Cell-to-cell coupling in early-stage bovine embryos: A preliminary report

Gap junction communication has been implicated in providing positional information within an embryo. This positional information is then used to direct the differentiation of the early embryo. To begin to gain an assessment of the cell-to-cell communication observed in the early bovine embryo, fluorescein (5%) was microinjected into single blastomeres of freshly collected embryos. Dye communication was not observed in any of the 8-to 16-cell stage embryos. Very limited dye coupling was observed in compact morula (18%) and expanded blastocysts (25%). Interestingly, none of the expanded blastocysts resulting from in vitro maturation and in vitro fertilization showed any dye coupling. The degree of coupling observed in the bovine embryo was less than that observed in compact morula mouse embryos, where almost all (95%) embryos showed dye coupling. This experimental data is discussed in context with previous electron microscopy data.     

Proximal-distal pattern formation in Drosophila: cell autonomous requirement for Distal-less gene activity in limb development

Limb development in the Drosophila embryo requires a pattern-forming system to organize positional information along the proximal–distal axis of the limb. This system must function in the context of the well characterized anterior–posterior and dorsal–ventral pattern-forming systems that are required to organize the body plan of the embryo. By genetic criteria the Distal-less gene appears to play a central role in limb development. Lack-of-function Distal-less mutations cause the deletion of a specific subset of embryonic peripheral sense organs that represent the evolutionary remnants of larval limbs. Distal-less activity is also required in the imaginal discs for the development of adult limbs. This requirement is cell autonomous and region specific within the developing limb primordium. Production of genetically mosaic imaginal discs, in which clones of cells lack Distal-less activity, indicates the existence of an organized proximal–distal positional information in very young imaginal disc primordia. We suggest that this graded positional information may depend on the activity of the Distal-less gene.     

Developmental regulation of the Hox genes during axial morphogenesis in the mouse

The Hox genes confer positional information to the axial and paraxial tissues as they emerge gradually from the posterior aspect of the vertebrate embryo. Hox genes are sequentially activated in time and space, in a way that reflects their organisation into clusters in the genome. Although this co-linearity of expression of the Hox genes has been conserved during evolution, it is a phenomenon that is still not understood at the molecular level. This review aims to bring together recent findings that have advanced our understanding of the regulation of the Hox genes during mouse embryonic development. In particular, we highlight the integration of these transducers of anteroposterior positional information into the genetic network that drives tissue generation and patterning during axial elongation.     

Inner cell allocation in the mouse morula: the role of oriented division during fourth cleavage

Two populations of blastomeres become positionally distinct during fourth cleavage in the mouse embryo; the inner cells become enclosed within the embryo and the outer cells form the enclosing layer. The segregation of these two cell populations is important for later development, because it represents the initial step in the divergence of placental and fetal lineages. The mechanism by which the inner cells become allocated has been thought to involve the oriented division of polarized 8-cell blastomeres, but this has never been examined in the intact embryo. By using the technique of time-lapse cinemicrography, we have been able for the first time to directly examine the division planes of 8-cell blastomeres during fourth cleavage, and find that there are three, rather than two, major division plane orientations; anticlinal (perpendicular to the outer surface of the blastomere), periclinal (parallel to the outer surface of the blastomere), and oblique (at an angle between the other two). The observed frequencies of each type of division plane orientation provide evidence that the inner cells of the morula must derive from oriented division of 8-cell blastomeres, in accordance with the polarization hypothesis. Analysis of fourth cleavage division plane orientation with respect to either lineage or division order reveals that it is not associated with lineage from either the 2- or the 4-cell stage, but has a slight statistical association with fourth cleavage division order. The lack of association between division plane orientation and lineage supports the prediction that packing patterns and intercellular interactions within the 8-cell embryo during compaction play a role in determining fourth cleavage division plane orientation and thus, the positional fate of the daughter 16-cell blastomeres.     

Specifying positional information in the embryo: looking beyond morphogens

Concentration gradients of small diffusible molecules called morphogens are key regulators of development, specifying position during pattern formation in the embryo. It is now becoming clear that additional or alternative mechanisms involving interactions among cells are also crucial for positional specification.     

Cell polarity and cell diversification during early mouse embryogenesis.

At the eight-cell stage of mouse development, the organization of blastomeres changes from radially symmetrical to polarized. This acquisition of cell polarity, followed by asymmetric divisions, leads to the formation of two phenotypically different cell types, which give rise to the first two cell lineages of the mouse blastocyst embryo, trophectoderm and the inner cell mass. Cell fate, controlled by positional information, is not irreversibly fixed during differentiation, providing the embryo with considerable developmental flexibility.     

Hierarchical regulation of organ differentiation during embryogenesis in rice

In plants, genetic mechanisms leading to shoot and root formation are almost unknown. Because basic body organization of such organisms is established during embryogenesis, induction and isolation of embryonic mutants is a promising approach to the study of plant development. The study of available embryonic mutants of rice indicates the existence of three major developmental processes taking place during embryogenesis before morphogenetic events start: determination of organ differentiation, positional regulation of organs and size regulation of the embryo. The consideration of specific rice mutants supports the existence of two types of mutations in each regulatory process, one affecting the embryo as a whole and the second concerning more restricted embryonal regions. A hierarchical type of control of rice embryogenesis is suggested.     

mRNA localization studies suggest that murine FGF-5 plays a role in gastrulation.

During gastrulation in the mouse, the pluripotent embryonic ectoderm cells form the three primary germ layers, ectoderm, mesoderm and endoderm. Little is known about the mechanisms responsible for these processes, but evidence from previous studies in amphibians, as well as expression studies in mammals, suggest that signalling molecules of the Fibroblast Growth Factor (FGF) family may play a role in gastrulation. To determine whether this might be the case for FGF-5 in the mouse embryo, we carried out RNA in situ hybridization studies to determine when and where in the early postimplantation embryo the Fgf-5 gene is expressed. We chose to study this particular member of the FGF gene family because we had previously observed that its pattern of expression in cultures of teratocarcinoma cell aggregates is consistent with the proposal that Fgf-5 plays a role in gastrulation in vivo. The results reported here show that Fgf-5 expression increases dramatically in the pluripotent embryonic ectoderm just prior to gastrulation, is restricted to the cells forming the three primary germ layers during gastrulation, and is not detectable in any cells in the embryo once formation of the primary germ layers is virtually complete. Based on this provocative expression pattern and in light of what is known about the functions in vitro of other members of the FGF family, we hypothesize that in the mouse embryo Fgf-5 functions in an autocrine manner to stimulate the mobility of the cells that contribute to the embryonic germ layers or to render them competent to respond to other inductive or positional signals.     

Separate elements cause lineage restriction and specify boundaries of Hox-1.1 expression.

The Hox genes are a class of putative developmental control genes that are thought to be involved in the specification of positional identity along the anteroposterior axis of the vertebrate embryo. It is apparent from their expression pattern that their regulation is dependent upon positional information. In a previous analysis of the Hox-1.1 promoter in transgenic mice, we identified sequences that were sufficient to establish transgene expression in a specific region of the embryo. The construct used, however, did not contain enough regulatory sequences to reproduce all aspects of Hox-1.1 expression. In particular, neither a posterior boundary nor a restriction of expression to prevertebrae was achieved. Here we show correct regulation by Hox-1.1 sequences in transgenic mice and identify the elements responsible for different levels of control. Concomitant with the subdivision of mesodermal cells into different lineages during gastrulation and organogenesis, Hox-1.1 expression is restricted to successively smaller sets of cells. Distinct elements are required at different stages of development to execute this developmental programme. One position-responsive element (130 bp nontranslated leader) was shown to be crucial for the restriction of expression not only along the anteroposterior axis of the embryo, setting the posterior border, but also along the dorsoventral axis of the neural tube and to the lineage giving rise to the prevertebrae. Thus, Hox-1.1 expression is established in a specific region of the embryo and in a specific lineage of the mesoderm by restricting the activity of the promoter by the combined effect of several regulatory elements.     

Visualisation and Quantification of Morphogen Gradient Formation in the Zebrafish

During embryonic development, signalling molecules known as morphogens act in a concentration-dependent manner to provide positional information to responding tissues. In the early zebrafish embryo, graded signalling by members of the nodal family induces the formation of mesoderm and endoderm, thereby patterning the embryo into three germ layers. Nodal signalling has also been implicated in the establishment of the dorso-ventral axis of the embryo. Although one can infer the existence of nodal gradients by comparing gene expression patterns in wild-type embryos and embryos in which nodal signalling is diminished or augmented, real understanding can only come from directly observing the gradients. One approach is to determine local ligand concentrations in the embryo, but this is technically challenging, and the presence of inhibitors might cause the effective concentration of a ligand to differ from its actual concentration. We have therefore taken two approaches to visualise a direct response to nodal signalling. In the first, we have used transgenic embryos to study the nuclear accumulation of a Smad2-Venus fusion protein, and in the second we have used bimolecular fluorescence complementation to visualise the formation of a complex between Smad2 and Smad4. This has allowed us to visualise, in living embryos, the formation of a graded distribution of nodal signalling activity. We have quantified the formation of the gradient in time and space, and our results not only confirm that nodal signalling patterns the embryo into three germ layers, but also shed light on its role in patterning the dorso-ventral axis and highlight unexpected complexities of mesodermal patterning.     

Expression of the homeobox Chick-en gene in chick/quail chimeras with inverted mes-metencephalic grafts

The homeobox gene Chick-en, sharing homologies to the engrailed gene of Drosophila, is expressed, during early steps of development, in a restricted area of the chick embryo including mes-metencephalic neuroepithelia. The expression of the Chick-en gene has been analyzed in chick/quail chimeric embryos in which a portion of the 2-day-old mes-metencephalic neuroepithelium has been transplanted in an inverted position. By means of a monoclonal antibody, "Mab 4D9," recognizing engrailed proteins, it is shown that the expression of the Chick-en gene is regulated in the inverted neuroepithelium according to its new position in the host neural tube. The regulation takes place within 20 hr after transplantation. These results, together with previous data demonstrating that the phenotypic expression of the inverted neuroepithelium depends, also, on its new position in the host neural tube, strongly suggest that the engrailed protein could play an important role in the positional specification of the mes-metencephalic neuroepithelium.     

Positional behaviour of olive baboons (Papio anubis) and its relationship to maintenance and social activities

A troop of olive baboons (Papio anubis) was studied over a four week period. Total observation time was 140 hours, 118 hours of which were spent gathering quantitative data on positional behaviour and the maintenance and social behaviour with which it is associated. The contribution of each activity on a time basis to the positional repertoire is described and some data is presented on the development of positional activities in infants. The contribution of positional activities to various maintenance and social activities is also described quantitatively. The complex of positional, maintenance, social and environmental factors involved especially in feeding, foraging, play, and grooming are described and discussed. The similarities between the adaptive response shown during the study period and that of other ground living cercopithecines is also discussed.     

Genes that control dorsoventral polarity affect gene expression along the anteroposterior axis of the Drosophila embryo

At least 13 genes control the establishment of dorsoventral polarity in the Drosophila embryo and more than 30 genes control the anteroposterior pattern of body segments. Each group of genes is thought to control pattern formation along one body axis, independently of the other group. We have used the expression of the fushi tarazu (ftz) segmentation gene as a positional marker to investigate the relationship between the dorsoventral and anteroposterior axes. The ftz gene is normally expressed in seven transverse stripes. Changes in the striped pattern in embryos mutant for other genes (or progeny of females homozygous for maternal-effect mutations) can reveal alterations of cell fate resulting from such mutations. We show that in the absence of any of ten maternal-effect dorsoventral polarity gene functions, the characteristic stripes of ftz protein are altered. Normally there is a difference between ftz stripe spacing on the dorsal and ventral sides of the embryo; in dorsalized mutant embryos the ftz stripes appear to be altered so that dorsal-type spacing occurs on all sides of the embryo. These results indicate that cells respond to dorsoventral positional information in establishing early patterns of gene expression along the anteroposterior axis and that there may be more significant interactions between the different axes of positional information than previously determined.     

Abdominal-B is essential for proper sexually dimorphic development of the Drosophila gonad

Sexual dimorphism requires the integration of positional information in the embryo with the sex determination pathway. Homeotic genes are a major source of positional information responsible for patterning along the anterior-posterior axis in embryonic development, and are likely to play a critical role in sexual dimorphism. Here, we investigate the role of homeotic genes in the sexually dimorphic development of the gonad in Drosophila. We have found that Abdominal-B (ABD-B) is expressed in a sexually dimorphic manner in the embryonic gonad. Furthermore, Abd-B is necessary and sufficient for specification of a sexually dimorphic cell type, the male-specific somatic gonadal precursors (msSGPs). In Abd-B mutants, the msSGPs are not specified and male gonads now resemble female gonads with respect to these cells. Ectopic expression of Abd-B is sufficient to induce formation of extra msSGPs in additional segments of the embryo. Abd-B works together with abdominal-A to pattern the non-sexually dimorphic somatic gonad in both sexes, while Abd-B alone specifies the msSGPs. Our results indicate that Abd-B acts at multiple levels to regulate gonad development and that Abd-B class homeotic genes are conserved factors in establishing gonad sexual dimorphism in diverse species.     

Selective embryo abortion in a perennial tree-legume: a case for maternal advantage of reduced seed number per fruit

In this study, I analyzed time-course of embryo abortion, positional bias in seed maturation and maternal costs of seed packaging in Cercis canadensis. While basal embryos experience similar rates of abortion as those in other positions during the first week of development, abortion rates peak during the second week. Head start in resource sequestration by stigmatic embryos may explain high rates of basal embryo abortion. Similar seed packaging costs and seed mass for single and multi-seeded pods suggests that maternal parent regulates pod size in accordance with seed numbers per pod investing equally in the surviving offspring. Competition during early developmental period results in the abortion of less competitive embryos allowing for optimal resource investment.     

Does the Bicoid gradient matter?

The generation and interpretation of positional information are key processes in developmental systems. In this issue, Chen et?al. report discoveries made in the Drosophila embryo that give new insights into how positional information can be produced by patterning gradients.     

Positional Information,Positional Error,and Readout Precision in Morphogenesis: A Mathematical Framework

The concept of positional information is central to our understanding of how cells determine their location in a multicellular structure and thereby their developmental fates. Nevertheless, positional information has neither been defined mathematically nor quantified in a principled way. Here we provide an information-theoretic definition in the context of developmental gene expression patterns and examine the features of expression patterns that affect positional information quantitatively. We connect positional information with the concept of positional error and develop tools to directly measure information and error from experimental data. We illustrate our framework for the case of gap gene expression patterns in the early Drosophila embryo and show how information that is distributed among only four genes is sufficient to determine developmental fates with nearly single-cell resolution. Our approach can be generalized to a variety of different model systems; procedures and examples are discussed in detail.