Epithelial fusions in the embryo

Morphogenesis in the embryo involves the bending, folding and fusing of epithelial tissues to create the final complex shapes of the various organs and structures in the body. One essential process that occurs frequently during development is the drawing together and fusion of epithelial edges. Drosophila dorsal closure is perhaps the most genetically tractable of this type of movement, and several recent advances have revealed much about the signals regulating the dynamic actin cytoskeletal machineries that underlie the zippering-closed of this hole in the embryonic fly. It is now clear that there are intriguing parallels with more complex morphogenetic tissue movements in vertebrates.     

Getting an embryo into shape

Formation of a multicellular organism is a complex process involving differentiation and morphogenesis. During early vertebrate development, the radial symmetric organization of the egg is transferred into a bilateral symmetric organism with three distinct body axes: anteroposterior (AP), dorsoventral, and left–right. Due to cellular movements and proliferation, the body elongates along the AP axis. How are these processes coupled? Two recent publications now indicate that cell migration as well as orientated cell divisions contribute to axis elongation. The processes are coupled through the planar cell polarity pathway. 1 At the same time, the AP axis is patterned independently of convergent extension. This process, however, is required for cell migration and represents a cue for polarized cell motility during gastrulation. Thus, it is AP polarity that instructs individual cells how to orientate with respect to the embryonic axis and provides positional information for the process of convergent extension. 2 BioEssays 26:1272–1275, 2004. © 2004 Wiley Periodicals, Inc.     

Epithelial cell shape: cadherins and small GTPases

Cadherins are cell-cell adhesion receptors that are essential for the establishment of the epithelial cell shape and maintenance of the differentiated epithelial phenotype. In order to show efficient adhesion, cadherin receptors require an association with actin filaments and the activity of RHO proteins. The RHO family of small GTPases is primarily involved in the reorganization of the cytoskeleton. In different cell types, each member of the family can induce specific types of organization of actin filaments: stress fibers (Rho), lamellae/ruffles (Rac), or filopodia (Cdc42). This review focuses on how the function of small GTPases may impinge on the regulation of cadherin-dependent adhesion. In particular, it discusses the impact that the above cytoskeletal structures induced by RHO proteins have on the development of epithelial morphology. Finally, the participation of small GTPase-interacting proteins is considered during the remodeling of cell shape that follows cell-cell contact formation.     

Proteins that shape DNA

During the last five years, considerable accumulation of data on nucleic acids metabolism leads to the discovery of a number of proteins designed to change the conformation of DNA and to "shape" it. Experimental results emphasize the importance of the conformation and the flexibility of DNA itself in such interactions. The mutual recognition of nucleic acids by proteins may be or not dependent on the nnucleotide sequence and in most cases is accompanied by conformational changes in the proteins involved. Among these are proteins that bind in stoichiometric amounts to DNA, proteins that promote the separation of the two strands in a duplex, and finally proteins that change the topology of DNA.     

Morphogenesis: joining the dots to shape an embryo

In the study of morphogenesis, how upstream signalling events are intricately linked to downstream cytoskeletal organisation is not entirely understood. Recent work in the Drosophila embryo has begun to shed light on this problem.     

The molecular machines that mediate microRNA maturation

MicroRNAs (miRNA) are small RNAs that regulate the translation of thousands of message RNAs and play a profound role in mammalian biology. Over the past 5 years, significant advances have been made towards understanding the pathways that generate miRNAs and the mechanisms by which miRNAs exert their regulatory functions. An emerging theme is that miRNAs are both generated by and utilized by large and complex macromolecular assemblies. Here, we review the biology of mammalian miRNAs with a focus on the macromolecular complexes that generate and control the biogenesis of miRNAs.     

Epithelial cell specialization at a limb segment boundary in the grasshopper embryo

Epithelial cells at the developing femur-trochanter limb segment boundary in the grasshopper embryo are specialized with respect to nonboundary cells. They are elongated, with the long axis oriented along the limb circumference. Some cells along the boundary preferentially bind an antibody (anti-HRP), and so are molecularly specialized as well. The specialized cells are the most proximal cells of the more distal segment.     

Trophectoderm mechanics direct epiblast shape upon embryo implantation

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Nuclear-cytoplasmic ratio and shape of erythroid cells of the reindeer embryo

With the beginning of normoblastic erythropoiesis the nuclear-cytoplasmic ratio (NCR) in the erythroid cells increases. From the chromatophilic erythroblasts with NCR from 1.0 and more erythrocytes possessing the concavo-concave form are produced as a result of the nucleus elimination.     

Epithelial and basement membrane responses to chick embryo primitive streak grafts

Chick embryo primitive streak grafts, placed beneath the epiblast of host embryos, tend to result in the formation of either a neural plate in response to anterior streak grafts, or in de-epithelialization in response to posterior grafts. Ultrastructural and immunocytochemical examination shows that both reactions are preceded by basement membrane disruption and early removal of fibronectin therefrom. This disruption does not occur in response to non-streak grafts. It is suggested that the disruption, evoked by primitive streak cells, is a prerequisite first step, allowing direct graft-epiblast cell contact. This contact elicits a specific cytoskeletal reaction determining the epiblast response.     

Environmental factors that shape biofilm formation

Cells respond to the environment and alter gene expression. Recent studies have revealed the social aspects of bacterial life, such as biofilm formation. Biofilm formation is largely affected by the environment, and the mechanisms by which the gene expression of individual cells affects biofilm development have attracted interest. Environmental factors determine the cell’s decision to form or leave a biofilm. In addition, the biofilm structure largely depends on the environment, implying that biofilms are shaped to adapt to local conditions. Second messengers such as cAMP and c-di-GMP are key factors that link environmental factors with gene regulation. Cell-to-cell communication is also an important factor in shaping the biofilm. In this short review, we will introduce the basics of biofilm formation and further discuss environmental factors that shape biofilm formation. Finally, the state-of-the-art tools that allow us investigate biofilms under various conditions are discussed.     

Epithelial effects on limb chondrogenesis involve extracellular matrix and cell shape

Collagen gel cultures of limb bud mesenchymal cells are normally permissive for chondrogenesis but become inhibitory for chondrogenesis when they are preconditioned by limb ectoderm. This inhibition is specific for cartilage differentiation, inasmuch as myoblast differentiation is unaffected and flattened, fibroblastic cells are more numerous on conditioned gels. The antichondrogenic effect of ectoderm-conditioned gels is not blocked by agents that elevate intracellular cyclic AMP levels and that promote chondrogenesis under other conditions. In contrast, the inhibitory effect of the ectoderm is alleviated when cultures are treated with cytochalasin D, a cytoskeleton-disrupting agent that causes the cells to remain spherical. These results suggest that ectoderm-conditioned collagen gels inhibit chondrogenesis through an effect on cell shape.