Cell motility driving mediolateral intercalation in explants of Xenopus laevis.

In Xenopus, convergence and extension are produced by active intercalation of the deep mesodermal cells between one another along the mediolateral axis (mediolateral cell intercalation), to form a narrower, longer array. The cell motility driving this intercalation is poorly understood. A companion paper shows that the endodermal epithelium organizes the outermost mesodermal cells immediately beneath it to undergo convergence and extension, and other evidence suggests that these deep cells are the most active participants in mediolateral intercalation (Shih, J. and Keller, R. (1992) Development 116, 887-899). In this paper, we shave off the deeper layers of mesodermal cells, which allows us to observe the protrusive activity of the mesodermal cells next to the organizing epithelium with high resolution video microscopy. These mesodermal cells divide in the early gastrula and show rapid, randomly directed protrusive activity. At the early midgastrula stage, they begin to express a characteristic sequence of behaviors, called mediolateral intercalation behavior (MIB): (1) large, stable, filiform and lamelliform protrusions form in the lateral and medial directions, thus making the cells bipolar; (2) these protrusions are applied directly to adjacent cell surfaces and exert traction on them, without contact inhibition; (3) as a result, the cells elongate and align parallel to the mediolateral axis and perpendicular to the axis of extension; (4) the elongate, aligned cells intercalate between one another along the mediolateral axis, thus producing a longer, narrower array. Explants of essentially a single layer of deep mesodermal cells, made at stage 10.5, converge and extend by mediolateral intercalation. Thus by stage 10.5 (early midgastrula), expression of MIB among deep mesodermal cells is physiologically and mechanically independent of the organizing influence of the endodermal epithelium, described previously (Shih, J. and Keller, R. (1992) Development 116 887-899), and is the fundamental cell motility underlying mediolateral intercalation and convergence and extension of the body axis.     

Patterns of cell motility in the organizer and dorsal mesoderm of Xenopus laevis.

In a companion paper (Shih, J. and Keller, R. (1992) Development 116, 901-914), we described a sequence of cell behaviors, called mediolateral intercalation behavior (MIB), that produces mediolateral cell intercalation, the process that drives convergence and extension of the axial and paraxial mesoderm of Xenopus. In this paper, we describe the pattern of expression of MIB in the mesoderm during gastrulation, using video image processing and recording of cell behavior in 'shaved', open-faced explants of the marginal zone. At midgastrula stage (10.5), MIB begins at two dorsolateral sites in the prospective anterior mesoderm and progresses medially along two arcs that lengthen toward and meet at the midline to form a single arc of cells expressing MIB, called the vegetal alignment zone (VgAZ). The notochordal-somitic mesodermal boundary forms within the VgAZ at stage 11, and then progresses animally and laterally, along the prospective anterior-posterior axis, eventually bounding a trapezoidal area the shape of the fate-mapped notochord. Meanwhile, from its origin in the VgAZ, MIB spreads in the prospective posterior direction along the lateral boundaries of both the notochordal and somitic mesoderm. From there it spreads medially in both tissues. Subsequently, vacuolation of notochord cells, and segmentation and expression of a somite-specific marker repeat the progression of mediolateral intercalation behavior. Thus cells in the posterior, medial regions of the notochordal and the somitic territories are the last to express mediolateral intercalation behavior and subsequent tissue differentiations. In explants that do not converge, these cells neither express mediolateral intercalation behavior nor differentiate. These facts suggest that progressions of MIB in the anterior-posterior and lateral-medial directions may be organized by signals emanating from the lateral somitic and notochordal boundaries. These signals may have limited range and may be dependent on convergence, driven by mediolateral cell intercalation, to bring cells within their range. In the embryo, the posterior progression of MIB results in arcs of convergence, anchored in the vegetal endoderm at each end, acting on the inside of the blastoporal lip to produce involution of the IMZ.     

The midline (notochord and notoplate) patterns the cell motility underlying convergence and extension of the Xenopus neural plate

We investigated the role of the dorsal midline structures, the notochord and notoplate, in patterning the cell motilities that underlie convergent extension of the Xenopus neural plate. In explants of deep neural plate with underlying dorsal mesoderm, lateral neural plate cells show a monopolar, medially directed protrusive activity. In contrast, neural plate explants lacking the underlying dorsal mesoderm show a bipolar, mediolaterally directed protrusive activity. Here, we report that "midlineless" explants consisting of the deep neural plate and underlying somitic mesoderm, but lacking a midline, show bipolar, mediolaterally oriented protrusive activity. Adding an ectopic midline to the lateral edge of these explants restores the monopolar protrusive activity over the entire extent of the midlineless explant. Monopolarized cells near the ectopic midline orient toward it, whereas those located near the original, removed midline orient toward this midline. This behavior can be explained by two signals emanating from the midline. We postulate that one signal polarizes neural plate deep cells and is labile and short-lived and that the second signal orients any polarized cells toward the midline and is persistent.     

Monopolar protrusive activity: a new morphogenic cell behavior in the neural plate dependent on vertical interactions with the mesoderm in Xenopus

We compared the type and patterning of morphogenic cell behaviors driving convergent extension of the Xenopus neural plate in the presence and absence of persistent vertical signals from the mesoderm by videorecording explants of deep neural tissue with involuted mesoderm attached and of deep neural tissue alone. In deep neural-over-mesoderm explants, neural plate cells express monopolar medially directed motility and notoplate cells express randomly oriented motility, two new morphogenic cell behaviors. In contrast, in deep neural explants (without notoplate), all cells express bipolar mediolateral cell motility. Deep neural-over-mesoderm and deep neural explants also differ in degree of neighbor exchange during mediolateral cell intercalation. In deep neural-over-mesoderm explants, cells intercalate conservatively, whereas in deep neural explants cells intercalate more promiscuously. Last, in both deep neural-over-mesoderm and deep neural explants, morphogenic cell behaviors differentiate in an anterior-to-posterior and lateral-to-medial progression. However, in deep neural-over-mesoderm explants, morphogenic behaviors first differentiate in intervals along the anteroposterior axis, whereas in deep neural explants, morphogenic behaviors differentiate continuously from the anterior end of the tissue posteriorly. These results describe new morphogenic cell behaviors driving neural convergent extension and also define roles for signals from the mesoderm, up to and beyond late gastrulation, in patterning these cell behaviors.     

The presumptive floor plate (notoplate) induces behaviors associated with convergent extension in medial but not lateral neural plate cells of Xenopus

In previous work (Elul, T., Keller, R., 2000. Monopolar protrusive activity: a new morphogenic cell behavior in the neural plate dependent on vertical interactions with the mesoderm in Xenopus. Dev. Biol. 224, 3-19; Ezin, A.M., Skoglund, P. Keller, R. 2003. The midline (notochord and notoplate) patterns the cell motility underlying convergence and extension of the Xenopus neural plate. Dev. Biol. 256, 100-114), the midline tissues of notochord and overlying notoplate were found to induce the monopolar, medially directed protrusive activity of deep neural cells. This behavior is thought to drive the mediolateral intercalation and convergent extension of the neural plate in Xenopus. Here we address the issue of whether the notochord, the notoplate, or both is essential for this induction. Our strategy was to remove the notochord, leaving the overlying notoplate intact, and determine whether it alone can induce the monopolar, medially directed cell behavior. We first establish that the notoplate (presumptive floor plate), when separated from the underlying notochord in the early neurula (stages 13-14), will independently mature into a floor plate as assayed three criteria: (1) continued expression of an early marker, sonic hedgehog, and a later, marker, F-spondin; (2) the display of the notoplate/floor plate-specific randomly oriented protrusive activity; (3) the characteristic lack of mixing of cells between the notoplate and lateral neural plate. Under these conditions, in the presence of a mature notoplate/floor plate and in the absence of the notochord, the characteristic monopolar, medially directed behavior occurred, but only locally near the midline. These results show that the notoplate/floor plate capacity to induce the medially directed motility is limited in range, and they suggest that the notochord is necessary for the normally observed longer range induction in lateral neural plate cells. This work helps to further the understanding of molecular and tissue interactions required for convergent extension.     

Cell rearrangement during gastrulation of Xenopus: direct observation of cultured explants.

We have analyzed cell behavior in the organizer region of the Xenopus laevis gastrula by making high resolution time-lapse recordings of cultured explants. The dorsal marginal zone, comprising among other tissues prospective notochord and somitic mesoderm, was cut from early gastrulae and cultured in a way that permits high resolution microscopy of the deep mesodermal cells, whose organized intercalation produces the dramatic movements of convergent extension. At first, the explants extend without much convergence. This initial expansion results from rapid radial intercalation, or exchange of cells between layers. During the second half of gastrulation, the explants begin to converge strongly toward the midline while continuing to extend vigorously. This second phase of extension is driven by mediolateral cell intercalation, the rearrangement of cells within each layer to lengthen and narrow the array. Toward the end of gastrulation, fissures separate the central notochord from the somitic mesoderm on each side, and cells in both tissues elongate mediolaterally as they intercalate. A detailed analysis of the spatial and temporal pattern of these behaviors shows that both radial and mediolateral intercalation begin first in anterior tissue, demonstrating that the anterior-posterior timing gradient so evident in the mesoderm of the neurula is already forming in the gastrula. Finally, time-lapse recordings of intact embryos reveal that radial intercalation takes places primarily before involution, while mediolateral intercalation begins as the mesoderm goes around the lip. We discuss the significance of these findings to our understanding of both the mechanics of gastrulation and the patterning of the dorsal axis.     

Mediolateral cell intercalation in the dorsal, axial mesoderm of Xenopus laevis

The pattern of mediolateral cell intercalation in mesodermal tissues during gastrulation and neurulation of Xenopus laevis was determined by tracing cells labeled with fluorescein dextran amine (FDA). Patches of the involuting marginal zone (IMZ) of early gastrula stage embryos, labeled by injection of FDA at the one-cell stage, were grafted to the corresponding regions of unlabeled host embryos. The host embryos were fixed at several stages, serially sectioned, and examined with fluorescence microscopy and three-dimensional reconstruction. Patterns of mixing of labeled and unlabeled cells show that mediolateral cell intercalation occurs in the posterior, dorsal mesoderm as this region undergoes convergent extension and differentiates into somites and notochord. In contrast, it does not occur in any dorsoventral sector of the anterior, leading edge of the mesodermal mantle. These results, taken with other evidence, suggest that the mesoderm of Xenopus consists of two subpopulations, each with a characteristic morphogenetic movement, cell behavior, and tissue fate. The migrating mesoderm (1) does not show convergent extension; (2) migrates and spreads on the blastocoel roof; (3) is dependent on this substratum for its morphogenesis; (4) shows little mediolateral intercalation; (5) consists of the anterior, early-involuting region of the mesodermal mantle; and (6) differentiates into head, heart, blood island, and lateral body wall mesoderm. The extending mesoderm (1) shows convergent extension; (2) is independent of the blastocoel roof in its morphogenesis; (3) shows extensive mediolateral intercalation; (4) consists of the posterior, late-involuting parts of the mesodermal mantle; and (5) differentiates into somite and notochord.     

Integrin alpha5beta1 and fibronectin regulate polarized cell protrusions required for Xenopus convergence and extension

BACKGROUND: Integrin recognition of fibronectin is required for normal gastrulation including the mediolateral cell intercalation behaviors that drive convergent extension and the elongation of the frog dorsal axis; however, the cellular and molecular mechanisms involved are unclear. RESULTS: We report that depletion of fibronectin with antisense morpholinos blocks both convergent extension and mediolateral protrusive behaviors in explant preparations. Both chronic depletion of fibronectin and acute disruptions of integrin alpha5beta1 binding to fibronectin increases the frequency and randomizes the orientation of polarized cellular protrusions, suggesting that integrin-fibronectin interactions normally repress frequent random protrusions in favor of fewer mediolaterally oriented ones. In the absence of integrin alpha5beta1 binding to fibronectin, convergence movements still occur but result in convergent thickening instead of convergent extension. CONCLUSIONS: These findings support a role for integrin signaling in regulating the protrusive activity that drives axial extension. We hypothesize that the planar spatial arrangement of the fibrillar fibronectin matrix, which delineates tissue compartments within the embryo, is critical for promoting productive oriented protrusions in intercalating cells.     

The planar cell polarity gene strabismus regulates convergence and extension and neural fold closure in Xenopus

We cloned Xenopus Strabismus (Xstbm), a homologue of the Drosophila planar cell or tissue polarity gene. Xstbm encodes four transmembrane domains in its N-terminal half and a PDZ-binding motif in its C-terminal region, a structure similar to Drosophila and mouse homologues. Xstbm is expressed strongly in the deep cells of the anterior neural plate and at lower levels in the posterior notochordal and neural regions during convergent extension. Overexpression of Xstbm inhibits convergent extension of mesodermal and neural tissues, as well as neural tube closure, without direct effects on tissue differentiation. Expression of Xstbm(DeltaPDZ-B), which lacks the PDZ-binding region of Xstbm, inhibits convergent extension when expressed alone but rescues the effect of overexpressing Xstbm, suggesting that Xstbm(DeltaPDZ-B) acts as a dominant negative and that both increase and decrease of Xstbm function from an optimum retards convergence and extension. Recordings show that cells expressing Xstbm or Xstbm(DeltaPDZ-B) fail to acquire the polarized protrusive activity underlying normal cell intercalation during convergent extension of both mesodermal and neural and that this effect is population size-dependent. These results further characterize the role of Xstbm in regulating the cell polarity driving convergence and extension in Xenopus.     

Planar cell polarity genes regulate polarized extracellular matrix deposition during frog gastrulation

The noncanonical wnt/planar cell polarity (PCP) pathway [1] regulates the mediolaterally (planarly) polarized cell protrusive activity and intercalation that drives the convergent extension movements of vertebrate gastrulation [2], yet the underlying mechanism is unknown. We report that perturbing expression of Xenopus PCP genes, Strabismus (Xstbm), Frizzled (Xfz7), and Prickle (Xpk), disrupts radially polarized fibronectin fibril assembly on mesodermal tissue surfaces, mediolaterally polarized motility, and intercalation. Polarized motility is restored in Xpk-perturbed explants but not in Xstbm- or Xfz7-perturbed explants cultured on fibronectin surfaces. The PCP complex, including Xpk, first regulates polarized surface assembly of the fibronectin matrix, which is necessary for mediolaterally polarized motility, and then, without Xpk, has an additional and necessary function in polarizing motility. These results show that the PCP complex regulates several cell polarities (radial, planar) and several processes (matrix deposition, motility), by indirect and direct mechanisms, and acts in several modes, either with all or a subset of its components, during vertebrate morphogenesis.     

Shaping the zebrafish notochord

Promptly after the notochord domain is specified in the vertebrate dorsal mesoderm, it undergoes dramatic morphogenesis. Beginning during gastrulation, convergence and extension movements change a squat cellular array into a narrow, elongated one that defines the primary axis of the embryo. Convergence and extension might be coupled by a highly organized cellular intermixing known as mediolateral intercalation behavior (MIB). To learn whether MIB drives early morphogenesis of the zebrafish notochord, we made 4D recordings and quantitatively analyzed both local cellular interactions and global changes in the shape of the dorsal mesodermal field. We show that MIB appears to mediate convergence and can account for extension throughout the dorsal mesoderm. Comparing the notochord and adjacent somitic mesoderm reveals that extension can be regulated separately from convergence. Moreover, mutational analysis shows that extension does not require convergence. Hence, a cellular machine separate from MIB that can drive dorsal mesodermal extension exists in the zebrafish gastrula. The likely redundant control of morphogenesis may provide for plasticity at this critical stage of early development.     

XGAP, an ArfGAP, is required for polarized localization of PAR proteins and cell polarity in Xenopus gastrulation

To dissect the molecular mechanisms underlying convergent extension (CE), a prominent set of cell movements during Xenopus gastrulation, we performed a functional expression screen and identified a GTPase-activating protein for ADP ribosylation factors (ArfGAP), which we termed XGAP. We demonstrated that XGAP is required to confine or restrict the cellular protrusive activity to the mediolateral ends of cells, where XGAP is normally localized, and therefore for the proper intercalation of cells participating in CE. We also demonstrated that a C-terminal conserved domain of XGAP, but not its GAP activity, is required and sufficient for this intracellular localization and function. We further showed that XGAP physically interacts with the known polarity proteins 14-3-3epsilon, aPKC, and PAR-6 and directs them to the mediolateral ends of dorsal mesoderm cells during gastrulation. We propose that XGAP controls CE through the restriction and maintenance of partitioning-defective (PAR) proteins in the regions that harbor protrusive activity.     

Cell rearrangement and segmentation in Xenopus: direct observation of cultured explants

We make use of a novel system of explant culture and high resolution video-film recording to analyse for the first time the cell behaviour underlying convergent extension and segmentation in the somitic mesoderm of Xenopus. We find that a sequence of activities sweeps through the somitic mesoderm from anterior to posterior during gastrulation and neurulation, beginning with radial cell intercalation or thinning, continuing with mediolateral intercalation and cell elongation, and culminating in segmentation and somite rotation. Radial intercalation at the posterior tip lengthens the tissue, while mediolateral intercalation farther anterior converges it toward the midline. This extension of the somitic mesoderm helps to elongate the dorsal side of intact neurulae. By separating tissues, we demonstrate that cell rearrangement is independent of the notochord, but radial intercalation - and thus the bulk of extension - requires the presence of an epithelium, either endodermal or ectodermal. Segmentation, on the other hand, can proceed in somitic mesoderm isolated at the end of gastrulation. Finally, we discuss the relationship between cell rearrangement and segmentation.     

Eph regulates dorsoventral asymmetry of the notochord plate and convergent extension-mediated notochord formation

Background

The notochord is a signaling center required for the patterning of the vertebrate embryic midline, however, the molecular and cellular mechanisms involved in the formation of this essential embryonic tissue remain unclear. The urochordate Ciona intestinalis develops a simple notochord from 40 specific postmitotic mesodermal cells. The precursors intercalate mediolaterally and establish a single array of disk-shaped notochord cells along the midline. However, the role that notochord precursor polarization, particularly along the dorsoventral axis, plays in this morphogenetic process remains poorly understood.

Methodology/Principal Findings

Here we show that the notochord preferentially accumulates an apical cell polarity marker, aPKC, ventrally and a basement membrane marker, laminin, dorsally. This asymmetric accumulation of apicobasal cell polarity markers along the embryonic dorsoventral axis was sustained in notochord precursors during convergence and extension. Further, of several members of the Eph gene family implicated in cellular and tissue morphogenesis, only Ci-Eph4 was predominantly expressed in the notochord throughout cell intercalation. Introduction of a dominant-negative Ci-Eph4 to notochord precursors diminished asymmetric accumulation of apicobasal cell polarity markers, leading to defective intercalation. In contrast, misexpression of a dominant-negative mutant of a planar cell polarity gene Dishevelled preserved asymmetric accumulation of aPKC and laminin in notochord precursors, although their intercalation was incomplete.

Conclusions/Significance

Our data support a model in which in ascidian embryos Eph-dependent dorsoventral polarity of notochord precursors plays a crucial role in mediolateral cell intercalation and is required for proper notochord morphogenesis.     

Segmental lineage restrictions in the chick embryo spinal cord depend on the adjacent somites.

We have investigated whether the developing spinal cord is intrinsically segmented in its rostrocaudal (anteroposterior) axis by mapping the spread of clones derived from single labelled cells within the neural tube of the chick embryo. A single cell in the ventrolateral neural tube of the trunk was marked in situ with the fluorescent tracer lysinated rhodamine dextran (LRD) and its descendants located after two days of further incubation. We find that clones derived from cells labelled before overt segmentation of the adjacent mesoderm do not respect any boundaries within the neural tube. Those derived from cells marked after mesodermal segmentation, however, never cross an invisible boundary aligned with the middle of each somite, and tend to be elongated along the mediolateral axis of the neural tube. When the somite pattern is surgically disturbed, neighbouring clones derived from neuroectodermal cells labelled after somite formation behave like clones derived from younger cells: they no longer respect any boundaries, and are not elongated mediolaterally. These results indicate that periodic lineage restrictions do exist in the developing spinal cord of the chick embryo, but their maintenance requires the presence of the adjacent somite mesoderm.     

N-cadherin is required for the polarized cell behaviors that drive neurulation in the zebrafish

Through the direct analysis of cell behaviors, we address the mechanisms underlying anterior neural tube morphogenesis in the zebrafish and the role of the cell adhesion molecule N-cadherin (N-cad) in this process. We demonstrate that although the mode of neurulation differs at the morphological level between amphibians and teleosts, the underlying cellular mechanisms are conserved. Contrary to previous reports, the zebrafish neural plate is a multi-layered structure, composed of deep and superficial cells that converge medially while undergoing radial intercalation, to form a single cell-layered neural tube. Time-lapse recording of individual cell behaviors reveals that cells are polarized along the mediolateral axis and exhibit protrusive activity. In N-cad mutants, both convergence and intercalation are blocked. Moreover, although N-cad-depleted cells are not defective in their ability to form protrusions, they are unable to maintain them stably. Taken together, these studies uncover key cellular mechanisms underlying neural tube morphogenesis in teleosts, and reveal a role for cadherins in promoting the polarized cell behaviors that underlie cellular rearrangements and shape the vertebrate embryo.     

Xenopus Dishevelled signaling regulates both neural and mesodermal convergent extension: parallel forces elongating the body axis

During amphibian development, non-canonical Wnt signals regulate the polarity of intercalating dorsal mesoderm cells during convergent extension. Cells of the overlying posterior neural ectoderm engage in similar morphogenetic cell movements. Important differences have been discerned in the cell behaviors associated with neural and mesodermal cell intercalation, raising the possibility that different mechanisms may control intercalations in these two tissues. In this report, targeted expression of mutants of Xenopus Dishevelled (Xdsh) to neural or mesodermal tissues elicited different defects that were consistent with inhibition of either neural or mesodermal convergent extension. Expression of mutant Xdsh also inhibited elongation of neural tissues in vitro in Keller sandwich explants and in vivo in neural plate grafts. Targeted expression of other Wnt signaling antagonists also inhibited neural convergent extension in whole embryos. In situ hybridization indicated that these defects were not due to changes in cell fate. Examination of embryonic phenotypes after inhibition of convergent extension in different tissues reveals a primary role for mesodermal convergent extension in axial elongation, and a role for neural convergent extension as an equalizing force to produce a straight axis. This study demonstrates that non-canonical Wnt signaling is a common mechanism controlling convergent extension in two very different tissues in the Xenopus embryo and may reflect a general conservation of control mechanisms in vertebrate convergent extension.     

The cytoplasmic tyrosine kinase Arg regulates gastrulation via control of actin organization

Coordinated cell movements are crucial for vertebrate gastrulation and are controlled by multiple signals. Although many factors are shown to mediate non-canonical Wnt pathways to regulate cell polarity and intercalation during gastrulation, signaling molecules acting in other pathways are less investigated and the connections between various signals and cytoskeleton are not well understood. In this study, we show that the cytoplasmic tyrosine kinase Arg modulates gastrulation movements through control of actin remodeling. Arg is expressed in the dorsal mesoderm at the onset of gastrulation, and both gain- and loss-of-function of Arg disrupted axial development in Xenopus embryos. Arg controlled migration of anterior mesendoderm, influenced cell decision on individual versus collective migration, and modulated spreading and protrusive activities of anterior mesendodermal cells. Arg also regulated convergent extension of the trunk mesoderm by influencing cell intercalation behaviors. Arg modulated actin organization to control dynamic F-actin distribution at the cell-cell contact or in membrane protrusions. The functions of Arg required an intact tyrosine kinase domain but not the actin-binding motifs in its carboxyl terminus. Arg acted downstream of receptor tyrosine kinases to regulate phosphorylation of endogenous CrkII and paxillin, adaptor proteins involved in activation of Rho family GTPases and actin reorganization. Our data demonstrate that Arg is a crucial cytoplasmic signaling molecule that controls dynamic actin remodeling and mesodermal cell behaviors during Xenopus gastrulation.     

Polarized basolateral cell motility underlies invagination and convergent extension of the ascidian notochord.

We use 3D time-lapse analysis of living embryos and laser scanning confocal reconstructions of fixed, staged, whole-mounted embryos to describe three-dimensional patterns of cell motility, cell shape change, cell rearrangement and tissue deformation that accompany formation of the ascidian notochord. We show that notochord formation involves two simultaneous processes occurring within an initially monolayer epithelial plate: The first is invagination of the notochord plate about the axial midline to form a solid cylindrical rod. The second is mediolaterally directed intercalation of cells within the plane of the epithelial plate, and then later about the circumference of the cylindrical rod, that accompanies its extension along the anterior/posterior (AP) axis. We provide evidence that these shape changes and rearrangements are driven by active extension of interior basolateral notochord cell edges directly across the faces of their adjacent notochord neighbors in a manner analogous to leading edge extension of lamellapodia by motile cells in culture. We show further that local edge extension is polarized with respect to both the AP axis of the embryo and the apicobasal axis of the notochord plate. Our observations suggest a novel view of how active basolateral motility could drive both invagination and convergent extension of a monolayer epithelium. They further reveal deep similarities between modes of notochord morphogenesis exhibited by ascidians and other chordate embryos, suggesting that cellular mechanisms of ascidian notochord formation may operate across the chordate phylum.