial and temporal patterns of cell division in the early Xenopus embryo, concentrating on the period between the midblastula transition and the early tailbud stage. Mitotic cells were identified using an antibody recognising phosphorylated histone H3. At least four observations are of interest. First, axial mesodermal cells, including prospective notochord, stop dividing after involution and may not divide thereafter. Second, cell division is more pronounced in the neural plate than in nonneural ectoderm, and the pattern of cell division becomes further refined as neurogenesis proceeds. Third, cells in the cement gland cease proliferation completely as they begin to accumulate pigment. Finally, the precursors of peripheral sensory organs such as the ear and olfactory placode undergo active cell proliferation when they arise from the sensorial layer of the ectoderm. These observations and others should provide a platform to study the relationship between the regulation of developmental processes and the cell cycle during Xenopus embryogenesis.     

Analysis of cell movements and fate mapping during early embryogenesis in Drosophila melanogaster

Four spatially differentiated surface regions, called aeropyle crown, flat, stripe, and micropyle, are found on the mature eggshell (chorion). Specializations of the apical surfaces of the secretory follicular epithelial cells are implicated in the formation of regional patterns on the chorion. Some of these specializations are restricted to cells overlying certain regions; others are shared by more than one region. Differences between regions are more apparent on the surface than within the bulk of the chorion. Evidence is presented that distinct cell populations, corresponding to the regions, are present long before the start of choriogenesis. One hundred eighty-six chorion-specific polypeptides have been resolved by two-dimensional gel electrophoresis. Fifteen of these are found entirely or predominantly in the aeropyle crown and stripe regions, while eight others are restricted to the aeropyle crown region. Certain of the spatially restricted components are quite unusual in their amino acid compositions when compared with previously analyzed chorion components. Others are closely related, although clearly distinct.     

Organisation and assembly of the surface membrane during early cleavage of the mouse embryo

Summary An attempt was made to understand the ways in which ‘newly inserted’ membrane was organised in relation to existing membrane during early cleavage of the mouse embryo by (i) monitoring the redistribution of a variety of surface-binding ligands (applied to the embryo during the previous cell cycle) and (ii) analysing the localisation of newly synthesised lipid at defined stages during the second cell cycle. The membrane dynamics of the embryo appear similar to those of somatic cells during cytokinesis and/or motility, and are consistent with previous suggestions (Pratt 1985) that the main cytocortical domains of the polarised 8-cell blastomere may start to diverge during early cleavage as a result of localised assembly and reorganisation of the embryo cytocortex.     

Cell cycle control and early embryogenesis: Xenopus laevis maturation and early embryonic cell cycles

Our understanding of how meiotic maturation is regulated in Xenopus laevis continues to flourish. Premature initiation of maturation is prevented by the cAMP-dependent protein kinase, which inhibits the synthesis of Mos and potently blocks activation of cdc25. The autoamplification of maturation promoting factor (MPF) activity can be explained by the ability of MPF to directly activate cdc25. Later, in Meiosis II, the contribution of Mos to cytostatic factor (CSF) appears to be mediated through its activation of the mitogen-activated protein kinase, and cdk2 has been added to the active components of CSF. A model is presented illustrating the pathways of meiotic reinitiation, and indicating gaps in our knowledge.     

DNA-binding proteins of Xenopus laevis: synthesis during oogenesis-ovulation and early embryogenesis

DNA-binding proteins of Xenopus laevis synthesized during two periods of early development (oogenesis-ovulation and early embryogenesis) were co-chromatographed on DNA-cellulose. Proteins with an affinity for DNA were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Most of the proteins eluted from DNA-cellulose with 0.6 M NaCl had mol. wts less than 40 000; some of these proteins were synthesized to a greater extent by developing embryos than by oocytes. A DNA-binding protein or group of proteins with a mol. wt of approx. 70 000 was synthesized during oogenesis-ovulation but not during embryogenesis. Differential labeling of developing embryos with $\text{[}{}^3\text{H}\text{]}\text{tryptophan}$ and $\text{[}{}^{14}\text{C}\text{]}\text{lysine}$ indicated that some of the low mol. wt DNA-binding proteins are histones. Some of these proteins also incorporated monosodium $\text{[}{}^{32}\text{P}\text{]}\text{phosphate}$. A greater fraction of the proteins synthesized by oocytes and developing embryos were bound to DNA-histone-cellulose than to DNA-cellulose. A group of low mol. wt proteins made during oogenesis-ovulation were bound more to DNA-histone-cellulose than were proteins with similar mol. wts made by developing embryos.     

Changes in oligosaccharide expression on plasma membrane of the mouse oocyte during fertilisation and early cleavage

It has been reported that various structural and functional changes occur on the surface of the plasma membrane of the ovum and embryo during fertilisation and cleavage in preparation for implantation. Glycoproteins are thought to be one of the factors in cell attachment. Thus, we investigated the changes in glycoprotein expression on the cell surface membrane of the mouse embryo by using lectins. Among seven types of lectin (ConA, WGA, UEA-I, MPA, LCA, DBA and PNA), the fluorescent intensities of ConA and WGA markedly increased from unfertilised ova to blastocysts. By quantitative analysis using immuno-scanning electron microscopy, the numbers of ConA-gold particles were small until 4-cell cleavage, but increased significantly at the blastocyst stage. In contrast, an increased number of WGA-gold particles was detected even at the 4-cell stage, and this increase continued to the blastocyst stage. From the above observations, we conclude that the numbers of sugar chains bound to both ConA andWGA increases with blastocyst formation and earlier expression is observed with WGA. The present study dearly shows that glycoproteins on the cell membrane surface of the mouse embryo quantitatively increase at the time of implantation, and the possibility has been indicated that glycoproteins are involved in intercellular recognition and adhesion between the embryo and endometrial epithelium.     

斑马鱼Z-OTU蛋白在卵子发生和胚胎发育早期的表达(英文)

斑马鱼z-otu基因编码的蛋白可能具有DUBs活性,它包含OTU和TUDOR结构域,属于OTU相关蛋白酶家族的成员。该研究将原核表达的融合蛋白(OTU结构域和GST)纯化后免疫新西兰兔,获得多克隆抗体anti-Z-OTU,并利用该抗体对Z-OTU蛋白质在斑马鱼卵子发生和早期胚胎发育过程中的表达进行了分析。根据原位和整体免疫组织化学检测结果并结合以前的研究结论,分析并比较了z-otu基因的mRNA和蛋白质的分布,发现在卵子发生和早期胚胎发育过程中,z-otu基因的mRNA和蛋白质表达模式存在明显差异:mRNA仅在卵子发生早期表达,卵母细胞受精后才重新开始表达,而其蛋白在卵子发生过程中均表达;在卵子发生过程中,mRNA分布于细胞质中,而蛋白质先分布于细胞核中,然后向细胞质迁移,接着又向卵母细胞生发泡(germinal vesicle,GV)集中。推测Z-OTU蛋白类似于其他具有去泛素化酶活性的OTU相关蛋白酶,对于卵母细胞减数分裂过程中生发泡破裂、生发泡迁移及维持胚胎的分裂是必需的。     

The action of cytochalasin B during egg cleavage in Xenopus laevis: dependence on cell membrane permeability

By exposing Xenopus eggs during the first cleavage to cytochalasin B (CCB) for successive periods of 4 min, it has been shown that CCB sensitivity becomes manifest approximately 7 min after the onset of furrow formation. However, even before this time furrow regression can be induced by the injection of CCB under the membrane in the furrow. This shows that during the first 7 min of cleavage the operative contractile system is CCB sensitive. Using microelectrode techniques, electrical membrane characteristics (membrane potential and resistance) were measured continuously in normally cleaving eggs and in cleaving eggs injected with CCB. It was found that the onset of sensitivity to externally applied CCB coincides with a rapid alteration of the membrane potential and resistance. We have concluded that externally applied CCB can only enter the egg when the membrane permeability increases. No evidence has been found that CCB alters the ionic permeability of preexisting cell membrane.     

Nuclear localization of Prickle2 is required to establish cell polarity during early mouse embryogenesis

The establishment of trophectoderm (TE) manifests as the formation of epithelium, and is dependent on many structural and regulatory components that are commonly found and function in many epithelial tissues. However, the mechanism of TE formation is currently not well understood. Prickle1 (Pk1), a core component of the planar cell polarity (PCP) pathway, is essential for epiblast polarization before gastrulation, yet the roles of Pk family members in early mouse embryogenesis are obscure. Here we found that Pk2(-/-) embryos died at E3.0-3.5 without forming the blastocyst cavity and not maintained epithelial integrity of TE. These phenotypes were due to loss of the apical-basal (AB) polarity that underlies the asymmetric redistribution of microtubule networks and proper accumulation of AB polarity components on each membrane during compaction. In addition, we found GTP-bound active form of nuclear RhoA was decreased in Pk2(-/-) embryos during compaction. We further show that the first cell fate decision was disrupted in Pk2(-/-) embryos. Interestingly, Pk2 localized to the nucleus from the 2-cell to around the 16-cell stage despite its cytoplasmic function previously reported. Inhibiting farnesylation blocked Pk2's nuclear localization and disrupted AB cell polarity, suggesting that Pk2 farnesylation is essential for its nuclear localization and function. The cell polarity phenotype was efficiently rescued by nuclear but not cytoplasmic Pk2, demonstrating the nuclear localization of Pk2 is critical for its function.     

Changes in cell surface and cortical cytoplasmic organization during early embryogenesis in the preimplantation mouse embryo

Membrane topography and organization of cortical cytoskeletal elements and organelles during early embryogenesis of the mouse have been studied by transmission and scanning electron microscopy with improved cellular preservation. At the four- and early eight-cell stages, blastomeres are round, and scanning electron microscopy shows a uniform distribution of microvilli over the cell surface. At the onset of morphogenesis, a reorganization of the blastomere surface is observed in which microvilli becomes restricted to an apical region and the basal zone of intercellular contact. As the blastomeres spread on each other during compaction, many microvilli remain in the basal region of imminent cell-cell contacts, but few are present where the cells have completed spreading on each other. Microvilli on the surface of these embryos contain linear arrays of microfilaments with lateral cross bridges. Microtubules and mitochondria become localized beneath the apposed cell membranes during compaction. Arrays of cortical microtubules are aligned parallel to regions of apposed membranes. During cytokinesis, microtubules become redistributed in the region of the mitotic spindle, and fewer microvilli are present on most of the cell surface. The cell surface and cortical changes initiated during compaction are the first manifestations of cell polarity in embryogenesis. These and previous findings are interpreted as evidence that cell surface changes associated with trophoblast development appear as early as the eight-cell stage. Our observations suggest that morphogenesis involves the activation of a developmental program which coordinately controls cortical cytoplasmic and cell surface organization.     

Utilization of exogenous linolenic and oleic acids for plasma membrane phosphoglyceride systhesis in L-Fibroblasts

The inability of the strain L-fibroblast to synthesize quatitatively significant amounts of polyenoic fatty acid and the apparent lack of turnover of their phosphoglyceride acyl groups under the usual conditions of cell culture makes them especially well suited for studies concerning the effect of fatty acid unsaturation on biological membranes. Such cells grown in the absence of exogenous lipid sources have in their phosphoglycerides only traces of polyenoic fatty acid.By infusing fatty acid supplements into suspension cultures of logarithmically growing cultures of L-fibroblasts it is possible to increase singnificantly their phosphoglyceride polyenoic fatty content to as much as 50% of the total lipid phosphoglyceride fatty acids. The infusion of fatty acid supplements at a constant rate over a 48 h time period diminishes the toxic effects which may accompany single doses of unesterified fatty acid and reduces considerably the accumulation of cytoplasmic lipid droplets. Cultures supplemented in this way have virtually the same generation times as non-supplemented control cultures. The data show that alterations in surface membrane and homogenate polyenoic fatty acid composition are minimal when oleic acid is supplied to the culture. During exposure to large amounts of polyenoic fatty acid, however, the unsaturation of plasma membrane total phosphoglyceride fraction is less than that of the cell homogenate. This effect is more pronounced in the phosphatidylethanolamine than in the phosphatidylcholine fraction.