Chicken embryos share mammalian patterns of apoptosis in the posterior placodal area

In the posterior placodal area (PPA) of C57BL/6N mice and primate‐related Tupaia belangeri (Scandentia), apoptosis helps to establish morphologically separated otic and epibranchial placodes. Here, we demonstrate that basically identical patterns of apoptosis pass rostrocaudally through the Pax2⁺ PPA of chicken embryos. Interplacodal apoptosis eliminates unneeded cells either between the otic anlage and the epibranchial placodes 1, 2 and/or 3, respectively (type A), or between neighbouring epibranchial placodes (type B). These observations support the idea that in chicken embryos, as in mammals, interplacodal apoptosis serves to remove vestigial lateral line placodes (Washausen & Knabe, 2018, Biol Open 7 , bio031815). A special case represents the recently discovered Pax2^?/Sox2⁺ paratympanic organ (PTO) placode that has been postulated to be molecularly distinct from and developmentally independent of the ventrally adjacent first epibranchial (or ‘geniculate’) placode (O'Neill et al. 2012, Nat Commun 3 , 1041). We show that Sox2⁺ (PTO placodal) cells seem to segregate from the Pax2⁺ geniculate placode, and that absence of Pax2 in the mature PTO placode is due to secondary loss. We further report that, between Hamburger–Hamilton (HH) stages HH14 and HH26, apoptosis in the combined anlage of the first epibranchial and PTO placodes is almost exclusively found within and/or immediately adjacent to the dorsally located PTO placode. Hence, apoptosis appears to support decision‐making processes among precursor cells of the early developing PTO placode and, later, regression of the epibranchial placodes 2 and 3.     

Otic placode cell specification and proliferation are regulated by Notch signaling in avian development

Background: The entire inner ear including the cochlear‐vestibular ganglion arises from a simple epithelium, the otic placode. Precursors for the placode originate from a pool of progenitors located in ectoderm next to the future hindbrain, the pre‐otic field, where they are intermingled with future epibranchial and epidermal cells. While the importance of secreted proteins, such as FGFs and Wnts, in imparting otic identity has been well studied, how precursors for these different fates segregate locally is less well understood. Results: (1) The Notch ligand Delta1 and the Notch target Hes5‐2 are expressed in a part of pre‐otic field before otic commitment, indicative of active Notch signaling, and this is confirmed using a Notch reporter. (2) Loss and gain‐of‐function approaches reveal that Notch signaling regulates both proliferation and specification of pre‐otic progenitors. Conclusions: Our results identify a novel function of Notch signaling in cell fate determination in the pre‐otic field of avian embryos. Developmental Dynamics 244:839–851, 2015. © 2015 Wiley Periodicals, Inc.     

Plasticity of neural crest–placode interaction in the developing visceral nervous system

The reciprocal relationship between rhombomere (r)‐derived cranial neural crest (NC) and epibranchial placodal cells derived from the adjacent branchial arch is critical for visceral motor and sensory gangliogenesis, respectively. However, it is unknown whether the positional match between these neurogenic precursors is hard‐wired along the anterior–posterior (A/P) axis. Here, we use the interaction between r4‐derived NC and epibranchial placode‐derived geniculate ganglion as a model to address this issue. In Hoxa1^?/?b1^?/? embryos, r2 NC compensates for the loss of r4 NC. Specifically, a population of r2 NC cells is redirected toward the geniculate ganglion, where they differentiate into postganglionic (motor) neurons. Reciprocally, the inward migration of the geniculate ganglion is associated with r2 NC. The ability of NC and placodal cells to, respectively, differentiate and migrate despite a positional mismatch along the A/P axis reflects the plasticity in the relationship between the two neurogenic precursors of the vertebrate head. Developmental Dynamics 240:1880–1888, 2011. © 2011 Wiley‐Liss, Inc.     

Autotaxin controls caudal diencephalon‐mesencephalon development in the chick

The diencephalon is the embryonic anlagen of the higher integration centers of the brain. Recent studies have elucidated how the cells in the rostral diencephalon acquire their regional identities. However, the understanding of the mechanisms under which the caudal diencephalon is formed is still limited. Here we focus on the role of Autotaxin (ATX), a lysophospholipid‐generating exoenzyme, whose mRNA is detected in the caudal diencephalon. RNA interference against ATX altered the expression pattern of Pax6‐regualted genes, Tcf4, Lim1, and En1, implying that ATX is required for the maintenance of the regional identity of the caudal diencephalon and the diencephalon‐mesencephalon boundary (DMB). Furthermore, ATX‐RNAi inhibited neuroepithelial cell proliferation on both sides of the DMB. We propose a dual role of ATX in chick brain development, in which ATX not only contributes to the formation of caudal diencephalon as a short‐range signal, but also regulates the growth of mesencephalon as a long‐range signal. Developmental Dynamics 239:2647–2658, 2010. © 2010 Wiley‐Liss, Inc.     

The transcription factor D‐Pax2 regulates crystallin production during eye development in Drosophila melanogaster

The generation of a functioning Drosophila eye requires the coordinated differentiation of multiple cell types and the morphogenesis of eye‐specific structures. Here we show that D‐Pax2 plays a significant role in lens development through regulation of the Crystallin gene and because Crystallin is also expressed in D‐Pax2⁺ cells in the external sensory organs. Loss of D‐Pax2 function leads to loss of Crystallin expression in both eyes and bristles. A 2.3 kilobase (kb) upstream region of the Crystallin gene can drive GFP expression in the eye and is dependent on D‐Pax2. In addition, D‐Pax2 binds to an evolutionarily conserved site in this region that, by itself, is sufficient to drive GFP expression in the eye. However, mutation of this site does not greatly affect the regulatory region's function. The data indicate that D‐Pax2 acts to promote lens development by controlling the production of the major protein component of the lens. Whether this control is direct or indirect remains unresolved. Developmental Dynamics 238:2530–2539, 2009. © 2009 Wiley‐Liss, Inc.     

The role of Zic genes in inner ear development in the mouse: Exploring mutant mouse phenotypes

Background: Murine Zic genes (Zic1–5) are expressed in the dorsal hindbrain and in periotic mesenchyme (POM) adjacent to the developing inner ear. Zic genes are involved in developmental signaling pathways in many organ systems, including the ear, although their exact roles haven't been fully elucidated. This report examines the role of Zic1, Zic2, and Zic4 during inner ear development in mouse mutants in which these Zic genes are affected. Results: Zic1/Zic4 double mutants don't exhibit any apparent defects in inner ear morphology. By contrast, inner ears from Zic2^kd/kd and Zic2^Ku/Ku mutants have severe but variable morphological defects in endolymphatic duct/sac and semicircular canal formation and in cochlear extension in the inner ear. Analysis of otocyst patterning in the Zic2^Ku/Ku mutants by in situ hybridization showed changes in the expression patterns of Gbx2 and Pax2. Conclusions: The experiments provide the first genetic evidence that the Zic genes are required for morphogenesis of the inner ear. Zic2 loss‐of‐function doesn't prevent initial otocyst patterning but leads to molecular abnormalities concomitant with morphogenesis of the endolymphatic duct. Functional hearing deficits often accompany inner ear dysmorphologies, making Zic2 a novel candidate gene for ongoing efforts to identify the genetic basis of human hearing loss. Developmental Dynamics 243:1487–1498, 2014. © 2014 Wiley Periodicals, Inc.     

<Emphasis Type="Italic">Pax7</Emphasis> and superior collicular polarity: insights from <Emphasis Type="Italic">Pax6 (Sey)</Emphasis> mutant mice

Pax genes are important modulators of CNS development. Pax7 and Pax6 polarise the neural tube and regionalise the brain. Pax7 is pivotal in specifying the superior colliculus/tectum, an important centre for integration of visuomotor responses and a target for Pax6 ⁺ retinal ganglion cell axons during retinocollicular mapping. Whilst initial Pax7-specification of the mesencephalon is well-established, a role in regulating polarity within the maturing mouse superior colliculus is yet to be defined, although already detailed for the chick tectum. We therefore quantified Pax7 cellular distribution and expression levels at three functionally distinct stages of superior collicular development, and analysed Pax7 expression in response to aberrant axonal input and altered forebrain/midbrain boundary placement in Pax6 mutant mice. Comparative expression profiles of ephrin-A2 and its co-localisation with Pax7 were determined in wildtype and Pax6 mutant mice. Results indicate that graded Pax7 expression in wildtype mice is perturbed in Pax6 mutant mice; changes manifest as a shift in polarity, loss of graded expression and dramatically reduced protein levels during RGC synaptogenesis. Ephrin-A2 expression is similarly altered. These results implicate Pax7 as an important determinant of polarity within the mouse superior colliculus, and suggest a role in retinotopic mapping.     

Differential contribution of Neurog1 and Neurog2 on the formation of cranial ganglia along the anterior-posterior axis

Background: The neural crest (NC) and placode are transient neurogenic cell populations that give rise to cranial ganglia of the vertebrate head. The formation of the anterior NC‐ and placode‐derived ganglia has been shown to depend on the single activity of either Neurog1 or Neurog2. The requirement of the more posterior cranial ganglia on Neurog1 and Neurog2 is unknown. Results: Here we show that the formation of the NC‐derived parasympathetic otic ganglia and placode‐derived visceral sensory petrosal and nodose ganglia are dependent on the redundant activities of Neurog1 and Neurog2. Tamoxifen‐inducible Cre lineage labeling of Neurog1 and Neurog2 show a dynamic spatiotemporal expression profile in both NC and epibranchial placode that correlates with the phenotypes of the Neurog‐mutant embryos. Conclusion: Our data, together with previous studies, suggest that the formation of cranial ganglia along the anterior‐posterior axis is dependent on the dynamic spatiotemporal activities of Neurog1 and/or Neurog2 in both NC and epibranchial placode. Developmental Dynamics 241:229–241, 2012. © 2011 Wiley Periodicals, Inc.     

Expression profiles suggest a role for Pax7 in the establishment of tectal polarity and map refinement

The role for Pax7 in establishing tectal polarity and map refinement was authenticated by gene expression studies in vivo and in vitro. Throughout development (stages E2–E12 were examined) a rostral^low-caudal^high and dorsal^high-ventral^low Pax7 expression gradient was detected immunohistochemically in the chick optic tectum, indicating a role for Pax7 in establishing tectal polarity. Chick retino-recipient tectal cells positive for Pax7 also co-expressed ephrin-A2, a molecule involved in the establishment and refinement of the retinotopic map. In vitro, PAX7 up-regulated ephrin-A2 when transfected into undifferentiated P19 cells; cells became negative for both Pax7 and ephrin-A2 protein following treatment with anti-sense oligonucleotides. These results suggest that in addition to being involved in the early establishment of tectal polarity, Pax7 plays a later role in retino-tectal map formation and refinement.