Shroom induces apical constriction and is required for hingepoint formation during neural tube closure

Gastrulation Movements: the Logic and the Nuts and Bolts

Gastrulation, the period during the early development of animals when major cell and tissue movements remodel an initially unstructured group of cells, requires coordinated control of different types of cellular activities in different cell populations. A hierarchy of genetic control mechanisms, involving cell signaling and transcriptional regulation, sets up the embryonic axes and specify the territories of the future germ layers. Cells in these territories modulate their cytoskeleton and their adhesive behavior, resulting in shape changes and movement. Similarities among different species in patterning and cell biological mechanisms are beginning to allow us to recognize general, conserved principles and speculate on possible ancestral mechanisms of gastrulation.

The interface between cell and developmental biology

Cell differentiation, morphology, migration, polarity, intercellular communication and adhesion are all cellular processes that control embryo morphogenesis and lie at the interface of cell and developmental biology. The interface between these two fields is best illustrated, however, in studies of axiation and cytoskeletal remodeling during development. Recent advances reveal novel mechanisms for axiation, including the role of RNA and protein degradation in regulating the timely expression of morphogenetic signals. Significant progress has also been made in identifying components of the cytoskeleton and the extracellular matrix that mediate embryonic cell migration and polarity. Cellular processes at the interface of cell and developmental biology are overseen by the Wnt signaling cascade that coordinates both axiation and cytoskeletal remodeling during development.

Compartmentalized morphogenesis in epithelia: From cell to tissue shape

During development, embryonic tissues are shaped in a species-specific manner. Yet, across species, general classes of tissue remodeling events occur, such as tissue infolding and tissue elongation. The spatiotemporal control of these morphogenetic processes is responsible for the organization of different body plans, as well as for organogenesis. Cell morphogenesis in a mesenchyme contributes to the shaping of embryonic tissues. Epithelial cells, despite that they need to maintain an apicobasal organization, play an equally important role during morphogenesis. Moving from apical to basal, we review compartmentalized cellular rearrangements underlying tissue remodeling in Drosophila and compare them with those found in other organisms. Contractile activity at the apical surface triggers tissue folding and invagination. The regulation of adhesion at adherens junctions controls polarized neighbor exchange during intercalation and tissue elongation. Basolateral protrusive activity underlies other cases of intercalation. These localized cell shape changes are spatially regulated by developmental signals. Some signals define a local change in cell behavior (e.g., apical constriction), others orient a dynamic process in the plane of the tissue (e.g., junction remodeling).

Strategies of vertebrate neurulation and a re-evaluation of teleost neural tube formation

The vertebrate neural tube develops by two distinct mechanisms. Anteriorly, in the brain and future trunk (cervicothoracic) region, 'primary neurulation' occurs, where an epithelial sheet rolls or bends into a tube. Posteriorly, in the future lumbar and tail region, the neural tube forms by 'secondary neurulation', where a mesenchymal cell population condenses to form a solid rod that undergoes transformation to an epithelial tube. Teleost neurulation has been described as different from that of other vertebrates. This is principally because the teleost trunk neural tube initially forms a solid rod (the neural keel) that later develops a lumen. This process has also been termed secondary neurulation. However, this description is not accurate since the teleost neural tube derives from an epithelial sheet that folds. This best fits the description of primary neurulation. It has also been suggested that teleost neurulation is primitive, however, both primary and secondary neurulation are found in groups with a more ancient origin than the teleosts. The similarity between neurulation in teleosts and other vertebrates indicates that this group includes viable models (such as the zebrafish) for understanding human neural tube development.

A Rho GTPase Signaling Pathway Is Used Reiteratively in Epithelial Folding and Potentially Selects the Outcome of Rho Activation

A single Rho GTPase family member is capable of initiating several different processes, including cell cycle regulation, cytokinesis, cell migration, and transcriptional regulation . It is not clear, however, how the Rho protein selects which of these processes to initiate. Guanine nucleotide exchange factors (GEFs), proteins that activate Rho GTPases, could be important in making this selection. We show here that in vivo, DRhoGEF2, a GEF that is ubiquitously expressed and specific for Rho1, is reiteratively required for epithelial folding and invagination, but not for other processes regulated by Rho. The limitation of DRhoGEF2 function supports the hypothesis that the GEF selects the outcome of Rho activation. DRhoGEF2 exerts its effects in gastrulation through the regulation of Myosin II to orchestrate coordinated apical cell constriction. Apical myosin localization is also regulated by Concertina (Cta), a Galpha(12/13) family member that is thought to activate DRhoGEF2 and is itself activated by a putative ligand, Folded gastrulation (Fog). Fog and Cta also play a role in the morphogenetic events requiring DRhoGEF2, suggesting the existence of a conserved signaling pathway in which Fog, Cta, and DRhoGEF2 locally activate Myosin for epithelial invagination and folding.

Parallels between tissue repair and embryo morphogenesis

Wound healing involves a coordinated series of tissue movements that bears a striking resemblance to various embryonic morphogenetic episodes. There are several ways in which repair recapitulates morphogenesis. We describe how almost identical cytoskeletal machinery is used to repair an embryonic epithelial wound as is involved during the morphogenetic episodes of dorsal closure in Drosophila and eyelid fusion in the mouse foetus. For both naturally occurring and wound-activated tissue movements, JNK signalling appears to be crucial, as does the tight regulation of associated cell divisions and adhesions. In the embryo, both morphogenesis and repair are achieved with a perfect end result, whereas repair of adult tissues leads to scarring. We discuss whether this may be due to the adult inflammatory response, which is absent in the embryo.

Formation and remodeling of epithelial polarity

Polarity is a fundamental property of all eukaryotic cells that underlies many developmental processes. A recent EMBO workshop (March 27-31) organized by Thomas Lecuit, Norbert Perrimon, and Keith Mostov brought cell and developmental biologists together on the Mediterranean coast near Marseille, France, to share views on how epithelium polarity is established and remodeled during development and disease. Participants witnessed and celebrated the emerging convergence of intellectual and experimental approaches to address how individual cells acquire polarity and form polarized tissues in the context of developing embryos.