Zic1 and Zic4 regulate zebrafish roof plate specification and hindbrain ventricle morphogenesis

During development, the lumen of the neural tube develops into a system of brain cavities or ventricles, which play important roles in normal CNS function. We have established that the formation of the hindbrain (4th) ventricle in zebrafish is dependent upon the pleiotropic functions of the genes implicated in human Dandy Walker Malformation, Zic1 and Zic4. Using morpholino knockdown we show that zebrafish Zic1 and Zic4 are required for normal morphogenesis of the 4th ventricle. In Zic1 and/or Zic4 morphants the ventricle does not open properly, but remains completely or partially fused from the level of rhombomere (r) 2 towards the posterior. In the absence of Zic function early hindbrain regionalization and neural crest development remain unaffected, but dorsal hindbrain progenitor cell proliferation is significantly reduced. Importantly, we find that Zic1 and Zic4 are required for development of the dorsal roof plate. In Zic morphants expression of roof plate markers, including lmx1b.1 and lmx1b.2, is disrupted. We further demonstrate that zebrafish Lmx1b function is required for both hindbrain roof plate development and 4th ventricle morphogenesis, confirming that roof plate formation is a critical component of ventricle development. Finally, we show that dorsal rhombomere boundary signaling centers depend on Zic1 and Zic4 function and on roof plate signals, and provide evidence that these boundary signals are also required for ventricle morphogenesis. In summary, we conclude that Zic1 and Zic4 control zebrafish 4th ventricle morphogenesis by regulating multiple mechanisms including cell proliferation and fate specification in the dorsal hindbrain.     

A simple model for early morphogenesis

Despite the large amount of knowledge which continues to accumulate about early developmental events, very little is known about the processes which control them. Part of the problem may lie in that workers applying different approaches and techniques have different points of view and appear to be reluctant to read each others' literature. My aim in this paper is not to give a generative, formal model for early development, but rather to suggest several connecting strands between the physiological, biochemical, cell biological and experimental embryological approaches which may stimulate new research in fields between those already exploited.     

Expression and function of mouse Sox17 gene in the specification of gallbladder/bile-duct progenitors during early foregut morphogenesis

In early-organogenesis-stage mouse embryos, the posteroventral foregut endoderm adjacent to the heart tube gives rise to liver, ventral pancreas and gallbladder. Hepatic and pancreatic primordia become specified in the posterior segment of the ventral foregut endoderm at early somite stages. The mechanisms for demarcating gallbladder and bile duct primordium, however, are poorly understood. Here, we demonstrate that the gallbladder and bile duct progenitors are specified in the paired lateral endoderm domains outside the heart field at almost the same timing as hepatic and pancreatic induction. In the anterior definitive endoderm, Sox17 reactivation occurs in a certain population within the most lateral domains posterolateral to the anterior intestinal portal (AIP) lip on both the left and right sides. During foregut formation, the paired Sox17-positive domains expand ventromedially to merge in the midline of the AIP lip and become localized between the liver and pancreatic primordia. In Sox17-null embryos, these lateral domains are missing, resulting in a complete loss of the gallbladder/bile-duct structure. Chimera analyses revealed that Sox17-null endoderm cells in the posteroventral foregut do not display any gallbladder/bile-duct molecular characters. Our findings show that Sox17 functions cell-autonomously to specify gallbladder/bile-duct in the mouse embryo.     

Shape self-regulation in early lung morphogenesis

The arborescent architecture of mammalian conductive airways results from the repeated branching of lung endoderm into surrounding mesoderm. Subsequent lung's striking geometrical features have long raised the question of developmental mechanisms involved in morphogenesis. Many molecular actors have been identified, and several studies demonstrated the central role of Fgf10 and Shh in growth and branching. However, the actual branching mechanism and the way branching events are organized at the organ scale to achieve a self-avoiding tree remain to be understood through a model compatible with evidenced signaling. In this paper we show that the mere diffusion of FGF10 from distal mesenchyme involves differential epithelial proliferation that spontaneously leads to branching. Modeling FGF10 diffusion from sub-mesothelial mesenchyme where Fgf10 is known to be expressed and computing epithelial and mesenchymal growth in a coupled manner, we found that the resulting laplacian dynamics precisely accounts for the patterning of FGF10-induced genes, and that it spontaneously involves differential proliferation leading to a self-avoiding and space-filling tree, through mechanisms that we detail. The tree's fine morphological features depend on the epithelial growth response to FGF10, underlain by the lung's complex regulatory network. Notably, our results suggest that no branching information has to be encoded and that no master routine is required to organize branching events at the organ scale. Despite its simplicity, this model identifies key mechanisms of lung development, from branching to organ-scale organization, and could prove relevant to the development of other branched organs relying on similar pathways.     

Shaping the zebrafish heart: From left-right axis specification to epithelial tissue morphogenesis

Although vertebrates appear bilaterally symmetric on the outside, various internal organs, including the heart, are asymmetric with respect to their position and/or their orientation based on the left/right (L/R) axis. The L/R axis is determined during embryo development. Determination of the L/R axis is fundamentally different from the determination of the anterior-posterior or the dorsal-ventral axis. In all vertebrates a ciliated organ has been described that induces a left-sided gene expression program, which includes Nodal expression in the left lateral plate mesoderm. To have a better understanding of organ laterality it is important to understand how L/R patterning induces cellular responses during organogenesis. In this review, we discuss the current understanding of the mechanisms of L/R patterning during zebrafish development and focus on how this affects cardiac morphogenesis. Several recent studies have provided unprecedented insights into the intimate link between L/R signaling and the cellular responses that drive morphogenesis of this organ.     

Topological determination of early morphogenesis in Metazoa

This paper presents a topological interpretation of some developmental events through the use of well-known concepts and theorems of combinatorial geometry. The organization of early embryo using a simulation of cleavage considering only blastomere contacts is examined. Each blastomere is modeled as a topological cell and whole embryo—as cell packing. The egg cleavage results in a pattern of cellular contacts on the surface of each blastomere and whole embryo, a discrete morphogenetic field. We find topological distinctions between different types of early egg cleavage and suggest a topological classification of cleavage. Blastulation and gastrulation may be related to an inevitable emergence of discrete curvature that directs development in three-dimensional space. The relationship between local and global orders in metazoan development, i.e., between local morphogenetic processes and integral developmental patterns, is established. Thus, this methodology reveals a topological imperative: a certain set of topological rules that constrains and directs biological morphogenesis.     

beta-Catenin is required for specification of proximal/distal cell fate during lung morphogenesis

The lungs are divided, both structurally and functionally, into two distinct components, the proximal airways, which conduct air, and the peripheral airways, which mediate gas exchange. The mechanisms that control the specification of these two structures during lung development are currently unknown. Here we show that beta-catenin signaling is required for the formation of the distal, but not the proximal, airways. When the gene for beta-catenin was conditionally excised in epithelial cells of the developing mouse lung prior to embryonic day 14.5, the proximal lung tubules grew and differentiated appropriately. The mice, however, died at birth because of respiratory failure. Analysis of the lungs by in situ hybridization and immunohistochemistry, using molecular markers of the epithelial and mesenchymal components of both proximal and peripheral airways, showed that the lungs were composed primarily of proximal airways. These observations establish, for the first time, both the sites and timing of specification of the proximal and peripheral airways in the developing lung, and that beta-catenin is one of the essential components of this specification.     

Liver specification: a new role for Wnts in liver development

Secreted Wnt proteins control a diverse array of developmental decisions. A recent analysis of the zebrafish mutant prometheus points to a previously unknown role for Wnts during liver specification.     

Desmin expression during early mouse tongue morphogenesis.

Occipital somites provide progenitor cells for craniofacial muscle development including the tongue musculature. Serum-derived factors are assumed to be pre-requisite for myogenesis in vitro. To test these assertions, we designed experiments to determine whether early mouse tongue development in general, and desmin localization in particular, were expressed during the development of embryonic mouse first branchial arch explants cultured in serumless, chemically-defined medium. Immunohistochemical techniques determined the chronology and positions of desmin expression during early craniofacial development. Occipital somites expressed desmin at E9 (9 days +/- 2 h post-fertilization, 18-20 somites). A discrete cell migration pathway initiating in the somites and terminating in the lateral lingual processes of the tongue primordium was defined based upon desmin expression patterns in E9-E11 embryos and computer-assisted three dimensional reconstructions. The in vitro model system was permissive for tongue morphogenesis, allowing development and fusion of the lateral lingual processes with the tuberculum impar. During culture myoblasts were not observed to fuse into myotubes with sarcomeric assembly, even though explant myoblasts produced muscle-specific protein. E10 explants cultured for 9 days demonstrated a five-fold increase in cell number that expressed desmin (P less than 0.05) when compared to the E10 starting material. We interpret these results to indicate that the tongue myogenic cell lineage was determined between E8 and E11, and that this resident population expanded within explants cultured in serumless medium by several explanations: (i) cells other than progenitor myoblasts (e.g., satellite cells) were induced to become myoblasts, and/or (ii) progenitor myoblasts within the original explants expanded by cell division in the absence of serum factors.(ABSTRACT TRUNCATED AT 250 WORDS)