Development of cranial placodes: insights from studies in chick

Tlx3 mediates neuronal differentiation and proper condensation of the developing trigeminal ganglion

The trigeminal ganglion, the largest of the vertebrate cranial ganglia, is comprised of sensory neurons that relay sensations of pain, touch, and temperature to the brain. These neurons are derived from two embryonic cell types, the neural crest and ectodermal placodes, whose interactions are critical for proper ganglion formation. While the T-cell leukemia homeobox 3 (Tlx3) gene is known to be expressed in placodally-derived sensory neurons and necessary for their differentiation, little was known about Tlx3 expression and/or function in the neural crest-derived component of the developing trigeminal ganglion. By combining lineage labeling with in situ hybridization in the chick embryo, we show that neural crest-derived cells that contribute to the cranial trigeminal ganglion express Tlx3 at a time point that coincides with the onset of ganglion condensation. Importantly, loss of Tlx3 function in vivo diminishes the overall size and abundance of neurons within the trigeminal ganglion. Conversely, ectopic expression of Tlx3 in migrating cranial neural crest results in their premature neuronal differentiation. Taken together, our results demonstrate a critical role for Tlx3 in neural crest-derived cells during chick trigeminal gangliogenesis.

Cadherin-7 mediates proper neural crest cell-placodal neuron interactions during trigeminal ganglion assembly

The cranial trigeminal ganglia play a vital role in the peripheral nervous system through their relay of sensory information from the vertebrate head to the brain. These ganglia are generated from the intermixing and coalescence of two distinct cell populations: cranial neural crest cells and placodal neurons. Trigeminal ganglion assembly requires the formation of cadherin-based adherens junctions within the neural crest cell and placodal neuron populations; however, the molecular composition of these adherens junctions is still unknown. Herein, we aimed to define the spatio-temporal expression pattern and function of Cadherin-7 during early chick trigeminal ganglion formation. Our data reveal that Cadherin-7 is expressed exclusively in migratory cranial neural crest cells and is absent from trigeminal neurons. Using molecular perturbation experiments, we demonstrate that modulation of Cadherin-7 in neural crest cells influences trigeminal ganglion assembly, including the organization of neural crest cells and placodal neurons within the ganglionic anlage. Moreover, alterations in Cadherin-7 levels lead to changes in the morphology of trigeminal neurons. Taken together, these findings provide additional insight into the role of cadherin-based adhesion in trigeminal ganglion formation, and, more broadly, the molecular mechanisms that orchestrate the cellular interactions essential for cranial gangliogenesis.

Expression Pattern of Axin2 During Chicken Development

Canonical Wnt-signalling is well understood and has been extensively described in many developmental processes. The regulation of this signalling pathway is of outstanding relevance for proper development of the vertebrate and invertebrate embryo. Axin2 provides a negative-feedback-loop in the canonical Wnt-pathway, being a target gene and a negative regulator. Here we provide a detailed analysis of the expression pattern in the development of the chicken embryo. By performing in-situ hybridization on chicken embryos from stage HH 04+ to HH 32 we detected a temporally and spatially restricted dynamic expression of Axin2. In particular, data about the expression of Axin2 mRNA in early embryogenesis, somites, neural tube, limbs, kidney and eyes was obtained.

Tissue growth constrained by extracellular matrix drives invagination during optic cup morphogenesis

In the early embryo, the eyes form initially as relatively spherical optic vesicles (OVs) that protrude from both sides of the brain tube. Each OV grows until it contacts and adheres to the overlying surface ectoderm (SE) via an extracellular matrix (ECM) that is secreted by the SE and OV. The OV and SE then thicken and bend inward (invaginate) to create the optic cup (OC) and lens vesicle, respectively. While constriction of cell apices likely plays a role in SE invagination, the mechanisms that drive OV invagination are poorly understood. Here, we used experiments and computational modeling to explore the hypothesis that the ECM locally constrains the growing OV, forcing it to invaginate. In chick embryos, we examined the need for the ECM by (1) removing SE at different developmental stages and (2) exposing the embryo to collagenase. At relatively early stages of invagination (Hamburger-Hamilton stage HH14[Formula: see text]), removing the SE caused the curvature of the OV to reverse as it 'popped out' and became convex, but the OV remained concave at later stages (HH15) and invaginated further during subsequent culture. Disrupting the ECM had a similar effect, with the OV popping out at early to mid-stages of invagination (HH14[Formula: see text] to HH14[Formula: see text]). These results suggest that the ECM is required for the early stages but not the late stages of OV invagination. Microindentation tests indicate that the matrix is considerably stiffer than the cellular OV, and a finite-element model consisting of a growing spherical OV attached to a relatively stiff layer of ECM reproduced the observed behavior, as well as measured temporal changes in OV curvature, wall thickness, and invagination depth reasonably well. Results from our study also suggest that the OV grows relatively uniformly, while the ECM is stiffer toward the center of the optic vesicle. These results are consistent with our matrix-constraint hypothesis, providing new insight into the mechanics of OC (early retina) morphogenesis.

The molecular basis of craniofacial placode development

The sensory organs of the vertebrate head originate from simple ectodermal structures known as cranial placodes. All cranial placodes derive from a common domain adjacent to the neural plate, the preplacodal region, which is induced at the border of neural and non-neural ectoderm during gastrulation. Induction and specification of the preplacodal region is regulated by the fibroblast growth factor, bone morphogenetic protein, WNT, and retinoic acid signaling pathways, and characterized by expression of the EYA and SIX family of transcriptional regulators. Once the preplacodal region is specified, different combinations of local signaling molecules and placode-specific transcription factors, including competence factors, promote the induction of individual cranial placodes along the neural axis of the head region. In this review, we summarize the steps of cranial placode development and discuss the roles of the main signaling molecules and transcription factors that regulate these steps during placode induction, specification, and development. For further resources related to this article, please visit the WIREs website.

Lens placode planar cell polarity is dependent on Cdc42-mediated junctional contraction inhibition

Development of the ocular lens commences with the formation of the lens placode, an epithelial structure that thickens and subsequently bends inward in a process called invagination. Invagination is observed during the development of many embryonic structures, but the spectrum of morphogenetic events driving this process are, in most cases, not fully understood. A characteristic commonly found in embryonic tissues undergoing epithelial reorganization is planar polarity, a property where cells are geometrically and/or molecularly orientated in a specific direction along the plane of an epithelium. Planar polarity is known to drive the morphogenesis of several epithelial structures, however its role during invagination events is less clear. We have found that at the onset of invagination, cells of the lens placode become geometrically planar polarized such that they are orientated toward a central point in the lens placode. Further investigation revealed that this is due to contraction of radially orientated junctions and the elongation of those circumferentially orientated. Radial junctions have an elevated localization of actomyosin and their contraction is dependent on the F-actin and Rho-kinase binding protein, Shroom3. Elongation of circumferential junctions is dependent upon Cdc42, a Rho-GTPase known to regulate polarity via the Par-complex. We determined that Cdc42 and members of the Par-complex inhibit Shroom3-induced contractility and promote anisotropic placode cell geometry through inhibition of junctional contraction. We postulate that invagination of the lens placode requires careful orchestration of these opposing processes which are mediated by the planar polarization of junctional proteins.

Apical constriction and epithelial invagination are regulated by BMP activity

Epithelial invagination is a morphological process in which flat cell sheets transform into three-dimensional structures through bending of the tissue. It is accompanied by apical constriction, in which the apical cell surface is reduced in relation to the basal cell surface. Although much is known about the intra-cellular molecular machinery driving apical constriction and epithelial invagination, information of how extra-cellular signals affect these processes remains insufficient. In this study we have established several in vivo assays of placodal invagination to explore whether the external signal BMP regulates processes connected to epithelial invagination. By inhibiting BMP activity in prospective cranial placodes, we provide evidence that BMP signals are required for RhoA and F-actin rearrangements, apical constriction, cell elongation and epithelial invagination. The failure of placode invagination after BMP inhibition appears to be a direct consequence of disrupted apical accumulation of RhoA and F-actin, rather than changes in cell death or proliferation. In addition, our results show that epithelial invagination and acquisition of placode-specific identities are two distinct and separable developmental processes. In summary, our results provide evidence that BMP signals promote epithelial invagination by acting upstream of the intracellular molecular machinery that drives apical constriction and cell elongation.

Summary: We describe a novel role for BMP activity in promoting a direct and cell type-independent mechanism for apical constriction, cell elongation and epithelial invagination, separate from acquisition of placode-specific identities.

The mouse Foxi3 transcription factor is necessary for the development of posterior placodes

The inner ear develops from the otic placode, one of the cranial placodes that arise from a region of ectoderm adjacent to the anterior neural plate called the pre-placodal domain. We have identified a Forkhead family transcription factor, Foxi3, that is expressed in the pre-placodal domain and down-regulated when the otic placode is induced. We now show that Foxi3 mutant mice do not form otic placodes as evidenced by expression changes in early molecular markers and the lack of thickened placodal ectoderm, an otic cup or otocyst. Some preplacodal genes downstream of Foxi3-Gata3, Six1 and Eya1-are not expressed in the ectoderm of Foxi3 mutant mice, and the ectoderm exhibits signs of increased apoptosis. We also show that Fgf signals from the hindbrain and cranial mesoderm, which are necessary for otic placode induction, are received by pre-placodal ectoderm in Foxi3 mutants, but do not initiate otic induction. Finally, we show that the epibranchial placodes that develop in close proximity to the otic placode and the mandibular division of the trigeminal ganglion are missing in Foxi3 mutants. Our data suggest that Foxi3 is necessary to prime pre-placodal ectoderm for the correct interpretation of inductive signals for the otic and epibranchial placodes.

Differential expression pattern of Annexin A6 in chick neural crest and placode cells during cranial gangliogenesis

The cranial trigeminal and epibranchial ganglia are components of the peripheral nervous system that possess an important somatosensory role. These ganglia arise from the intermixing and coalescence of two different migratory cell types, neural crest cells and neurogenic placodes cells, and thus typify the phenomena of cell migration and intercellular interactions for their creation. The underlying molecular mechanisms of ganglia formation, however, are still poorly understood. To address this, we have analyzed the spatio-temporal expression profile of Annexin A6 during chick gangliogenesis, as Annexin proteins play important, conserved roles in ganglia development and physiology. We observe Annexin A6 protein in cranial neural crest cells prior to, during and after their emergence from the neural tube. Fully migratory cranial neural crest cells, however, are devoid of Annexin A6. Interestingly, we note Annexin A6 protein in trigeminal and epibranchial placode cells as these cells ingress from the ectoderm to initiate ganglia formation. This expression is also maintained in the sensory placodes later on when they coalesce with neural crest cells to assemble the cranial ganglia. These results suggest that the dynamic expression of Annexin A6 in various embryonic cell types may allow Annexin A6 to serve distinct functions throughout embryonic development.

Migratory neural crest cell αN-catenin impacts chick trigeminal ganglia formation

Neural crest cells are an embryonic cell population that is crucial for proper vertebrate development. Initially localized to the dorsal neural folds, premigratory neural crest cells undergo an epithelial-to-mesenchymal transition (EMT) and migrate to their final destinations in the developing embryo. Together with epidermally-derived placode cells, neural crest cells then form the cranial sensory ganglia of the peripheral nervous system. Our prior work has shown that αN-catenin, the neural subtype of the adherens junction α-catenin protein, regulates cranial neural crest cell EMT by controlling premigratory neural crest cell cadherin levels. Although αN-catenin down-regulation is critical for initial neural crest cell EMT, a potential role for αN-catenin in later neural crest cell migration, and formation of the cranial ganglia, has not been examined. In this study, we show for the first time that migratory neural crest cells that will give rise to the cranial trigeminal ganglia express αN-catenin and Cadherin-7. αN-catenin loss- and gain-of-function experiments reveal effects on the migratory neural crest cell population that include subsequent defects in trigeminal ganglia assembly. Moreover, αN-catenin perturbation in neural crest cells impacts the placode cell contribution to the trigeminal ganglia and also changes neural crest cell Cadherin-7 levels and localization. Together, these results highlight a novel function for αN-catenin in migratory neural crest cells that form the trigeminal ganglia.

From placode to labyrinth: culture of the chicken inner ear

The inner ear transduces the mechanical stimuli that are associated with sound and balance perception. Missteps during its formation often result in deafness, and thus understanding otic development has a profound clinical relevance. The intricate complexity of the inner ear is derived from a simple epithelial sheet during embryogenesis. Study of this process in vitro has provided insight into the mechanisms of otic induction, patterning and differentiation. This article details methods for the culture of otic placode, otocyst, and basilar papilla, providing a toolkit for the investigation of multiple facets of otic organogenesis, for regeneration studies and for setting up small molecule screens to identify possible therapeutic targets.