Developmental Fate of Artificially Disordered Somite Cells after Heteroplastic Transplantation

A disordered somite pattern could be produced artificially when the segmental lateral plate of chickembryo was replaced by dissociated cells of quail segmental pate.The artificially disordered somitepattern formed at either place was used in our work as a model to analyze the mechanism of thedevelopment and differentiation of somite on chick embryo.Our conclusions include the following:1.Although the formation of somites from the dissociated segmental plate cells does not requirespecial environment,the development and differentiation of the somltes require a special environmentwhich is related to the neural tube and notochord.The effect of this special environmental factor maydecrease gradually with the increase of the distance from neural tube to lateral plate.2.The somites located on paraxial area at different distances to the axis have different fates indevelopment.3.The formation of epithelial vesicles is the property of somite cells and the epithelial vesicle is thestructural basis of somite differentiation.If and factor interferes with the differentiation of thesomite,the epithelial vesicle of the somite will be degenerated within certain period of time.4.During resegmentation of the somite,the number,size and arrangement of sclerotome in situ donot depend on the somite from which they are derived.5.Somite cells do not transdifferentiate into kidney tubule directly from their original epithelialvesicles,but are reorganized from the free cells dispersed from the disrupted somites.6.The establishment of cell commitment may involve several steps.Before commitment isestablished the of cell commitment is labile.7.The differentiation of sclerotome starts with the rupture of epithelial wall of somites and thedirection of its movement depends not only on the notochord but also on their position with respectto the neural tube and notochord.8.The disordered somite pattern doesn't influence the segmentation of dorsal root ganglia in situ,but causes the formation of the ectopic dorsal root ganglia.Key Words:Somite differentiation;Artificial disordered somite pattern;Chimeral somite;Resegmentation of sclerotome;Distribution of dorsal root ganglia     

Genetic analysis of interactions between the somitic muscle, cartilage and tendon cell lineages during mouse development

Proper formation of the musculoskeletal system requires the coordinated development of the muscle, cartilage and tendon lineages arising from the somitic mesoderm. During early somite development, muscle and cartilage emerge from two distinct compartments, the myotome and sclerotome, in response to signals secreted from surrounding tissues. As the somite matures, the tendon lineage is established within the dorsolateral sclerotome, adjacent to and beneath the myotome. We examine interactions between the three lineages by observing tendon development in mouse mutants with genetically disrupted muscle or cartilage development. Through analysis of embryos carrying null mutations in Myf5 and Myod1, hence lacking both muscle progenitors and differentiated muscle, we identify an essential role for the specified myotome in axial tendon development, and suggest that absence of tendon formation in Myf5/Myod1 mutants results from loss of the myotomal FGF proteins, which depend upon Myf5 and Myod1 for their expression, and are required, in turn, for induction of the tendon progenitor markers. Our analysis of Sox5/Sox6 double mutants, in which the chondroprogenitors are unable to differentiate into cartilage, reveals that the two cell fates arising from the sclerotome, axial tendon and cartilage are alternative lineages, and that cartilage differentiation is required to actively repress tendon development in the dorsolateral sclerotome.     

Regulating mechanical tension at compartment boundaries in Drosophila

During animal development, cells with similar function and fate often stay together and sort out from cells with different fates. In Drosophila wing imaginal discs, cells of anterior and posterior fates are separated by a straight compartment boundary. Separation of anterior and posterior cells requires the homeodomain-containing protein Engrailed, which is expressed in posterior cells. Engrailed induces the expression of the short-range signaling molecule Hedgehog in posterior cells and confines Hedgehog signal transduction to anterior cells. Transduction of the Hedgehog signal in anterior cells is required for the separation of anterior and posterior cells. Previous work showed that this separation of cells involves a local increase in mechanical tension at cell junctions along the compartment boundary. However, how mechanical tension was locally increased along the compartment boundary remained unknown. A recent paper now shows that the difference in Hedgehog signal transduction between anterior and posterior cells is necessary and sufficient to increase mechanical tension. The local increase in mechanical tension biases junctional rearrangements during cell intercalations to maintain the straight shape of the compartment boundary. These data highlight how developmental signals can generate patterns of mechanical tension important for tissue organization.     

Compartmentalization of the somite and myogenesis in chick embryos are influenced by wnt expression

Muscles of the body and bones of the axial skeleton derive from specialized regions of somites. Somite development is influenced by adjacent structures. In particular, the dorsal neural tube and the overlying ectoderm have been shown to be necessary for the induction of myogenic precursor cells in the dermomyotome. Members of the Wnt family of signaling molecules, which are expressed in the dorsal neural tube and the ectoderm, are postulated to be responsible for this process. It is shown here that ectopically implanted Wnt-1-, -3a-, and -4-expressing cells alter the process of somite compartmentalization in vivo. An enlarged dorsal compartment results from the implantation of Wnt-expressing cells ventrally between the neural tube/notochord and epithelial somites, at the expense of the ventral compartment, the sclerotome. Thus, ectopic Wnt expression is able to override the influence of ventralizing signals arising from notochord and floor plate. This shift of the border between the two compartments was identified by an increase in the domain of Pax-3 expression and a complete loss of Pax-1 expression in somites close to the ectopic Wnt signal. The expanded expression of MyoD and desmin provides evidence that it is the myotome which increases as a result of Wnt signaling. Paraxis expression is also drastically amplified after implantation of Wnt-expressing cells indicating that Wnts are involved in the formation and maintenance of somite epithelium and suggesting that Paraxis is activated through Wnt signaling pathways. Taken together these results suggest that ectopic Wnts disturb the normal balance of signaling molecules within the somite, resulting in an enhanced recruitment of somitic cells into the myogenic lineage.     

Divergent regulation of Wnt-mediated development of the dorsomedial and ventrolateral dermomyotomal lips

The dermomyotome is the dorsal compartment of the somite which gives rise to multiple cell fates including skeletal muscle, connective tissue, and endothelia. It consists of a pseudostratified, roughly rectangular epithelial sheet, the margins of which are called the dermomyotomal lips. The dermomyotomal lips are blastema-like epithelial growth zones, which continuously give rise to resident dermomyotomal cells and emigrating muscle precursor cells, which populate the subjacent myotomal compartment. Wnt signaling has been shown to regulate both dermomyotome formation and maintenance of the dermomyotomal lips. Whereas the epithelialization of the dermomyotome is regulated via canonical, β-catenin-dependent Wnt signaling, the downstream signaling mechanisms suppressing epithelial-mesenchymal transition (EMT) in the mature dermomyotomal lips have been unknown. Here, we present evidence that dermomyotomal lip sustainment is differentially regulated. Whereas the dorsomedial dermomyotomal lip is maintained by canonical Wnt signaling, development of the ventrolateral dermomyotomal lip is regulated by non-canonical, PCP-like Wnt signaling. We discuss our results in the light of the different developmental prerequisites in the dorsomedial and ventrolateral lips, respectively, thus providing a new perspective on the regulation of dermomyotomal EMT.     

The forkhead genes, Foxc1 and Foxc2, regulate paraxial versus intermediate mesoderm cell fate

During vertebrate embryogenesis, the newly formed mesoderm is allocated to the paraxial, intermediate, and lateral domains, each giving rise to different cell and tissue types. Here, we provide evidence that the forkhead genes, Foxc1 and Foxc2, play a role in the specification of mesoderm to paraxial versus intermediate fates. Mouse embryos lacking both Foxc1 and Foxc2 show expansion of intermediate mesoderm markers into the paraxial domain, lateralization of somite patterning, and ectopic and disorganized mesonephric tubules. In gain of function studies in the chick embryo, Foxc1 and Foxc2 negatively regulate intermediate mesoderm formation. By contrast, their misexpression in the prospective intermediate mesoderm appears to drive cells to acquire paraxial fate, as revealed by expression of the somite markers Pax7 and Paraxis. Taken together, the data indicate that Foxc1 and Foxc2 regulate the establishment of paraxial versus intermediate mesoderm cell fates in the vertebrate embryo.     

Patterning of angiogenesis in the zebrafish embryo

Little is known about how vascular patterns are generated in the embryo. The vasculature of the zebrafish trunk has an extremely regular pattern. One intersegmental vessel (ISV) sprouts from the aorta, runs between each pair of somites, and connects to the dorsal longitudinal anastomotic vessel (DLAV). We now define the cellular origins, migratory paths and cell fates that generate these metameric vessels of the trunk. Additionally, by a genetic screen we define one gene, out of bounds (obd), that constrains this angiogenic growth to a specific path. We have performed lineage analysis, using laser activation of a caged dye and mosaic construction to determine the origin of cells that constitute the ISV. Individual angioblasts destined for the ISVs arise from the lateral posterior mesoderm (LPM), and migrate to the dorsal aorta, from where they migrate between somites to their final position in the ISVs and dorsal longitudinal anastomotic vessel (DLAV). Cells of each ISV leave the aorta only between the ventral regions of two adjacent somites, and migrate dorsally to assume one of three ISV cell fates. Most dorsal is a T-shaped cell, based in the DLAV and branching ventrally; the second constitutes a connecting cell; and the third an inverted T-shaped cell, based in the aorta and branching dorsally. The ISV remains between somites during its ventral course, but changes to run mid-somite dorsally. This suggests that the pattern of ISV growth ventrally and dorsally is guided by different cues. We have also performed an ENU mutagenesis screen of 750 mutagenized genomes and identified one mutation, obd that disrupts this pattern. In obd mutant embryos, ISVs sprout precociously at abnormal sites and migrate anomalously in the vicinity of ventral somite. The dorsal extent of the ISV is less perturbed. Precocious sprouting can be inhibited in a VEGF morphant, but the anomalous site of origin of obd ISVs remains. In mosaic embryos, obd somite causes adjacent wild-type endothelial cells to assume the anomalous ISV pattern of obd embryos. Thus, the launching position of the new sprout and its initial trajectory are directed by inhibitory signals from ventral somites. Zebrafish ISVs are a tractable system for defining the origins and fates of vessels, and for dissecting elements that govern patterns of vessel growth.     

Sclerotomal origin of vascular smooth muscle cells and pericytes in the embryo

We previously demonstrated that progenitors of both endothelium and smooth muscle cells in the aortic wall originated from the somite in the trunk of the embryo. However whether the contribution to vascular Smooth Muscle Cells (vSMC) is restricted to the aorta or encompasses other vessels of the trunk is not known. Moreover, the somitic compartment that gives rise to vSMC is yet to be defined. Quail-chick orthotopic transplantations of either the segmental plate or the dorsal or ventral halves from single somites demonstrate that 1° vSMC of the body wall including those of the limbs originate from the somite. 2° Like vSMC, aortic pericytes originate from the somite. 3° The sclerotome is the somite compartment that gives rise to vSMC and pericytes. PAX1 and FOXC2, two molecular markers of the sclerotomal compartment, are expressed by vSMC and pericytes during the earliest phases of vascular wall formation. Later on, PDGFR-β and MYOCARDIN are also expressed by these cells. In contrast, the dermomyotome gives rise to endothelium but never to cells in the vascular wall. Taken together, out data point out to the critical role of the somite in vessel formation and demonstrate that vSMC and endothelial cells originate from two independent somitic compartments.     

The plant apoplasm is an important recipient compartment for nematode secreted proteins

Similarly to microbial pathogens, plant-parasitic nematodes secrete into their host plants proteins that are essential to establish a functional interaction. Identifying the destination of nematode secreted proteins within plant cell compartment(s) will provide compelling clues on their molecular functions. Here the fine localization of five nematode secreted proteins was analysed throughout parasitism in Arabidopsis thaliana. An immunocytochemical method was developed that preserves both the host and the pathogen tissues, allowing the localization of nematode secreted proteins within both organisms. One secreted protein from the amphids and three secreted proteins from the subventral oesophageal glands involved in protein degradation and cell wall modification were secreted in the apoplasm during intercellular migration and to a lower extent by early sedentary stages during giant cell formation. Conversely, another protein produced by both subventral and dorsal oesophageal glands in parasitic stages accumulated profusely at the cell wall of young and mature giant cells. In addition, secretion of cell wall-modifying proteins by the vulva of adult females suggested a role in egg laying. The study shows that the plant apoplasm acts as an important destination compartment for proteins secreted during migration and during sedentary stages of the nematode.     

Drosophila wing development in the absence of dorsal identity

The developing wing disc of Drosophila is divided into distinct lineage-restricted compartments along both the anterior/posterior (A/P) and dorsal/ventral (D/V) axes. At compartment boundaries, morphogenic signals pattern the disc epithelium and direct appropriate outgrowth and differentiation of adult wing structures. The mechanisms by which affinity boundaries are established and maintained, however, are not completely understood. Compartment-specific adhesive differences and inter-compartment signaling have both been implicated in this process. The selector gene apterous (ap) is expressed in dorsal cells of the wing disc and is essential for D/V compartmentalization, wing margin formation, wing outgrowth and dorsal-specific wing structures. To better understand the mechanisms of Ap function and compartment formation, we have rescued aspects of the ap mutant phenotype with genes known to be downstream of Ap. We show that Fringe (Fng), a secreted protein involved in modulation of Notch signaling, is sufficient to rescue D/V compartmentalization, margin formation and wing outgrowth when appropriately expressed in an ap mutant background. When Fng and alphaPS1, a dorsally expressed integrin subunit, are co-expressed, a nearly normal-looking wing is generated. However, these wings are entirely of ventral identity. Our results demonstrate that a number of wing development features, including D/V compartmentalization and wing vein formation, can occur independently of dorsal identity and that inter-compartmental signaling, refined by Fng, plays the crucial role in maintaining the D/V affinity boundary. In addition, it is clear that key functions of the ap selector gene are mediated by only a small number of downstream effectors.     

Developmental regulation of somite derivatives: muscle,cartilage and tendon

Recent research has broadened significantly our understanding of how the somite, a specialized mesodermal structure found in vertebrate embryos, gives rise to the cartilage, muscle and tendon cell lineages. The specification of somite derivatives involves the action of patterning signals secreted from adjacent tissue combined with the activation, in particular somitic compartments, of genes promoting cell lineage specification.     

EGF receptor attenuates Dpp signaling and helps to distinguish the wing and leg cell fates in Drosophila

Wing and leg precursors of Drosophila are recruited from a common pool of ectodermal cells expressing the homeobox gene Dll. Induction by Dpp promotes this cell fate decision toward the wing and proximal leg. We report here that the receptor tyrosine kinase EGFR antagonizes the wing-promoting function of Dpp and allows recruitment of leg precursor cells from uncommitted ectodermal cells. By monitoring the spatial distribution of cells responding to Dpp and EGFR, we show that nuclear transduction of the two signals peaks at different position along the dorsoventral axis when the fates of wing and leg discs are specified and that the balance of the two signals assessed within the nucleus determines the number of cells recruited to the wing. Differential activation of the two signals and the cross talk between them critically affect this cell fate choice.     

Cell fate choices in Drosophila tracheal morphogenesis

The Drosophila tracheal system is a branched tubular structure that supplies air to target tissues. The elaborate tracheal morphology is shaped by two linked inductive processes, one involving the choice of cell fates, and the other a guided cell migration. We will describe the molecular basis for these processes, and the allocation of cell fate decisions to four temporal hierarchies. First, tracheal placodes are specified within the embryonic ectoderm. Subsequently, branch fates are allocated within the tracheal placodes, prior to migration. Localized presentation of the FGF ligand, Branchless, to tracheal cells that express the FGF receptor, Breathless, guides migration. Once cell migration is initiated, distinct cell fates are determined within each migrating branch. Finally, inhibitory feedback mechanisms ensure the correct assignment of these fates. Tracheal cell fate choices are determined by signaling cascades triggered by signals emanating from the tracheal cells, as well as by ligands produced by adjacent tissues.     

A gradient of Shh establishes mutually repressing somitic cell fates induced by Nkx3.2 and Pax3

Wnt and Sonic Hedgehog (Shh) signals are known to pattern the somite into dermomyotomal, myotomal and sclerotomal cell fates. By employing explants of presomitic mesoderm cultured with constant levels of Wnt3a conditioned medium and increasing levels of Shh, we found that differing levels of Shh signaling elicit differing responses from somitic cells: the lowest level of Shh signaling allows dermomyotomal gene expression, intermediate levels induce loss of dermomyotomal markers and activation of myogenic differentiation, and higher levels induce loss of myotomal markers and activation of sclerotomal gene expression. In addition, we have found that in the presence of high levels of Wnt signaling, instead of inducing sclerotomal markers, Shh signals act to maintain the expression of dermomyotomal and myotomal markers. One of the sclerotomal genes induced by high levels of Shh signaling is Nkx3.2. Forced expression of Nkx3.2 blocks somitic expression of the dermomyotomal marker Pax3 both in vitro and in vivo. Conversely, forced expression of Pax3 in somites can block Shh-mediated induction of sclerotomal gene expression and chondrocyte differentiation in vitro. Thus we propose that varying levels of Shh signaling act in a morphogen-like manner to elicit differing responses from somitic cells, and that Pax3 and Nkx3.2 set up mutually repressing cell fates that promote either dermomyotome/myotome or sclerotome differentiation, respectively.     

Extrinsic influences on limb muscle organisation

Skeletal muscle within the vertebrate limb originates from the somite. Much work has focussed upon the role of secreted signalling molecules of the Hedgehog, fibroblast growth factor (FGF), bone morphogenetic protein (BMP), and Wnt families plus their associated antagonists in establishing somitic cell types, yet there is no consensus on how these signals combine to influence muscle patterning. When somitic cells migrate into the limb bud, they become subject to a new set of guidance and patterning cues. Here we discuss the possible roles played by signalling proteins, particularly Hedgehogs, in guiding the cells of the limb musculature to their fate.