Patterns of cell division and interkinetic nuclear migration in the chick embryo hindbrain

Segmentation and patterning of the vertebrate hindbrain

During early development, the hindbrain is sub-divided into rhombomeres that underlie the organisation of neurons and adjacent craniofacial tissues. A gene regulatory network of signals and transcription factors establish and pattern segments with a distinct anteroposterior identity. Initially, the borders of segmental gene expression are imprecise, but then become sharply defined, and specialised boundary cells form. In this Review, we summarise key aspects of the conserved regulatory cascade that underlies the formation of hindbrain segments. We describe how the pattern is sharpened and stabilised through the dynamic regulation of cell identity, acting in parallel with cell segregation. Finally, we discuss evidence that boundary cells have roles in local patterning, and act as a site of neurogenesis within the hindbrain.

A single cell transcriptome atlas of the developing zebrafish hindbrain

Segmentation of the vertebrate hindbrain leads to the formation of rhombomeres, each with a distinct anteroposterior identity. Specialised boundary cells form at segment borders that act as a source or regulator of neuronal differentiation. In zebrafish, there is spatial patterning of neurogenesis in which non-neurogenic zones form at boundaries and segment centres, in part mediated by Fgf20 signalling. To further understand the control of neurogenesis, we have carried out single cell RNA sequencing of the zebrafish hindbrain at three different stages of patterning. Analyses of the data reveal known and novel markers of distinct hindbrain segments, of cell types along the dorsoventral axis, and of the transition of progenitors to neuronal differentiation. We find major shifts in the transcriptome of progenitors and of differentiating cells between the different stages analysed. Supervised clustering with markers of boundary cells and segment centres, together with RNA-seq analysis of Fgf-regulated genes, has revealed new candidate regulators of cell differentiation in the hindbrain. These data provide a valuable resource for functional investigations of the patterning of neurogenesis and the transition of progenitors to neuronal differentiation.

Actomyosin regulation by Eph receptor signaling couples boundary cell formation to border sharpness

The segregation of cells with distinct regional identity underlies formation of a sharp border, which in some tissues serves to organise a boundary signaling centre. It is unclear whether or how border sharpness is coordinated with induction of boundary-specific gene expression. We show that forward signaling of EphA4 is required for border sharpening and induction of boundary cells in the zebrafish hindbrain, which we find both require kinase-dependent signaling, with a lesser input of PDZ domain-dependent signaling. We find that boundary-specific gene expression is regulated by myosin II phosphorylation, which increases actomyosin contraction downstream of EphA4 signaling. Myosin phosphorylation leads to nuclear translocation of Taz, which together with Tead1a is required for boundary marker expression. Since actomyosin contraction maintains sharp borders, there is direct coupling of border sharpness to boundary cell induction that ensures correct organisation of signaling centres.

Rho kinase-dependent apical constriction counteracts M-phase apical expansion to enable mouse neural tube closure

Cellular generation of mechanical forces required to close the presumptive spinal neural tube, the 'posterior neuropore' (PNP), involves interkinetic nuclear migration (INM) and apical constriction. Both processes change the apical surface area of neuroepithelial cells, but how they are biomechanically integrated is unknown. Rho kinase (Rock; herein referring to both ROCK1 and ROCK2) inhibition in mouse whole embryo culture progressively widens the PNP. PNP widening is not caused by increased mechanical tension opposing closure, as evidenced by diminished recoil following laser ablation. Rather, Rock inhibition diminishes neuroepithelial apical constriction, producing increased apical areas in neuroepithelial cells despite diminished tension. Neuroepithelial apices are also dynamically related to INM progression, with the smallest dimensions achieved in cells positive for the pan-M phase marker Rb phosphorylated at S780 (pRB-S780). A brief (2 h) Rock inhibition selectively increases the apical area of pRB-S780-positive cells, but not pre-anaphase cells positive for phosphorylated histone 3 (pHH3⁺). Longer inhibition (8 h, more than one cell cycle) increases apical areas in pHH3⁺ cells, suggesting cell cycle-dependent accumulation of cells with larger apical surfaces during PNP widening. Consequently, arresting cell cycle progression with hydroxyurea prevents PNP widening following Rock inhibition. Thus, Rock-dependent apical constriction compensates for the PNP-widening effects of INM to enable progression of closure.This article has an associated First Person interview with the first authors of the paper.

Yap/Taz-TEAD activity links mechanical cues to progenitor cell behavior during zebrafish hindbrain segmentation

Cells perceive their microenvironment through chemical and physical cues. However, how mechanical signals are interpreted during embryonic tissue deformation resulting in specific cell behaviors is largely unknown. The Yap/Taz family of transcriptional co-activators has emerged as an important regulator of tissue growth and regeneration, responding to physical cues from the extracellular matrix, cell shape changes and actomyosin cytoskeleton. In this study, we demonstrated the role of Yap/Taz-TEAD activity as a sensor of mechanical signals in the regulation of the progenitor behavior of boundary cells during zebrafish hindbrain compartmentalization. Monitoring of in vivo Yap/Taz-activity during hindbrain segmentation indicated that boundary cells responded to mechanical cues in a cell-autonomous manner through Yap/Taz-TEAD activity. Cell-lineage analysis revealed that Yap/Taz-TEAD boundary cells decreased their proliferative activity when Yap/Taz-TEAD activity ceased, which preceded changes in their cell fate from proliferating progenitors to differentiated neurons. Functional experiments demonstrated the pivotal role of Yap/Taz-TEAD signaling in maintaining progenitor features in the hindbrain boundary cell population.

Formin homology 2 domain-containing 3 (Fhod3) controls neural plate morphogenesis in mouse cranial neurulation by regulating multidirectional apical constriction

Neural tube closure requires apical constriction during which contraction of the apical F-actin network forces the cell into a wedged shape, facilitating the folding of the neural plate into a tube. However, how F-actin assembly at the apical surface is regulated in mammalian neurulation remains largely unknown. We report here that formin homology 2 domain-containing 3 (Fhod3), a formin protein that mediates F-actin assembly, is essential for cranial neural tube closure in mouse embryos. We found that Fhod3 is expressed in the lateral neural plate but not in the floor region of the closing neural plate at the hindbrain. Consistently, in Fhod3-null embryos, neural plate bending at the midline occurred normally, but lateral plates seemed floppy and failed to flex dorsomedially. Because the apical accumulation of F-actin and constriction were impaired specifically at the lateral plates in Fhod3-null embryos, we concluded that Fhod3-mediated actin assembly contributes to lateral plate-specific apical constriction to advance closure. Intriguingly, Fhod3 expression at the hindbrain was restricted to neuromeric segments called rhombomeres. The rhombomere-specific accumulation of apical F-actin induced by the rhombomere-restricted expression of Fhod3 was responsible for the outward bulging of rhombomeres involving apical constriction along the anteroposterior axis, as rhombomeric bulging was less prominent in Fhod3-null embryos than in the wild type. Fhod3 thus plays a crucial role in the morphological changes associated with neural tube closure at the hindbrain by mediating apical constriction not only in the mediolateral but also in the anteroposterior direction, thereby contributing to tube closure and rhombomere segmentation, respectively.

Radial WNT5A-Guided Post-mitotic Filopodial Pathfinding Is Critical for Midgut Tube Elongation

The early midgut undergoes intensive elongation, but the underlying cellular and molecular mechanisms are unknown. The early midgut epithelium is pseudostratified, and its nuclei travel between apical and basal surfaces in concert with cell cycle. Using 3D confocal imaging and 2D live imaging, we profiled behaviors of individual dividing cells. As nuclei migrate apically for mitosis, cells maintain a basal process (BP), which splits but is inherited by only one daughter. After mitosis, some daughters directly use the inherited BP as a "conduit" to transport the nucleus basally, while >50% of daughters generate a new basal filopodium and use it as a path to return the nucleus. Post-mitotic filopodial "pathfinding" is guided by mesenchymal WNT5A. Without WNT5A, some cells fail to tether basally and undergo apoptosis, leading to a shortened midgut. Thus, these studies reveal previously unrecognized strategies for efficient post-mitotic nuclear trafficking, which is critical for early midgut elongation.

Cerebellar networks and neuropathology of cerebellar developmental disorders

The cerebellar system is a series of axonal projections and synaptic circuits as networks, similar to those of the limbic system and those subserving the propagation and spread of seizures. Three principal cerebellar networks are identified and cerebellar disease often affects components of the networks other than just the cerebellar cortex. Contemporary developmental neuropathology of the cerebellum is best considered in the context of alterations of developmental processes: embryonic segmentation and genetic gradients along the three axes of the neural tube, individual neuronal and glial cell differentiation, migration, synaptogenesis, and myelination. Precisely timed developmental processes may be delayed or precocious rhombencephalosynapsis and pontocerebellar hypoplasia exemplify opposite gradients in the horizontal axis. Chiari II malformation may be reconsidered as a disorder of segmentation rather than simply due to mechanical forces upon normally developing hindbrain structures. Cellular nodules in the roof of the fourth ventricle are heterotopia of histologically differentiated but architecturally disoriented and disorganized neurons and glial cells; they often are less mature immunocytochemically than similar cells in adjacent normal folia. Cell rests are nodules of undifferentiated neuroepithelial cells. Both are frequent in human fetuses and neonates. Axonal projections from heterotopia to adjacent cerebellar folia or nuclei are few or absent, hence these nodules are clinically silent despite neuronal differentiation.

Cellular systems for epithelial invagination

Epithelial invagination is a fundamental module of morphogenesis that iteratively occurs to generate the architecture of many parts of a developing organism. By changing the physical properties such as the shape and/or position of a population of cells, invagination drives processes ranging from reconfiguring the entire body axis during gastrulation, to forming the primordia of the eyes, ears and multiple ducts and glands, during organogenesis. The epithelial bending required for invagination is achieved through a variety of mechanisms involving systems of cells. Here we provide an overview of the different mechanisms, some of which can work in combination, and outline the circumstances in which they apply.

This article is part of the themed issue ‘Systems morphodynamics: understanding the development of tissue hardware’.

Notch-dependent epithelial fold determines boundary formation between developmental fields in the Drosophila antenna

Compartment boundary formation plays an important role in development by separating adjacent developmental fields. Drosophila imaginal discs have proven valuable for studying the mechanisms of boundary formation. We studied the boundary separating the proximal A1 segment and the distal segments, defined respectively by Lim1 and Dll expression in the eye-antenna disc. Sharp segregation of the Lim1 and Dll expression domains precedes activation of Notch at the Dll/Lim1 interface. By repressing bantam miRNA and elevating the actin regulator Enable, Notch signaling then induces actomyosin-dependent apical constriction and epithelial fold. Disruption of Notch signaling or the actomyosin network reduces apical constriction and epithelial fold, so that Dll and Lim1 cells become intermingled. Our results demonstrate a new mechanism of boundary formation by actomyosin-dependent tissue folding, which provides a physical barrier to prevent mixing of cells from adjacent developmental fields.

During development, boundary formation between adjacent developmental fields is important to maintain the integrity of complex organs and tissues. We examined how boundaries become established between adjacent developmental fields—which are defined by expression of distinct selector genes and developmental fates—using the Drosophila eye-antennal disc as a model. We show that boundary formation is a progressive process. We focused our analysis on the antennal A1 fold that separates the A1 and A2-Ar segments, corresponding to the evolutionarily conserved segregation between coxopodite and telopodite segments of arthropod appendages. We describe a clear temporal and causal sequence of events from selector gene expression to establishment of a lineage-restricting boundary. We found that Notch activation at the boundary between adjacent fields of selector gene expression triggers actomyosin-mediated cell apical constriction, which induces the formation of an epithelial fold and prevents intermixing of cells from adjacent fields. Our findings describe a novel mechanism by which epithelial fold provides a physical barrier for cell segregation.

Cellular and Molecular Underpinnings of Neuronal Assembly in the Central Auditory System during Mouse Development

During development, the organization of the auditory system into distinct functional subcircuits depends on the spatially and temporally ordered sequence of neuronal specification, differentiation, migration and connectivity. Regional patterning along the antero-posterior axis and neuronal subtype specification along the dorso-ventral axis intersect to determine proper neuronal fate and assembly of rhombomere-specific auditory subcircuits. By taking advantage of the increasing number of transgenic mouse lines, recent studies have expanded the knowledge of developmental mechanisms involved in the formation and refinement of the auditory system. Here, we summarize several findings dealing with the molecular and cellular mechanisms that underlie the assembly of central auditory subcircuits during mouse development, focusing primarily on the rhombomeric and dorso-ventral origin of auditory nuclei and their associated molecular genetic pathways.

A new role of hindbrain boundaries as pools of neural stem/progenitor cells regulated by Sox2

Background

Compartment boundaries are an essential developmental mechanism throughout evolution, designated to act as organizing centers and to regulate and localize differently fated cells. The hindbrain serves as a fascinating example for this phenomenon as its early development is devoted to the formation of repetitive rhombomeres and their well-defined boundaries in all vertebrates. Yet, the actual role of hindbrain boundaries remains unresolved, especially in amniotes.

Results

Here, we report that hindbrain boundaries in the chick embryo consist of a subset of cells expressing the key neural stem cell (NSC) gene Sox2. These cells co-express other neural progenitor markers such as Transitin (the avian Nestin), GFAP, Pax6 and chondroitin sulfate proteoglycan. The majority of the Sox2⁺ cells that reside within the boundary core are slow-dividing, whereas nearer to and within rhombomeres Sox2⁺ cells are largely proliferating. In vivo analyses and cell tracing experiments revealed the contribution of boundary Sox2⁺ cells to neurons in a ventricular-to-mantle manner within the boundaries, as well as their lateral contribution to proliferating Sox2⁺ cells in rhombomeres. The generation of boundary-derived neurospheres from hindbrain cultures confirmed the typical NSC behavior of boundary cells as a multipotent and self-renewing Sox2⁺ cell population. Inhibition of Sox2 in boundaries led to enhanced and aberrant neural differentiation together with inhibition in cell-proliferation, whereas Sox2 mis-expression attenuated neurogenesis, confirming its significant function in hindbrain neuronal organization.

Conclusions

Data obtained in this study deciphers a novel role of hindbrain boundaries as repetitive pools of neural stem/progenitor cells, which provide proliferating progenitors and differentiating neurons in a Sox2-dependent regulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0277-y) contains supplementary material, which is available to authorized users.

Novel sites of expression of the bHLH gene NSCL1 in the developing nervous system

We report on novel sites of expression of the bHLH transcription factor NSCL1 in the developing forebrain, hindbrain and spinal cord in chick and mouse. In the hindbrain in particular, NSCL1 is the first bHLH transcription factor detected so far in rhombomere boundaries and its expression is coincident with boundary formation and maintenance. Novel sites of expression of this gene include the hippocampus, septum, tectum and hypothalamic nuclei. NSCL1 is thus expressed in various neuronal populations that are either not actively proliferating or postmitotic.

Signals and mechanics shaping compartment boundaries in Drosophila

During animal development groups of cells with similar fates and functions often stay together and separate from cells with different fates. An example for this cellular behavior is the formation of compartments, groups of cells with similar fates that are separated by sharp boundaries from neighboring groups of cells. Compartments play important roles during patterning by serving as units of growth and gene expression. Boundaries between compartments are associated with organizers that secrete signaling molecules instructing growth and differentiation throughout the tissue. The straight shape of the boundary between compartments is important for maintaining the position and shape of the organizer and thus for precise patterning. The straight shape of compartment boundaries, however, is challenged by cell divisions and cell intercalations that take place in many developing tissues. Early work established a role for selector genes and signaling pathways in setting up and keeping boundaries straight. Recent work in Drosophila has now begun to further unravel the physical and cellular mechanisms that maintain compartment boundaries. Key to the separation of compartments is a local increase of actomyosin-dependent mechanical tension at cell junctions along the boundary. Increased mechanical tension acts as a barrier to cell mixing during cell division and influences cell rearrangements during cell intercalations along the compartment boundary in a way that the straight shape of the boundary is maintained. An important question for the future is how the signaling pathways that maintain the straight shape of compartment boundaries control mechanical tension along these boundaries.

Crypto-rhombomeres of the mouse medulla oblongata, defined by molecular and morphological features

The medulla oblongata is the caudal portion of the vertebrate hindbrain. It contains major ascending and descending fiber tracts as well as several motor and interneuron populations, including neural centers that regulate the visceral functions and the maintenance of bodily homeostasis. In the avian embryo, it has been proposed that the primordium of this region is subdivided into five segments or crypto-rhombomeres (r7-r11), which were defined according to either their parameric position relative to intersomitic boundaries (Cambronero and Puelles, in J Comp Neurol 427:522-545, 2000) or a stepped expression of Hox genes (Marín et al., in Dev Biol 323:230-247, 2008). In the present work, we examine the implied similar segmental organization of the mouse medulla oblongata. To this end, we analyze the expression pattern of Hox genes from groups 3 to 8, comparing them to the expression of given cytoarchitectonic and molecular markers, from mid-gestational to perinatal stages. As a result of this approach, we conclude that the mouse medulla oblongata is segmentally organized, similarly as in avian embryos. Longitudinal structures such as the nucleus of the solitary tract, the dorsal vagal motor nucleus, the hypoglossal motor nucleus, the descending trigeminal and vestibular columns, or the reticular formation appear subdivided into discrete segmental units. Additionally, our analysis identified an internal molecular organization of the migrated pontine nuclei that reflects a differential segmental origin of their neurons as assessed by Hox gene expression.

Hox proteins drive cell segregation and non-autonomous apical remodelling during hindbrain segmentation

Hox genes encode a conserved family of homeodomain transcription factors regulating development along the major body axis. During embryogenesis, Hox proteins are expressed in segment-specific patterns and control numerous different segment-specific cell fates. It has been unclear, however, whether Hox proteins drive the epithelial cell segregation mechanism that is thought to initiate the segmentation process. Here, we investigate the role of vertebrate Hox proteins during the partitioning of the developing hindbrain into lineage-restricted units called rhombomeres. Loss-of-function mutants and ectopic expression assays reveal that Hoxb4 and its paralogue Hoxd4 are necessary and sufficient for cell segregation, and for the most caudal rhombomere boundary (r6/r7). Hox4 proteins regulate Eph/ephrins and other cell-surface proteins, and can function in a non-cell-autonomous manner to induce apical cell enlargement on both sides of their expression border. Similarly, other Hox proteins expressed at more rostral rhombomere interfaces can also regulate Eph/ephrins, induce apical remodelling and drive cell segregation in ectopic expression assays. However, Krox20, a key segmentation factor expressed in odd rhombomeres (r3 and r5), can largely override Hox proteins at the level of regulation of a cell surface target, Epha4. This study suggests that most, if not all, Hox proteins share a common potential to induce cell segregation but in some contexts this is masked or modulated by other transcription factors.

A novel role for Pax6 in the segmental organization of the hindbrain

Complex patterns and networks of genes coordinate rhombomeric identities, hindbrain segmentation and neuronal differentiation and are responsible for later brainstem functions. Pax6 is a highly conserved transcription factor crucial for neuronal development, yet little is known regarding its early roles during hindbrain segmentation. We show that Pax6 expression is highly dynamic in rhombomeres, suggesting an early function in the hindbrain. Utilization of multiple gain- and loss-of-function approaches in chick and mice revealed that loss of Pax6 disrupts the sharp expression borders of Krox20, Kreisler, Hoxa2, Hoxb1 and EphA and leads to their expansion into adjacent territories, whereas excess Pax6 reduces these expression domains. A mutual negative cross-talk between Pax6 and Krox20 allows these genes to be co-expressed in the hindbrain through regulation of the Krox20-repressor gene Nab1 by Pax6. Rhombomere boundaries are also distorted upon Pax6 manipulations, suggesting a mechanism by which Pax6 acts to set hindbrain segmentation. Finally, FGF signaling acts upstream of the Pax6-Krox20 network to regulate Pax6 segmental expression. This study unravels a novel role for Pax6 in the segmental organization of the early hindbrain and provides new evidence for its significance in regional organization along the central nervous system.

Boundary formation in the development of the vertebrate hindbrain

The formation of a sharp interface of adjacent subdivisions is important for establishing the precision of tissue organization, and at specific borders it serves to organize key signaling centers. We discuss studies of vertebrate hindbrain development that have given important insights into mechanisms that underlie the formation and maintenance of sharp borders. The hindbrain is subdivided into a series of segments with distinct anteroposterior identity that underlies the specification of distinct neuronal cell types. During early stages of segmentation, cell identity switching contributes to the refinement of borders and enables homogenous territories to be maintained despite intermingling of cells between segments. At later stages, there is a specific restriction to cell intermingling between segments that is mediated by Eph receptor and ephrin signaling. Eph-ephrin signaling can restrict cell intermingling and sharpen borders through multiple mechanisms, including the regulation of cell adhesion and contact inhibition of cell migration.

Regional differences in actomyosin contraction shape the primary vesicles in the embryonic chicken brain

In the early embryo, the brain initially forms as a relatively straight, cylindrical epithelial tube composed of neural stem cells. The brain tube then divides into three primary vesicles (forebrain, midbrain, hindbrain), as well as a series of bulges (rhombomeres) in the hindbrain. The boundaries between these subdivisions have been well studied as regions of differential gene expression, but the morphogenetic mechanisms that generate these constrictions are not well understood. Here, we show that regional variations in actomyosin-based contractility play a major role in vesicle formation in the embryonic chicken brain. In particular, boundaries did not form in brains exposed to the nonmuscle myosin II inhibitor blebbistatin, whereas increasing contractile force using calyculin or ATP deepened boundaries considerably. Tissue staining showed that contraction likely occurs at the inner part of the wall, as F-actin and phosphorylated myosin are concentrated at the apical side. However, relatively little actin and myosin was found in rhombomere boundaries. To determine the specific physical mechanisms that drive vesicle formation, we developed a finite-element model for the brain tube. Regional apical contraction was simulated in the model, with contractile anisotropy and strength estimated from contractile protein distributions and measurements of cell shapes. The model shows that a combination of circumferential contraction in the boundary regions and relatively isotropic contraction between boundaries can generate realistic morphologies for the primary vesicles. In contrast, rhombomere formation likely involves longitudinal contraction between boundaries. Further simulations suggest that these different mechanisms are dictated by regional differences in initial morphology and the need to withstand cerebrospinal fluid pressure. This study provides a new understanding of early brain morphogenesis.

Expression of hindbrain boundary markers is regulated by FGF3

Compartment boundaries act as organizing centers that segregate adjacent areas into domains of gene expression and regulation, and control their distinct fates via the secretion of signalling factors. During hindbrain development, a specialized cell-population forms boundaries between rhombomeres. These boundary cells demonstrate unique morphological properties and express multiple genes that differs them from intra-rhombomeric cells. Yet, little is known regarding the mechanisms that controls the expression or function of these boundary markers.Multiple components of the FGF signaling system, including ligands, receptors, downstream effectors as well as proteoglycans are shown to localize to boundary cells in the chick hindbrain. These patterns raise the possibility that FGF signaling plays a role in regulating boundary properties. We provide evidence to the role of FGF signaling, particularly the boundary-derived FGF3, in regulating the expression of multiple markers at hindbrain boundaries. These findings enable further characterization of the unique boundary-cell population, and expose a new function for FGFs as regulators of boundary-gene expression in the chick hindbrain.

Notch-mediated repression of bantam miRNA contributes to boundary formation in the Drosophila wing

Subdivision of proliferating tissues into adjacent compartments that do not mix plays a key role in animal development. The Actin cytoskeleton has recently been shown to mediate cell sorting at compartment boundaries, and reduced cell proliferation in boundary cells has been proposed as a way of stabilizing compartment boundaries. Cell interactions mediated by the receptor Notch have been implicated in the specification of compartment boundaries in vertebrates and in Drosophila, but the molecular effectors remain largely unidentified. Here, we present evidence that Notch mediates boundary formation in the Drosophila wing in part through repression of bantam miRNA. bantam induces cell proliferation and we have identified the Actin regulator Enabled as a new target of bantam. Increased levels of Enabled and reduced proliferation rates contribute to the maintenance of the dorsal-ventral affinity boundary. The activity of Notch also defines, through the homeobox-containing gene cut, a distinct population of boundary cells at the dorsal-ventral (DV) interface that helps to segregate boundary from non-boundary cells and contributes to the maintenance of the DV affinity boundary.

The dorsal neural tube: a dynamic setting for cell fate decisions

The dorsal neural tube first generates neural crest cells that exit the neural primordium following an epithelial-to-mesenchymal conversion to become sympathetic ganglia, Schwann cells, dorsal root sensory ganglia, and melanocytes of the skin. Following the end of crest emigration, the dorsal midline of the neural tube becomes the roof plate, a signaling center for the organization of dorsal neuronal cell types. Recent lineage analysis performed before the onset of crest delamination revealed that the dorsal tube is a highly dynamic region sequentially traversed by fate-restricted crest progenitors. Furthermore, prospective roof plate cells were shown to originate ventral to presumptive crest and to progressively relocate dorsalward to occupy their definitive midline position following crest delamination. These data raise important questions regarding the mechanisms of cell emigration in relation to fate acquisition, and suggest the possibility that spatial and/or temporal information in the dorsal neural tube determines initial segregation of neural crest cells into their derivatives. In addition, they emphasize the need to address what controls the end of neural crest production and consequent roof plate formation, a fundamental issue for understanding the separation between central and peripheral lineages during development of the nervous system.

Clonal analysis in mice underlines the importance of rhombomeric boundaries in cell movement restriction during hindbrain segmentation

BACKGROUND

Boundaries that prevent cell movement allow groups of cells to maintain their identity and follow independent developmental trajectories without the need for ongoing instructive signals from surrounding tissues. This is the case of vertebrate rhombomeric boundaries. Analysis in the developing chick hindbrain provided the first evidence that rhombomeres are units of cell lineage. The appearance of morphologically visible rhombomeres requires the segment restricted expression of a series of transcription factors, which position the boundaries and prefigure where morphological boundaries will be established. When the boundaries are established, when the cells are committed to a particular rhombomere and how they are organized within the hindbrain are important questions to our understanding of developmental regionalization.

METHODOLOGY/PRINCIPAL FINDINGS

Sophisticated experimental tools with high-resolution analysis have allowed us to explore cell lineage restriction within the hindbrain in mouse embryos. This novel strategy is based on knock-in alleles of ubiquitous expression and allows unrestricted clonal analysis of cell lineage from the two-cell stage to the adult mouse. Combining this analysis with statistical and mathematical tools we show that there is lineage compartmentalization along the anteroposterior axis from very early stages of mouse embryonic development.

CONCLUSIONS

Our results show that the compartment border coincides with the morphological boundary in the mouse hindbrain. The restriction of the cells to cross rhombomeric boundaries seen in chick is also observed in mouse. We show that the rhombomeric boundaries themselves are involved in cell movement restriction, although an underlying pre-pattern during early embryonic development might influence the way that cell populations organize.

Boundary cells regulate a switch in the expression of FGF3 in hindbrain rhombomeres

Background

During formation of the vertebrate central nervous system, the hindbrain is organized into segmental units, called rhombomeres (r). These cell-lineage restricted segments are separated by a subpopulation of cells known as boundary cells. Boundary cells display distinct molecular and cellular properties such as an elongated shape, enriched extracellular matrix components and a reduced proliferation rate compared to intra-rhombomeric cells. However, little is known regarding their functions and the mechanisms that regulate their formation.

Results

Hindbrain boundary cells express several signaling molecules, such as FGF3, which at earlier developmental stages is transiently expressed in specific rhombomeres. We show that chick embryos that lack boundary cells due to overexpression of truncated EphA4 receptor in the hindbrain have continued segmental expression of FGF3 at stages when it is normally restricted to hindbrain boundaries. Furthermore, surgical ablation of the boundary between r3 and r4, or blocking of the contact of r4 with boundary cells, results in sustained FGF3 expression in this segment.

Conclusion

These findings suggest that boundary cells are required for the downregulation of segmental FGF3, presumably mediated by a soluble factor(s) that emanates from boundaries. We propose that this new function of boundary cells enables a switch in gene expression that may be required for stage-specific functions of FGF3 in the developing hindbrain.

Btg1 and Btg2 gene expression during early chick development

Btg/Tob genes encode for a new family of proteins with antiproliferative functions, which are also able to stimulate cell differentiation. Btg1 and Btg2 are the most closely related members in terms of gene sequence. We analyzed their expression patterns in avian embryos by in situ hybridization, from embryonic day 1 to 3. Btg1 was distinctively expressed in the Hensen's node, the notochord, the cardiogenic mesoderm, the lens vesicle, and in the apical ectodermal ridge and mesenchyme of the limb buds. On the other hand, Btg2 expression domains included the neural plate border, presomitic mesoderm, trigeminal placode, and mesonephros. Both genes were commonly expressed in the myotome, epibranchial placodes, and dorsal neural tube. The results suggest that Btg1 and Btg2 are involved in multiple developmental processes. Overlapping expression of Btg1 and Btg2 may imply redundant functions, but unique expression patterns suggest also differential regulation and function.

Expression study of cadherin7 and cadherin20 in the embryonic and adult rat central nervous system

BACKGROUND

Vertebrate classic cadherins are divided into type I and type II subtypes, which are individually expressed in brain subdivisions (e.g., prosomeres, rhombomeres, and progenitor domains) and in specific neuronal circuits in region-specific manners. We reported previously the expression of cadherin19 (cad19) in Schwann cell precursors. Cad19 is a type II classic cadherin closely clustered on a chromosome with cad7 and cad20. The expression patterns of cad7 and cad20 have been reported previously in chick embryo but not in the developing and adult central nervous system of mammals. In this study, we identified rat cad7 and cad20 and analyzed their expression patterns in embryonic and adult rat brains.

RESULTS

Rat cad7 protein showed 92% similarity to chick cad7, while rat cad20 protein had 76% similarity to Xenopus F-cadherin. Rat cad7 mRNA was initially expressed in the anterior neural plate including presumptive forebrain and midbrain regions, and then accumulated in cells of the dorsal neural tube and in rhombomere boundary cells of the hindbrain. Expression of rat cad20 mRNA was specifically localized in the anterior neural region and rhombomere 2 in the early neural plate, and later in longitudinally defined ventral cells of the hindbrain. The expression boundaries of cad7 and cad20 corresponded to those of region-specific transcription factors such as Six3, Irx3 and Otx2 in the neural plate, and Dbx2 and Gsh1 in the hindbrain. At later stages, the expression of cad7 and cad20 disappeared from neuroepithelial cells in the hindbrain, and was almost restricted to postmitotic cells, e.g. somatic motor neurons and precerebellar neurons. These results emphasized the diversity of cad7 and cad20 expression patterns in different vertebrate species, i.e. birds and rodents.

CONCLUSION

Taken together, our findings suggest that the expression of cad7 and cad20 demarcates the compartments, boundaries, progenitor domains, specific nuclei and specific neural circuits during mammalian brain development.

Hox gene colinear expression in the avian medulla oblongata is correlated with pseudorhombomeric domains

The medulla oblongata (or caudal hindbrain) is not overtly segmented, since it lacks observable interrhombomeric boundaries. However, quail-chick fate maps showed that it is formed by 5 pseudorhombomeres (r7-r11) which were empirically found to be delimited consistently at planes crossing through adjacent somites (Cambronero and Puelles, 2000). We aimed to reexamine the possible segmentation or rostrocaudal regionalisation of this brain region attending to molecular criteria. To this end, we studied the expression of Hox genes from groups 3 to 7 correlative to the differentiating nuclei of the medulla oblongata. Our results show that these genes are differentially expressed in the mature medulla oblongata, displaying instances of typical antero-posterior (3' to 5') Hox colinearity. The different sensory and motor columns, as well as the reticular formation, appear rostrocaudally regionalised according to spaced steps in their Hox expression pattern. The anterior limits of the respective expression domains largely fit boundaries defined between the experimental pseudorhombomeres. Therefore the medulla oblongata shows a Hox-related rostrocaudal molecular regionalisation comparable to that found among rhombomeres, and numerically consistent with the pseudorhombomere list. This suggests that medullary pseudorhombomeres share some AP patterning mechanisms with the rhombomeres present in the rostral, overtly-segmented hindbrain, irrespective of variant boundary properties.

Analysis of Lrrn1 expression and its relationship to neuromeric boundaries during chick neural development

Background

The Drosophila leucine-rich repeat proteins Tartan (TRN) and Capricious (CAPS) mediate cell affinity differences during compartition of the wing imaginal disc. This study aims to identify and characterize the expression of a chick orthologue of TRN/CAPS and examine its potential function in relation to compartment boundaries in the vertebrate central nervous system.

Results

We identified a complementary DNA clone encoding Leucine-rich repeat neuronal 1 (Lrrn1), a single-pass transmembrane protein with 12 extracellular leucine-rich repeats most closely related to TRN/CAPS. Lrrn1 is dynamically expressed during chick development, being initially localized to the neural plate and tube, where it is restricted to the ventricular layer. It becomes downregulated in boundaries following their formation. In the mid-diencephalon, Lrrn1 expression prefigures the position of the anterior boundary of the zona limitans intrathalamica (ZLI). It becomes progressively downregulated from the presumptive ZLI just before the onset of expression of the signalling molecule Sonic hedgehog (Shh) within the ZLI. In the hindbrain, downregulation at rhombomere boundaries correlates with the emergence of specialized boundary cell populations, in which it is subsequently reactivated. Immunocolocalization studies confirm that Lrrn1 protein is endocytosed from the plasma membrane and is a component of the endosomal system, being concentrated within the early endosomal compartment.

Conclusion

Chick Lrrn1 is expressed in ventricular layer neuroepithelial cells and is downregulated at boundary regions, where neurogenesis is known to be delayed, or inhibited. The timing of Lrrn1 downregulation correlates closely with the activation of signaling molecule expression at these boundaries. This expression is consistent with the emergence of secondary organizer properties at boundaries and its endosomal localisation suggests that Lrrn1 may regulate the subcellular localisation of specific components of signalling or cell-cell recognition pathways in neuroepithelial cells.

Persistent and high levels of Hes1 expression regulate boundary formation in the developing central nervous system

The developing central nervous system is partitioned into compartments by boundary cells, which have different properties than compartment cells, such as forming neuron-free zones, proliferating more slowly and acting as organizing centers. We now report that in mice the bHLH factor Hes1 is persistently expressed at high levels by boundary cells but at variable levels by non-boundary cells. Expression levels of Hes1 display an inverse correlation to those of the proneural bHLH factor Mash1, suggesting that downregulation of Hes1 leads to upregulation of Mash1 in non-boundary regions,whereas persistent and high Hes1 expression constitutively represses Mash1 in boundary regions. In agreement with this notion, in the absence of Hes1 and its related genes Hes3 and Hes5, proneural bHLH genes are ectopically expressed in boundaries, resulting in ectopic neurogenesis and disruption of the organizing centers. Conversely, persistent Hes1 expression in neural progenitors prepared from compartment regions blocks neurogenesis and reduces cell proliferation rates. These results indicate that the mode of Hes1 expression is different between boundary and non-boundary cells, and that persistent and high levels of Hes1 expression constitutively repress proneural bHLH gene expression and reduce cell proliferation rates,thereby forming boundaries that act as the organizing centers.

Rhombomere boundaries are Wnt signaling centers that regulate metameric patterning in the zebrafish hindbrain

The vertebrate hindbrain develops from a series of segments (rhombomeres) distributed along the anteroposterior axis. We are studying the roles of Wnt and Delta-Notch signaling in maintaining rhombomere boundaries as organizing centers in the zebrafish hindbrain. Several wnt genes (wnt1, wnt3a, wnt8b, and wnt10b) show elevated expression at rhombomere boundaries, whereas several delta genes (dlA, dlB, and dlD) are expressed in transverse stripes flanking rhombomere boundaries. Partial disruption of Wnt signaling by knockdown of multiple wnt genes, or the Wnt mediator tcf3b, ablates boundaries and associated cell types. Expression of dlA is chaotic, and cell types associated with rhombomere centers are disorganized. Similar patterning defects are observed in segmentation mutants spiel-ohne-grenzen (spg) and valentino (val), which fail to form rhombomere boundaries due to faulty interactions between adjacent rhombomeres. Stripes of wnt expression are variably disrupted, with corresponding disturbances in metameric patterning. Mutations in dlA or mind bomb (mib) disrupt Delta-Notch signaling and cause a wide range of patterning defects in the hindbrain. Stripes of wnt1 are initially normal but subsequently dissipate, and metameric patterning becomes increasingly disorganized. Driving wnt1 expression using a heat-shock construct partially rescues metameric patterning in mib mutants. Thus, rhombomere boundaries act as Wnt signaling centers required for precise metameric patterning, and Delta signals from flanking cells provide feedback to maintain wnt expression at boundaries. Similar feedback mechanisms operate in the Drosophila wing disc and vertebrate limb bud, suggesting coaptation of a conserved signaling module that spatially organizes cells in complex organ systems.

The rostral and caudal boundaries of the diencephalon

Knowledge of nature and features of the boundaries between the main neural regions seems to be essential to understand the rules of brain regionalization. On the light of several current and classical criteria used to define cerebral boundaries, we examine the features of the places recognized as rostral and caudal boundaries in the developing diencephalon and provide new images about the glial features of these boundaries. One demonstrated property of some embryonic boundaries is the prevention of the crossing cells in the early ventricular zone (clonal restriction), while the intermediate zone seems to lack it. Data available so far indicate that the early boundary between diencephalon and mesencephalon (d/m) is a clonal restriction limit, but not between diencephalon and telencephalon (d/t). Later, while diencephalic nuclei form, cellular dispersion does not occur through the alar part of d/m, but it achieves in the corresponding d/t alar portion. The relationship between origin, migration, and cell-type specification of neural cells is being the object of special attention in the telencephalon, where specific cellular fenotipes can migrate to distant regions following non-radial routes. Such is the case of most GABAergic interneurons of avian and mammalian pallium and oligodendrocytes of the forebrain. In this regard, little attention has been devoted to the diencephalon, where this type of migration, specially those through the rostral boundary, has been reported by different authors. We introduce increasing evidence about non-conventional neuronal migration in the developing diencephalon and compare the reported migratory behavior with respect to both boundaries.

Molecular profiling indicates avian branchiomotor nuclei invade the hindbrain alar plate

It is generally believed that the spinal cord and hindbrain consist of a motor basal plate and a sensory alar plate. We now have molecular markers for these territories. The relationship of migrating branchiomotor neurons to molecularly defined alar and basal domains was examined in the chicken embryo by mapping the expression of cadherin-7 and cadherin-6B, in comparison to genetic markers for ventrodorsal patterning (Otp, Pax6, Pax7, Nkx2.2, and Shh) and motoneuron subpopulations (Phox2b and Isl1). We show cadherin-7 is expressed in a complete radial domain occupying a lateral region of the hindbrain basal plate. The cadherin-7 domain abuts the medial border of Pax7 expression; this common limit defines, or at least approximates, the basal/alar boundary. The hindbrain branchiomotor neurons originate in the medial part of the basal plate, close to the floor plate. Their cadherin-7-positive axons grow into the alar plate and exit the hindbrain close to the corresponding afferent nerve root. The cadherin-7-positive neuronal cell bodies later translocate laterally, following this axonal trajectory, thereby passing through the cadherin-7-positive basal plate domain. Finally, the cell bodies traverse the molecularly defined basal/alar boundary and move into positions within the alar plate. After the migration has ended, the branchiomotor neurons switch expression from cadherin-7 to cadherin-6B. These findings demonstrate that a specific subset of primary motor neurons, the branchiomotor neurons, migrate into the alar plate of the chicken embryo. Consequently, the century-old concept that all primary motor neurons come to reside in the basal plate should be revised.

Fgfr1-dependent boundary cells between developing mid- and hindbrain

Signaling molecules regulating development of the midbrain and anterior hindbrain are expressed in distinct bands of cells around the midbrain-hindbrain boundary. Very little is known about the mechanisms responsible for the coherence of this signaling center. One of the fibroblast growth factor (FGF) receptors, Fgfr1, is required for establishment of a straight border between developing mid- and hindbrain. Here we show that the cells close to the border have unique features. Unlike the cells further away, these cells express Fgfr1 but not the other FGF receptors. The cells next to the midbrain-hindbrain boundary express distinct cell cycle regulators and proliferate less rapidly than the surrounding cells. In Fgfr1 mutants, these cells fail to form a coherent band at the boundary. The slowly proliferating boundary cells are necessary for development of the characteristic isthmic constriction. They may also contribute to compartmentalization of this brain region.

Wnt1 regulates neurogenesis and mediates lateral inhibition of boundary cell specification in the zebrafish hindbrain

The formation of localised signalling centres is essential for patterning of a number of tissues during development. Previous work has revealed that a distinct population of boundary cells forms at the interface of segments in the vertebrate hindbrain, but the role of these cells is not known. We have investigated the function of the Wnt1 signalling molecule that is expressed by boundary and roof plate cells in the zebrafish hindbrain. Knockdown of wnt1 or of tcf3b, a mediator of Wnt signalling, leads to ectopic expression of boundary cell markers, rfng and foxb1.2, in non-boundary regions of the hindbrain. Ectopic boundary marker expression also occurs following knockdown of rfng, a modulator of Notch signalling required for wnt1 expression at hindbrain boundaries. We show that the boundary and roof plate expression of wnt1 each contribute to upregulation of proneural and delta gene expression and neurogenesis in non-boundary regions, which in turn blocks ectopic boundary marker expression. Boundary cells therefore play a key role in the regulation of cell differentiation in the zebrafish hindbrain. The network of genes underlying the regulation of neurogenesis and lateral inhibition of boundary cell formation by Wnt1 has a striking similarity to mechanisms at the dorsoventral boundary in the Drosophila wing imaginal disc.

Connexin 43 expression in glial cells of developing rhombomeres of Xenopus laevis

Connexin 43 is a gap junctional protein found predominantly in astrocytes. In the mammalian nervous system, it appears to play an organizational role during neural development. In the current study, conducted on the frog, Xenopus laevis, we found that connexin 43 occurs in glial cells during development of rhombomeres and that its expression is spatially and temporally regulated. We used neural (2G9) and cell proliferation (BrdU) markers to identify the overall organization of Xenopus rhombomeres and then tracked expression of connexin 43 and glial fibrillary acidic protein, an intermediate filament protein known to mark glia during rhombomeric development. 2G9 was expressed in rhombomeric centers (ventricular concavities) and outlying neuropil regions, whereas BrdU-labeled cells marked boundary regions (ventricular convexities), as early as stage 35/36. These labeling patterns persisted through premetamorphic stages of hindbrain development. At stage 47, 2G9-labeled profiles were highlighted by the presence of connexin 43, and at stage 49/50, connexin 43-labeled profiles, i.e., rhombomeric centers and neuropil, as well as rhombomeric boundaries, not labeled by connexin 43, became immunoreactive to glial fibrillary acidic protein. Cells of rhombomeric center regions and their processes in the outlying neuropil co-expressed glial fibrillary acidic protein and connexin 43 at a time that is characterized by the emergence of hindbrain auditory neural circuitry. Glial fibrillary acidic protein positive glial cells that appeared at rhombomeric boundaries never expressed connexin 43, but rather appeared to physically bisect ventricular convexities into adjacent rhombomeric regions. Thus, glial cells that express connexin 43 in developing rhombomeric centers may be similar to radial glia, assisting in formation of neural circuitry, while glial cells that do not express connexin 43, situated at rhombomeric boundaries, may be involved in demarcating adjacent rhombomeres.

Notch Activation Regulates the Segregation and Differentiation of Rhombomere Boundary Cells in the Zebrafish Hindbrain

During segmentation of the vertebrate hindbrain, a distinct population of boundary cells forms at the interface between each segment. Little is known regarding mechanisms that regulate the formation or functions of these cells. We have investigated a potential role of Notch signaling and find that in the zebrafish hindbrain, radical fringe is expressed in boundary cells and delta genes are expressed adjacent to boundaries, consistent with a sustained activation of Notch in boundary cells. Mosaic expression experiments reveal that activation of the Notch/Su(H) pathway regulates cell affinity properties that segregate cells to boundaries. In addition, Notch signaling correlates with a delayed neurogenesis at hindbrain boundaries and is required to inhibit premature neuronal differentiation of boundary cells. These findings reveal that Notch activation couples the regulation of location and differentiation in hindbrain boundary cells. Such coupling may be important for these cells to act as a stable signaling center.

A method for pulse and chase BrdU-labeling of early chick embryos

Pulse and chase BrdU labeling of early chick embryos represents a serious technical problem due to the hindrance of removing unincorporated BrdU after the pulse. We have developed a simple method that allows BrdU washout and control of pulse/chase duration. In this method, BrdU pulses are carried out in ovo. Afterwards, embryos are removed from the yolk, BrdU is washed out, and the embryos are maintained in a wholemount culture. Under these conditions, HH8-12 embryos continue with their normal development for at least 30 h. Morphological development of the nervous system and cell cycle kinetics of precursor cells seem to be normally maintained in cultured embryos. To prove the feasibility of the method, it has been applied to determine the onset of TUJ1 expression. TUJ1 is frequently considered an early neuronal marker, yet some reports have shown its expression in dividing progenitor cells and differentiating neurons. The application of this new method demonstrates that TUJ1 is expressed in newborn neurons as early as 1 h after cell cycle exit.

Interkinetic nuclear movement in the vertebrate neuroepithelium: encounters with an old acquaintance

The vertebrate neuroepithelium is a highly efficient structure with respect to the process of neurogenesis. The orthogonal arrangement of the nuclei with respect to the surface of the epithelium facilitates the dramatic increase in cell density necessary to produce a high number of neurons. Moreover, the spatial organization of the neuroepithelium reflects the segregation of cells that are transiently embarked upon distinct functions, thereby avoiding any interference between these populations in terms of their physiological activities. Two main regions can be distinguished: an apical neurogenetic zone, where lateral inhibitory signals and neurogenic gene expression can be observed, and a basally located pre-neurogenetic zone, which contains cells replicating their DNA and able to receive the signals that will modify their fate.

Mnb/Dyrk1A is transiently expressed and asymmetrically segregated in neural progenitor cells at the transition to neurogenic divisions

The Minibrain (Mnb) gene encodes a new family of protein kinases that is evolutionarily conserved from insects to humans. In Drosophila, Mnb is involved in postembryonic neurogenesis. In humans, MNB has been mapped within the Down's Syndrome (DS) critical region of chromosome 21 and is overexpressed in DS embryonic brain. In order to study a possible role of Mnb on the neurogenesis of vertebrate brain, we have cloned the chick Mnb orthologue and studied the spatiotemporal expression of Mnb in proliferative regions of the nervous system. In early embryos, Mnb is expressed before the onset of neurogenesis in the three general locations where neuronal precursors are originated: neuroepithelia of the neural tube, neural crest, and cranial placodes. Mnb is transiently expressed during a single cell cycle of neuroepithelial progenitor (NEP) cells. Mnb expression precedes and widely overlaps with the expression of Tis21, an antiproliferative gene that has been reported to be expressed in the onset of neurogenic divisions of NEP cells. Mnb transcription begins in mitosis, continues during G(1), and stops before S-phase. Very interestingly, we have found that Mnb mRNA is asymmetrically localized during the mitosis of these cells and inherited by one of the sibling cells after division. We propose that Mnb defines a transition step between proliferating and neurogenic divisions of NEP cells.

Boundary formation in the hindbrain: Eph only it were simple

Segmentation of the vertebrate hindbrain into rhombomeres is a key step in the development of a complex pattern of differentiated neurons from a homogeneous neuroepithelium. Many of the transcription factors important for establishing the segmental plan and assigning rhombomere identity are now known. However, the downstream effectors that bring about the formation of rhombomere boundaries are only just being characterized. Here we discuss molecules that could be responsible for segregating populations of cells from different rhombomeres. We focus on recent work demonstrating that the Eph family of receptor tyrosine kinases and their ligands, the ephrins, function in rhombomere-specific cell sorting and initiation of a structural boundary. We discuss the contributions of two mechanisms -- cell sorting and plasticity -- to the formation of rhombomere boundaries.

Stabilizing the regionalisation of the developing vertebrate central nervous system

During embryonic development, a number of tissues are patterned by their subdivision into domains with distinct regional identity. An important question is how sharp interfaces are established and maintained between adjacent domains despite the potential for scrambling due to cell intermingling during tissue growth. Two mechanisms have been found to underlie the maintenance of sharp interfaces: the specific restriction of cell mixing across boundaries, or the switching of identity of cells that cross between domains. We review the evidence for these mechanisms at distinct boundaries in the developing vertebrate central nervous system, and discuss what is known about their molecular mediators.

Cellular dynamics during evagination of the thyroid primordium in the chick embryo

The thyroid forms as an outpouching of the ventral pharynx. Evidence supports the conclusion that formation of the thyroid pit is mediated by changes in the cytoskeleton that cause constriction of cell apices. However, it seems unlikely that a relatively flat epithelial sheet can be converted into a pit without either distortions of the surface or considerable rearrangement of cells to reduce surface area. Possible cellular rearrangements were investigated by tracing the movements of individual cells by using time-lapse video microscopy. Changes in shape of the primordium were investigated by marking with carbon and DiI and by scanning electron microscopy. Cell movements occurred only over short distances, mostly shifts relative to a neighbor, especially at the edge of the pit. Instead, cells rearranged into clusters that piled up at the edge of the pit and then tilted inside. Adjacent rings of pharyngeal cells were annexed by the growing thyroid, undergoing rearrangement into clusters, piling up at the edge, and moving inside the pit. The consequence was the formation of a series of shelf-like extensions within the cavity, representing successive generations of cell rings moving inside. These results have implications for the formation of other organs by evagination.

Boundary formation and compartition in the avian diencephalon

The diencephalon comprises three functionally distinct regions: synencephalon, dorsal thalamus, and ventral thalamus. Patterning of the diencephalon has been proposed to involve subdivision of its anteroposterior axis into segments, neuromeres or prosomeres (Bergquist and Kallen, 1954; Vaage, 1969; Figdor and Stern, 1993; Rubenstein et al., 1994; Redies et al., 2000; Yoon et al., 2000). However, the number and sequence of diencephalic neuromeres, or even their existence, are uncertain. We have examined the proposed subdivisions by morphology, gene expression, acquisition of boundary-specific phenotypes, and cell lineage restriction. We find that at stage 16 in chick the diencephalon is divided into synencephalon and parencephalon. The synencephalon exhibits neuromeric morphology, expresses Prox, and acquires neuromere boundary properties at its interface with both the midbrain and the parencephalon. Although the mesencephalic/synencephalic boundary restricts cell mixing, the synencephalic/parencephalic boundary does not. Similarly, there is no lineage restriction between the parencephalon and the more rostral forebrain (secondary prosencephalon). Subdivision of the parencephalon into ventral and dorsal thalamus involves the formation of a narrow intraparencephalic territory, the zona limitans intrathalamica (zli). This is correlated with the acquisition of cell lineage restriction at both anterior and posterior borders of the zli, the appearance of boundary-specific properties, and Gbx2 and Dlx2 expression in dorsal thalamic and ventral thalamic territories, respectively. At stage 22, the synencephalon is divided into two domains, distinguished by differential gene expression and tissue morphology, but associated with neither a boundary phenotype nor cell lineage restriction. Our results suggest that the diencephalon does not have an overt segmental pattern.