Patterning the early Xenopus embryo

Developmental biology teachers use the example of the frog embryo to introduce young scientists to the wonders of vertebrate development, and to pose the crucial question, 'How does a ball of cells become an exquisitely patterned embryo?'. Classical embryologists also recognized the power of the amphibian model and used extirpation and explant studies to explore early embryo polarity and to define signaling centers in blastula and gastrula stage embryos. This review revisits these early stages of Xenopus development and summarizes the recent explosion of information on the intrinsic and extrinsic factors that are responsible for the first phases of embryonic patterning.     

Vascular morphogenesis in the zebrafish embryo

During embryonic development, the vertebrate vasculature is undergoing vast growth and remodeling. Blood vessels can be formed by a wide spectrum of different morphogenetic mechanisms, such as budding, cord hollowing, cell hollowing, cell wrapping and intussusception. Here, we describe the vascular morphogenesis that occurs in the early zebrafish embryo. We discuss the diversity of morphogenetic mechanisms that contribute to vessel assembly, angiogenic sprouting and tube formation in different blood vessels and how some of these complex cell behaviors are regulated by molecular pathways.     

Patterning lessons from a dorsalized embryo

A paper by Nunes da Fonseca and colleagues in this issue of Developmental Cell shows that, to pattern its dorsoventral axis, the beetle Tribolium utilizes many of the same genes used in flies, but in very different ways: rather than relying on maternal information, it uses Dorsal and Dpp as part of two coordinated ancestral self-organized systems.     

Patterning the zebrafish diencephalon by the conserved zinc-finger protein Fezl

The forebrain constitutes the most anterior part of the central nervous system, and is functionally crucial and structurally conserved in all vertebrates. It includes the dorsally positioned telencephalon and eyes, the ventrally positioned hypothalamus, and the more caudally located diencephalon [from rostral to caudal: the prethalamus, the zona limitans intrathalamica (ZLI), the thalamus and the pretectum]. Although antagonizing Wnt proteins are known to establish the identity of the telencephalon and eyes, it is unclear how various subdivisions are established within the diencephalon--a complex integration center and relay station of the vertebrate brain. The conserved forebrain-specific zinc-finger-containing protein Fezl plays a crucial role in regulating neuronal differentiation in the vertebrate forebrain. Here, we report a new and essential role of zebrafish Fezl in establishing regional subdivisions within the diencephalon. First, reduced activity of fezl results in a deficit of the prethalamus and a corresponding expansion of the ZLI. Second, Gal4-UAS-mediated fezl overexpression in late gastrula is capable of expanding the prethalamus telencephalon and hypothalamus at the expense of the ZLI and other fore- and/or mid-brain regions. Such altered brain regionalization is preceded by the early downregulation of wnt expression in the prospective diencephalon. Finally, fezl overexpression is able to restore the anterior forebrain and downregulate wnt expression in Headless- and/or Tcf3 (also known as Tcf7l1a)-deficient embryos. Our findings reveal that Fezl is crucial for establishing regional subdivisions within the diencephalon and may also play a role in the development of the telencephalon and hypothalamus.     

Apoptosis in the developing zebrafish embryo.

Apoptosis is a major part of the normal development of many organ systems and tissues. The zebrafish (Danio rerio) has become a useful model for studying early development, and recent advances in techniques used to label apoptotic cells have made it possible to visualize apoptotic cells in this model system. We have used the in situ terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) to describe the temporal and spatial distribution of apoptotic cells during normal development of the zebrafish embryo from 12 to 96 h postfertilization. By counting labeled apoptotic cells, we have demonstrated transient high rates of cell death in various structures during development, and we have correlated these peaks with previously described developmental changes in these structures. Our analysis has focused on the nervous system and associated sensory organs including the olfactory organ, retina, lens, cornea, otic vesicle, lateral line organs, and Rohon-Beard neurons. Apoptosis is also described in other non-neural structures such as the notochord, somites, muscle, tailbud, and fins.     

Organization of hindbrain segments in the zebrafish embryo

To learn how neural segments are structured in a simple vertebrate, we have characterized the embryonic zebrafish hindbrain with a library of monoclonal antibodies. Two regions repeat in an alternating pattern along a series of seven segments. One, the neuromere centers, contains the first basal plate neurons to develop and the first neuropil. The other region, surrounding the segment boundaries, contains the first neurons to develop in the alar plate. The projection patterns of these neurons differ: those in the segment centers have descending axons, while those in the border regions form ventral commissures. A row of glial fiber bundles forms a curtain-like structure between each center and border region. Specific features of the individual hindbrain segments in the series arise within this general framework. We suggest that a cryptic simplicity underlies the eventual complex structure that develops from this region of the CNS.     

Patterning the zebrafish heart tube: acquisition of anteroposterior polarity.

The patterning of an internal organ, like the heart, is little understood. Central to this patterning is the formation, or the acquisition, of an anteroposterior (A-P) axis. We have approached the question of how the heart tube acquires polarity in the zebrafish, Brachydanio rerio, which offers numerous advantages for studying cardiac morphogenesis. During the early stages of organogenesis in the fish, the heart tube lies in an A-P orientation with the venous end lying anteriorly and the arterial end lying posteriorly. High doses (10(-6)-10(-5)M) of retinoic acid (RA) cause truncation of the body axis, as they do in Xenopus. Low doses of retinoic acid (10(-8)-10(-7) M), which do not appear to affect the rest of the embryo, have pronounced effects upon heart tube morphogenesis, causing it to shrink progressively along the A-P axis. To investigate this further, we identified monoclonal antibodies that distinguish between the zebrafish cardiac chambers and used them to show that the RA-induced cardiac truncation always begins at the arterial end of the heart tube. There is a continuous gradient of sensitivity from the arterial to the venous end, such that increasing RA exposure causes the progressive and sequential deletion first of the bulbus arteriosus and then, in order, of the ventricle, the atrium, and the sinus venosus. As exposure increases, parts of chambers are deleted before entire chambers; thus, the sensitivity to RA appears to be independent of chamber boundaries. The analysis of the heart tube's sensitivity to RA and its timing suggest that polarity is established during or shortly after initial commitment to the cardiac lineage.     

Patterning the embryo in higher plants: Emerging pathways and challenges

Embryogenesis, which establishes the basic body plan for the post-embryonic organs after stereotyped cell divisions, initiates the first step of the plant life cycle. Studies in the last two decades indicate that embryogenesis is a precisely controlled process, and any defect would result in abnormalities. Here we discuss the recent progresses, with a focus on the cellular pathways governing early embryogenesis in the model species Arabidopsis.     

Scube2 mediates Hedgehog signalling in the zebrafish embryo

The Hedgehog family of secreted morphogens specifies the fate of a large number of different cell types within invertebrate and vertebrate embryos, including the muscle cell precursors of the embryonic myotome of zebrafish. Formation of Hedgehog-sensitive muscle fates is disrupted within homozygous zebrafish mutants of the "you"-type class, the majority of which disrupt components of the Hedgehog (HH) signal transduction pathway. We have undertaken a phenotypic and molecular characterisation of one of these mutants, you, which we show results from mutations within the zebrafish orthologue of the mammalian gene scube2. This gene encodes a member of the Scube family of proteins, which is characterised by several protein motifs including EGF and CUB domains. Epistatic and molecular analyses position Scube2 function upstream of Smoothened (Smoh), the signalling component of the HH receptor complex, suggesting that Scube2 may act during HH signal transduction prior to, or during, receipt of the HH signal at the plasma membrane. In support of this model we show that scube2 has homology to cubilin, which encodes an endocytic receptor involved in protein trafficking suggesting a possible mode of function for Scube2 during HH signal transduction.     

Expression of the zebrafish Staufen gene in the embryo and adult

Staufen, a double stranded RNA binding protein, has been shown to be involved in creating and maintaining cellular asymmetry in the Drosophila oocyte, neuroblast, and mammalian neuron. Staufen binds to the 3' UTR of specific mRNAs and acts in their localization and anchoring to various subcellular domains. Staufen's molecular interactions during development have been limited to investigations in Drosophila melanogaster. Since a vertebrate Staufen has not been studied in a developmental system, the aim of this study was to clone and characterize a staufen orthologue gene in the vertebrate developmental model, zebrafish. The zebrafish staufen-like sequence shows a 64% homology to the human staufen with a 81.2% homology in the highly conserved double stranded RNA binding domain (dsRBDs). Staufen maps on the LN54 radiation hybrid panel to linkage group 6, 16.25 cR from Z265 between fb22h06 and fi16e01. Northern blot and in situ hybridization showed that staufen is expressed both maternally and zygotically. Zygotically expressed staufen is localized to the developing nervous system and at 24 h is highly concentrated in the subventricular zone of the developing brain. Maternally expressed staufen is dispersed in the mature oocyte and early embryo. In the adult, staufen is expressed in specific brain nuclei, the testis, neurons and Leydig cells.