A molecular analysis of neurogenic placode and cranial sensory ganglion development in the shark, Scyliorhinus canicula

Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway

Pax6 is involved in the control of neuronal specification, migration, and differentiation in the olfactory epithelium and in the generation of different interneuron subtypes in the olfactory bulb. Whether these roles are conserved during evolution is not known. Cartilaginous fish are extremely useful models for assessing the ancestral condition of brain organization because of their phylogenetic position. To shed light on the evolution of development of the olfactory system in vertebrates and on the involvement of Pax6 in this process, we analyzed by in situ hybridization and immunohistochemistry the expression pattern of Pax6 in the developing olfactory system in a basal vertebrate, the lesser spotted dogfish Scyliorhinus canicula. This small shark is becoming an important fish model in studies of vertebrate development. We report Pax6 expression in cells of the olfactory epithelium and olfactory bulb, and present the first evidence in vertebrates of strings of Pax6-expressing cells extending along the developing olfactory nerve. The results indicate the olfactory epithelium as the origin of these cells. These data are compatible with a role for Pax6 in the development of the olfactory epithelium and fibers, and provide a basis for future investigations into the mechanisms that regulate development of the olfactory system throughout evolution.

Electrosensory ampullary organs are derived from lateral line placodes in cartilaginous fishes

Ampullary organ electroreceptors excited by weak cathodal electric fields are used for hunting by both cartilaginous and non-teleost bony fishes. Despite similarities of neurophysiology and innervation, their embryonic origins remain controversial: bony fish ampullary organs are derived from lateral line placodes, whereas a neural crest origin has been proposed for cartilaginous fish electroreceptors. This calls into question the homology of electroreceptors and ampullary organs in the two lineages of jawed vertebrates. Here, we test the hypothesis that lateral line placodes form electroreceptors in cartilaginous fishes by undertaking the first long-term in vivo fate-mapping study in any cartilaginous fish. Using DiI tracing for up to 70 days in the little skate, Leucoraja erinacea, we show that lateral line placodes form both ampullary electroreceptors and mechanosensory neuromasts. These data confirm the homology of electroreceptors and ampullary organs in cartilaginous and non-teleost bony fishes, and indicate that jawed vertebrates primitively possessed a lateral line placode-derived system of electrosensory ampullary organs and mechanosensory neuromasts.

A timeline of pharyngeal endoskeletal condensation and differentiation in the shark, Scyliorhinus canicula, and the paddlefish, Polyodon spathula

The lesser-spotted dogfish (Scyliorhinus canicula) and the North American paddlefish (Polyodon spathula) are two emerging model systems for the study of vertebrate craniofacial development. Notably, both of these taxa have retained plesiomorphic aspects of pharyngeal endoskeletal organization, relative to more commonly used models of vertebrate craniofacial development (e.g. zebrafish, chick and mouse), and are therefore well suited to inform the pharyngeal endoskeletal patterning mechanisms that functioned in the last common ancestor of jawed vertebrates. Here, we present a histological overview of the condensation and chondrogenesis of the most prominent endoskeletal elements of the jaw, hyoid and gill arches - the palatoquadrate/Meckel's cartilage, the hyomandibula/ceratohyal, and the epi-/ceratobranchial cartilages, respectively - in embryonic series of S. canicula and P. spathula. Our observations provide a provisional timeline and anatomical framework for further molecular developmental and functional investigations of pharyngeal endoskeletal differentiation and patterning in these phylogenetically informative taxa.

The origin and evolution of the ectodermal placodes

Many of the features that distinguish the vertebrates from other chordates are found in the head. Prominent amongst these differences are the paired sense organs and associated cranial ganglia. Significantly, these structures are derived developmentally from the ectodermal placodes. It has therefore been proposed that the emergence of the ectodermal placodes was concomitant with and central to the evolution of the vertebrates. More recent studies, however, indicate forerunners of the ectodermal placodes can be readily identified outside the vertebrates, particularly in urochordates. Thus the evolutionary history of the ectodermal placodes is deeper and more complex than was previously appreciated with the full repertoire of vertebrate ectodermal placodes, and their derivatives, being assembled over a protracted period rather than arising collectively with the vertebrates.

Electrosensory ampullary organs are derived from lateral line placodes in bony fishes

Electroreception is an ancient subdivision of the lateral line sensory system, found in all major vertebrate groups (though lost in frogs, amniotes and most ray-finned fishes). Electroreception is mediated by 'hair cells' in ampullary organs, distributed in fields flanking lines of mechanosensory hair cell-containing neuromasts that detect local water movement. Neuromasts, and afferent neurons for both neuromasts and ampullary organs, develop from lateral line placodes. Although ampullary organs in the axolotl (a representative of the lobe-finned clade of bony fishes) are lateral line placode-derived, non-placodal origins have been proposed for electroreceptors in other taxa. Here we show morphological and molecular data describing lateral line system development in the basal ray-finned fish Polyodon spathula, and present fate-mapping data that conclusively demonstrate a lateral line placode origin for ampullary organs and neuromasts. Together with the axolotl data, this confirms that ampullary organs are ancestrally lateral line placode-derived in bony fishes.

Overview of the transcriptome profiles identified in hagfish, shark, and bichir: current issues arising from some nonmodel vertebrate taxa

Because of their crucial phylogenetic positions, hagfishes, sharks, and bichirs are recognized as key taxa in our understanding of vertebrate evolution. The expression patterns of the regulatory genes involved in developmental patterning have been analyzed in the context of evolutionary developmental studies. However, in a survey of public sequence databases, we found that the large-scale sequence data for these taxa are still limited. To address this deficit, we used conventional Sanger DNA sequencing and a next-generation sequencing technology based on 454 GS FLX sequencing to obtain expressed sequence tags (ESTs) of the Japanese inshore hagfish (Eptatretus burgeri; 161,482 ESTs), cloudy catshark (Scyliorhinus torazame; 165,819 ESTs), and gray bichir (Polypterus senegalus; 34,336 ESTs). We deposited the ESTs in a newly constructed database, designated the "Vertebrate TimeCapsule." The ESTs include sequences from genes that can be effectively used in evolutionary developmental studies; for instance, several encode cartilaginous extracellular matrix proteins, which are central to an understanding of the ways in which evolutionary processes affected the skeletal elements, whereas others encode regulatory genes involved in craniofacial development and early embryogenesis. Here, we discuss how hagfishes, sharks, and bichirs contribute to our understanding of vertebrate evolution, we review the current status of the publicly available sequence data for these three taxa, and we introduce our EST projects and newly developed database.

Molecular analysis of neurogenic placode development in a basal ray-finned fish

Neurogenic placodes are transient, thickened patches of embryonic vertebrate head ectoderm that give rise to the paired peripheral sense organs and most neurons in cranial sensory ganglia. We present the first analysis of gene expression during neurogenic placode development in a basal actinopterygian (ray-finned fish), the North American paddlefish (Polyodon spathula). Pax3 expression in the profundal placode confirms its homology with the ophthalmic trigeminal placode of amniotes. We report the conservation of expression of Pax2 and Pax8 in the otic and/or epibranchial placodes, Phox2b in epibranchial placode-derived neurons, Sox3 during epibranchial and lateral line placode development, and NeuroD in developing cranial sensory ganglia. We identify Sox3 as a novel marker for developing fields of electrosensory ampullary organs and for ampullary organs themselves. Sox3 is also the first molecular marker for actinopterygian ampullary organs. This is consistent with, though does not prove, a lateral line placode origin for actinopterygian ampullary organs.

Regionalization of the shark hindbrain: a survey of an ancestral organization

Cartilaginous fishes (chondrichthyans) represent an ancient radiation of vertebrates currently considered the sister group of the group of gnathostomes with a bony skeleton that gave rise to land vertebrates. This out-group position makes chondrichthyans essential in assessing the ancestral organization of the brain of jawed vertebrates. To gain knowledge about hindbrain evolution we have studied its development in a shark, the lesser spotted dogfish Scyliorhinus canicula by analyzing the expression of some developmental genes and the origin and distribution of specific neuronal populations, which may help to identify hindbrain subdivisions and boundaries and the topology of specific cell groups. We have characterized three developmental periods that will serve as a framework to compare the development of different neuronal systems and may represent a suitable tool for comparing the absolute chronology of development among vertebrates. The expression patterns of Pax6, Wnt8, and HoxA2 genes in early embryos of S. canicula showed close correspondence to what has been described in other vertebrates and helped to identify the anterior rhombomeres. Also in these early embryos, the combination of Pax6 with protein markers of migrating neuroblasts (DCX) and early differentiating neurons (general: HuC/D; neuron type specific: GAD, the GABA synthesizing enzyme) revealed the organization of S. canicula hindbrain in both transverse segmental units corresponding to visible rhombomeres and longitudinal columns. Later in development, when the interrhombomeric boundaries fade away, accurate information about S. canicula hindbrain subdivisions was achieved by comparing the expression patterns of Pax6 and GAD, serotonin (serotoninergic neurons), tyrosine hydroxylase (catecholaminergic neurons), choline acetyltransferase (cholinergic neurons), and calretinin (a calcium-binding protein). The patterns observed revealed many topological correspondences with other vertebrates and led to reconsideration of the current view of the elasmobranch hindbrain segmentation as peculiar among vertebrates.

Holocephalan embryos provide evidence for gill arch appendage reduction and opercular evolution in cartilaginous fishes

Chondrichthyans possess endoskeletal appendages called branchial rays that extend laterally from their hyoid and gill-bearing (branchial) arches. Branchial ray outgrowth, like tetrapod limb outgrowth, is maintained by Sonic hedgehog (Shh) signaling. In limbs, distal endoskeletal elements fail to form in the absence of normal Shh signaling, whereas shortened duration of Shh expression correlates with distal endoskeletal reduction in naturally variable populations. Chondrichthyans also exhibit natural variation with respect to branchial ray distribution--elasmobranchs (sharks and batoids) possess a series of ray-supported septa on their hyoid and gill arches, whereas holocephalans (chimaeras) possess a single hyoid arch ray-supported operculum. Here we show that the elongate hyoid rays of the holocephalan Callorhinchus milii grow in association with sustained Shh expression within an opercular epithelial fold, whereas Shh is only transiently expressed in the gill arches. Coincident with this transient Shh expression, branchial ray outgrowth is initiated in C. milii but is not maintained, yielding previously unrecognized vestigial gill arch branchial rays. This is in contrast to the condition seen in sharks, where sustained Shh expression corresponds to the presence of fully formed branchial rays on the hyoid and gill arches. Considered in light of current hypotheses of chondrichthyan phylogeny, our data suggest that the holocephalan operculum evolved in concert with gill arch appendage reduction by attenuation of Shh-mediated branchial ray outgrowth, and that chondrichthyan branchial rays and tetrapod limbs exhibit parallel developmental mechanisms of evolutionary reduction.

From shared lineage to distinct functions: the development of the inner ear and epibranchial placodes

The inner ear and the epibranchial ganglia constitute much of the sensory system in the caudal vertebrate head. The inner ear consists of mechanosensory hair cells, their neurons, and structures necessary for sound and balance sensation. The epibranchial ganglia are knots of neurons that innervate and relay sensory signals from several visceral organs and the taste buds. Their development was once thought to be independent, in line with their independent functions. However, recent studies indicate that both systems arise from a morphologically distinct common precursor domain: the posterior placodal area. This review summarises recent studies into the induction, morphogenesis and innervation of these systems and discusses lineage restriction and cell specification in the context of their common origin.

The evolution of gnathostome development: Insight from chondrichthyan embryology

Chondrichthyans (cartilaginous fishes) represent one of the two lineages of gnathostomes, the other being the osteicthyans (bony fishes). Classical studies on chondrichthyan embryology have strongly impacted our views of vertebrate body plan evolution, while recent studies highlight oviparous chondrichthyans as emerging vertebrate model systems that are amenable to experimental embryological manipulation. Here, we review three particular areas of interest in the field of chondrichthyan developmental biology-gastrulation, neural development, and appendage patterning-and we discuss recent findings within a broader chondrichthyan-osteichthyan comparative framework. In some cases, comparative studies of chondrichthyan and osteichthyan development reveal conserved patterns of gene expression in common developmental contexts. Studies of chondrichthyan gastrulation reveal conserved patterns of developmental gene expression, despite highly divergent modes of mesendoderm internalization, while molecular characterization of chondrichthyan neurogenic placodes indicates a conservation of placode transcription factor expression across gnathostome phylogeny. In other cases, comparative studies of chondrichthyan and osteichthyan development yield evidence of shared patterning mechanisms functioning in different developmental contexts. This is exemplified by studies on the development of chondrichthyan appendages-paired fins, median fins, and gill rays. These have demonstrated that a retinoic acid-responsive Shh-expressing signaling center functions to pattern the endoskeleton of gnathostome paired fins and chondrichthyan gill rays, while expression patterns of Tbx18 and HoxD family members are shared by gnathostome paired fins and chondrichthyan median fins. These findings fuel novel hypotheses of developmental genetic homology, and demonstrate how comparative studies of gnathostome development can provide insight into the evolutionary processes that underlie morphological diversity.

The Pax3 and Pax7 paralogs cooperate in neural and neural crest patterning using distinct molecular mechanisms, in Xenopus laevis embryos

Pax3 and Pax7 paralogous genes have functionally diverged in vertebrate evolution, creating opportunity for a new distribution of roles between the two genes and the evolution of novel functions. Here we focus on the regulation and function of Pax7 in the brain and neural crest of amphibian embryos, which display a different pax7 expression pattern, compared to the other vertebrates already described. Pax7 expression is restricted to the midbrain, hindbrain and anterior spinal cord, and Pax7 activity is important for maintaining the fates of these regions, by restricting otx2 expression anteriorly. In contrast, pax3 displays broader expression along the entire neuraxis and Pax3 function is important for posterior brain patterning without acting on otx2 expression. Moreover, while both genes are essential for neural crest patterning, we show that they do so using two distinct mechanisms: Pax3 acts within the ectoderm which will be induced into neural crest, while Pax7 is essential for the inducing activity of the paraxial mesoderm towards the prospective neural crest.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Apoptosis and proliferation in the trigeminal placode

The neurogenic trigeminal placode develops from the crescent-shaped panplacodal primordium which delineates the neural plate anteriorly. We show that, in Tupaia belangeri, the trigeminal placode is represented by a field of focal ectodermal thickenings which over time changes positions from as far rostral as the level of the forebrain to as far caudal as opposite rhombomere 3. Delamination proceeds rostrocaudally from the ectoderm adjacent to the rostral midbrain, and contributes neurons to the trigeminal ganglion as well as to the ciliary ganglion/oculomotor complex. Proliferative events are centered on the field prior to the peak of delamination. They are preceded, paralleled and, finally, outnumbered by apoptotic events which proceed rostrocaudally from non-delaminating to delaminating parts of the field. Apoptosis persists upon regression of the placode, thereby exhibiting a massive “wedge” of apoptotic cells which includes the postulated position of the “ventrolateral postoptic placode” (Lee et al. in Dev Biol 263:176–190, 2003), merges with groups of lens-associated apoptotic cells, and disappears upon lens detachment. In conjunction with earlier work (Washausen et al. in Dev Biol 278:86–102, 2005) our findings suggest that apoptosis contributes repeatedly to the disintegration of the panplacodal primordium, to the elimination of subsets of premigratory placodal neuroblasts, and to the regression of placodes.

Chordate roots of the vertebrate nervous system: expanding the molecular toolkit

The vertebrate brain is highly complex with millions to billions of neurons. During development, the neural plate border region gives rise to the neural crest, cranial placodes and, in anamniotes, to Rohon-Beard sensory neurons, whereas the boundary region of the midbrain and hindbrain develops organizer properties. Comparisons of developmental gene expression and neuroanatomy between vertebrates and the basal chordate amphioxus, which has only thousands of neurons and lacks a neural crest, most placodes and a midbrain-hindbrain organizer, indicate that these vertebrate features were built on a foundation already present in the ancestral chordate. Recent advances in genomics have provided insights into the elaboration of the molecular toolkit at the invertebrate-vertebrate transition that may have facilitated the evolution of these vertebrate characteristics.

Embryonic development of glial cells and myelin in the shark, Chiloscyllium punctatum

Glial cells are responsible for a wide range of functions in the nervous system of vertebrates. The myelinated nervous systems of extant elasmobranchs have the longest independent history of all gnathostomes. Much is known about the development of glia in other jawed vertebrates, but research in elasmobranchs is just beginning to reveal the mechanisms guiding neurodevelopment. This study examines the development of glial cells in the bamboo shark, Chiloscyllium punctatum, by identifying the expression pattern of several classic glial and myelin proteins. We show for the first time that glial development in the bamboo shark (C. punctamum) embryo follows closely the one observed in other vertebrates and that neural development seems to proceed at a faster rate in the PNS than in the CNS. In addition, we observed more myelinated tracts in the PNS than in the CNS, and as early as stage 32, suggesting that the ontogeny of myelin in sharks is closer to osteichthyans than agnathans.

Activation of Pax3 target genes is necessary but not sufficient for neurogenesis in the ophthalmic trigeminal placode

Vertebrate cranial neurogenic placodes are relatively simple model systems for investigating the control of sensory neurogenesis. The ophthalmic trigeminal (opV) placode, for which the earliest specific marker is the paired domain homeodomain transcription factor Pax3, forms cutaneous sensory neurons in the ophthalmic lobe of the trigeminal ganglion. We previously showed that Pax3 expression in avian opV placode cells correlates with specification and commitment to a Pax3+, cutaneous sensory neuron fate. Pax3 can act as a transcriptional activator or repressor, depending on the cellular context. We show using mouse Splotch(2H) mutants that Pax3 is necessary for the normal neuronal differentiation of opV placode cells. Using an electroporation construct encoding a Pax3-Engrailed fusion protein, which represses Pax3 target genes, we show that activation of Pax3 target genes is required cell-autonomously within chick opV placode cells for expression of the opV placode markers FGFR4 and Ngn2, maintenance of the preplacodal marker Eya2, expression of Pax3 itself (suggesting that Pax3 autoregulates), neuronal differentiation and delamination. Mis-expression of Pax3 in head ectoderm is sufficient to induce FGFR4 and Ngn2 expression, but neurons do not differentiate, suggesting that additional signals are necessary to enable Pax3+ cells to differentiate as neurons. Mis-expression of Pax3 in the Pax2+ otic and epibranchial placodes also downregulates Pax2 and disrupts otic vesicle closure, suggesting that Pax3 is sufficient to alter the identity of these cells. Overall, our results suggest that activation of Pax3 target genes is necessary but not sufficient for neurogenesis in the opV placode.

The Dogfish Scyliorhinus canicula: A Reference in Jawed Vertebrates

INTRODUCTIONDue to their large size and long generation times, chondrichthyans have been largely ignored by geneticists. However, their key phylogenetic position makes them ideal subjects to study the molecular bases of the important morphological and physiological innovations that characterize jawed vertebrates. Such analyses are crucial to understanding the origin of the complex genetic mechanisms unraveled in osteichthyans. The small spotted dogfish Scyliorhinus canicula, a representative of the largest order of extant sharks, presents a number of advantages in this context. Due to its relatively small size among sharks, its abundance, and easy maintenance, the dogfish has been an important model in comparative anatomy and physiology for more than a century. Recently, revived interest has occurred with the development of large-scale transcriptomic and genomic resources, together with the establishment of facilities allowing massive egg and embryo production. These new tools open the way to molecular analyses of the elaborate physiological and sensory systems used by sharks. They also make it possible to take advantage of unique characteristics of these species, such as organ zonation, in analyses of cell proliferation and differentiation. Finally, they offer important perspectives to evolutionary developmental biology that will provide a better understanding of the origin and diversifications of jawed vertebrates. The dogfish whole-genome sequence, which may shortly become accessible, should establish this species as an essential shark reference, complementary to other chondrichthyan models. These analyses are likely to reveal an organism of an underestimated complexity, far from the primitive prototypical gnathostome anticipated in gradistic views.

Expression of FoxC, FoxF, FoxL1, and FoxQ1 genes in the dogfish Scyliorhinus canicula defines ancient and derived roles for Fox genes in vertebrate development

In the human genome, members of the FoxC, FoxF, FoxL1, and FoxQ1 gene families are found in two paralagous clusters. Here we characterize all four gene families in the dogfish Scyliorhinus canicula, a member of the cartilaginous fish lineage that diverged before the radiation of osteichthyan vertebrates. We identify two FoxC genes, two FoxF genes, and single FoxQ1 and FoxL1 genes, demonstrating cluster duplication preceded the radiation of gnathostomes. The expression of all six genes was analyzed by in situ hybridization. The results show conserved expression of FoxL1, FoxF, and FoxC genes in different compartments of the mesoderm and of FoxQ1 in pharyngeal endoderm and its derivatives, confirming these as ancient sites of Fox gene expression, and also illustrate multiple cases of lineage-specific expression domains. Comparison to invertebrate chordates shows that the majority of conserved vertebrate expression domains mark tissues that are part of the primitive chordate body plan.

Lateral line, otic and epibranchial placodes: developmental and evolutionary links?

Two embryonic cell populations, the neural crest and cranial ectodermal placodes, between them give rise to many of the unique characters of vertebrates. Neurogenic placode derivatives are vital for sensing both external and internal stimuli. In this speculative review, we discuss potential developmental and evolutionary relationships between two placode series that are usually considered to be entirely independent: lateral line placodes, which form the mechanosensory and electroreceptive hair cells of the anamniote lateral line system as well as their afferent neurons, and epibranchial placodes (geniculate, petrosal and nodose), which form Phox2b(+) visceral sensory neurons with input from both the external and internal environment. We illustrate their development using molecular data we recently obtained in shark embryos, and we describe their derivatives, including the possible geniculate placode origin of a mechanosensory sense organ associated with the first pharyngeal pouch/cleft (the anamniote spiracular organ/amniote paratympanic organ). We discuss how both lateral line and epibranchial placodes can be related in different ways to the otic placode (which forms the inner ear and its afferent neurons), and how both are important for protective somatic reflexes. Finally, we put forward a highly speculative proposal about the original function of the cells whose evolutionary descendants today include the derivatives of the lateral line, otic and epibranchial placodes, namely that they produced sensory receptors and neurons for Phox2b-dependent protective reflex circuits. We hope this review will stimulate both debate and a fresh look at possible developmental and evolutionary relationships between these seemingly disparate and independent placodes.

Historical hypotheses regarding segmentation of the vertebrate head

The morphology of the vertebrate head is extremely complex and comprises numerous iterative structures that arise from each of the embryonic germ layers. The search for a fundamental plan uniting all of these serial structures spans ∼200 years. The earliest attempt to identify a common plan was J. W. Goethe's vertebral theory of skull organization, in which the skull was interpreted as being formed by a series of trunk vertebrae. This theory was rejected by T. H. Huxley in the 1858 Croonian Lecture and was replaced by the segmented mesodermal model of Francis Balfour, which was elaborated subsequently by A. Marshall, Gavin de Beer, and Edwin Goodrich. This model assumes that the head of the earliest vertebrates consisted of eight segments. It further assumes that each segment contained dorsal muscles arising from the somitic mesoderm, and ventral muscles arising from lateral plate mesoderm, except for the first segment, which lacked ventral muscles derived from the lateral plate mesoderm. The muscles of each head segment were believed to be innervated by two pairs of cranial nerves, homologous to the dorsal and ventral spinal nerves of lampreys. The validity of this theory, known as the Goodrich model, came into question, however, after the discovery that the branchiomeric muscles associated with each pharyngeal arch do not arise from lateral plate mesoderm, as initially proposed by Marshall and subsequently accepted by Goodrich and de Beer, but, rather, arise from paraxial mesoderm. Furthermore, segmentation of the brain into some 14 neuromeres cannot be accommodated by any model involving eight segments. Finally, there is also clear evidence that at least one, if not two, additional series of placodally derived sensory nerves occurs in the head and has no counterpart in the trunk. At present, there is no theory of segmentation that can account for all cephalic iterative structures.

Fine-grained fate maps for the ophthalmic and maxillomandibular trigeminal placodes in the chick embryo

Vertebrate cranial ectodermal placodes are transient, paired thickenings of embryonic head ectoderm that are crucial for the formation of the peripheral sensory nervous system: they give rise to the paired peripheral sense organs (olfactory organs, inner ears and anamniote lateral line system), as well as the eye lenses, and most cranial sensory neurons. Here, we present the first detailed spatiotemporal fate-maps in any vertebrate for the ophthalmic trigeminal (opV) and maxillomandibular trigeminal (mmV) placodes, which give rise to cutaneous sensory neurons in the ophthalmic and maxillomandibular lobes of the trigeminal ganglion. We used focal DiI and DiO labelling to produce eight detailed fate-maps of chick embryonic head ectoderm over approximately 24 h of development, from 0-16 somites. OpV and mmV placode precursors arise from a partially overlapping territory; indeed, some individual dyespots labelled both opV and mmV placode-derived cells. OpV and mmV placode precursors are initially scattered within a relatively large region of ectoderm adjacent to the neural folds, intermingled both with each other and with future epidermal cells, and with geniculate and otic placode precursors. Although the degree of segregation increases with time, there is no clear border between the opV and mmV placodes even at the 16-somite stage, long after neurogenesis has begun in the opV placode, and when neurogenesis is just beginning in the mmV placode. Finally, we find that occasional cells in the border region between the opV placode and mmV placode express both Pax3 (an opV placode specific marker) and Neurogenin1 (an mmV placode specific marker), suggesting that a few cells are responding to both opV and mmV placode-inducing signals. Overall, our results fill a large gap in our knowledge of the early stages of development of both the opV and mmV placodes, providing an essential framework for subsequent studies of the molecular control of their development.

Molecular evolution of the vertebrate mechanosensory cell and ear

The molecular basis of mechanosensation, mechanosensory cell development and mechanosensory organ development is reviewed with an emphasis on its evolution. In contrast to eye evolution and development, which apparently modified a genetic program through intercalation of genes between the master control genes on the top (Pax6, Eya1, Six1) of the hierarchy and the structural genes (rhodopsin) at the bottom, the as yet molecularly unknown mechanosensory channel precludes such a firm conclusion for mechanosensors. However, recent years have seen the identification of several structural genes which are involved in mechanosensory tethering and several transcription factors controlling mechanosensory cell and organ development; these warrant the interpretation of available data in very much the same fashion as for eye evolution: molecular homology combined with potential morphological parallelism. This assertion of molecular homology is strongly supported by recent findings of a highly conserved set of microRNAs that appear to be associated with mechanosensory cell development across phyla. The conservation of transcription factors and their regulators fits very well to the known or presumed mechanosensory specializations which can be mostly grouped as variations of a common cellular theme. Given the widespread distribution of the molecular ability to form mechanosensory cells, it comes as no surprise that structurally different mechanosensory organs evolved in different phyla, presenting a variation of a common theme specified by a conserved set of transcription factors in their cellular development. Within vertebrates and arthropods, some mechanosensory organs evolved into auditory organs, greatly increasing sensitivity to sound through modifications of accessory structures to direct sound to the specific sensory epithelia. However, while great attention has been paid to the evolution of these accessory structures in vertebrate fossils, comparatively less attention has been spent on the evolution of the inner ear and the central auditory system. Recent advances in our molecular understanding of ear and brain development provide novel avenues to this neglected aspect of auditory neurosensory evolution.