The developmental expression of foxl2 in the dogfish Scyliorhinus canicula

The FoxL2 genes are a subfamily of the Fox (forkhead box) gene family. FOXL2 is mutated in the disorder Blepharophimosis, Ptosis, and Epicanthus Inversus Syndrome (BPES), which is characterized by eyelid malformations, and Premature Ovarian Failure (POF). In the mouse expression is seen in the perioptic mesenchyme, developing eyelids, ovary and pituitary. We have isolated a foxl2 cDNA from the dogfish Scyliorhinus canicula (also known as the lesser spotted catshark), allowing the characterisation of this gene's sequence and expression from a lineage that diverged early in the evolution of gnathostomes. Molecular phylogenetic analysis strongly grouped this sequence with the gnathostomes within the FoxL2 subfamily. We demonstrate the early expression of Scyliorhinus canicula foxl2 in the mandibular head mesoderm and later in continuous populations of mandibular arch cells and mandibular head mesenchyme cells around the developing pituitary. As development proceeds expression decreases in the mesenchyme of the head but is seen in the mesenchyme around the eye and later in the developing eyelids. Additionally expression is seen in regions of pharyngeal arch mesoderm and in ectoderm from which gill buds will form. This expression is maintained in the developing and elongating gill buds. Thus, S. canicula foxl2 is a marker for the mandibular mesoderm and gill buds and its expression is conserved in the perioptic mesenchyme, developing eyelids and pituitary.     

Differential expression of Eya1 and Eya2 during chick early embryonic development

Eyes absent is essential for compound eye formation in Drosophila. Its mammalian homologues of Eya are involved in the development of sensory organs, skeletal muscles and kidneys. Mutations of EYA1 in human cause branchio-oto-renal syndrome, with abnormalities in branchial derivatives, ear and kidney. For an insight into the function of Eya1 and Eya2 in early development, we performed whole-mount in situ hybridization and compared the expression patterns of these two genes in the developing chick embryos. Eya1 was first expressed in the primitive streak at Hamburger and Hamilton stage 4 (HH4) and appeared in the ectoderm and head mesenchyme distinct from migrating neural crest cells at HH6-HH11. At HH15 and HH17, the olfactory, otic and vagal/nodose placodes and cranial ganglia were positive for Eya1. In contrast, Eya2 was already expressed in the endoderm at HH4, and appeared in the endoderm and prospective placodal region at HH6-HH11. Eya2 expression was observed in pharyngeal clefts and pouches as well as cranial placodes at HH15 and HH17. These results indicate differential expression of Eya1 and Eya2, both spatially and temporally, in chick during early development. The expression patterns are somewhat different from those of other species such as Xenopus, zebrafish and mouse. The results suggest distinct and unique functions for Eya1 and Eya2 in early chick development.     

Xenopus galectin-VIa shows highly specific expression in cement glands and is regulated by canonical Wnt signaling

Anterior-posterior neural patterning of Xenopus embryo is determined during gastrulation and then followed by differentiation of neural structures including brain and eye. The cement gland is a mucus-secreting neural organ located in the anterior end of the neural plate. This study analyzed expression patterns of Xenopus galectin-VIa (Xgalectin-VIa) by whole-mount in situ hybridization, and found highly restricted expression of this gene in the cement gland region. These patterns were similar to those of XAG-1 and XCG, known cement gland-specific genes. In addition, Xgalectin-VIa was expressed in the dorsal edge of eye vesicles, the otic vesicle, and in part of the hatching gland at the tadpole stage. Although the spatial expression pattern was similar, the temporal expression of Xgalectin-VIa differed from that of XAG-1 and XCG. RT-PCR analysis showed only weak Xgalectin-VIa expression in early neurula embryos, whereas both XAG-1 and CGS were strongly expressed at that stage. We also showed that Xgalectin-VIa expression is repressed by enhancement of Wnt signaling and increased by its inhibition. Furthermore, Xgalectin-VIa expression was activated by neural-gene inducer Xotx2, as is the case for XAG-1 and CGS. Together, these results indicated that Xgalectin-VIa possesses different features from other cement gland genes and is a novel and useful marker of the cement gland in developing embryos.     

Platelet-derived growth factor-A and its receptor are expressed in separate, but adjacent cell layers of the mouse embryo.

The localized developmental expression of murine platelet-derived growth factor A (PDGF-A) was compared to that of its receptor (Pdgfra). Our in situ hybridization study included germ layers of primitive streak embryos, early axial structures (dermatome, myotome, sclerotome, floor plate), the skin and some of its derivatives (hair and mammary gland), the developing forelimb, the branchial arches and various sense organs (otic vesicle, olfactory epithelium and the eye). We report that PDGF-A and Pdgfra are expressed in separate, but adjacent cell layers in these structures and that in most, the ligand is expressed in the epithelium, whereas the receptor in the mesenchyme. This localization corresponds to classical experimental evidence for developmental interactions across cell layers. We suggest that the spatio-temporal regulation of PDGF-A and Pdgfra, and other related systems, represents one model for the spatial regulation of receptor-ligand interactions.     

The Sine oculis/Six class family of homeobox genes in jellyfish with and without eyes: development and eye regeneration

The development of visual organs is regulated in Bilateria by a network of genes where members of the Six and Pax gene families play a central role. To investigate the molecular aspects of eye evolution, we analyzed the structure and expression patterns of cognate members of the Six family genes in jellyfish (Cnidaria, Hydrozoa), representatives of a basal, non-bilaterian phylum where complex lens eyes with spherical lens, an epidermal cornea, and a retina appear for the first time in evolution. In the jellyfish Cladonema radiatum, a species with well-developed lens eyes in the tentacle bulbs, Six1/2-Cr and Six3/6-Cr, are expressed in the eye cup. Six4/5-Cr is mainly expressed in the manubrium, the feeding, and sex organ. All three Six genes are expressed in different subsets of epidermal nerve cells, possibly of the RFamide type which are part of a net connecting the different eyes with each other and the effector organs. Furthermore, expression is found in other tissues, notably in the striated muscle. During eye regeneration, expression of Six1/2-Cr and Six3/6-Cr is upregulated, but not of Six4/5-Cr. In Podocoryne carnea, a jellyfish without eyes, Six1/2-Pc and Six3/6-Pc are also expressed in the tentacle bulbs, Six1/2-Pc additionally in the manubrium and striated muscle, and Six3/6-Pc in the mechanosensory nematocytes of the tentacle. The conserved gene structure and expression patterns of all Cladonema Six genes suggest broad conservation of upstream regulatory mechanisms in eye development.     

Expression of evolutionarily conserved eye specification genes during Drosophila embryogenesis

Eye specification in Drosophila is thought be controlled by a set of seven nuclear factors that includes the Pax6 homolog, Eyeless. This group of genes is conserved throughout evolution and has been repeatedly recruited for eye specification. Several of these genes are expressed within the developing eyes of vertebrates and mutations in several mouse and human orthologs are the underlying causes of retinal disease syndromes. Ectopic expression in Drosophila of any one of these genes is capable of inducing retinal development, while loss-of-function mutations delete the developing eye. These nuclear factors comprise a complex regulatory network and it is thought that their combined activities are required for the formation of the eye. We examined the expression patterns of four eye specification genes, eyeless (ey), sine oculis (so), eyes absent (eya), and dachshund (dac) throughout all time points of embryogenesis and show that only eyeless is expressed within the embryonic eye anlagen. This is consistent with a recently proposed model in which the eye primordium acquires its competence to become retinal tissue over several time points of development. We also compare the expression of Ey with that of a putative antennal specifying gene Distal-less (Dll). The expression patterns described here are quite intriguing and raise the possibility that these genes have even earlier and wide ranging roles in establishing the head and visual field.     

Planarian Gtsix3, a member of the Six/so gene family,is expressed in brain branches but not in eye cells

Six/sine oculis (Six/so) class genes, with representatives in vertebrates and invertebrates, include members with key developmental roles in the anterior part of the central nervous system (CNS) and eye. Having characterized the role of the first planarian gene of the Six/so family in eye development, we attempted to identify novel genes of this family related to the platyhelminth eye genetic network. We isolated a new Six/so gene in the planarian Girardia tigrina, Gtsix-3, which belongs to the Six3/6 class. Whole mount in situ hybridization revealed Gtsix3 expression in a stripe surrounding the cephalic ganglia in adults. This spatial pattern corresponds to the cephalic branches, the nerve cells that connect the CNS with the marginal sensory organs located continuously at the edge of the head. During head regeneration, Gtsix-3 shows delayed activation compared to other head genes, with an initial two spot pattern that later evolves to a continuous lateral expression in the new regenerated cephalic ganglia with a final reduction to the adult pattern. However, Gtsix-3 is not activated in tail regeneration and no eye expression is observed at any regenerative stage. These findings provide a new marker for the developing anterior nervous system and evidence the complexity of planarian brain.     

Planarian Gtsix3, a member of the Six/so gene family, is expressed in brain branches but not in eye cells

Six/sine oculis (Six/so) class genes, with representatives in vertebrates and invertebrates, include members with key developmental roles in the anterior part of the central nervous system (CNS) and eye. Having characterized the role of the first planarian gene of the Six/so family in eye development, we attempted to identify novel genes of this family related to the platyhelminth eye genetic network. We isolated a new Six/so gene in the planarian Girardia tigrina, Gtsix-3, which belongs to the Six3/6 class. Whole mount in situ hybridization revealed Gtsix3 expression in a stripe surrounding the cephalic ganglia in adults. This spatial pattern corresponds to the cephalic branches, the nerve cells that connect the CNS with the marginal sensory organs located continuously at the edge of the head. During head regeneration, Gtsix-3 shows delayed activation compared to other head genes, with an initial two spot pattern that later evolves to a continuous lateral expression in the new regenerated cephalic ganglia with a final reduction to the adult pattern. However, Gtsix-3 is not activated in tail regeneration and no eye expression is observed at any regenerative stage. These findings provide a new marker for the developing anterior nervous system and evidence the complexity of planarian brain.     

Sonic hedgehog regulates otic capsule chondrogenesis and inner ear development in the mouse embryo

Development of the cartilaginous capsule of the inner ear is dependent on interactions between otic epithelium and its surrounding periotic mesenchyme. During these tissue interactions, factors endogenous to the otic epithelium influence the differentiation of the underlying periotic mesenchyme to form a chondrified otic capsule. We report the localization of Sonic hedgehog (Shh) protein and expression of the Shh gene in the tissues of the developing mouse inner ear. We demonstrate in cultures of periotic mesenchyme that Shh alone cannot initiate otic capsule chondrogenesis. However, when Shh is added to cultured periotic mesenchyme either in combination with otic epithelium or otic epithelial-derived fibroblast growth factor (FGF2), a significant enhancement of chondrogenesis occurs. Addition of Shh antisense oligonucleotide (AS) to cultured periotic mesenchyme with added otic epithelium decreases levels of endogenous Shh and suppresses the chondrogenic response of the mesenchyme cells, while supplementation of Shh AS-treated cultures with Shh rescues cultures from chondrogenic inhibition. We demonstrate that inactivation of Shh by targeted mutation produces anomalies in the developing inner ear and its surrounding capsule. Our results support a role for Shh as a regulator of otic capsule formation and inner ear development during mammalian embryogenesis.