Left/right patterning signals and the independent regulation of different aspects of situs in the chick embryo

Left-right asymmetry of fly wings and the evolution of body axes

The body plan of Drosophila, and presumably that of other insects, develops under the control of anterio-posterior and dorsal ventral axes, but no evidence for a left-right axis has yet been found. We used geometric morphometrics to study the wings in three species of flies: Drosophila melanogaster, Musca domestica and Glossina palpalis gambiensis. In all three species, we found that both size and shape showed subtle, but statistically significant directional asymmetry. For size, these asymmetries were somewhat inconsistent within and between species, but for shape, highly significant directional asymmetry was found in all samples examined. These systematic left-right differences imply the existence of a left-right axis that conveys distinct positional identities to the wing imaginal discs on either body side. Hence, the wing discs of Drosophila may be a new model to study the developmental genetics of left-right asymmetry. The asymmetries of shape were similar among species, suggesting that directional asymmetry has been evolutionarily conserved since the three lineages diverged. We discuss the implications of this evolutionary conservatism in conjunction with results from earlier studies that showed a lack of genetic variation for directional asymmetry in Drosophila.

Recent progress in the molecular genetics of congenital heart defects

Congenital heart defects (CHD) constitute the single most common anatomic class of birth defects and are a major cause of infant mortality. Correlation of normal and pathological embryology/anatomy has led to the formulation of mechanistic models, but there is limited understanding of the genetic basis for the inferred embryological processes. Most evidence points to extensive etiologic heterogeneity and a re-evaluation of simple multifactorial models is required. The recent identification of several genes responsible for congenital heart defects in the context of more complex clinical disorders provides significant entry points for the genetic analysis of human heart development. The association of aneusomies (particularly microdeletion syndromes) with specific cardiac lesions provides further strong support for mechanistic classification. Studies in the mouse are laying the groundwork for a comprehensive genetic model of cardiac organogenesis. Nevertheless, the basis for the large majority of CHD, especially isolated defects, remains obscure. Dissection of the genetic components of CHD is one of the greatest challenges in medical genetics for the coming decades.

A Cascade of Gene Action Controlling Heart Asymmetry and Torsion in Embryonic Development

Lateralized looping of the heart tube, with associated torsion of the embryo axis, is the first structural sign of the consistent left-right asymmetrical organization that characterizes all vertebrate embryos. Rare failures or reversals of this asymmetry in humans lead to clinically important syndromes of malformation in heart and great blood vessels. Recently, elements of the genetic control sequence underlying this left-right aspect of development have been uncovered. The normal sequence of transmission for asymmetry information can now be traced to a point close to the actual execution of right-hand looping in the heart tube, with the hope that the further sequence of gene activity, within the heart itself and directing these events, may soon be uncovered.

Cloning and expression pattern of chicken Pitx2: a new component in the SHH signaling pathway controlling embryonic heart looping

Asymmetry along the left-right axis of the embryo is a vital feature of vertebrate embryogenesis. In this study, we report the isolation and characterization of a bicoid-related homeobox gene, cPitx2, which displays left-right asymmetric expression during early chick embryogenesis. Asymmetric expression of cPitx2 is first detected at stage 7 and is restricted to mesodermal tissues on the left side of the embryo including the left sided lateral mesoderm, the left sided precardiac mesoderm, and the left half epimyocardium of the primitive heart. cPitx2 is also detected in the presumptive blood islands and endothelia of the embryonic blood vessels. Implantation of Sonic hedgehog (SHH) protein soaked beads on the right side of embryos induced ectopic cPitx2 expression on that side. Based on these observations, we suggest that cPitx2 is a component in SHH signaling pathway and plays a role in determining left-right asymmetry and in vasculogenesis during avian embryogenesis.

Notochord repression of endodermal Sonic hedgehog permits pancreas development

Notochord signals to the endoderm are required for development of the chick dorsal pancreas. Sonic hedgehog (SHH) is normally absent from pancreatic endoderm, and we provide evidence that notochord, in contrast to its effects on adjacent neuroectoderm where SHH expression is induced, represses SHH expression in adjacent nascent pancreatic endoderm. We identify activin-betaB and FGF2 as notochord factors that can repress endodermal SHH and thereby permit expression of pancreas genes including Pdx1 and insulin. Endoderm treatment with antibodies that block hedgehog activity also results in pancreatic gene expression. Prevention of SHH expression in prepancreatic dorsal endoderm by intercellular signals, like activin and FGF, may be critical for permitting early steps of chick pancreatic development.

The left-right coordinator: the role of Vg1 in organizing left-right axis formation

The asymmetries of internal organs are consistently oriented along the left-right axis in all vertebrates, and perturbations of left-right orientation lead to significant congenital disease. We propose a model in which a "left-right coordinator" interacts with the Spemann organizer to coordinate the evolutionarily conserved three-dimensional asymmetries in the embryo. The Vg1 cell-signaling pathway plays a central role in left-right coordinator function. Antagonists of Vg1 alter left-right development; antagonists of other members of the TGFbeta family do not. Cell-lineage directed expression of Vg1 protein can fully invert the left-right axis (situs inversus), can randomize left-right asymmetries, or can "rescue" a perturbed left-right axis in conjoined twins to normal orientation (situs solitus), indicating that Vg1 can mimic left-right coordinator activity. These are the first molecular manipulations in any vertebrate by which the left-right axis can be reliably controlled.

The transfer of left-right positional information during chick embryogenesis

The earliest known left-right asymmetric genes are expressed at Hensen's node during chick gastrulation. Gene expression following reorientation of the node shows asymmetry is instructed by adjacent tissue, hence left-right information originates outside the node. Subsequently, the node signals back to the lateral tissue, initiating a cascade leading to left-sided expression of nodal in the lateral plate mesoderm. Loss of nodal expression in the presence of blocking antibodies confirms that Sonic hedgehog is the key signal conveying left-right information from the node; however, manipulation of explant cultures suggests that the induction of nodal requires secondary signals produced in the paraxial mesoderm. These experiments establish the time of action of these signals to and from Hensen's node in establishing left-right asymmetry.

Knowing in your heart what's right

During heart formation in all vertebrate species, the linear heart tube undergoes rightward looping followed by the formation of the atrial and ventricular chambers. The direction of cardiac looping is determined in part by the asymmetric expression of members of the transforming growth factor beta family across the left-right axis of the embryo. The basic helix-loop-helix (bHLH) transcription factors dHAND and eHAND are expressed in the heart tube within specific cardiogenic precursors destined to form the right and left ventricular regions, respectively, and loss-of-function experiments have demonstrated the importance of these factors for looping morphogenesis and ventricular development. We propose a model in which the HAND gene products interpret asymmetric positional information in the developing heart and participate in transcriptional programmes that control development of the right and left ventricular compartments of the heart.

Left-right asymmetry in vertebrates

All vertebrates develop with a distinct, invariant, left-right (L-R) body asymmetry. Recently, a number of molecules have been shown to be expressed asymmetrically with respect to the L-R axis during early embryogenesis. The temporal expression patterns of these molecules, in combination with embryo manipulation and genetic experiments, indicate that they may represent elements of a conserved pathway governing embryonic handedness.