Evidence from reversal of handedness in C. elegans embryos for early cell interactions determining cell fates

Many animals with overall bilateral symmetry also exhibit some left-right asymmetries with generally invariant handedness. Therefore, the left-right embryonic axis must have a consistent polarity, whose origins and subsequent effects on development are not understood. Caenorhabditis elegans exhibits such left-right asymmetries at all developmental stages. The embryonic cell lineage is asymmetric as well: although the animal is generally bilaterally symmetric, many of its contralaterally analogous cells arise from different lineages on the two sides of the embryo. I accomplished reversal of embryonic handedness by micromanipulation at the 6-cell stage, which resulted in mirror-image but otherwise normal development into healthy, fertile animals with all the usual left-right asymmetries reversed. This result demonstrates that in the 6-cell embryo the pair of anterior (AB) blastomeres on the right is equivalent to the pair on the left, and that the extensive differences in fates between lineally homologous derivatives of these cells on the two sides of the animal must be dictated by cell interactions, most of which are likely to occur early in embryogenesis.     

The SIL gene is required for mouse embryonic axial development and left-right specification.

The establishment of the main body axis and the determination of left-right asymmetry are fundamental aspects of vertebrate embryonic development. A link between these processes has been revealed by the frequent finding of midline defects in humans with left-right anomalies. This association is also seen in a number of mutations in mouse and zebrafish, and in experimentally manipulated Xenopus embryos. However, the severity of laterality defects accompanying abnormal midline development varies, and the molecular basis for this variation is unknown. Here we show that mouse embryos lacking the early-response gene SIL have axial midline defects, a block in midline Sonic hedgehog (Shh) signalling and randomized cardiac looping. Comparison with Shh mutant embryos, which have axial defects but normal cardiac looping, indicates that the consequences of abnormal midline development for left-right patterning depend on the time of onset, duration and severity of disruption of the normal asymmetric patterns of expression of nodal, lefty-2 and Pitx2.     

Control of segment number in vertebrate embryos

The vertebrate body axis is subdivided into repeated segments, best exemplified by the vertebrae that derive from embryonic somites. The number of somites is precisely defined for any given species but varies widely from one species to another. To determine the mechanism controlling somite number, we have compared somitogenesis in zebrafish, chicken, mouse and corn snake embryos. Here we present evidence that in all of these species a similar 'clock-and-wavefront' mechanism operates to control somitogenesis; in all of them, somitogenesis is brought to an end through a process in which the presomitic mesoderm, having first increased in size, gradually shrinks until it is exhausted, terminating somite formation. In snake embryos, however, the segmentation clock rate is much faster relative to developmental rate than in other amniotes, leading to a greatly increased number of smaller-sized somites.     

Retinoic acid signalling links left-right asymmetric patterning and bilaterally symmetric somitogenesis in the zebrafish embryo

During embryogenesis, cells are spatially patterned as a result of highly coordinated and stereotyped morphogenetic events. In the vertebrate embryo, information on laterality is conveyed to the node, and subsequently to the lateral plate mesoderm, by a complex cascade of epigenetic and genetic events, eventually leading to a left-right asymmetric body plan. At the same time, the paraxial mesoderm is patterned along the anterior-posterior axis in metameric units, or somites, in a bilaterally symmetric fashion. Here we characterize a cascade of laterality information in the zebrafish embryo and show that blocking the early steps of this cascade (before it reaches the lateral plate mesoderm) results in random left-right asymmetric somitogenesis. We also uncover a mechanism mediated by retinoic acid signalling that is crucial in buffering the influence of the flow of laterality information on the left-right progression of somite formation, and thus in ensuring bilaterally symmetric somitogenesis.     

Function of Rieger syndrome gene in left-right asymmetry and craniofacial development.

Rieger syndrome, an autosomal dominant disorder, includes ocular, craniofacial and umbilical abnormalities. The pitx2 homeobox gene, which is mutated in Rieger syndrome, has been proposed to be the effector molecule interpreting left-right axial information from the early embryonic trunk to each organ. Here we have used gene targeting in mice to generate a loss-of-function allele that would be predicted to result in organ randomization or isomerization. Although pitx2-/- embryos had abnormal cardiac morphogenesis, mutant hearts looped in the normal direction. Pitx2-/- embryos had correctly oriented, but arrested, embryonic rotation and right pulmonary isomerism. They also had defective development of the mandibular and maxillary facial prominences, regression of the stomodeum and arrested tooth development. Fgf8 expression was absent, and Bmp4 expression was expanded in the branchial-arch ectoderm. These data reveal a critical role for pitx2 in left-right asymmetry but indicate that pitx2 may function at an intermediate step in cardiac morphogenesis and embryonic rotation.     

An unconventional myosin in Drosophila reverses the default handedness in visceral organs

The internal organs of animals often have left-right asymmetry. Although the formation of the anterior-posterior and dorsal-ventral axes in Drosophila is well understood, left-right asymmetry has not been extensively studied. Here we find that the handedness of the embryonic gut and the adult gut and testes is reversed (not randomized) in viable and fertile homozygous Myo31DF mutants. Myo31DF encodes an unconventional myosin, Drosophila MyoIA (also referred to as MyoID in mammals; refs 3, 4), and is the first actin-based motor protein to be implicated in left-right patterning. We find that Myo31DF is required in the hindgut epithelium for normal embryonic handedness. Disruption of actin filaments in the hindgut epithelium randomizes the handedness of the embryonic gut, suggesting that Myo31DF function requires the actin cytoskeleton. Consistent with this, we find that Myo31DF colocalizes with the cytoskeleton. Overexpression of Myo61F, another myosin I (ref. 4), reverses the handedness of the embryonic gut, and its knockdown also causes a left-right patterning defect. These two unconventional myosin I proteins may have antagonistic functions in left-right patterning. We suggest that the actin cytoskeleton and myosin I proteins may be crucial for generating left-right asymmetry in invertebrates.     

Noggin and retinoic acid transform the identity of avian facial prominences.

The signals that determine body part identity in vertebrate embryos are largely unknown, with some exceptions such as those for teeth and digits. The vertebrate face is derived from small buds of tissue, facial prominences, that surround the embryonic oral cavity. In chicken embryos, the skeleton of the upper beak is derived from the frontonasal mass and maxillary prominences. Here we show that bone morphogenetic proteins (Bmps) and the vitamin A derivative, retinoic acid (RA), are used to specify the identity of the frontonasal mass and maxillary prominences. Implanting two beads adjacent to the stage-15 presumptive maxillary field, one soaked in the Bmp antagonist Noggin and one soaked in RA, induces a duplicate set of frontonasal mass skeletal elements in place of maxillary bones. We also show that the duplicated beak is due to transformation of the maxillary prominence into a second frontonasal mass and not due to ectopic migration of cells or splitting of the normal frontonasal mass. Thus the levels of Bmp and RA determine whether specific regions of the face form maxillary or frontonasal mass derivatives.     

视黄酸对斑马鱼胚胎发育的影响

作者首次通过Ｂｒｄｕ－ａｎｔｉ－Ｂｒｄｕ法标记处于Ｓ期细胞核等系列实验表明，视黄酸对斑马鱼早期胚胎的中枢神经系统影响较大，主要表现为由前胸缺损而引起的小头畸形、无眼或无心脏，但后脑及脊髓部分的原始反射弧仍存在，而对尾芽只作用于顶端生长区．另外，利用低浓度的视黄酸还诱导出了过早出现的爪蟾前肢及火鲑鱼晚期尾芽胚的腹鳍，说明视黄酸对软骨的发生依浓度不同而有促进与抑制两方面的作用。     

Type ID unconventional myosin controls left-right asymmetry in Drosophila

Breaking left-right symmetry in Bilateria embryos is a major event in body plan organization that leads to polarized adult morphology, directional organ looping, and heart and brain function. However, the molecular nature of the determinant(s) responsible for the invariant orientation of the left-right axis (situs choice) remains largely unknown. Mutations producing a complete reversal of left-right asymmetry (situs inversus) are instrumental for identifying mechanisms controlling handedness, yet only one such mutation has been found in mice (inversin) and snails. Here we identify the conserved type ID unconventional myosin 31DF gene (Myo31DF) as a unique situs inversus locus in Drosophila. Myo31DF mutations reverse the dextral looping of genitalia, a prominent left-right marker in adult flies. Genetic mosaic analysis pinpoints the A8 segment of the genital disc as a left-right organizer and reveals an anterior-posterior compartmentalization of Myo31DF function that directs dextral development and represses a sinistral default state. As expected of a determinant, Myo31DF has a trigger-like function and is expressed symmetrically in the organizer, and its symmetrical overexpression does not impair left-right asymmetry. Thus Myo31DF is a dextral gene with actin-based motor activity controlling situs choice. Like mouse inversin, Myo31DF interacts and colocalizes with beta-catenin, suggesting that situs inversus genes can direct left-right development through the adherens junction.     

FGF-induced vesicular release of Sonic hedgehog and retinoic acid in leftward nodal flow is critical for left-right determination

The precise specification of left-right asymmetry is an essential process for patterning internal organs in vertebrates. In mouse embryonic development, the symmetry-breaking process in left-right determination is initiated by a leftward extraembryonic fluid flow on the surface of the ventral node. However, it is not known whether the signal transduction mechanism of this flow is chemical or mechanical. Here we show that fibroblast growth factor (FGF) signalling triggers secretion of membrane-sheathed objects 0.3-5 microm in diameter termed 'nodal vesicular parcels' (NVPs) that carry Sonic hedgehog and retinoic acid. These NVPs are transported leftward by the fluid flow and eventually fragment close to the left wall of the ventral node. The silencing effects of the FGF-receptor inhibitor SU5402 on NVP secretion and on a downstream rise in Ca2+ were sufficiently reversed by exogenous Sonic hedgehog peptide or retinoic acid, suggesting that FGF-triggered surface accumulation of cargo morphogens may be essential for launching NVPs. Thus, we propose that NVP flow is a new mode of extracellular transport that forms a left-right gradient of morphogens.     

微注射人TCP10L基因对斑马鱼形态的影响

利用模式生物斑马鱼,通过微注射的方法,研究了人肝脏特异性转录因子TCP10L对斑马鱼早期胚胎发育的影响.结果显示:人TCP10L基因对斑马鱼胚胎发育有一定的毒性.表现为早期的脊索发育异常,个体严重畸形;当血液循环出现后,表现为血流延缓和围心腔水肿等症状.该项研究表明:人肝脏特异性转录因子TCP10L在斑马鱼胚胎中的过表达会引起显著的形态异常和心血管障碍,并且这种影响与其是否在斑马鱼肝内的特异表达有关,为进一步探明该转录因子在人肝脏内发挥的功能奠定了基础.     

花背蟾蜍和中华大蟾蜍胚胎染色体组型分析

本研究用空气干燥制片法,以胚胎细胞为材料,对花背蟾蜍和中华大蟾蜍从囊胚期到尾芽期胚胎染色体的组型及其变化进行了比较分析。结果表明:两种蟾蜍发育各期的胚胎染色体均与其成体有基本相同的染色体组型。染色体绝对长度由囊胚列原肠晚期逐渐增长,神经胚期开始下降,尾芽期显著缩短。囊胚期染色体数目大多为四倍体。四倍体的出现率随发育时期的进展而逐渐减少,到神经胚期恢复正常。囊胚期分裂相中有额外随体,其出现率在不同染色体及其长、短臂上均无一定的规律。     

视黄酸对热带爪蟾胚胎致畸效应的表型特征

取典型的胚胎致畸剂——全反式视黄酸对热带爪蟾(Xenopus tropicalis)胚胎进行24,36和48 h暴露.结果表明,2,10和50 μg·L^-1视黄酸暴露对胚胎的存活率没有影响,对胚胎的生长和发育却有明显的抑制作用,并导致所有胚胎出现畸形现象,说明视黄酸对热带爪蟾胚胎具有极强的致畸性,同时显示了热带爪蟾胚胎对视黄酸的敏感性.视黄酸主要引起胚胎脑部缩小、眼睛色素减少、围心腔水肿和尾巴弯曲等多种畸形类型. 24,36和48 h后,暴露组脑部畸形指数和眼睛畸形指数较对照组均有明显增加,并表现出较好的浓度-效应关系,而时间-效应关系不明显,表明视黄酸对脑和眼部的影响主要发生在暴露的前24小时;48 h后尾部的畸形指数相对于24 h有明显的升高.将视黄酸导致的胚胎畸形类型与污染物三丁基锡对比,表明视黄酸致畸特征图谱在三丁基锡致畸机制的研究中有重要的参考价值.由此可见,有可能将FETAX实验发展为一种基于致畸机制的发育毒性检测方法.     

Regulation of vertebrate left-right asymmetries by extracellular matrix.

The vertebrate body is organized along three geometric axes: anterior-posterior, dorsal-ventral and left-right. Left-right axis formation, displayed in heart and gut development, is the least understood, even though it has been studied for many years. In Xenopus laevis gastrulae, a fibronectin-rich extracellular matrix is deposited on the basal surface of ectoderm cells over which cardiac and visceral primordia move during development. Here I report experiments in which localized perturbation of a small patch of extracellular matrix by microsurgery was correlated with localized randomization of left-right asymmetries. Global perturbation of the extracellular matrix by microinjection of Arg-Gly-Asp peptides or heparinase into the blastocoel resulted in global randomization of left-right asymmetries. From these observations, I suggest that left-right axial information is contained in the extracellular matrix early in development and is independently transmitted to cardiac and visceral primordia.     

cdx4 mutants fail to specify blood progenitors and can be rescued by multiple hox genes

Organogenesis is dependent on the formation of distinct cell types within the embryo. Important to this process are the hox genes, which are believed to confer positional identities to cells along the anteroposterior axis. Here, we have identified the caudal-related gene cdx4 as the locus mutated in kugelig (kgg), a zebrafish mutant with an early defect in haematopoiesis that is associated with abnormal anteroposterior patterning and aberrant hox gene expression. The blood deficiency in kgg embryos can be rescued by overexpressing hoxb7a or hoxa9a but not hoxb8a, indicating that the haematopoietic defect results from perturbations in specific hox genes. Furthermore, the haematopoietic defect in kgg mutants is not rescued by scl overexpression, suggesting that cdx4 and hox genes act to make the posterior mesoderm competent for blood development. Overexpression of cdx4 during zebrafish development or in mouse embryonic stem cells induces blood formation and alters hox gene expression. Taken together, these findings demonstrate that cdx4 regulates hox genes and is necessary for the specification of haematopoietic cell fate during vertebrate embryogenesis.     

Cloning and roles of goldfish maternal factor β-Catenin cDNA in embryonic development

Interaction between nucleus and cytoplasm has been focused in the field of animal embryonic development, in which study of maternal factors is required positively. β-Catenin, an important maternal factor in early embryogenesis, has been analyzed in its expression pattern and functions in this paper. We have cloned goldfish β-Catenin cDNA gene and compared it with zebrafish β-Catenin cDNA. High homology was found in cDNA and in amino acid sequences between them, 93% (2227/2384 bp) and 98.5% (768/780 aa) respectively. The expression pattern of β-Catenin by in situ hybridization and the roles of β-Catenin on embryonic development by co-injection of anti-sense RNA and reporter gene, EGFP have been investigated in the whole process of goldfish embryonic development. The results suggest that β-Catenin presents dynamic distribution, mainly locates at body axis, dorsal tissues, head and tail structures after being fertilized. The loss of β-Catenin activity would cause serious destruction of embryo in dorsal tissues and in anteroposterior axes, and leads embryos to die before larva get hatched.     

Determination of left-right patterning of the mouse embryo by artificial nodal flow

Substantial insight has recently been achieved into the mechanisms responsible for the generation of left-right (L-R) asymmetry in the vertebrate body plan. However, the mechanism that underlies the initial breaking of symmetry has remained unclear. In the mouse, a leftward fluid flow on the ventral side of the node caused by the vortical motion of cilia (referred to as nodal flow) is implicated in symmetry breaking, but direct evidence for the role of this flow has been lacking. Here we describe the development of a system in which mouse embryos are cultured under an artificial fluid flow and with which we have examined how flow affects L-R patterning. An artificial rightward flow that was sufficiently rapid to reverse the intrinsic leftward nodal flow resulted in reversal of situs in wild-type embryos. The artificial flow was also able to direct the situs of mutant mouse embryos with immotile cilia. These results provide the first direct evidence for the role of mechanical fluid flow in L-R patterning.     

哈密翼龙及其3D胚胎化石研究

 翼龙是地球上第一类飞向天空也是唯一绝灭的飞行脊椎动物，人类对其产卵繁殖、生长发育和生活习性等方面的了解还十分有限。本文介绍了一件超过200枚哈密翼龙蛋、胚胎和骨骼化石三位一体保存的重要标本，包括16枚翼龙蛋含有三维立体的胚胎化石。针对这件全世界首次发现的3D翼龙胚胎，研究提出哈密翼龙具有相对早熟的胚胎发育模式，其后肢发育速度较前肢快，孵化之后只能走不能飞；胚胎发育期间牙齿尚未萌出，出生后还需要父母照料；从胚胎到亚成年都具有快速生长的骨骼结构；显示哈密翼龙具有群居的生活习性，白垩纪的湖泊风暴导致其集群死亡并快速埋藏。     

斑马鱼胚胎发育的功能染色体组

随着人类和其他物种染色体组测序工作的完成 ,人类科学最大的任务就是阐明数以万计的基因的生物功能。人类和动物生命周期都从受精卵开始 ,然后一步一步地发育成具有多元组织和器官的生物体。在胚胎形成过程中 ,伴随着基因生成物的协同运作 ,基因按其固有的程序陆续显现出影响 ,从而决定并实现整个人体程序。如果使用适当的动物做模型的话 ,可以加速胚胎功能染色体组的研究。斑马鱼就是这项研究一个很好的模型。斑马鱼最大的优点就是产卵多、体外胚胎发育、体积小、容易养活 ,除此以外 ,很多的分子、细胞、胚胎和基因操作在斑马鱼身上都很容…