FGF-dependent mechanosensory organ patterning in zebrafish.

During development, organ primordia reorganize to form repeated functional units. In zebrafish (Danio rerio), mechanosensory organs called neuromasts are deposited at regular intervals by the migrating posterior lateral line (pLL) primordium. The pLL primordium is organized into polarized rosettes representing proto-neuromasts, each with a central atoh1a-positive focus of mechanosensory precursors. We show that rosettes form cyclically from a progenitor pool at the leading zone of the primordium as neuromasts are deposited from the trailing region. fgf3/10 signals localized to the leading zone are required for rosette formation, atoh1a expression, and primordium migration. We propose that the fibroblast growth factor (FGF) source controls primordium organization, which, in turn, regulates the periodicity of neuromast deposition. This previously unrecognized mechanism may be applicable to understanding segmentation and morphogenesis in other organ systems.

Evolutionary changes in sensory precursor formation in arthropods: embryonic development of leg sensilla in the spider Cupiennius salei.

We describe here for the first time the development of mechanosensory organs in a chelicerate, the spider Cupiennius salei. It has been shown previously that the number of external sense organs increases with each moult. While stage 1 larvae do not have any external sensory structures, stage 2 larvae show a stereotyped pattern of touch sensitive 'tactile hairs' on their legs. We show that these mechanosensory organs develop during embryogenesis. In contrast to insects, groups of sensory precursors are recruited from the leg epithelium, rather than single sensory organ progenitors. The groups increase by proliferation, and neural cells delaminate from the cluster, which migrate away to occupy a position proximal to the accessory cells of the sense organ. In addition, we describe the development of putative internal sense organs, which do not differentiate until larval stage 2. We show by RNA interference that, similar to Drosophila, proneural genes are responsible for the formation and subtype identity of sensory organs. Furthermore, we demonstrate an additional function for proneural genes in the coordinated invagination and migration of neural cells during sensory organ formation in the spider.

Development of Johnston's organ in Drosophila.

Hearing is a specialized mechanosensory modality that is refined during evolution to meet the particular requirements of different organisms. In the fruitfly, Drosophila, hearing is mediated by Johnston's organ, a large chordotonal organ in the antenna that is exquisitely sensitive to the near-field acoustic signal of courtship songs generated by male wing vibration. We summarize recent progress in understanding the molecular genetic determinants of Johnston's organ development and discuss surprising differences from other chordotonal organs that likely facilitate hearing. We outline novel discoveries of active processes that generate motion of the antenna for acute sensitivity to the stimulus. Finally, we discuss further research directions that would probe remaining questions in understanding Johnston's organ development, function and evolution.

Stretching, mechanotransduction, and proinflammatory cytokines in the fetal membranes.

In the third trimester of normal pregnancy, the human fetal membranes become increasingly distended and use mechanotransduction and its downstream signaling to remodel and function. Their overdistension either by multifetal pregnancy or by polyhydramnios often leads to preterm birth, but the mechanism is unclear. Stretching of the fetal membranes in vitro upregulates several cytokines and enzymes that can drive collagen degradation, leading to membrane rupture. The sensitivity of this response appears to be specific for different cell types and is likely to result from differential activation of some key transcription factors and cofactors. Few cytokines in the fetal membranes respond to stretch: the most robust of these is pre-B-cell colony-enhancing factor (PBEF). This is constitutively expressed and protects the amnion cells from apoptosis caused by chronic static distension. However, it can also be stimulated by inflammation, infection, and hypoxia and upregulates a number of proinflammatory cytokines, chemokines, and enzymes important in the initiation of parturition. Therefore, it is proposed here that PBEF functions in normal pregnancy to protect the amnion cells as they become increasingly stretched, but if stimulated, it can initiate key events leading to parturition.

The molecular diversity of Dscam is functionally required for neuronal wiring specificity in Drosophila.

Alternative splicing of Dscam generates an enormous molecular diversity with maximally 38,016 different receptors. Whether this large diversity is required in vivo is currently unclear. We examined the role of Dscam in neuron-target recognition of single mechanosensory neurons, which connect with different target cells through multiple axonal branches. Analysis of Dscam null neurons demonstrated an essential role of Dscam for growth and directed extension of axon branches. Expression of randomly chosen single isoforms could not rescue connectivity but did restore basic axonal extension and rudimentary branching. Moreover, two Dscam alleles were generated that each reduced the maximally possible Dscam diversity to 22,176 isoforms. Reduction of Dscam diversity resulted in specific connectivity defects of mechanosensory neurons. Furthermore, the observed allele-specific phenotypes suggest functional differences among isoforms. Our findings provide evidence that a very large number of structurally unique receptor isoforms is required to ensure fidelity and precision of neuronal connectivity.

Cell cycle diversity involves differential regulation of Cyclin E activity in the Drosophila bristle cell lineage.

In the Drosophila bristle lineage, five differentiated cells arise from a precursor cell after a rapid sequence of asymmetric cell divisions (one every 2 hours). We show that, in mitotic cells, this rapid cadence of cell divisions is associated with cell cycles essentially devoid of the G1-phase. This feature is due to the expression of Cyclin E that precedes each cell division, and the differential expression of the S-transition negative regulator, Dacapo. Thus, apart from endocycles (G/S), which occurred in two out of five terminal cells, two other cell cycles coexist in this lineage: (1) an atypical cell cycle (S/G2/M), in which the S-phase is initiated during the preceding telophase; and (2) a canonical cell cycle (G1/S/G2/M) with a brief G1 phase. These two types of cell cycle result from either the absence or very transient expression of Dap, respectively. Finally, we show that the fate determinant factor, Tramtrack, downregulates Cyclin E expression and is probably involved in the exit of the cells from the cell cycle.

Target-determined expression of alpha3 isoform of the Na+,K+-ATPase in the somatic nervous system of rat.

Factors that determine the differential expression of isoforms of Na(+),K(+)-ATPase in the nervous system of vertebrates are not understood. To address this question we studied the expression of alpha(3) Na(+),K(+)-ATPase in the L5 dorsal root ganglia (DRG) of developing rat, the normal adult rat, and the adult rat after peripheral axotomy. During development, the first alpha(3) Na(+),K(+)-ATPase-positive DRG neurons appear by embryonic day 21. At birth, the L5 DRG have a full complement (14 +/- 2%) of these neurons. By 15 days after sciatic nerve transection in adult rat, the number of alpha(3) Na(+),K(+)-ATPase-positive DRG neurons and small myelinated L5 ventral root axons decreases to about 35% of control counts. These results combined with data from the literature suggest that the expression of alpha(3) Na(+),K(+)-ATPase by rat somatic neurons is determined by target-muscle spindle-derived factors.

Regulation of latent sensory hair cell precursors by glia in the zebrafish lateral line.

The lateral line is a placodally derived mechanosensory organ in anamniotes that detects the movement of water. In zebrafish embryos, a migrating primordium deposits seven to nine clusters of sensory hair cells, or neuromasts, at intervals along the trunk. Postembryonically, neuromasts continue to be added. We show that some secondary neuromasts arise from a pool of latent precursors that are deposited by the primordium between primary neuromasts. Interneuromast cells lie adjacent to the lateral line nerve and associated glia. These cells remain quiescent while they are juxtaposed with the glia; however, when they move away from the nerve they increase proliferation and form neuromasts. If glia are manually removed or genetically ablated by mutations in cls/sox10, hypersensitive (hps), or rowgain (rog), neuromasts precociously differentiate. Transplantation of wt glia into mutants rescues the appropriate temporal differentiation of interneuromast cells. Our studies reveal a role for glia in regulating sensory hair cell precursors.

Genetic analysis of an overlapping functional requirement for L1- and NCAM-type proteins during sensory axon guidance in Drosophila.

L1- and NCAM-type cell adhesion molecules represent distinct protein families that function as specific receptors for different axon guidance cues. However, both L1 and NCAM proteins promote axonal growth by inducing neuronal tyrosine kinase activity and are coexpressed in subsets of axon tracts in arthropods and vertebrates. We have studied the functional requirements for the Drosophila L1- and NCAM-type proteins, Neuroglian (Nrg) and Fasciclin II (FasII), during postembryonic sensory axon guidance. The rescue of the Neuroglian loss-of-function (LOF) phenotype by transgenically expressed L1- and NCAM-type proteins demonstrates a functional interchangeability between these proteins in Drosophila photoreceptor pioneer axons, where both proteins are normally coexpressed. In contrast, the ectopic expression of Fasciclin II in mechanosensory neurons causes a strong enhancement of the axonal misguidance phenotype. Moreover, our findings demonstrate that this functionally redundant specificity to mediate axon guidance has been conserved in their vertebrate homologs, L1-CAM and NCAM.

Lateral line receptors: where do they come from developmentally and where is our research going?

The lateral line system is composed of both mechanoreceptors, which exhibit little variation in structure between taxonomic groups, and electroreceptors, which exhibit considerably more variation. Cathodally sensitive ampullary electroreceptors are the primitive condition and are found in agnathans, chondrichthyans, and most osteichthyans. Aquatic amphibians also have ampullary electroreceptors for at least part of their life cycle. The more recently evolved anodally sensitive ampullary electroreceptors and tuberous electroreceptors are only found in four groups of teleost fishes. The basic ontogenetic unit of lateral line development is the dorsolateral placode. Primitively, there are six pairs of placodes, which pass through sequential stages of development into lateral line receptors. There is no question about the origin of primitive mechanoreceptors or electroreceptors, however, we do not have a good understanding of the origin of teleost mechanoreceptors and their ampullary or tuberous electroreceptors; do they come exclusively from dorsolateral placodes or from neural crest or even general ectoderm? A second intriguing lateral line question is how certain teleost fish groups evolved tuberous electroreceptors. Electroreception appears to have re-evolved at least twice in teleosts after being lost during the neopterygian radiation. It has been suggested that the development of tuberous electroreceptors might be due to changes in placodal patterning or a change in the general ectoderm that placodes arise from. Unfortunately, our understanding of lateral line origins in fishes is very sketchy, and, if we are to answer such an evolutionary question, we first need more complete information about lateral line development in a variety of fishes, which can then be combined with gene expression data to better interpret lateral line receptor development.

Mutual exclusion of sensory bristles and tendons on the notum of dipteran flies.

BACKGROUND: Genes of the achaete-scute complex encode transcription factors whose activity regulates the development of neural cells. The spatially restricted expression of achaete-scute on the mesonotum of higher flies governs the development and positioning of the large sensory bristles. On the scutum the bristles are arranged into conserved patterns, based on an ancestral arrangement of four longitudinal rows. This pattern appears to date back to the origin of cyclorraphous flies about 100-140 million years ago. The origin of the four-row bauplan, which is independent of body size, and the reasons for its conservation, are not known. RESULTS: We report that tendons for attachment of the indirect flight muscles are invariably located between the bristle rows of the scutum throughout the Diptera. Tendon development depends on the activity of a transcription factor encoded by the gene stripe. In Drosophila, stripe and achaete-scute have separate expression domains, leading to spatial segregation of tendon precursors and bristle precursors. Furthermore the products of these genes act antagonistically: ectopic sr expression prevents bristle development and ectopic sc expression prevents normal muscle attachment. The product of stripe acts downstream of Achaete-Scute and interferes with the development of bristle precursors. CONCLUSIONS: The pattern of flight muscles has changed little throughout the Diptera and we argue that the sites of muscle attachment may have constrained the positioning of bristles during the course of evolution. This could account for the pattern of four bristle rows on the scutum.

Towards a molecular understanding of Drosophila hearing.

The Drosophila auditory system is presented as a powerful new genetic model system for understanding the molecular aspects of development and physiology of hearing organs. The fly's ear resides in the antenna, with Johnston's organ serving as the mechanoreceptor. New approaches using electrophysiology and laser vibrometry have provided useful tools to apply to the study of mutations that disrupt hearing. The fundamental developmental processes that generate the peripheral nervous system are fairly well understood, although specific variations of these processes for chordotonal organs (CHO) and especially for Johnston's organ require more scrutiny. In contrast, even the fundamental physiologic workings of mechanosensitive systems are still poorly understood, but rapid recent progress is beginning to shed light. The identification and analysis of mutations that affect auditory function are summarized here, and prospects for the role of the Drosophila auditory system in understanding both insect and vertebrate hearing are discussed.

Altered whisker patterns induced by ectopic expression of Shh are topographically represented by barrels.

Barrels in the somatosensory cortex are segregated columns, which somatotopically relate to facial whiskers. The barrel pattern is assumed to be determined by an extrinsic mechanism (the domino theory). This theory is based on whisker lesion experiments and developmental observations regarding the serial establishment of the somatotopic pattern in which pattern formations are relayed from the periphery to the central nervous system. However, the barrel pattern is possibly determined by an intrinsic mechanism, especially in its primitive form. In order to investigate the definitive mechanism, we established an experimental system in which the cortical barrel pattern can be altered, not by using a lesion paradigm, but by epigenetically changing the whisker pattern. Sonic hedgehog (Shh) plays a pivotal role in whisker development. We transfected an adenovirus harboring chicken Shh (Ad-cShh) to mouse embryos (E9.5-E11.5) using an in utero surgical technique. When Ad-cShh was expressed in the epidermis, Bmp4, Ptch, Ptch2 and Gli1 were induced ectopically in the interfollicular region. In contrast, the expression of Bmp2 and Shh itself was unaltered. At a suitable dose of Ad-cShh, some pups displayed supernumerary whiskers or a disordered whisker pattern. The barrel patterns of these mice after the critical period were topographic representations of the contralateral side of the new whisker patterns when visualized by a cytochrome oxidase or Nissle staining method, supporting the instructive role of the extrinsic mechanism.

Studies of mechanosensation using the fly.

Mechanosensation requires the transduction of mechanical stimuli into neuronal impulses. It encompasses not only the sense of touch but also proprioception and hearing. In contrast to sight, smell and taste, relatively little is known about the molecular machinery of mechanosensation. It is already clear, however, that important aspects are conserved across phyla, from Caenorhabditis elegans to humans. Drosophila melanogaster is well placed to make a significant contribution to this field. Its advantages include a sequenced genome allied with powerful genetic techniques, and the ability to conduct electrophysiological recording from mechanoreceptor neurons. For human geneticists, it is expected that Drosophila studies will provide a source of candidate genes whose human homologues can be examined for roles in mechanosensory development, function and disease.

Developmental timing of hair follicle and dorsal skin innervation in mice.

The innervation of hair follicles offers an intriguing, yet hardly studied model for the dissection of the stepwise innervation during cutaneous morphogenesis. We have used immunofluorescence and a panel of neuronal markers to characterize the developmental choreography of C57BL/6 mouse backskin innervation. The development of murine skin innervation occurs in successive waves. The first cutaneous nerve fibers appeared before any morphological evidence of hair follicle development at embryonic day 15 (E15). Stage 1 and 2 developing hair follicles were already associated with nerve fibers at E16. These fibers approached a location where later in development the follicular (neural) network A (FNA) is located on fully developed pelage hair follicles. Prior to birth (E18), some nerve fibers had penetrated the epidermis, and an additional set of perifollicular nerve fibers arranged itself around the isthmus and bulge region of stage 5 hair follicles, to develop into the follicular (neural) network B (FNB). By the day of birth (P1), the neuropeptides substance P and calcitonin gene-related peptide became detectable in subcutaneous and dermal nerve fibers first. Newly formed hair follicles on E18 and P1 displayed the same innervation pattern seen in the first wave of hair follicle development. Just prior to epidermal penetration of hair shafts (P5), peptide histidine methionine-IR nerve fibers became detectable and epidermal innervation peaked; such innervation decreased after penetration (P7- P17). Last, tyrosine hydroxylase-IR and neuropeptide Y-IR became readily detectable. This sequence of developing innervation consistently correlates with hair follicle development, indicating a close interdependence of neuronal and epithelial morphogenesis.

Dcas is required for importin-alpha3 nuclear export and mechano-sensory organ cell fate specification in Drosophila.

We have studied the in vivo function and tissue specificity of Dcas, the Drosophila ortholog of CAS, the importin beta-like export receptor for importin alpha. While dcas mRNA is specifically expressed in the embryonic central nervous system, Dcas protein is maternally supplied to all embryonic cells and its nuclear/cytoplasmic distribution varies in different tissues and times in development. Unexpectedly, hypomorphic alleles of dcas show specific transformations in mechano-sensory organ cell identity, characteristic of mutations that increase Notch signaling. Dcas is essential for efficient importin-alpha3 nuclear export in mechano-sensory cells and the surrounding epidermal cells and is indirectly required for the import of one component of the Notch pathway, but not others tested. We interpret the specificity of the dcas phenotype as indicating that one or more Notch signaling components are particularly sensitive to a disruption in nuclear protein import. We propose that mutations in house keeping genes often cause specific developmental phenotypes, such as those observed in many human genetic disorders.

Postembryonic development of the posterior lateral line in zebrafish.

We examine how the posterior lateral line of the zebrafish grows and evolves from the simple midbody line present at the end of embryogenesis into the complex adult pattern. Our results suggest that secondary neuromasts do not form through budding from the embryonic line, but rather new waves of neuromasts are added anteroposteriorly. We propose that the developmental module that builds the embryonic pattern of neuromasts is used repeatedly during postembryonic development and that additional (secondary) primordia generate the additional neuromasts. We show that differentiated neuromasts migrate ventrally, and eventually generate "stitches" by successive bisections. We also examine the repatterning of the terminal neuromasts, which anticipates the up-bending of the tail leading to the highly asymmetrical caudal fin of the adult (which develops exclusively from the ventral part of the tail). Because terminal repatterning affects all aspects of tail formation, including its sensory development, we speculate that terminal axis bending may have become intimately associated with the terminal Hox genes before the appearance of the tetrapod lineage.

Cell migration in the postembryonic development of the fish lateral line.

We examine at the cellular level the postembryonic development of the posterior lateral line in the zebrafish. We show that the first wave of secondary neuromasts is laid down by a migrating primordium, primII. This primordium originates from a cephalic region much like the primordium that formed the primary line during embryogenesis. PrimII contributes to both the lateral and the dorsal branches of the posterior lateral line. Once they are deposited by the primordium, the differentiating neuromasts induce the specialisation of overlying epidermal cells into a pore-forming annulus, and the entire structure begins to migrate ventrally across the epithelium. Thus the final two-dimensional pattern depends on the combination of two orthogonal processes: anteroposterior waves of neuromast formation and dorsoventral migration of individual neuromasts. Finally, we examine how general these migratory processes can be by describing two fish species with very different adult patterns, Astyanax fasciatus (Mexican blind cavefish) and Oryzias latipes (medaka). We show that their primary patterns are nearly identical to that observed in zebrafish embryos, and that their postembryonic growth relies on the same combination of migratory processes that we documented in the case of the zebrafish.

Development of sensory systems in zebrafish (Danio rerio).

Zebrafish possess all of the classic sensory modalities: taste, tactile, smell, balance, vision, and hearing. For each sensory system, this article provides a brief overview of the system in the adult zebrafish followed by a more detailed overview of the development of the system. By far the majority of studies performed in each of the sensory systems of the zebrafish have involved some aspect of molecular biology or genetics. Although molecular biology and genetics are not major foci of the paper, brief discussions of some of the mutant strains of zebrafish that have developmental defects in each specific sensory system are included. The development of the sensory systems is only a small sampling of the work being done using zebrafish and provides a mere glimpse of the potential of this model for the study of vertebrate development, physiology, and human disease.

Distal-less expression in embryos of Limulus polyphemus (Chelicerata, Xiphosura) and Lepisma saccharina (Insecta, Zygentoma) suggests a role in the development of mechanoreceptors, chemoreceptors, and the CNS.

The homeobox gene Distal-less (Dll) is well known for its participation in the development of arthropod limbs and their derivatives. Dll activity has been described for all groups of arthropods, but also for molluscs, echinoderms and vertebrates. Generally, Dll participates in the establishment of the proximo-distal-axis and differentiation along this axis. During our investigation of the expression pattern in the silverfish Lepisma saccharina and the horseshoe crab Limulus polyphemus, we found several expressions in late stages which cannot be explained with the "normal" limb-specific function. The antenna, cerci and terminal filament of the silverfish show a striped expression; single cells on the labrum, mandibles, maxillary palps and anal valves are also strongly stained by the Dll antibody. In addition to cell groups in the developing ganglia of the CNS, in the coxal endites and several nerve cells in femur and the trochanter of the prosomal limbs, the whole prosomal shield of Limulus polyphemus is surrounded by Dll-positive cell clusters. Furthermore, the lateral processes of the opisthosoma and the edges of the opisthosomal appendages are Dll positive. To get an indication of the cell fate of these regions, we examined hatched larvae and juvenile stages of both species with the SEM. We found a striking correlation of these Dll-positive areas and different sense organs, especially mechanoreceptors. Since many sense organs in arthropods are situated on the limbs, interpretation of the Dll expression in limbs is problematical. This has critical implications for comparative analysis of Dll expression patterns between arthropods and for the claim of homology between limb-like structures. Furthermore, we discuss the possibility of convergent appendage evolution in various bilaterian groups based on the improvement of spatial sensory resolution.