Pattern formation: regional specification in the early C. elegans embryo

Influence of early fate decisions at the two-cell stage on the derivation of mouse embryonic stem cell lines

The first event of differentiation in mammalian embryogenesis is the segregation of the inner cell mass and trophectoderm lineages in the blastocyst. Cellular and molecular events related to this process are still a controversial issue. During the years it was thought that first allocation of blastomeres before the blastocyst stage was done in the late eight-cell stage with the formation of inner and outer cells. Lately, many studies have pointed out that individual blastomeres at the four-cell stage differ in their developmental properties according to their position within the embryo. In this report, we wanted to elucidate whether these early decisions influence the production of mouse embryonic stem cell lines, so that a selective isolation of blastomeres at the four-cell stage to derive the lines could improve the efficiency of the derivation process. Results from blastomere tracking experiments support the idea of a different developmental potential of blastomeres within the four-cell stage embryo. However, we also show a high plasticity in the developmental pattern of blastomeres once isolated from the embryo, thus making all four-cell stage blastomeres equally competent to derive ESC lines.

Multiple regulatory elements with spatially and temporally distinct activities control the expression of the epithelial differentiation gene lin-26 in C. elegans

Epithelial differentiation is a very early event during development of most species. The nematode Caenorhabditis elegans, with its well-defined and invariant lineage, offers the possibility to link cell lineage, cell fate specification and gene regulation during epithelial differentiation. Here, we focus on the regulation of the gene lin-26, which is required for proper differentiation of epithelial cells in the ectoderm and mesoderm (somatic gonad). lin-26 expression starts in early embryos and remains on throughout development, in many cell types originating from different sublineages. Using GFP reporters and mutant rescue assays, we performed a molecular dissection of the lin-26 promoter and could identify almost all elements required to establish its complex spatial and temporal expression. Most of these elements act redundantly, or synergistically once combined, to drive expression in cells related by function. We also show that lin-26 promoter elements mediate activation in the epidermis (hypodermis) by the GATA factor ELT-1, or repression in the foregut (pharynx) by the FoxA protein PHA-4. Taken together, our data indicate that lin-26 regulation is achieved to a large extent through tissue-specific cis-regulatory elements.

The worm's sense of smell

Animals sense their chemical environment using multiple chemosensory neuron types, each of which exhibits characteristic response properties. The chemosensory neurons of the nematode Caenorhabditis elegans provide an excellent system in which to explore the developmental mechanisms giving rise to this functional diversity. In this review, we discuss the principles underlying the patterning, generation, differentiation, and diversification of chemosensory neuron subtypes in C. elegans. Current knowledge of the molecular mechanisms underlying each of these individual steps is derived from work in different model organisms. It is essential to describe the complete developmental pathways in each organism to determine whether functional diversification in chemosensory systems is achieved via conserved or novel mechanisms. Such a complete description may be possible in C. elegans.

Ovarian differentiation and human embryo quality. 1. Molecular and morphogenetic homologies between oocytes and embryos in Drosophila, C. elegans, Xenopus and mammals

Knowledge on the formation of oocytes and follicles in Drosophila, C. elegans and Xenopus, and the genetic regulation of polarities and embryo growth, has been related to comparable data in mammalian oocytes and embryos. Initially, details of the nature of the regulatory processes in the non-mammals are described, with considerable attention being paid to the role of individual genes and their specific functions. The molecular genetic aspects of these developmental processes are discussed in detail. Attention then turns to mammals, to identify, describe and evaluate their homologies with the lower animals and flies. Several of these homologies are described, including genes regulating primary ovarian failure and various aspects of early embryonic growth. The polarized distribution of genes in mammalian oocytes and embyros is discussed, together with the implications in the form of differentiation in the early embryo. Morphogenetic systems operative during follicle maturation, fertilization and cleavage are described and related to similar processes in lower forms. These events include ooplasmic and pronuclear rotations, the form of ooplasmic inheritance in early blastomeres and the establishment of embryonic axes. Models of early mammalian development are considered.

Patterning the C. elegans embryo: moving beyond the cell lineage

The Caenorhabditis elegans embryo undergoes a series of stereotyped cell cleavages that generates the organs and tissues necessary for an animal to survive. Here we review two models of embryonic patterning, one that is lineage-based, and one that focuses on domains of organ and tissue precursors. Our evolving view of C. elegans embryogenesis suggests that this animal develops by mechanisms that are qualitatively similar to those used by other animals.

Digging out roots: pattern formation, cell division, and morphogenesis in plants

The analysis of plant development by genetic, molecular, and surgical approaches has accumulated a large body of data, and yet it remains a challenge to uncover the basic mechanisms that are operating. Early steps of development, when the zygote and its daughter cells organize the embryonic plant, are poorly understood despite considerable efforts toward the identification of relevant genes. Reported cases of genetic redundancy suggest that the difficulty in uncovering patterning genes may reflect overlapping gene activities. Our current knowledge on plant embryo development still leaves open whether mechanisms for axis formation and subsequent pattern formation are fundamentally different in animals and plants. Axis formation may follow the general principle of establishing a peripheral asymmetric cue and mobilizing the cytoskeleton toward this cue--in the case of plants possibly located in the cell wall--but the molecules involved may be entirely different. Embryonic pattern formation involves the establishment of different domains, but although there are candidates, it is not clear whether genes that define these domains are identified yet. Pattern formation continues postembryonically in the meristem, and the flexibility of this process may be explained by a feed-forward system of patterning cues originating from more mature cells. Control of cell division and differentiation, which is important in the meristems--regions of continuous development--has been studied intensively and appears to involve short-range signaling and transmembrane receptor kinase activation. Finally, although high importance of control of cell division rates and planes for plant morphogenesis have been often inferred, recent genetic studies as well as comparative morphological data point to a less decisive role of cell division and to global controls of as yet unknown nature.

Complexity of developmental control: analysis of embryonic cell lineage specification in Caenorhabditis elegans using pes-1 as an early marker

In the early Caenorhabditis elegans embryo five somatic founder cells are born during the first cleavages. The first of these founder cells, named AB, gives rise to 389 of the 558 nuclei present in the hatching larva. Very few genes directly involved in the specification of the AB lineage have been identified so far. Here we describe a screen of a large collection of maternal-effect embryonic lethal mutations for their effect on the early expression of a pes-1::lacZ fusion gene. This fusion gene is expressed in a characteristic pattern in 14 of the 32 AB descendants present shortly after the initiation of gastrulation. Of the 37 mutations in 36 genes suspected to be required specifically during development, 12 alter the expression of the pes-1::lacZ marker construct. The gene expression pattern alterations are of four types: reduction of expression, variable expression, ectopic expression in addition to the normal pattern, and reduction of the normal pattern together with ectopic expression. We estimate that approximately 100 maternal functions are required to establish the pes-1 expression pattern in the early embryo.

Control of cell division in the root epidermis of Arabidopsis thaliana

The formation of the root epidermis in Arabidopsis thaliana provides a simple model to study mechanisms underlying patterning in plants. In this paper we have analyzed the relationships between cell fate specification and the pattern of cell division that occur in the root epidermis. Using clonal analysis, the two cell types of the developing root epidermis, trichoblasts and atrichoblasts, were distinguished by different rates of cell division, highest in trichoblasts. This character appears to be dependent on TTG which controls epidermal cell fate specification. The ability of epidermal cells to undergo longitudinal divisions which are involved in the control of the radial symmetry was shown to be controlled in a cell-specific manner by TTG. The control of the rate and the orientation of cell division in the root meristem epidermal layer thus appear to be under the control of cell fate specification mechanisms.

Binary specification of the embryonic lineage in Caenorhabditis elegans

In Caenorhabditis elegans, the early embryo contains five somatic founder cells (known as AB, MS, E, C and D) which give rise to very different lineages. Two simply produce twenty intestinal (E) or muscle (D) cells each, whereas the remainder produce a total of 518 cells which collectively contribute in a complex pattern to a variety of tissues. A central problem in embryonic development is to understand how the developmental potential of blastomeres is restricted to permit the terminal expression of such complex differentiation patterns. Here we identify a gene, lit-1, that appears to play a central role in controlling the asymmetry of cell division during embryogenesis in C. elegans. Mutants in lit-1 suggest that its product controls up to six consecutive binary switches which cause one of the two equivalent cells produced at each cleavage to assume a posterior fate. Most blastomere identities in C. elegans may therefore stem from a process of stepwise binary diversification.

Cell communities and robustness in development

The robustness of patterning events in development is a key feature that must be accounted for in proposed models of these events. When considering explicitly cellular systems, robustness can be exhibited at different levels of organization. Consideration of two widespread patterning mechanisms suggests that robustness at the level of cell communities can result from variable development at the level of individual cells; models of these mechanisms show how interactions between participating cells guarantee community-level robustness. Cooperative interactions enhance homogeneity within communities of like cells and the sharpness of boundaries between communities of distinct cells, while competitive interactions amplify small inhomogeneities within communities of initially equivalent cells, resulting in fine-grained patterns of cell specialization.

Activin has direct long-range signalling activity and can form a concentration gradient by diffusion

BACKGROUND

Activin has strong mesoderm-inducing properties in the early Xenopus embryo, and has a long-range signalling activity that activates genes in cells distant from a source in a concentration-dependent way. It has not yet been established what mechanism of signal transmission accounts for this and other examples of long-range signalling in vertebrates. Nor is it known whether activin itself acts on distant cells or whether other kinds of molecules are used for long-range signalling. Here we have used a well characterised model system, involving animal caps of Xenopus blastulae treated with activin or transforming growth factor beta, to analyze some fundamental properties of long-range signalling and of the formation of a morphogen gradient.

RESULTS

We find that cells distant from the source of activin require functional activin receptors to activate Xbrachyury, a result suggesting that activin itself acts directly on distant cells and that other secondary signalling molecules are not required. We also find that the signals can be transmitted across a tissue that cannot respond to it; this argues against a relay process. We provide direct evidence that labelled activin forms a concentration gradient emanating from its source and extending to the distant cells that express Xbrachyury. Lastly, we show that there is no inherent polarity in the responding tissue that influences either the direction or rate of signalling.

CONCLUSIONS

The long-range signalling mechanism by which activin initiates the transcription of genes in a concentration-dependent manner depends on a process of rapid diffusion and the establishment of an activin gradient across the tissue. It cannot be explained by a relay or wave propagation mechanism. Activin itself is the signalling molecule to which distant cells respond.

MEX-3 is a KH domain protein that regulates blastomere identity in early C. elegans embryos

After the first division of the C. elegans embryo, the posterior blastomere can produce numerous muscles while the anterior blastomere cannot. We show here that maternal-effect lethal mutations in the gene mex-3 cause descendants of the anterior blastomere to produce muscles by a pattern of development similar to that of a descendant of the wild-type posterior blastomere. mex-3 encodes a probable RNA-binding protein that is distributed unequally in early embryos and that is a component of germline-specific granules called P granules. We propose that MEX-3 contributes to anterior-posterior asymmetry by regulating one or more mRNAs involved in specifying the fate of the posterior blastomere.