Hensen's Node Research Articles

Expression and evolution of the Tiki1 and Tiki2 genes in vertebrates.

Tiki1 is a Wnt protease and antagonist specifically expressed in the Spemann-Mangold Organizer and is required for head formation in Xenopus embryos. Here we report neighbor-joining phylogenetic analysis of vertebrate Tiki genes and their mRNA expression patterns in chick, mouse, and rabbit embryos. Tiki1 and Tiki2 orthologues are highly conserved, and exhibit similar but also different developmental expression patterns among the vertebrate/mammalian species analyzed. The Tiki1 gene is noticeably absent in the rodent lineage, but is present in lagomorphs and all other vertebrate/mammalian species examined. Expression in Hensen's node, the equivalent of the Xenopus Organizer, was observed for Chick Tiki2 and Rabbit Tiki1 and Tiki2. Mouse Tiki2 was detected at low levels at gastrulation and head fold stages, but not in the node. Mouse Tiki2 and chick Tiki1 display similar expression in the dorsal spinal cord. Chick Tiki1 expression was also detected in the surface ectoderm and maxillary bud, while chick Tiki2 was found in the anterior intestinal portal, head mesenchyme and primitive atrium. Our expression analyses provide evidence that Tiki1 and Tiki2 are evolutionarily conserved among vertebrate species and their expression in the Organizer and other regions suggests contributions of these Wnt inhibitors to embryonic patterning, as well as organogenesis. Our analyses further reveal mis-regulation of TIKI1 and TIKI2 in human cancer and diseases.

The contribution of different alpha-amylase isoenzymes of the commodity grain spring wheat in the formation of falling number values

The participation of various isoenzymes of alpha-amylase in the formation of falling number values of the commodity grain of wheat grown in the Republic of Kazakhstan was investigated. It was found that active isoenzymes alpha-AMY1 and alpha-AMY2 of the embryonic shield were present in the grain with an index over 200. A significant decrease in the falling number depended mainly on the synthesis of alpha-AMY1 and alpha-AMY2 isoenzymes in the aleurone layer. In the grain, isoenzymes with high isoelectric points (p1 > or = 7.3) were found; these isoenzymes belong to alpha-amylase or late maturing or alpha-amylase of practically mature grains. It was discovered that the exogenous hormone (gibberellic acid) induced synthesis of alpha-amylase isoenzymes of scutellum, whole caryopses, and aleurone. It was shown that the impact of exogenous gibberellic acid on the activity and structure of alpha-amylase is reduced in grain with a low falling number.

Identification and expression profiles of prdm1 in medaka Oryzias latipes

Mouse Prdm1, also known as Blimp1, plays important roles in maturation and survival of lymphoid cells, as well as in organogenesis of muscle, limb, sensor organs and primordial germ cells. The homologues of mouse prdm1 have been identified in a diverse of animals including zebrafish and fugu. Here, we report the identification and expression profiles of two homologues of prdm1, namely prdm1a and prdm1b in medaka, Oryzias latipes. The transcripts of prdm1a and prdm1b were detectable in all the tissues including immune organs such as gill, spleen, kidney, liver and intestine that we have checked on. The transcripts of prdm1a could be detected in the embryonic shield at mid-gastrula stage and later in the somite, eye, otic vesicle, branchial arches, fin, intestine and cloaca during embryogenesis using in situ hybridization. Moreover, the expression of prdm1a in the liver of both medaka and zebrafish could be up-regulated by the immune stimuli including lipopolysaccharide, polyI:C and the grass carp reovirus, similarly to the up-regulation of IL1B. These results indicate that Prdm1a may play important roles in embryogenesis and also in immune response in fish.

Establishment of visceral left-right asymmetry in mammals: The role of ciliary action and leftward fluid flow in the region of Hensen’s node

During individual development of vertebrates, the anteroposterior, dorsoventral, and left-right axes of the body are established. Although the vertebrates are bilaterally symmetric outside, their internal structure is asymmetric. Of special interest is the insight into establishment of visceral left-right asymmetry in mammals, since it has not only basic but also an applied medical significance. As early as 1976, it was hypothesized that the ciliary action could be associated with the establishment of left-right asymmetry in mammals. Currently, the majority of researchers agree that the ciliary action in the region of Hensen's node and the resulting leftward laminar fluid flow play a key role in the loss of bilateral symmetry and triggering of expression of the genes constituting the Nodal-Ptx2 signaling cascade, specific of the left side of the embryo. The particular mechanism underlying this phenomenon is still insufficiently clear. There are three competing standpoints on how leftward fluid flow induces expression of several genes in the left side of the embryo. The morphogen gradient hypothesis postulates that the leftward flow creates a high concentration of a signaling biomolecule in the left side of Hensen's node, which, in turn, stimulates triggering of.gene expression of the Nodal-Ptx2 cascade. The biomechanical hypothesis (or two-cilia model) states that the immotile cilia located in the periphery of Hensen's node act as mechanosensors, activate mechanosensory ion channels, and trigger calcium signaling in the left side of the embryo. Finally, the "shuttle-bus model" holds that leftward fluid flow carries the lipid vesicles, which are crashed when colliding immotile cilia in the periphery of Hensen's node to release the contained signaling biomolecules. It is also noteworthy that the association between the ciliary action and establishment of asymmetry has been recently discovered in representatives of the lower invertebrates. In this paper, the author considers evolution of concepts on the mechanisms underlying establishment of visceral left-right asymmetry since 1976 until the present and critically reexamines the current concepts in this field of science. According to the author, serious arguments favoring the biomechanical hypothesis for determination of left-right asymmetry in mammals have been obtained.

Identification and migration of primordial germ cells in Atlantic salmon, Salmo salar: Characterization of Vasa, Dead End, and Lymphocyte antigen 75 genes

No information exists on the identification of primordial germ cells (PGCs) in the super-order Protacanthopterygii, which includes the Salmonidae family and Atlantic salmon (Salmo salar L.), one of the most commercially important aquatic animals worldwide. In order to identify salmon PGCs, we cloned the full-length cDNA of vasa, dead end (dnd), and lymphocyte antigen 75 (ly75/CD205) genes as germ cell marker candidates, and analyzed their expression patterns in both adult and embryonic stages of Atlantic salmon. Semi-quantitative RT-PCR results showed that salmon vasa and dnd were specifically expressed in testis and ovary, and vasa, dnd, and ly75 mRNA were maternally deposited in the egg. vasa mRNA was consistently detected throughout embryogenesis while dnd and ly75 mRNA were gradually degraded during cleavages. In situ analysis revealed the localization of vasa and dnd mRNA and Ly75 protein in PGCs of hatched larvae. Whole-mount in situ hybridization detected vasa mRNA during embryogenesis, showing a distribution pattern somewhat different to that of zebrafish; specifically, at mid-blastula stage, vasa-expressing cells were randomly distributed at the central part of blastodisc, and then they migrated to the presumptive region of embryonic shield. Therefore, the typical vasa localization pattern of four clusters during blastulation, as found in zebrafish, was not present in Atlantic salmon. In addition, salmon PGCs could be specifically labeled with a green fluorescence protein (GFP) using gfp-rt-vasa 3′-UTR RNA microinjection for further applications. These findings may assist in understanding PGC development not only in Atlantic salmon but also in other salmonids.

Severe Sirenomelia following frozen-thawed embryo transfer cycle

Sirenomelia is a rare congenital syndrome caused by anomalous development of the caudal region of the fetal body, resulting in a single limb or fused lower limbs [1]. Other anomalies associated with Sirenomelia include a single umbilical artery, bilateral renal agenesis or dysgenesis, absent bladder, absent rectum, imperforate anus, and absent external genitalia, lumbosacral and pelvic malformations, sacral agenesis, malformed vertebrae and hemivertebrae, and corresponding anomalies of the central nervous system [2]. We present a 37-year-old female who was referred for routine antenatal follow-up care after conceiving via a frozen-thawed embryo transfer cycle. She reported an 8-year period of infertility, including three unsuccessful cycles of in vitro fertilization. She had no past medical or surgical history. On a follow-up ultrasound at 11 weeks, a fetal cystic hygroma and absence of one lower extremity were noted. FISH results of chorionic villus sampling confirmed that chromosomes 13, 18, and 21 were normal, as were the sex chromosomes. Following termination of the pregnancy at 13 weeks, external examination of the fetus revealed a single, severely hypoplastic lower extremity engaged posteriorly and cranially (Fig. 1). The external genitalia were not apparent. Multiple anomalies were reported on computed tomographic scan and pathologic examination: a single umbilical artery, renal agenesis, absence of the ureters and bladder, bilateral hypoplasia of the renal arteries; anorectal atresia, and a blind-ending ileum. This combination of findings confirmed the diagnosis of Sirenomelia. Many theories have been proposed as a means of explaining Sirenomelia. At present, there are two acceptable hypotheses: the defective blastogenesis hypothesis and the vascular steal hypothesis [3]. According to the defective blastogenesis hypothesis, Sirenomelia is a primary defect of blastogenesis that occurs during the final stages of gastrulation at the tailbud stage, in conjunction with the primary embryonic process [4]. The phenotypic variability depends on the intensity, the time of initiation, and the duration of the underlying event (days 1-28) [4]. The defects of the primary embryonic period lead to failure of axis formation, mid-line, primitive node and streak, gastrulation, segmentation of the paraxial mesoderm, and defects in the determination of laterality [5]. Although frozen-thawed embryo transfers have not been associated with an increased risk of congenital abnormalities [6], the possibility exists that this anomaly may have been caused by defective blastogenesis due to the stress exerted during freezing and thawing of the embryo. According to the vascular steal hypothesis, a single umbilical artery emerging from the fetal abdominal aorta immediately below the celiac branch leads to insufficient blood supply, resulting in deficient development of the caudal mesoderm, kidneys, large intestine and genitalia [7]. S. Bodur (&) Department of Obstetrics and Gynecology, Maresal Cakmak Military Hospital, Erzurum 25070, Turkey e-mail: serkan_bodur@yahoo.com; bodurs@mail.magee.edu

Circadian clock genes Bmal1 and Clock during early chick development

The circadian clock is a well-described temporal organizer in adult organisms. Despite the particularly evident need for temporal control during embryo development, the effect of environmental cues is still greatly neglected. Few studies have reported circadian clock gene expression in early embryonic stages. However, nothing is known about circadian clock gene expression and function in the first stages of avian embryogenesis. In this work, the presence and spatial distribution of core circadian clock Bmal1 and Clock transcripts were thoroughly characterized during the first 50 hr of chick development using reverse transcriptase-polymerase chain reaction (RT-PCR), single and double whole-mount in situ hybridization and subsequent cross-section histology analysis. RT-PCR detected both Bmal1 and Clock transcripts since the egg is laid and until the embryo reaches the 22-somite stage. Whole-mount in situ hybridization showed that Bmal1 and Clock are expressed in the Hensen's node and primitive streak at early gastrula stage. Later, both mRNAs are present in the developing nervous system, optic vesicle, notochord, foregut, and somites. Clock was further identified in the developing heart. Noticeably, Bmal1 and Clock are expressed with a "salt and pepper" pattern, suggesting the existence of nonentrained oscillatory transcription which could play a nondependent dark/light function during chick embryo development.

Morphogenetic consequences of partial removal of blastomere cytoplasm during early embryonic development of the loach, Misgurnus fossilis L.

The development of loach embryos is successfully regulated (normalized) after partial removal of the cytoplasm from one blastomere at the two- or four-cell stage or complete removal of one or two blastomeres at the stage of 8-16 cells. Using time-lapse video imaging and morphometric analysis, it has been shown that this regulation is a two-stage process. At the first stage, the ratio between the volumes of the blastodisk and yolk sac is rapidly (within one or two cell cycles) restored almost to the initial level; at the second stage, morphogenesis of the embryo is modified according to its new structural features acquired after the operation. After several rounds of cytokinesis, the cytoplasm remaining in the operated blastomere fuses with the marginal yolk syncytium (periblast),which at the blastula stage forms a distinct extension at the operation site. This extension marks the site of embryonic shield formation. The results of morphometric analysis show that restoration of the initial blastoderm volume in operated embryos leads to a reduction of active tension at the blastoderm--yolk boundary and an increase in the ratio of blastoderm surface to its volume at the moment of epiboly initiation. As a result, the convergence of blastoderm cells to the operation site and the embryonic shield formation begin at a lesser degree of epiboly, compared to the control.

Noggin4 expression during chick embryonic development

We describe here the expression pattern of Noggin4 during the early development of the chick embryo (Gallus gallus). The expression of this gene starts with the onset of gastrulation (stage HH4), in two bilateral bands along the primitive streak, with a local maximum around Hensen's node. By the end of gastrulation, Noggin4 transcripts are distributed diffusely throughout the epiblast, with the highest concentration in the head ectoderm. Interestingly, the expression of Noggin4 during the first half of gastrulation demonstrates a clear left-right asymmetry in Hensen's node, being much more intensive in its right anterior portion. During neurulation, Noggin4 is expressed mainly in the neuroectoderm, with the most intensive expression in the head and lateral neural folds. In mesoderm derivatives, expression is seen in somites but not in the notochord. In general, primarily ectodermal and diffusive expression of Noggin4 in chick embryo, with a maximum in the anterior neurectoderm, resembles that of its ortholog in Xenopus, which indicates a conservative function of this gene in evolution.

BMP inhibition by DAN in Hensen's node is a critical step for the establishment of left–right asymmetry in the chick embryo

During left–right (L–R) axis formation, Nodal is expressed in the node and has a central role in the transfer of L–R information in the vertebrate embryo. Bone morphogenetic protein (BMP) signaling also has an important role for maintenance of gene expression around the node. Several members of the Cerberus/Dan family act on L–R patterning by regulating activity of the transforming growth factor-β (TGF-β) family. We demonstrate here that chicken Dan plays a critical role in L–R axis formation. Chicken Dan is expressed in the left side of the node shortly after left-handed Shh expression and before the appearance of asymmetrically expressed genes in the lateral plate mesoderm (LPM). In vitro experiments revealed that DAN inhibited BMP signaling but not NODAL signaling. SHH had a positive regulatory effect on Dan expression while BMP4 had a negative effect. Using overexpression and RNA interference-mediated knockdown strategies, we demonstrate that Dan is indispensable for Nodal expression in the LPM and for Lefty-1 expression in the notochord. In the perinodal region, expression of Dan and Nodal was independent of each other. Nodal up-regulation by DAN required NODAL signaling, suggesting that DAN might act synergistically with NODAL. Our data indicate that Dan plays an essential role in the establishment of the L–R axis by inhibiting BMP signaling around the node.

Extracellular matrix fluctuations during early embryogenesis

Extracellular matrix (ECM) movements and rearrangements were studied in avian embryos during early stages of development. We show that the ECM moves as a composite material, whereby distinct molecular components as well as spatially separated layers exhibit similar displacements. Using scanning wide field and confocal microscopy we show that the velocity field of ECM displacement is smooth in space and that ECM movements are correlated even at locations separated by several hundred micrometers. Velocity vectors, however, strongly fluctuate in time. The autocorrelation time of the velocity fluctuations is less than a minute. Suppression of the fluctuations yields a persistent movement pattern that is shared among embryos at equivalent stages of development. The high resolution of the velocity fields allows a detailed spatio-temporal characterization of important morphogenetic processes, especially tissue dynamics surrounding the embryonic organizer (Hensen's node).

Distinct steps of neural induction revealed by Asterix, Obelix and TrkC, genes induced by different signals from the organizer.

The amniote organizer (Hensen's node) can induce a complete nervous system when grafted into a peripheral region of a host embryo. Although BMP inhibition has been implicated in neural induction, non-neural cells cannot respond to BMP antagonists unless previously exposed to a node graft for at least 5 hours before BMP inhibitors. To define signals and responses during the first 5 hours of node signals, a differential screen was conducted. Here we describe three early response genes: two of them, Asterix and Obelix, encode previously undescribed proteins of unknown function but Obelix appears to be a nuclear RNA-binding protein. The third is TrkC, a neurotrophin receptor. All three genes are induced by a node graft within 4–5 hours but they differ in the extent to which they are inducible by FGF: FGF is both necessary and sufficient to induce Asterix, sufficient but not necessary to induce Obelix and neither sufficient nor necessary for induction of TrkC. These genes are also not induced by retinoic acid, Noggin, Chordin, Dkk1, Cerberus, HGF/SF, Somatostatin or ionomycin-mediated Calcium entry. Comparison of the expression and regulation of these genes with other early neural markers reveals three distinct “epochs”, or temporal waves, of gene expression accompanying neural induction by a grafted organizer, which are mirrored by specific stages of normal neural plate development. The results are consistent with neural induction being a cascade of responses elicited by different signals, culminating in the formation of a patterned nervous system.

Isolation and Characterization of Node/Notochord-Like Cells from Mouse Embryonic Stem Cells

The homeobox gene Noto is expressed in the node and its derivative the notochord. Here we use a targeted Noto-GFP reporter to isolate and characterize node/notochord-like cells derived from mouse embryonic stem cells. We find very few Noto-expressing cells after spontaneous differentiation. However, the number of Noto-expressing cells was increased when using Activin A to induce a Foxa2- and Brachyury-expressing progenitor population, whose further differentiation into Noto-expressing cells was improved by simultaneous inhibition of BMP, Wnt, and retinoic acid signaling. Noto-GFP(+) cells expressed the node/notochord markers Noto, Foxa2, Shh, Noggin, Chordin, Foxj1, and Brachyury; showed a vacuolarization characteristic of notochord cells; and can integrate into midline structures when grafted into Hensen's node of gastrulating chicken embryos. The ability to generate node/notochord-like cells in vitro will aid the biochemical characterization of these developmentally important structures.

Secondary neurulation: Fate‐mapping and gene manipulation of the neural tube in tail bud

The body tail is a characteristic trait of vertebrates, which endows the animals with a variety of locomotive functions. During embryogenesis, the tail develops from the tail bud, where neural and mesodermal tissues make a major contribution. The neural tube in the tail bud develops by the process known as secondary neurulation (SN), where mesenchymal cells undergo epithelialization and tubulogenesis. These processes contrast with the well known primary neurulation, which is achieved by invagination of an epithelial cell sheet. In this study we have identified the origin of SN-undergoing cells, which is located caudo-medially to Hensen's node of early chicken embryo. This region is distinctly fate-mapped from tail-forming mesoderm. The identification of the presumptive SN region has allowed us to target this region with exogenous genes using in ovo electroporation techniques. The SN-transgenesis has further enabled an exploration of molecular mechanisms underlying mesenchymal-to-epithelial transition during SN, where activity levels of Cdc42 and Rac1 are critical. This is the first demonstration of molecular and cellular analyses of SN, which can be performed at a high resolution separately from tail-forming mesoderm.

Regulation of programmed cell death during neural induction in the chick embryo

To study early responses to neural inducing signals from the organizer (Hensen's node), a differential screen was performed in primitive streak stage chick embryos, comparing cells that had or had not been exposed to a node graft for 5 hours. Three of the genes isolated have been implicated in Programmed Cell Death (PCD): Defender Against Cell Death (Dad1), Polyubiquitin II (UbII) and Ferritin Heavy chain (fth1). We therefore explored the potential involvement of PCD in neural induction. Dad1, UbII and fth1 are expressed in partly overlapping domains during early neural plate development, along with the pro-apoptotic gene Cas9 and the death effector Cas3. Dad1 and UbII are induced by a node graft within 3 hours. TUNEL staining revealed that PCD is initially random, but both during normal development and following neural induction by a grafted node, it becomes concentrated at the border of the forming neural plate and anterior non-neural ectoderm and downregulated from the neural plate itself. PCD was observed in regions of Caspase expression that are free from Dad1, consistent with the known anti-apoptotic role of Dad1. However, gain- and loss-of-function of any of these genes had no detectable effect on cell identity or on neural plate development. This study reveals that early development of the neural plate is accompanied by induction of putative pro- and anti-apoptotic genes in distinct domains. We suggest that the neural plate is protected against apoptosis, confining cell death to its border and adjacent non-neural ectoderm.

FoxA transcription factors are essential for the development of dorsal axial structures

In vertebrates, embryonic structures present at the dorsal midline, prechordal plate, notochord, hypochord and floor plate share a common embryonic origin. In zebrafish, they derive from a pool of progenitors located within the embryonic shield at the onset of gastrulation. The molecular mechanisms responsible for the common development of these structures remain unknown. Based on their spatial and temporal expression, transcription factors of the Forkhead box A (FoxA) family appeared to be good candidates to play such a role. In agreement with this hypothesis, we found that simultaneous knockdown of FoxA2 and FoxA3 abolish the formation of all axial derivatives, while overexpression of these transcription factors strongly enlarges dorsal mesodermal territories. We establish that, in FoxA2–FoxA3 double morphants, precursors of axial tissues are correctly induced at early gastrula stage, but their dorsal midline identity is not maintained during development and we found that progenitors of these tissues are cell-autonomously re-specified to form muscle fibers as well as cells of the ventral neural tube. Our study provides the first example of a specific loss of all dorsal midline tissues and demonstrates that members of the FoxA family have redundant functions essential to maintain the axial identity of prechordal plate, notochord, floor plate and hypochord progenitors during gastrulation.

Spinal Epidural Teratoma: Review of Spinal Teratoma With Consideration on the Pathogenesis: Case Report

Of the 170 cases of teratomas in the spinal canal reported in the English literature, only 13 were spinal epidural teratomas (SETs). We present a case of SET, review the characteristics of SETs in comparison with spinal intradural teratomas (SITs), and investigate the pathogenesis of spinal teratoma. A 17-month-old boy visited our clinic with paraparesis. A multicystic mass was noted in the left epidural space from T8 to T10 and the left paraspinal area. Complete resection of the tumor, including the paraspinal portion, was accomplished by laminoplastic laminotomy. The tumor was well encapsulated and filled with heterogeneous contents. The tumor was suspected to originate from the left T9 root. On histopathological examination, the tumor was found to be a mature teratoma. The clinical characteristics of SETs and SITs show similarities in age, male preponderance, location, associated anomalies, and pathology. SETs and SITs are probably 2 different types of a single disease entity. Pluripotent somatic cell from the Hensen's node and caudal cell mass is the probable origin of spinal teratoma. However, originating from misplaced primordial germ cell could not be excluded.

Embryonic development of the pacific cod Gadus macrocephalus (Gadidae)

Incubation of pacific cod eggs was divided into eight series, in which temperatures were set at −0.04°C to +4.03°C and warm and cold conditions alternated. The morphological changes that took place during the embryogenesis were described in detail using the results of the incubation. Twenty-two morphological characters that could be identified easily and that characterized the morphogenesis were defined in the course of development. The results of the incubation and data from the literature showed that the duration of the embryonic period in the Pacific cod’s lifecycle grew exponentially as water temperature decreased. It was found during the experiment that developing cod eggs survived low water temperatures up to freezing, as well as abrupt warming or cooling (over 3°C). According to the widely accepted Rass scale, the first stage of the Pacific cod embryogenesis takes 21% of its total duration, the second stage 23%, the third, 17%, and the fourth, 39%. However, at a temperature below 0°C, the relative duration of the stages of cleavage and embryonic shield was slightly shortened, whereas the mature embryo stage extended to almost half of the embryogenesis period. A more comprehensive analysis of temperature effects on embryogenesis revealed that the reduction of the rate of embryogenesis upon a temperature decrease occurred mostly at later stages of embryo growth. Modeling of development using defined morphological characters showed that the duration of embryogenesis grew linearly as the incubation temperature dropped in the first half of the embryogenesis and exponentially in the second half. A function was selected that described the obtained results most satisfactorily and that could be used for estimating the duration of the entire embryogenesis or any its stages within the range of water temperatures typical for Pacific cod.

Wnt‐3 and Wnt‐3a play different region‐specific roles in neural crest development in avians

Wnt signalling regulates cell proliferation and cell fate determination during embryogenesis. However, little is known about the developmental role of one Wnt family member, Wnt-3, during avian development. To investigate the possible functions of Wnt-3, its expression pattern was determined using whole-mount in situ hybridization. Wnt-3 is expressed in important signalling centres, including the dorsal neural tube, Hensen's node and the AER (apical ectodermal ridge). Most interestingly, Wnt-3 is expressed in the dorsal neural tube as a gradient, with the strongest expression anterior in the trunk. Furthermore, this study showed that Wnt-3 and Wnt-3a play a different role in neural crest lineages derived from different axial level of neural tube. Wnt-3 might be involved in proliferation of neural crest lineages, whereas Wnt-3a plays an important role in melanogenesis in vagal. However, both Wnt-3 and Wnt-3a cause a significant increase in melanogenesis in the trunk neural crest lineage.

Notch signalling regulates the contribution of progenitor cells from the chick Hensen's node to the floor plate and notochord

Hensen's node of the chick embryo contains multipotent self-renewing progenitor cells that can contribute to either the floor plate or the notochord. Floor plate cells are a population of epithelial cells that lie at the ventral midline of the developing neural tube, whereas the notochord is a rod of axial mesoderm that lies directly beneath the floor plate. These two tissues serve as a source of a potent signalling morphogen, sonic hedgehog (Shh), which patterns the dorsoventral axis of the neural tube. We show, through both gain- and loss-of-function approaches, that Notch signalling promotes the contribution of chick axial progenitor cells to the floor plate and inhibits contribution to the notochord. Thus, we propose that Notch regulates the allocation of appropriate numbers of progenitor cells from Hensen's node of the chick embryo to the notochord and the floor plate.