An anterior/posterior communication compartment border in engrailed wing discs: possible implications for Drosophila pattern formation

Our previous studies have suggested that all the known lineage compartment borders in the wing imaginal disc of Drosophila are coincident with boundaries of reduced gap junctional communication (communication compartment borders). Since engrailed discs have a disrupted anterior/posterior (A/P) lineage border (G. Morata and P. A. Lawrence, 1975, Nature (London) 255, 614-617), it was of great interest to determine if their A/P communication restriction boundary is similarly disrupted. Examination of gap-junction-mediated exchange of small fluorescent molecules between cells in the engrailed wing disc revealed a boundary of restricted communication that appeared to be identical to the wild-type A/P communication restriction boundary. This result suggests that lineage compartments are not required for the formation of A/P communication restrictions. Furthermore, we suggest that perhaps communication compartments are the domains within which information is provided for specifying the formation of lineage compartments.     

Hox genes and morphological identity: axial versus lateral patterning in the vertebrate mesoderm

The successful organization of the vertebrate body requires that local information in the embryo be translated into a functional, global pattern. Somite cells form the bulk of the musculoskeletal system. Heterotopic transplants of segmental plate along the axis from quail to chick were performed to test the correlation between autonomous morphological patterning and Hox gene expression in somite subpopulations. The data presented strengthen the correlation of Hox gene expression with axial specification and focus on the significance of Hox genes in specific derivatives of the somites. We have defined two anatomical compartments of the body based on the embryonic origin of the cells making up contributing structures: the dorsal compartment, formed from purely somitic cell populations; and the ventral compartment comprising cells from somites and lateral plate. The boundary between these anatomical compartments is termed the somitic frontier. Somitic tissue transplanted between axial levels retains both original Hox expression and morphological identity in the dorsal compartment. In contrast, migrating lateral somitic cells crossing the somitic frontier do not maintain donor Hox expression but apparently adopt the Hox expression of the lateral plate and participate in the morphology appropriate to the host level. Dorsal and ventral compartments, as defined here, have relevance for experimental manipulations that influence somite cell behavior. The correlation of Hox expression profiles and patterning behavior of cells in these two compartments supports the hypothesis of independent Hox codes in paraxial and lateral plate mesoderm.     

Compartments and organising boundaries in the Drosophila eye: the role of the homeodomain Iroquois proteins.

The Drosophila eye is patterned by a dorsal-ventral organising centre mechanistically similar to those in the fly wing and the vertebrate limb bud. Here we show how this organising centre in the eye is initiated - the first event in retinal patterning. Early in development the eye primordium is divided into dorsal and ventral compartments. The dorsally expressed homeodomain Iroquois genes are true selector genes for the dorsal compartment; their expression is regulated by Hedgehog and Wingless. The organising centre is then induced at the interface between the Iroquois-expressing and non-expressing cells at the eye midline. It was previously thought that the eye develops by a mechanism distinct from that operating in other imaginal discs, but our work establishes the importance of lineage compartments in the eye and thus supports their global role as fundamental units of patterning.     

msh specifies dorsal cell fate in the Drosophila wing

Drosophila limbs develop from imaginal discs that are subdivided into compartments. Dorsal-ventral subdivision of the wing imaginal disc depends on apterous activity in dorsal cells. Apterous protein is expressed in dorsal cells and is responsible for (1) induction of a signaling center along the dorsal-ventral compartment boundary (2) establishment of a lineage restriction boundary between compartments and (3) specification of dorsal cell fate. Here, we report that the homeobox gene msh (muscle segment homeobox) acts downstream of apterous to confer dorsal identity in wing development.     

Engrailed-1 misexpression in chick embryos prevents apical ridge formation but preserves segregation of dorsal and ventral ectodermal compartments

Using lineage tracers, we recently showed dorsal and ventral ectodermal compartments along the sides of the body in chick embryos. The compartments are formed both in presumptive limb-forming regions where they position the apical ridge and also in presumptive interlimb (flank). Here we show, using a novel technique combining fate mapping and in situ hybridisation, that the ventral compartment coincides with the Engrailed-1 (En-1) domain of expression. This coincidence suggests that En-1 could maintain the ventral compartment and be necessary for apical ridge formation. To test this hypothesis, we ectopically expressed En-1 via retroviral transfer and then examined limb development and cell lineage restriction in the ectoderm. En-1 misexpression can completely prevent formation of both normal limbs and ectopic limbs induced in the flank by application of FGF-2. In both cases, there are no morphological signs of apical ectodermal ridge formation and expression of ridge-associated genes is undetectable. In striking contrast, the lineage restriction between dorsal and ventral ectoderm is not altered. Therefore, En-1 is involved in the regulation of ridge formation but not compartment maintenance.     

Morphogen gradients in vertebrate limb development.

The developing limb is an excellent model for pattern formation in vertebrate embryos. Signalling by the polarizing region controls limb pattern across the antero-posterior axis of the chick limb. It was suggested first on theoretical grounds that signalling by the polarizing region could involve a morphogen gradient. Embryological manipulations provided evidence consistent with this model and, more recently, signalling molecules associated with the polarizing region have been identified and tested for their role as morphogens. It is still not clear whether any of the known molecules act directly as a morphogen. The extension of the morphogen model to patterning along the other axes of the limb has been proposed but this may not be applicable.     

The state of engrailed expression is not clonally transmitted during early Drosophila development.

In Drosophila embryos, boundaries of lineage restriction separate groups of cells, or compartments. Engrailed is essential for specification of the posterior compartment of each segment, and its expression is thought to mark this compartment. Using a new photo-activatable lineage tracer, we followed the progeny of single embryonic cells marked at the blastoderm stage. No clones straddled the anterior edges of engrailed stripes (the parasegment border). However, posterior cells of each stripe lose engrailed expression, producing mixed clones. We suggest that stable expression of engrailed by cells at the anterior edge of the stripe reflects, not cell-intrinsic mechanisms, but proximity to cells that produce Wingless, an extracellular signal needed for maintenance of engrailed expression. If control of posterior cell fate parallels control of engrailed expression, cell fate is initially responsive to cell environment and cell fate determination is a later event.     

Development of the Drosophila genital disc requires interactions between its segmental primordia

In both sexes, the Drosophila genital disc comprises three segmental primordia: the female genital primordium derived from segment A8, the male genital primordium derived from segment A9 and the anal primordium derived from segments A10-11. Each segmental primordium has an anterior (A) and a posterior (P) compartment, the P cells of the three segments being contiguous at the lateral edges of the disc. We show that Hedgehog (Hh) expressed in the P compartment differentially signals A cells at the AP compartment border and A cells at the segmental border. As in the wing imaginal disc, cell lineage restriction of the AP compartment border is defined by Hh signalling. There is also a lineage restriction barrier at the segmental borders, even though the P compartment cells of the three segments converge in the lateral areas of the disc. Lineage restriction between segments A9 and A10-11 depends on factors other than the Hh, En and Hox genes. The segmental borders, however, can be permeable to some morphogenetic signals. Furthermore, cell ablation experiments show that the presence of all primordia (either the anal or the genital primordium) during development are required for normal development of genital disc. Collectively, these findings suggest that interaction between segmental primordia is required for the normal development of the genital disc.     

Vertebrate limb development--the early stages in chick and mouse.

More news this year about FGFs and their roles in vertebrate limb initiation; Wnt signalling is shown for the first time to be another component of the signalling cascade involved in early limb formation. Ectodermal compartments that control apical ridge formation were previously described in chick embryos and are now shown to exist in mouse embryos; Engrailed1 is expressed in the ventral ectodermal compartment but experiments in both chick and mouse show that it is not responsible for compartment specification.     

Local extrinsic signals determine muscle and endothelial cell fate and patterning in the vertebrate limb

Both the muscle and endothelium of the vertebrate limb derive from somites. We have used replication-defective retroviral vectors to analyze the lineage relationships of these somite-derived cells in the chick. We find that myogenic precursors in the somites or proximal limb are not committed to forming slow or fast muscle fibers, particular anatomical muscles, or muscles within specific proximal/distal or dorsal/ventral limb regions. Somitic endothelial precursors are uncommitted to forming endothelium in particular proximal/distal or dorsal/ventral limb regions. Surprisingly, we also find that myogenic and endothelial cells are derived from a common somitic precursor. Thus, local extrinsic signals are critical for determining muscle and endothelial patterning as well as cell fate in the limb.     

心脏的左右不对称发育

心脏是脊椎动物发育过程中最早形成的器官之一,心管向右环化打破了左右对称的格局,是左右分化的第一个重要标志.不对称的心管环化和心脏腔室的形态发生是一个相当复杂的过程,人们对其分子机制,特别是心脏定位和不对称发育机理的了解还相当有限.为了探讨心脏的左右不对称发育,重点从形态学和分子水平对近期的研究作了简要的概述.     

Left-right asymmetry in embryonic development: a comprehensive review

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well characterized, the left-right (LR) axis has only recently begun to be understood at the molecular level. The mechanisms which ensure invariant LR asymmetry of the heart, viscera, and brain represent a thread connecting biomolecular chirality to human cognition, along the way involving fundamental aspects of cell biology, biophysics, and evolutionary biology. An understanding of LR asymmetry is important not only for basic science, but also for the biomedicine of a wide range of birth defects and human genetic syndromes. This review summarizes the current knowledge regarding LR patterning in a number of vertebrate and invertebrate species, discusses several poorly understood but important phenomena, and highlights some important open questions about the evolutionary origin and conservation of mechanisms underlying embryonic asymmetry.     

Existence of gradient in cell adhesiveness along the developing Xenopus hind limb bud, shown by a cellular sorting-out experiment in vitro

To examine the possibility of a difference in cell adhesiveness along the developing Xenopus hind limb bud axes, single mesenchymal cells from developing hind limb buds were cultured, allowing them to form an aggregate in a gyratory culture system. By observing the distribution of cells within aggregates, it was found that sorting-out occurred between cells from different positions and different stages. Cells derived from more distal positions tended to be situated interiorly in the aggregates. According to Steinberg's differential adhesion hypothesis, these results support the idea that there is a graded difference in cell adhesiveness along the proximo-distal axis of the developing limb, with adhesiveness increasing distally. Although similar sorting-out was observed between anterior and posterior cell populations, it could not be determined which cell populations were definitely more cohesive. These properties may be correlated with the experimentally demonstrated 'positional value', which should be different among cells located at different positions along the axes of the developing vertebrate limb bud.     

Lineage analysis in the chicken inner ear shows differences in clonal dispersion for epithelial, neuronal, and mesenchymal cells

The epithelial components of the vertebrate inner ear and its associated ganglion arise from the otic placode. The cell types formed include neurons, hair-cell mechanoreceptors, supporting cells, secretory cells that make endolymphatic fluid or otolithic membranes, and simple epithelial cells lining the fluid-filled cavities. The epithelial sheet is surrounded by an inner layer of connective and vascular tissues and an outer capsule of bone. To explore the mechanisms of cell fate specification in the ear, retrovirus-mediated lineage analysis was performed after injecting virus into the chicken otocyst on embryonic days 2.5-5.5. Because lineage analysis might reveal developmental compartments, an effort was made to study clonal dispersion by sampling infected cells from different parts of the same ear, including the auditory ganglion, cochlea, saccule, utricle, and semicircular canals. Lineage relationships were confirmed for 75 clones by amplification and sequencing of a variable DNA tag carried by each virus. While mesenchymal clones could span different structural parts of the ear, epithelial clones did not. The circumscribed epithelial clones indicated that their progenitors were not highly migratory. Ganglion cell clones, in contrast, were more dispersed. There was no evidence for a common lineage between sensory cells and their associated neurons, a prediction based on a proposal that the ear sensory organs and fly mechanosensory organs are evolutionarily homologous. As expected, placodal derivatives were unrelated to adjacent mesenchymal cells or to nonneuronal cells of the ganglion. Within the otic capsule, fibroblasts and cartilage cells could be related by lineage.     

Establishment of compartments in the developing leg imaginal discs ofDrosophila melanogaster

Summary By X-irradiation ofM/M ⁺ embryos and larvae to induce mitotic recombination, clones ofM ⁺/M⁺ genotype were obtained (Fig. 1). Since such cells grow faster than the surroundingM/M ⁺-cells they can fill large areas within the compartments of an imaginal disc.The present studies concentrated mainly on the three leg discs. Clones were induced by doses of 1000 r at ages ranging from 3±0.5 h after oviposition to 144 h.All clones induced later than the blastoderm stage were absolutely restricted to either the anterior or the posterior compartment of a disc. The border between the anterior and posterior compartment runs as a straight line along the entire leg and at the distal end separates the two claws (Figs. 5, 6, 7). A further subdivision of the anterior compartment is indicated by clones initiated in the second larval instar (Fig. 11). A parallel subdivision could not be detected in the posterior compartment. Irradiation in the early third instar led to clones which were restricted to single longitudinal bristle rows and leg segments. But no clear-cut compartment borders could be found; in particular a proximo-distal separation appears to be absent.Among the 318 clones induced at the blastoderm stage eleven extended from the wing into the second leg (Fig. 8), or from the haltere into the third leg.With the exception of 3 clones that apparently occupied the anterior as well as the posterior compartment of a wing or a leg, all clones remained confined to either the anterior or the posterior compartment.Frequently clones overlapped left and right forelegs (Fig. 9). Intersegmental overlaps were not observed.The results show that the earliest compartment borders appear in all thoracic discs. This suggests that compartmentalization is a fundamental process common to all discs.Supported bySchweizerischer Nationalfonds Gesuch Nr. 3.480-0.75     

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.     

Interplay between the molecular signals that control vertebrate limb development

Vertebrate limbs display three obvious axes of asymmetry. These three axes are referred to as proximal-distal (Pr-D; shoulder to digit tips), anterior-posterior (A-P; thumb to little finger), and dorsal-ventral (D-V; back of hand to palm). At a molecular level, it is now possible to define the signals that control patterning of each of the three axes of the developing limb. These signals do not work in isolation though but rather their activity must be integrated such that the various limb elements are coordinately formed with relation to these three axes. This review will provide an overview of the intricate medley amongst the molecular signals that serve to establish and coordinate patterning information along the three primary axes of the limb.