CRABP-I Expression Patterns in the Developing Chick Inner Ear.

The vertebrate inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions, regarded as an excellent system for analyzing events that occur during development, such as patterning, morphogenesis, and cell specification. Retinoic acid (RA) is involved in all these development processes. Cellular retinoic acid-binding proteins (CRABPs) bind RA with high affinity, buffering cellular free RA concentrations and consequently regulating the activation of precise specification programs mediated by particular regulatory genes. In the otic vesicle, strong CRABP-I expression was detected in the otic wall's dorsomedial aspect, where the endolymphatic apparatus develops, whereas this expression was lower in the ventrolateral aspect, where part of the auditory system forms. Thus, CRABP-I proteins may play a role in the specification of the dorsal-to-ventral and lateral-to-medial axe of the otic anlagen. Regarding the developing sensory patches, a process partly involving the subdivision of a ventromedial pro-sensory domain, the CRABP-I gene displayed different levels of expression in the presumptive territory of each sensory patch, which was maintained throughout development. CRABP-I was also relevant in the acoustic-vestibular ganglion and in the periotic mesenchyme. Therefore, CRABP-I could protect RA-sensitive cells in accordance with its dissimilar concentration in specific areas of the developing chick inner ear.

Expression patterns of Irx genes in the developing chick inner ear.

The vertebrate inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. The molecular patterning of the developing otic epithelium creates various positional identities, consequently leading to the stereotyped specification of each neurosensory and non-sensory element of the membranous labyrinth. The Iroquois (Iro/Irx) genes, clustered in two groups (A: Irx1, Irx2, and Irx4; and B: Irx3, Irx5, and Irx6), encode for transcriptional factors involved directly in numerous patterning processes of embryonic tissues in many phyla. This work presents a detailed study of the expression patterns of these six Irx genes during chick inner ear development, paying particular attention to the axial specification of the otic anlagen. The Irx genes seem to play different roles at different embryonic periods. At the otic vesicle stage (HH18), all the genes of each cluster are expressed identically. Both clusters A and B seem involved in the specification of the lateral and posterior portions of the otic anlagen. Cluster B seems to regulate a larger area than cluster A, including the presumptive territory of the endolymphatic apparatus. Both clusters seem also to be involved in neurogenic events. At stages HH24/25-HH27, combinations of IrxA and IrxB genes participate in the specification of most sensory patches and some non-sensory components of the otic epithelium. At stage HH34, the six Irx genes show divergent patterns of expression, leading to the final specification of the membranous labyrinth, as well as to cell differentiation.

Distinct and redundant expression and transcriptional diversity of MEIS gene paralogs during chicken development.

Members of the Meis family of TALE homeobox transcription factors are involved in many processes of vertebrate development and morphogenesis, showing extremely complex transcriptional and spatiotemporal expression patterns. In this work, we performed a comprehensive study of chicken Meis genes using multiple approaches. First, we assessed whether the chicken genome contains a Meis3 ortholog or harbors only two Meis genes; we gathered several lines of evidence pointing to a specific loss of the Meis3 ortholog in an early ancestor of birds. Next, we studied the transcriptional diversity generated from chicken Meis genes through alternative splicing during development. Finally, we performed a detailed analysis of chick Meis1/2 expression patterns during early embryogenesis and organogenesis. We show that the expression of both Meis genes begins at the gastrulation stage in the three embryonic layers, presenting highly dynamic patterns with overlapping as well as distinct expression domains throughout development.

Meis gene expression patterns in the developing chicken inner ear.

We are interested in stable gene network activities operating sequentially during inner ear specification. The implementation of this patterning process is a key event in the generation of functional subdivisions of the otic vesicle during early embryonic development. The vertebrate inner ear is a complex sensory structure that is a good model system for characterization of developmental mechanisms controlling patterning and specification. Meis genes, belonging to the TALE family, encode homodomain-containing transcription factors remarkably conserved during evolution, which play a role in normal and neoplastic development. To gain understanding of the possible role of homeobox Meis genes in the developing chick inner ear, we comprehensively analyzed their spatiotemporal expression patterns from early otic specification stages onwards. In the invaginating otic placode, Meis1/2 transcripts were observed in the borders of the otic cup, being absent in the portion of otic epithelium closest to the hindbrain. As development proceeds, Meis1 and Meis2 expressions became restricted to the dorsomedial otic epithelium. Both genes were strongly expressed in the entire presumptive domain of the semicircular canals, and more weakly in all associated cristae. The endolymphatic apparatus was labeled in part by Meis1/2. Meis1 was also expressed in the lateral wall of the growing cochlear duct, while Meis2 expression was detected in a few cells of the developing acoustic-vestibular ganglion. Our results suggest a possible role of Meis assigning regional identity in the morphogenesis, patterning, and specification of the developing inner ear.

Incipient forebrain boundaries traced by differential gene expression and fate mapping in the chick neural plate.

We correlated available fate maps for the avian neural plate at stages HH4 and HH8 with the progress of local molecular specification, aiming to determine when the molecular specification maps of the primary longitudinal and transversal domains of the anterior forebrain agree with the fate mapped data. To this end, we examined selected gene expression patterns as they normally evolved in whole mounts and sections between HH4 and HH8 (or HH10/11 in some cases), performed novel fate-mapping experiments within the anterior forebrain at HH4 and examined the results at HH8, and correlated grafts with expression of selected gene markers. The data provided new details to the HH4 fate map, and disclosed some genes (e.g., Six3 and Ganf) whose expression domains initially are very extensive and subsequently retract rostralwards. Apart from anteroposterior dynamics, some genes soon became downregulated at the prospective forebrain floor plate, or allowed to identify an early roof plate domain (dorsoventral pattern). Peculiarities of the telencephalon (initial specification and differentiation of pallium versus subpallium) are contemplated. The basic anterior forebrain subdivisions seem to acquire correlated specification and fate mapping patterns around stage HH8.

Raldh3 gene expression pattern in the developing chicken inner ear.

Retinoic acid (RA), an active metabolite of vitamin A, is a diffusible molecule that regulates the expression of several families of genes, playing a key role in specification processes during chordate development. With the aim of defining its possible role in the developing chick inner ear, we obtained in this work a detailed spatiotemporal distribution of the enzymes involved in its synthesis, the retinaldehyde dehydrogenases (RALH1-4). Our results showed that, in contrast to the mouse inner ear, Raldh3 expression was the only Raldh gene detected in the developing chick inner ear, where it appears as early as stage 18. During inner ear morphogenesis, Raldh3 expression was predominantly observed in the endolymphatic system. The Raldh3 expression pattern delimited totally or partially the Bmp4-positive presumptive territories of vestibular sensory epithelia by stage 24 and the basilar papilla at stage 34, suggesting a possible involvement of RA in their specification. In addition, several vestibular sensory areas showed some Raldh3-expressing cells close to the Raldh3-positive domain. These results suggest that the RA signaling pathway may play a role in the initial patterning of the otic epithelium and cell differentiation therein, providing local positional information. Having in mind this Raldh3 expression pattern, we discuss the regulatory interactions among the RA, bone morphogenetic protein, and fibroblast growth factor signaling pathways in the specification of otic sensory elements. Our investigation may underpin further experimental studies aimed at understanding the possible role of signaling pathways in patterning of the developing chick inner ear.

Quantitative analysis of neural plate thickness and cell density during gastrulation in the chick embryo.

We quantitatively analyzed the developing prospective neural and non-neural ectoderm during chicken gastrulation on semithin transverse sections. At stage PS8 (primitive streak stage 8 of Lopez-Sanchez et al. [C. Lopez-Sanchez, L. Puelles, V. Garcia-Martinez, L. Rodriguez-Gallardo, Morphological and molecular analysis of the early developing chick requires an expanded series of primitive streak stages, J. Morphol. 264 (2005) 105-116.], equivalent to stage HH4), the thickest area of the ectoderm agrees in extent with the fate-mapped neural plate we had reported previously. The thickness of the median ectoderm is constantly higher up to a distance of 250mum from Hensen's node, and thickness decreases along a mediolateral gradient with a further drop at the prospective lateral border of the neural plate. A higher cell density of the developing ectoderm also coincided with the prospective neural plate. We observed that cell death does not play an important role in the spatial definition of the neural plate.

Macrophages during retina and optic nerve development in the mouse embryo: relationship to cell death and optic fibres.

We compared the spatial and temporal patterns of distribution of macrophages, with patterns of naturally occurring cell death and optic fibre growth during early retina and optic nerve development, in the mouse. We used embryos between day 10 of embryogenesis (E10; before the first optic fibres are generated in the retina) and E13 (when the first optic fibres have crossed the chiasmatic anlage). The macrophages and optic axons were identified by immunocytochemistry, and the apoptotic cells were detected by the TUNEL technique, which specifically labels fragmented DNA. Cell death was observed in the retina and the optic stalk long before the first optic axons appeared in either region. Subsequently, specialized F4/80-positive phagocytes were detected in chronological and topographical coincidence with cell death, which disappeared progressively. As development proceeded, the pioneer ganglion cell axons reached the regions where the macrophages were located. As the number of optic fibres increased, the macrophages disappeared. Therefore, cell death, accompanied by macrophages, preceded the growth of fibres in the retina and the optic nerve. Moreover, these macrophages synthesized NGF and the optic axons were p75 neurotrophin receptor (p75(NTR))- and TrkA-positive. These findings suggest that macrophages may be involved in optic axon guidance and fasciculation.

Correlation of a chicken stage 4 neural plate fate map with early gene expression patterns.

A number of gene markers are currently claimed to allow positive or negative visualization of the early chick neural plate at stages 3d/4, when its fate becomes determined. Some markers labeled by various authors as either "neural" or "non-neural" indeed show ectodermal expression patterns roughly correlative with widespread yet vague ideas on the shape and size of the early neural plate, based on previous fate maps. However, for technical reasons, it is not clear how precisely these expression patterns correlate with any experimentally determined fate boundaries. An eventual mismatch between fate and marker interpretation might bear importantly on ideas about gene functions and causal hypotheses in issues such as the establishment of the neural/non-neural border or the earliest mechanisms of neural regionalization. In this review, we correlated a set of epiblastic and mesendodermal gene expression patterns with the novel neuroectoderm proportions suggested by our recent fate map of the chick neural plate at stages HH 3d/4 [P. Fernández-Garre, L. Rodriguez-Gallardo, V. Gallego-Diaz, I.S. Alvarez, L. Puelles, Fate map of the chicken neural plate at stage 4, Development 129 (2002) 2807-2822.]. This analysis suggests the existence of various nested subregions of the epiblast with boundaries codefined by given sets of gene patterns. No gene expression studied reproduces exactly or even approximately the entire neural plate shape, leading to a combinatorial hypothesis on its specification. This kind of analysis (fate and molecular maps), jointly with competence maps, provides the basis for understanding gene functions and the mechanisms of neural induction, specification and regionalization. Several gene patterns observed are consistent with precocious incipient regionalization of the neural plate along the dorsoventral and anteroposterior axes.

Agreement and disagreement among fate maps of the chick neural plate.

Fate maps are essential to understand embryonic development; they provide a background for deducing maps of differential cellular specification in the context of other experimental data and molecular expression patterns. Due to its accessibility, the chick neural plate has been fate-mapped many times, albeit without complete agreement with respect to its shape, extent and fated subdivisions. In this review, we first comment about avian neural plate fate maps reported since the early period of experimental embryology, referring to the different methods followed. We next review a perfected fate-mapping methodology, which recently allowed us rather precise delimitation of the chick neural plate at stages 3d/4. This leads to a general discussion about the apparent border of the neural plate and the prospective main rostrocaudal and longitudinal divisions of the neural tube.

Morphological and molecular analysis of the early developing chick requires an expanded series of primitive streak stages.

A detailed analysis of the gastrulating chick embryo was performed using three methods : time-lapse videotaping of embryos in culture, histological semithin sections, and in situ hybridization with 10 mRNA signals expressed during gastrulation. The results suggest that the gene expression pattern of Goosecoid, Hex, Crescent, and Bmp7 may be involved in the axial establishment of the temporal and spatial arrangement of cells forming the prechordal plate endoderm, and that Chordin, cNot1, Noggin, and Brachyury are precocious markers of cells coming from Hensen's node, which contribute to the rostralmost tip of the notochord, its arrowhead, the head process, and, later, the elongating notochord. These results explain several earlier descriptions based only on morphological analyses of the axial mesodermal structures characteristic of the gastrulation stages. The data, carefully observed and compared with the whole-mount observation in time-lapse video, show that the changes in cell populations, movements, and cell differentiation occur step-by-step over a precise temporal range, which requires the establishment of a subdivision of the stages usually employed. Knowledge of new aspects of avian gastrulation, including gene expression patterns, immunocytochemical analyses, and the great number of recent experiments based on microinjections or transplants of groups of cells to analyze processes of induction or regulation, need the support of a precisely defined scheme of primitive streak stages (PS-stages), and a correlation of these stages with other approaches to provide a finer resolution of the staging steps, and thus to facilitate a better understanding of the initial gastrulation period.

Fate map of the chicken neural plate at stage 4.

A detailed fate map was obtained for the early chick neural plate (stages 3d/4). Numerous overlapping plug grafts were performed upon New-cultured chick embryos, using fixable carboxyfluorescein diacetate succinimidyl ester to label donor chick tissue. The specimens were harvested 24 hours after grafting and reached in most cases stages 9-11 (early neural tube). The label was detected immunocytochemically in wholemounts, and cross-sections were later obtained. The positions of the graft-derived cells were classified first into sets of purely neural, purely non-neural and mixed grafts. Comparisons between these sets established the neural plate boundary at stages 3d/4. Further analysis categorized graft contributions to anteroposterior and dorsoventral subdivisions of the early neural tube, including data on the floor plate and the eye field. The rostral boundary of the neural plate was contained within the earliest expression domain of the Ganf gene, and the overall shape of the neural plate was contrasted and discussed with regard to the expression patterns of the genes Plato, Sox2, Otx2 and Dlx5 (and others reported in the literature) at stages 3d/4.