Reconstitution of the organizer is both sufficient and required to re-establish a fully patterned body plan in avian embryos

Cell-to-cell heterogeneity in Sox2 and Bra expression guides progenitor motility and destiny

Although cell-to-cell heterogeneity in gene and protein expression within cell populations has been widely documented, we know little about its biological functions. By studying progenitors of the posterior region of bird embryos, we found that expression levels of transcription factors Sox2 and Bra, respectively involved in neural tube (NT) and mesoderm specification, display a high degree of cell-to-cell heterogeneity. By combining forced expression and downregulation approaches with time-lapse imaging, we demonstrate that Sox2-to-Bra ratio guides progenitor's motility and their ability to stay in or exit the progenitor zone to integrate neural or mesodermal tissues. Indeed, high Bra levels confer high motility that pushes cells to join the paraxial mesoderm, while high levels of Sox2 tend to inhibit cell movement forcing cells to integrate the NT. Mathematical modeling captures the importance of cell motility regulation in this process and further suggests that randomness in Sox2/Bra cell-to-cell distribution favors cell rearrangements and tissue shape conservation.

Identification of emergent motion compartments in the amniote embryo

The tissue scale deformations (≥ 1 mm) required to form an amniote embryo are poorly understood. Here, we studied ∼400 μm-sized explant units from gastrulating quail embryos. The explants deformed in a reproducible manner when grown using a novel vitelline membrane-based culture method. Time-lapse recordings of latent embryonic motion patterns were analyzed after disk-shaped tissue explants were excised from three specific regions near the primitive streak: 1) anterolateral epiblast, 2) posterolateral epiblast, and 3) the avian organizer (Hensen's node). The explants were cultured for 8 hours-an interval equivalent to gastrulation. Both the anterolateral and the posterolateral epiblastic explants engaged in concentric radial/centrifugal tissue expansion. In sharp contrast, Hensen's node explants displayed Cartesian-like, elongated, bipolar deformations-a pattern reminiscent of axis elongation. Time-lapse analysis of explant tissue motion patterns indicated that both cellular motility and extracellular matrix fiber (tissue) remodeling take place during the observed morphogenetic deformations. As expected, treatment of tissue explants with a selective Rho-Kinase (p160ROCK) signaling inhibitor, Y27632, completely arrested all morphogenetic movements. Microsurgical experiments revealed that lateral epiblastic tissue was dispensable for the generation of an elongated midline axis- provided that an intact organizer (node) is present. Our computational analyses suggest the possibility of delineating tissue-scale morphogenetic movements at anatomically discrete locations in the embryo. Further, tissue deformation patterns, as well as the mechanical state of the tissue, require normal actomyosin function. We conclude that amniote embryos contain tissue-scale, regionalized morphogenetic motion generators, which can be assessed using our novel computational time-lapse imaging approach. These data and future studies-using explants excised from overlapping anatomical positions-will contribute to understanding the emergent tissue flow that shapes the amniote embryo.

Left-right asymmetry in the vertebrate embryo: from early information to higher-level integration

Although vertebrates seem to be essentially bilaterally symmetrical on the exterior, there are numerous interior left-right asymmetries in the disposition and placement of internal organs. These asymmetries are established during embryogenesis by complex epigenetic and genetic cascades. Recent studies in a range of model organisms have made important progress in understanding how this laterality information is generated and conveyed to large regions of the embryo. Both commonalities and divergences are emerging in the mechanisms that different vertebrates use in left-right axis specification. Recent evidence also provides intriguing links between the establishment of left-right asymmetries and the symmetrical elongation of the anterior-posterior axis.

Intercalary regeneration in planarians

How can a planarian regenerate its entire body from a small portion of its body? Neoblasts, the totipotent stem cells of planarian, are assumed to be able to produce all missing cell types. However, we do not know how the cell fate of these cells is controlled during regeneration. Our recent studies with molecular markers suggest that intercalary regeneration is the fundamental principle in planarian regeneration. Here, we introduce the intercalation induced by ectopic grafting along the anteroposterior (A-P), dorsoventral (D-V), and left-right (L-R) axes. Blastema formation is evoked by ectopic D-V interactions after wound closure. Intercalation between the blastema and stump induces rearrangement of the positional identities along the A-P axis. Consequently, totipotent stem cells change their differentiation patterns according to the newly rearranged positional identities along the A-P, D-V, and L-R axes. According to the classic view, the blastema is regarded as the place where undifferentiated cells accumulate and regenerative events occur. Here, we propose a new interpretation, i.e., that the blastema may work as a signaling center inducing intercalary regeneration. Also, the roles of molecules and genes involved in intercalary regeneration are discussed.

Mediolateral intercalation in planarians revealed by grafting experiments

We investigated how planarians organize their left-right axis by using ectopic grafting. Planarians have three body axes: anteroposterior (A-P), dorsoventral (D-V), and left-right (L-R). When a small piece is implanted into an ectopic region along the A-P and D-V axes, intercalary structures are always formed to compensate for positional gaps. There are two hypotheses regarding L-R axis formation in this organism: first, that the left and right sides of the animal may be recognized as different parts, and L-R intercalation can induce midline structures (asymmetry hypothesis); second, that both sides may have symmetrical positional values, and mediolateral (M-L) intercalation creates positional values along the L-R axis (symmetry hypothesis). We performed ectopic grafting experiments in the head region of the planarian, Dugesia japonica, to examine these hypotheses. A left lateral fragment containing a left auricle was implanted into the medial region of the host. Ectopic structures were always formed only on the left side of the graft, where lateral tissues abutted onto the medial tissues. However, no morphologic change was induced on the right side of the graft, where left-sided tissues faced onto right-sided tissues. Molecular marker analyses indicated that ectopic structures formed on the left side of the graft were induced by M-L intercalation, supporting the "symmetry hypothesis." When the midline tissues were implanted into a lateral region, they induced a complete ectopic head, demonstrating that M-L intercalation may be sufficient to establish the L-R axis in planarians.

Wherefore heart thou? Embryonic origins of cardiogenic mesoderm

The developing heart in avian embryos has been examined extensively over the past several decades using classic embryologic and, more recently, molecular and genetic approaches. Still, conflicting reports arise as to the location and regulation of early heart progenitors in the embryo. In addition, a new source of cardiomyocytes has been identified recently that contributes to the outflow tract after the heart initially forms. The focus of this review is the examination of the tissue interactions, signaling molecules, and gene regulatory mechanisms that, together, control heart formation from primary and secondary heart forming fields of the embryo. Early studies of the induction and regulation of the secondary heart field indicate that at least some of the events of primary cardiomyogenesis are recapitulated when the conotruncal myocardium is recruited into the heart. The consideration of classic embryologic studies of the heart forming fields in terms of modern molecular and genetic tools provides reinforcing evidence for the location of cardiac progenitors in the embryo. The accurate definition of early cardiac regulatory events provides a necessary foundation for the generation of new therapeutic sources of cardiomyocytes.

Cell interactions underlying notochord induction and formation in the chick embryo

The development of the notochord in the chick is traditionally associated with Hensen's node (the avian equivalent of the organizer). However, recent evidence has shown that two areas outside the node (called the inducer and responder) are capable of interacting after ablation of Hensen's node to form a notochord. It was not clear from these studies what effect (if any) signals from these areas had on normal notochord formation. A third area, the postnodal region, may also contribute to notochord formation, although this has also been questioned. Using transection and grafting experiments, we have evaluated the timing and cellular interactions involved in notochord induction and formation in the chick embryo. Our results indicate that the rostral primitive streak, including the node, is not required for formation of the notochord in rostral blastoderm isolates transected at stages 3a/b. In addition, neither the postnodal region nor the inducer is required for the induction and formation of the most rostral notochordal cells. However, inclusion of the inducer results in considerable elongation of the notochord in this experimental paradigm. Our results also demonstrate that the responder per se is not required for notochord formation, provided that at least the inducer and postnodal region are present, although in the absence of the responder, formation of the notochord occurs far less frequently. We also show that the node is not specified to form notochord until stage 4 and concomitant with this, the inducer loses its ability to induce notochord from the responder. The coincident timing of these changes in the node and inducer suggests that notochord specification and the activity of the inducer are regulated through a negative feedback loop. We propose a model relating our results to the induction of head and trunk organizer activity in the node.

Molecular genetic control of axis patterning during early embryogenesis of vertebrates

Formation of the axis and its subsequent patterning to establish the tube-within-a-tube body plan characteristic of vertebrates are initiated during gastrulation. In higher vertebrates (i.e., birds and mammals), gastrulation involves six key events: establishment of the rostrocaudal/mediolateral axis; formation and progression of the primitive streak and organizer; epiboly of the epiblast, ingression of prospective mesodermal and endodermal cells through the primitive streak, and migration of cells away from the primitive streak; regression of the primitive streak; establishment of the right-left axis; and formation of the tail bud. Over 50 years of study of these processes have provided a morphological framework for understanding how these events occur, and recent advances in imaging, microsurgical intervention, and cell tracking are beginning to elucidate the underlying cell behaviors that drive morphogenetic movements. Moreover, homotopic transplantation and dye microinjection studies are being used to generate high-resolution fate maps, and heterotopic transplantation studies are revealing the cell-cell interactions that are sufficient as well as required for mesodermal and ectodermal commitment. Additionally, the roles of the organizer and secondary signaling centers in establishing the body plan are being defined. With the advent of the molecular/genetic age, the molecular basis for axis formation is beginning to become understood. Thus, it is becoming clear that secreted growth factors/signaling molecules produced by localized signaling centers induce and pattern the axis, presumably through downstream activation of signal-transduction proteins and cascades of transcription factors.

Axis-inducing activities and cell fates of the zebrafish organizer

We have investigated axis-inducing activities and cellular fates of the zebrafish organizer using a new method of transplantation that allows the transfer of both deep and superficial organizer tissues. Previous studies have demonstrated that the zebrafish embryonic shield possesses classically defined dorsal organizer activity. When we remove the morphologically defined embryonic shield, embryos recover and are completely normal by 24 hours post-fertilization. We find that removal of the morphological shield does not remove all goosecoid- and floating head-expressing cells, suggesting that the morphological shield does not comprise the entire organizer region. Complete removal of the embryonic shield and adjacent marginal tissue, however, leads to a loss of both prechordal plate and notochord. In addition, these embryos are cyclopean, show a significant loss of floor plate and primary motorneurons and display disrupted somite patterning. Motivated by apparent discrepancies in the literature we sought to test the axis-inducing activity of the embryonic shield. A previous study suggested that the shield is capable of only partial axis induction, specifically being unable to induce the most anterior neural tissues. Contrary to this study, we find shields can induce complete secondary axes when transplanted into host ventral germ-ring. In induced secondary axes donor tissue contributes to notochord, prechordal plate and floor plate. When explanted shields are divided into deep and superficial fragments and separately transplanted we find that deep tissue is able to induce the formation of ectopic axes with heads but lacking posterior tissues. We conclude that the deep tissue included in our transplants is important for proper head formation.

Evidence that translation of smooth muscle alpha-actin mRNA is delayed in the chick promyocardium until fusion of the bilateral heart-forming regions

Heart development in the chick embryo proceeds from bilateral mesodermal primordia established during gastrulation. These primordia migrate to the midline and fuse into a single heart trough. During their migration as a cohesive sheet, the cells of the paired heart fields become epithelial and undergo cardiac differentiation, exhibiting organized myofibrils and rhythmic contractions near the time of their fusion. Between the stages of cardiomyoblast commitment and overt differentiation of cardiomyocytes, a significant time interval exists. Using a new riboprobe (usmaar) for whole-mount in situ hybridization in chick embryos, we report the earliest phases of smooth muscle alpha-actin (smaa) mRNA distribution during the precontractile developmental window. We show that ingressed heart-forming regions express smaa by the head-process stage (Hamburger and Hamilton stage 5). In addition, we used usmaar to study the formation and early morphogenesis of the heart. Consistent with fate mapping studies (Garcia-Martinez and Schoenwolf [1993] Dev. Biol. 159:706-719; Schoenwolf and Garcia-Martinez [1995] Cell Mol. Biol. Res. 41:233-240; Garcia-Martinez et al., in preparation), our results with this probe, combined with detailed histological and SEM analyses of the so-called cardiac crescent, demonstrate unequivocally that the heart arises from separated and paired heart rudiments, rather than from a single crescent-shaped rudiment (that is, prior to fusion of the paired heart rudiments to establish the straight-heart tube, the rostral midline of the cardiac crescent lacks mesodermal cells and consequently fails to label with usmaar). Smaa is also expressed in the splanchnic and somatic mesoderm, marking the earliest step in coelom formation. Consequently, we also used usmaar to describe formation of the pericardium. Finally, we provide evidence of a post-transcriptional level of control of smaa gene expression in the heart fields. Our results suggest that the expression of smaa may mark a primitive mesodermal state from which definitive cell types can be derived through inductive events.

Cell movements during gastrulation: come in and be induced

The conversion of an epithelial monolayer into a multilayered structure consisting of the three germ layers, ectoderm, mesoderm and endoderm, constitutes a conserved theme in the early development of animals. This is accomplished by morphogenetic movements that occur during gastrulation and serve not only to generate shape but also to ensure that cells receive the right signals at the right time. Recent evidence of the role of molecular interactions facilitated by cell movements in continuously defining the chick 'organizer' during gastrulation challenges the notion that it is a fixed cell population derived from an exclusive cell lineage.

Defining subregions of Hensen's node essential for caudalward movement, midline development and cell survival

Hensen's node, also called the chordoneural hinge in the tail bud, is a group of cells that constitutes the organizer of the avian embryo and that expresses the gene HNF-3(β). During gastrulation and neurulation, it undergoes a rostral-to-caudal movement as the embryo elongates. Labeling of Hensen's node by the quail-chick chimera system has shown that, while moving caudally, Hensen's node leaves in its wake not only the notochord but also the floor plate and a longitudinal strand of dorsal endodermal cells. In this work, we demonstrate that the node can be divided into functionally distinct subregions. Caudalward migration of the node depends on the presence of the most posterior region, which is closely apposed to the anterior portion of the primitive streak as defined by expression of the T-box gene Ch-Tbx6L. We call this region the axial-paraxial hinge because it corresponds to the junction of the presumptive midline axial structures (notochord and floor plate) and the paraxial mesoderm. We propose that the axial-paraxial hinge is the equivalent of the neuroenteric canal of other vertebrates such as Xenopus. Blocking the caudal movement of Hensen's node at the 5- to 6-somite stage by removing the axial-paraxial hinge deprives the embryo of midline structures caudal to the brachial level, but does not prevent formation of the neural tube and mesoderm located posteriorly. However, the whole embryonic region generated posterior to the level of Hensen's node arrest undergoes widespread apoptosis within the next 24 hours. Hensen's node-derived structures (notochord and floor plate) thus appear to produce maintenance factor(s) that ensures the survival and further development of adjacent tissues.

Molecular interactions continuously define the organizer during the cell movements of gastrulation

The organizer is a unique region in the gastrulating embryo that induces and patterns the body axis. It arises before gastrulation under the influence of the Nieuwkoop center. We show that during gastrulation, cell movements bring cells into and out of the chick organizer, Hensen's node. During these movements, cells acquire and lose organizer properties according to their position. A "node inducing center," which emits Vg1 and Wnt8C, is located in the middle of the primitive streak. Its activity is inhibited by ADMP produced by the node and by BMPs at the periphery. These interactions define the organizer as a position in the embryo, whose cellular makeup is constantly changing, and explain the phenomenon of organizer regeneration.