The second pharyngeal pouch is generated by dynamic remodeling of endodermal epithelium in zebrafish

Insights into Craniofacial Development and Anomalies: Exploring Fgf Signaling in Zebrafish Models

PURPOSE OF REVIEW

To illustrate the value of using zebrafish to understand the role of the Fgf signaling pathway during craniofacial skeletal development under normal and pathological conditions.

RECENT FINDINGS

Recent data obtained from studies on zebrafish have demonstrated the genetic redundancy of Fgf signaling pathway and have identified new molecular partners of this signaling during the early stages of craniofacial skeletal development. Studies on zebrafish models demonstrate the involvement of the Fgf signaling pathway at every stage of craniofacial development. They particularly emphasize the central role of Fgf signaling pathway during the early stages of the development, which significantly impacts the formation of the various structures making up the craniofacial skeleton. This partly explains the craniofacial abnormalities observed in disorders associated with FGF signaling. Future research efforts should focus on investigating zebrafish Fgf signaling during more advanced stages, notably by establishing zebrafish models expressing mutations responsible for diseases such as craniosynostoses.

Identification of a pharyngeal mucosal lymphoid organ in zebrafish and other teleosts: Tonsils in fish?

The constant exposure of the fish branchial cavity to aquatic pathogens causes local mucosal immune responses to be extremely important for their survival. Here, we used a marker for T lymphocytes/natural killer (NK) cells (ZAP70) and advanced imaging techniques to investigate the lymphoid architecture of the zebrafish branchial cavity. We identified a sub-pharyngeal lymphoid organ, which we tentatively named "Nemausean lymphoid organ" (NELO). NELO is enriched in T/NK cells, plasma/B cells, and antigen-presenting cells embedded in a network of reticulated epithelial cells. The presence of activated T cells and lymphocyte proliferation, but not V(D)J recombination or hematopoiesis, suggests that NELO is a secondary lymphoid organ. In response to infection, NELO displays structural changes including the formation of T/NK cell clusters. NELO and gill lymphoid tissues form a cohesive unit within a large mucosal lymphoid network. Collectively, we reveal an unreported mucosal lymphoid organ reminiscent of mammalian tonsils that evolved in multiple teleost fish families.

nkx2.3 is responsible for posterior pharyngeal cartilage formation by inhibiting Fgf signaling

Nkx2.3, a transcription factor, plays important roles in various developmental processes. However, the mechanisms underlying nkx2.3's regulation of pouch and pharyngeal arch development in zebrafish remain unclear. In this study, we demonstrated that knockdown or knockout of nkx2.3 resulted in the absence of posterior ceratobranchial cartilages in zebrafish. The absence of posterior pharyngeal cartilages is a consequence of the compromised proliferation and differentiation and survival of cranial neural crest cells (CNCCs). Notably, we found that nkx2.3 was not involved in endoderm pouch formation. Additionally, our findings suggested that nkx2.3 negatively regulated Fibroblast growth factor (Fgf) signaling, as overexpression of fgf8 could mimic the phenotype observed in nkx2.3 morphants, suppressing CNCC differentiation. Moreover, inhibiting Fgf signaling restored the abnormalities in posterior cartilages induced by nkx2.3 knockdown. These findings establish the essential role of nkx2.3 in the development of posterior ceratobranchial cartilages through the inhibition of fgf8.

Ripply suppresses Tbx6 to induce dynamic-to-static conversion in somite segmentation

The metameric pattern of somites is created based on oscillatory expression of clock genes in presomitic mesoderm. However, the mechanism for converting the dynamic oscillation to a static pattern of somites is still unclear. Here, we provide evidence that Ripply/Tbx6 machinery is a key regulator of this conversion. Ripply1/Ripply2-mediated removal of Tbx6 protein defines somite boundary and also leads to cessation of clock gene expression in zebrafish embryos. On the other hand, activation of ripply1/ripply2 mRNA and protein expression is periodically regulated by clock oscillation in conjunction with an Erk signaling gradient. Whereas Ripply protein decreases rapidly in embryos, Ripply-triggered Tbx6 suppression persists long enough to complete somite boundary formation. Mathematical modeling shows that a molecular network based on results of this study can reproduce dynamic-to-static conversion in somitogenesis. Furthermore, simulations with this model suggest that sustained suppression of Tbx6 caused by Ripply is crucial in this conversion.

Foxl2a and Foxl2b are involved in midbrain-hindbrain boundary development in zebrafish

Foxl2 plays conserved central function in ovarian differentiation and maintenance in several fish species. However, its expression pattern and function in fish embryogenesis are still largely unknown. In this study, we first presented a sequential expression pattern of zebrafish foxl2a and foxl2b during embryo development. They were predominantly expressed in the cranial paraxial mesoderm (CPM) and cranial venous vasculature (CVV) during somitogenesis and subsequently expressed in the pharyngeal arches after 48 h post-fertilization (hpf). Then, we compared the brain structures among zebrafish wildtype (WT) and three homozygous foxl2 mutants (foxl2a^-/-, foxl2b^-/- and foxl2a^-/-;foxl2b^-/-) and found the reduction of the fourth ventricle in the three foxl2 mutants, especially in foxl2a^-/-;foxl2b^-/- mutant. Finally, we detected several key transcription factors involved in the gene regulatory network of midbrain-hindbrain boundary (MHB) patterning, such as wnt1, en1b and pax2a. Their expression levels were obviously downregulated in MHB of foxl2a^-/- and foxl2a^-/-;foxl2b^-/- mutants. Thus, we suggest that Foxl2a and Foxl2b are involved in MHB and the fourth ventricle development in zebrafish. The current study provides insights into the molecular mechanism underlying development of brain ventricular system.

The conundrum of pharyngeal teeth origin: the role of germ layers, pouches, and gill slits

There are several competing hypotheses on tooth origins, with discussions eventually settling in favour of an 'outside-in' scenario, in which internal odontodes (teeth) derived from external odontodes (skin denticles) in jawless vertebrates. The evolution of oral teeth from skin denticles can be intuitively understood from their location at the mouth entrance. However, the basal condition for jawed vertebrates is arguably to possess teeth distributed throughout the oropharynx (i.e. oral and pharyngeal teeth). As skin denticle development requires the presence of ectoderm-derived epithelium and of mesenchyme, it remains to be answered how odontode-forming skin epithelium, or its competence, were 'transferred' deep into the endoderm-covered oropharynx. The 'modified outside-in' hypothesis for tooth origins proposed that this transfer was accomplished through displacement of odontogenic epithelium, that is ectoderm, not only through the mouth, but also via any opening (e.g. gill slits) that connects the ectoderm to the epithelial lining of the pharynx (endoderm). This review explores from an evolutionary and from a developmental perspective whether ectoderm plays a role in (pharyngeal) tooth and denticle formation. Historic and recent studies on tooth development show that the odontogenic epithelium (enamel organ) of oral or pharyngeal teeth can be of ectodermal, endodermal, or of mixed ecto-endodermal origin. Comprehensive data are, however, only available for a few taxa. Interestingly, in these taxa, the enamel organ always develops from the basal layer of a stratified epithelium that is at least bilayered. In zebrafish, a miniaturised teleost that only retains pharyngeal teeth, an epithelial surface layer with ectoderm-like characters is required to initiate the formation of an enamel organ from the basal, endodermal epithelium. In urodele amphibians, the bilayered epithelium is endodermal, but the surface layer acquires ectodermal characters, here termed 'epidermalised endoderm'. Furthermore, ectoderm-endoderm contacts at pouch-cleft boundaries (i.e. the prospective gill slits) are important for pharyngeal tooth initiation, even if the influx of ectoderm via these routes is limited. A balance between sonic hedgehog and retinoic acid signalling could operate to assign tooth-initiating competence to the endoderm at the level of any particular pouch. In summary, three characters are identified as being required for pharyngeal tooth formation: (i) pouch-cleft contact, (ii) a stratified epithelium, of which (iii) the apical layer adopts ectodermal features. These characters delimit the area in which teeth can form, yet cannot alone explain the distribution of teeth over the different pharyngeal arches. The review concludes with a hypothetical evolutionary scenario regarding the persisting influence of ectoderm on pharyngeal tooth formation. Studies on basal osteichthyans with less-specialised types of early embryonic development will provide a crucial test for the potential role of ectoderm in pharyngeal tooth formation and for the 'modified outside-in' hypothesis of tooth origins.

From Endoderm to Progenitors: An Update on the Early Steps of Thyroid Morphogenesis in the Zebrafish

The mechanisms underlying thyroid gland development have a central interest in biology and this review is aimed to provide an update on the recent advancements on the early steps of thyroid differentiation that were obtained in the zebrafish, because this teleost fish revealed to be a suitable organism to study the early developmental stages. Physiologically, the thyroid precursors fate is delineated by the appearance among the endoderm cells of the foregut of a restricted cell population expressing specific transcription factors, including pax2a, nkx2.4b, and hhex. The committed thyroid primordium first appears as a thickening of the pharyngeal floor of the anterior endoderm, that subsequently detaches from the floor and migrates to its final location where it gives rise to the thyroid hormone-producing follicles. At variance with mammalian models, thyroid precursor differentiation in zebrafish occurs early during the developmental process before the dislocation to the eutopic positioning of thyroid follicles. Several pathways have been implicated in these early events and nowadays there is evidence of a complex crosstalk between intrinsic (coming from the endoderm and thyroid precursors) and extrinsic factors (coming from surrounding tissues, as the cardiac mesoderm) whose organization in time and space is probably required for the proper thyroid development. In particular, Notch, Shh, Fgf, Bmp, and Wnt signaling seems to be required for the commitment of endodermal cells to a thyroid fate at specific developmental windows of zebrafish embryo. Here, we summarize the recent findings produced in the various zebrafish experimental models with the aim to define a comprehensive picture of such complicated puzzle.