Dermal morphogenesis controls lateral line patterning during postembryonic development of teleost fish

The lateral line system displays highly divergent patterns in adult teleost fish. The mechanisms underlying this variability are poorly understood. Here, we demonstrate that the lateral line mechanoreceptor, the neuromast, gives rise to a series of accessory neuromasts by a serial budding process during postembryonic development in zebrafish. We also show that accessory neuromast formation is highly correlated to the development of underlying dermal structures such as bones and scales. Abnormalities in opercular bone morphogenesis, in endothelin 1-knockdown embryos, are accompanied by stereotypic errors in neuromast budding and positioning, further demonstrating the tight correlation between the patterning of neuromasts and of the underlying dermal bones. In medaka, where scales form between peridermis and opercular bones, the lateral line displays a scale-specific pattern which is never observed in zebrafish. These results strongly suggest a control of postembryonic neuromast patterns by underlying dermal structures. This dermal control may explain some aspects of the evolution of lateral line patterns.     

The evolution of the amphibian lateral line system and its bearing on amphibian phylogeny

In modern amphibians that are aquatic the lateral line system is organized, by order, as follows: caecilians have electroreceptive ampullary organs and single rows of mechanoreceptive neuromast organs; generalized anurans have single rows of neuromasts that divide in a transverse plane to form secondary neuromasts or stitches, they do not have ampullary organs; generalized urodeles have ampullary organs, transverse stitches, and double or triple rows of neuromasts. Fossil evidence indicates that early amphibians had both ampullary organs and single rows of neuromasts embedded in bone. With time, receptors became epidermal in all three orders. Modern caecilians have retained the primitive receptor arrangement. I propose that the common ancestor of anurans and urodeles had transverse stitches, and that this character allies these two groups. Subsequent to the anuranurodele split, anurans lost their ampullary organs, perhaps concomitant with developing specializations for herbivory. Urodeles developed orthogonal neuromast couplets und triplets. In modern anurans und urodeles, transverse stitches are correlated with pond dwelling, while ampullary organs are correlated with carnivory, suggesting that the anuran-urodele ancestor(s) was a (were) pond-dwelling carnivore(s).     

Metamorphosis-related changes in the lateral line system of lampreys, <Emphasis Type="Italic">Petromyzon marinus</Emphasis>

Lamprey metamorphosis leads to considerable changes in morphology and behavior. We have recently reported that larval lampreys possess a functional lateral line system. Here we investigated metamorphic morphological changes in the lateral line system using light and electron microscopy. Functional modifications were studied by recording the trunk lateral line nerve activity of larvae and adults while stimulating neuromasts with approximately sinusoidal water motion. We found a general re-patterning of neuromasts on the head and trunk including an increase in numbers, redistribution within the pit lines, and shifts of the pit lines relative to external features. The trunk lateral line nerve response was qualitatively similar in adults and larvae. Both showed two neuronal populations responding to opposite directions of water flow. Magnitude of the response increased monotonically with stimulus amplitude. At low frequencies, the response lag relative to the stimulus maximum was approximately 220°, and the gain depended approximately linearly on frequency, confirming that superficial neuromasts are velocity detectors. Changes in phase lag with increasing stimulus frequency were steeper in larvae, suggesting slower afferent conductance. The response gain with frequency was smaller for adults, suggesting a narrower frequency discrimination range and decreased sensitivity. These changes may be adaptations for the active lifestyle of adult lampreys.     

The development of lateral line placodes: Taking a broader view

The lateral line system of anamniote vertebrates enables the detection of local water movement and weak bioelectric fields. Ancestrally, it comprises neuromasts – small sense organs containing mechanosensory hair cells – distributed in characteristic lines over the head and trunk, flanked on the head by fields of electroreceptive ampullary organs, innervated by afferent neurons projecting respectively to the medial and dorsal octavolateral nuclei in the hindbrain. Given the independent loss of the electrosensory system in multiple lineages, the development and evolution of the mechanosensory and electrosensory components of the lateral line must be dissociable. Nevertheless, the entire system arises from a series of cranial lateral line placodes, which exhibit two modes of sensory organ formation: elongation to form sensory ridges that fragment (with neuromasts differentiating in the center of the ridge, and ampullary organs on the flanks), or migration as collectives of cells, depositing sense organs in their wake. Intensive study of the migrating posterior lateral line placode in zebrafish has yielded a wealth of information concerning the molecular control of migration and neuromast formation in this migrating placode, in this cypriniform teleost species. However, our mechanistic understanding of neuromast and ampullary organ formation by elongating lateral line placodes, and even of other zebrafish lateral line placodes, is sparse or non-existent. Here, we attempt to highlight the diversity of lateral line development and the limits of the current research focus on the zebrafish posterior lateral line placode. We hope this will stimulate a broader approach to this fascinating sensory system.     

Larval lampreys possess a functional lateral line system

Morphology of larval lampreys’ neuromasts was found to be very similar to that of adults. Activity in the lateral line nerve, elicited by a vibrating ball, indicated a functional lateralis system. Analysis revealed at least two populations of afferents, responding to opposite directions of water flow, with adapting responses. The response magnitude increased monotonically with stimulus amplitude. Larval lampreys’ neuromasts were less sensitive than those of teleosts. At low frequencies the response showed a phase lead of 200–220° with respect to the maximum of the ball displacement and a gain that was approximately linearly proportional to frequency.     

Object localization through the lateral line system of fish: theory and experiment

Fish acquire information about their aquatic environment by means of their mechanosensory lateral-line system. This system consists of superficial and canal neuromasts that sense perturbations in the water surrounding them. Based on a hydrodynamic model presented here, we propose a mechanism through which fish can localize the source of these perturbations. In doing so we include the curvature of the fish body, a realistic lateral line canal inter-pore distance for the lateral-line canals, and the surface boundary layer. Using our model to explore receptor behavior based on experimental data of responses to dipole stimuli we suggest that superficial and canal neuromasts employ the same mechanism, hence provide the same type of input to the central nervous system. The analytical predictions agree well with spiking responses recorded experimentally from primary lateral-line nerve fibers. From this, and taking into account the central organization of the lateral-line system, we present a simple biophysical model for determining the distance to a source.     

An enhancer trap zebrafish line for lateral line development and regulation of six2b expression

Zebrafish lateral line system which is derived from neurogenic placodes has become a popular model for developmental biology since its formation involves cell migration, pattern formation, organogenesis, and hair cell regeneration. Transgenic lines play a crucial role in lateral line system study. Here, we identified an enhancer trap transgenic zebrafish line Et(gata2a:EGFP)189b (ET189b for short), which expressed enhanced green fluorescent protein (EGFP) in the pituitary, otic, and lateral line placodes and their derivatives. Especially, in neuromast, the accessory cells rather than hair cells were labeled by EGFP. Furthermore, we found the Tol2 transposon construct is integrated at the proximal upstream region of six2b gene locus. And EGFP expression of ET189b closely reflects the expression of endogenous six2b during development and after dkk1b over-expression. Taken together, our results indicated that ET189b is an ideal line for research on lateral line development and regulation of six2b expression.     

Effects of running water on brainstem lateral line responses in trout,<Emphasis Type="Italic"> Oncorhynchus mykiss</Emphasis>, to sinusoidal wave stimuli

We investigated how single units in the medial octavolateralis nucleus of the rainbow trout, Oncorhynchus mykiss, respond to a 50-Hz vibrating sphere in still and running water. Four types of units were distinguished. Type MI units (N=16) were flow-sensitive; their ongoing discharge rates either increased or decreased in running water, and as a consequence, responses of these units to the vibrating sphere were masked if the fish was exposed to water flow. Type MII units (N=7) were not flow-sensitive; their ongoing discharge rates were comparable in still and running water, and thus their responses to the vibrating sphere were not masked. Type MIII units (N=7) were also not flow-sensitive; nevertheless, their responses to the vibrating sphere were masked in running water. Type MIV units (N=14) were flow-sensitive, but their responses to the vibrating sphere were not masked. Our data confirm previous findings in the goldfish, Carassius auratus, indicating that the organization of the peripheral lateral line is reflected to a large degree in the medial octavolateralis nucleus. We compare data from goldfish and trout and discuss differences with respect to lateral line morphology, lifestyle and habitat of these species.Abbreviations CN canal neuromast - MON medial octavolateralis nucleus - SN superfical neuromast - a.c. alternating current - d.c. direct current     

Proneural gene requirement for hair cell differentiation in the zebrafish lateral line

The lateral line system comprises an array of mechanosensory organs, the neuromasts, distributed over the body surface. Each neuromast consists of a patch of mechanosensory hair cells surrounded by support cells. We show that, in the zebrafish, two proneural genes are essential for differentiation of the hair cells, neuroD (nrd) and atonal homolog 1 (ath1). Gene knockdown experiments demonstrate that loss of function of either gene, but not of the related proneural gene neurogenin1 (ngn1), abrogate the appearance of hair cell markers. This is in contrast to other sensory systems, such as the neurons of the lateral line ganglion, where nrd is regulated by ngn1 and not by ath1. Overexpression of ath1 can induce nrd, and the phenotype produced by loss of ath1 function can be partially rescued by injection of nrd mRNA. This supports the conclusion that the activation of nrd probably requires ath1 in the hair cell lineage, whereas in sensory neurons nrd activation requires ngn1. We propose that the emergence of two atonal homologs, ath1 and ngn1, allowed the cellular segregation of mechanoreception and signal transmission that were originally performed by a single cell type as found in insects.     

A zebrafish SKIV2L2-enhancer trap line provides a useful tool for the study of peripheral sensory circuit development

The zebrafish is an ideal model for elucidating the cellular and molecular mechanisms that underlie development of the peripheral nervous system. A transgenic line that selectively labels all the sensory circuits would be a valuable tool for such investigations. In this study, we describe such a line: the enhancer trap zebrafish line Tg(SKIV2L2:gfp)^j1775 which expresses green fluorescent protein (gfp) in the peripheral sensory ganglia. We show that this transgene marks all peripheral ganglia and sensory nerves, beginning at the time when the neurons are first extending their processes, but does not label the efferent nerves. The trapped reporter is inserted just upstream of a previously poorly described gene: lhfpl4 on LG6. The expression pattern of this gene by in situ hybridization reveals a different, but overlapping, pattern of expression compared to that of the transgene. This pattern also does not mimic that of the gene (skiv2l2), which provided the promoter element in the construct. These findings indicate that reporter expression is not dictated by an endogenous enhancer element, but instead arises through an unknown mechanism. Regardless, this reporter line should prove to be a valuable tool in the investigation of peripheral nervous system formation in the zebrafish.     

Duplicate sfrp1 genes in zebrafish: sfrp1a is dynamically expressed in the developing central nervous system, gut and lateral line

The secreted frizzled-related proteins (Sfrp) are a family of soluble proteins with diverse biological functions having the capacity to bind Wnt ligands, to modulate Wnt signalling, and to signal directly via the Wnt receptor, Frizzled. In an enhancer trap screen for embryonic expression in zebrafish we identified an sfrp1 gene. Previous studies suggest an important role for sfrp1 in eye development, however, no data have been reported using the zebrafish model. In this paper, we describe duplicate sfrp1 genes in zebrafish and present a detailed analysis of the expression profile of both genes. Whole mount in situ hybridisation analyses of sfrp1a during embryonic and larval development revealed a dynamic expression profile, including: the central nervous system, where sfrp1a was regionally expressed throughout the brain and developing eye; the posterior gut, from the time of endodermal cell condensation; the lateral line, where sfrp1a was expressed in the migrating primordia and interneuromast cells that give rise to the sensory organs. Other sites included the blastoderm, segmenting mesoderm, olfactory placode, developing ear, pronephros and fin-bud. We have also analysed sfrp1b expression during embryonic development. Surprisingly this gene exhibited a divergent expression profile being limited to the yolk syncytium under the elongating tail-bud, which later covered the distal yolk extension, and transiently in the tail-bud mesenchyme. Overall, our studies provide a basis for future analyses of these developmentally important factors using the zebrafish model.     

Characterization of zebrafish mutants with defects in bone calcification during development

Using the fluorescent dyes calcein and alcian blue, we stained the F3 generation of chemically (ENU) mutagenized zebrafish embryos and larvae, and screened for mutants with defects in bone development. We identified a mutant line, bone calcification slow (bcs), which showed delayed axial vertebra calcification during development. Before 4–5 days post-fertilization (dpf), the bcs embryos did not display obvious abnormalities in bone development (i.e., normal number, size and shape of cartilage and vertebrae). At 5–6 dpf, when vertebrae calcification starts, bcs embryos began to show defects. At 7 dpf, for example, in most of the bcs embryos examined, calcein staining revealed no signals of vertebrae mineralization, whereas during the same developmental stages, 2–14 mineralized vertebrae were observed in wild-type animals. Decreases in the number of calcified vertebrae were also observed in bcs mutants when examined at 9 and 11 dpf, respectively. Interestingly, by 13 dpf the defects in bcs mutants were no longer evident. There were no significant differences in the number of calcified vertebrae between wild-type and mutant animals. We examined the expression of bone development marker genes (e.g., Sox9b, Bmp2b, and Cyp26b1, which play important roles in bone formation and calcification). In mutant fish, we observed slight increases in Sox9b expression, no alterations in Bmp2b expression, but significant increases in Cyp26b1 expression. Together, the data suggest that bcs delays axial skeletal calcification, but does not affect bone formation and maturation.     

Germ line control of female sex determination in zebrafish

A major transition during development of the gonad is commitment from an undifferentiated “bi-potential” state to ovary or testis fate. In mammals, the oogonia of the developing ovary are known to be important for folliculogenesis. An additional role in promoting ovary fate or female sex determination has been suggested, however it remains unclear how the germ line might regulate this process. Here we show that the germ line is required for the ovary versus testis fate choice in zebrafish. When the germ line is absent, the gonad adopts testis fate. These germ line deficient testes have normal somatic structures indicating that the germ line influences fate determination of surrounding somatic tissues. In germ line deficient animals the expression of the ovary specific gene cyp19a1a fails to be maintained whereas the testis genes sox9a and amh remain expressed. Furthermore, we observed decreased levels of the ovary specific genes cyp19a1a and foxL2 in germ line deficient animals prior to morphological sex differentiation of the gonad. We propose that the germ line has a common role in female sex determination in fish and mammals. Additionally, we show that testis specification is sufficient for masculinization of the fish pointing to a direct role of hormone signaling from the gonad in directing sex differentiation of non-gonadal tissues.     

A study of the viscoelastic effect in a bone remodeling model

The present paper addresses the following question can a simple regulatory bone remodeling model predict effects of viscosity on the trabecular morphology? For that, we propose an extension of a previous bone remodeling model by taking into account the viscosity properties of the tissue. Zener’s law is used to describe the mechanical behavior of the bone and a specific law of the apparent bone density rate is proposed. Based on stability analysis, numerical simulations are then performed to investigate the viscosity role on simulations of the bone remodeling process. We show that the viscous contribution affects the evolution of the apparent bone density, by slowing down the adaptation process, which seems to be confirmed by simulations with real data obtained from rat tibia.     

Development of the lateral line system in the sea bass

Using light and electron microscopy, a study of the development of the lateral line system of the sea bass Dicentrarchus labrax , from embryo to adult, revealed that the first free neuromasts appeared on the head shortly before hatching and multiplied during the larval stage. They were aligned on the head and trunk in a pattern which corresponded to the location of future canals. The transition to the juvenile stage marked the start of important anatomical changes during which head and trunk canals were formed successively. Neuromasts, with a cupula and consisting of standard sensory cells and supporting cells, were characterized by bidirectional polarity. The exact location of the first neuromast formed in the embryo was identified and its differentiation monitored from primordium to eruption. This neuromast was distinguishable from the others by its radial polarity. Correlations were made between the development of the lateral line system and the behaviour of the sea bass.     

Activity of lateral line efferents in the axolotl (Ambystoma mexicanum)

Summary Activity of efferent fibers was recorded from the ramus ophthalmicus superficialis of the head lateral line nerve and the ramus medialis of the trunk lateral line nerve of the axolotl Ambystoma mexicanum. Baseline activity and activity evoked by sensory stimuli were examined. Electrical stimulation of selected branches was used to determine the conduction velocity and the branching pattern of efferent fibers. The influence of lesions at different levels in the CNS on efferent activity was studied.Up to 5 units with baseline activity were found in a single ramus of the lateral line nerve. Discharge rates were variable and highly irregular; they differed between units of the same branch. Bursting activity occurred in 62% of the units. Movements of the animal were accompanied by activity in up to 8 efferent units in a single nerve.Efferent activity could be elicited or modified by stimulation of visual, labyrinthine, somatosensory, and lateral line systems. Stimulation of the electrosensory system had no effect. Individual efferent neurons innervated different fields in the lateral line periphery. Conduction velocities of efferent fibers ranged from 5 to 12 m/s.Efferent units received input from various sources at different brain levels up to the diencephalon. These in puts determined the baseline activity. The mechanosensory input was mediated at the medullary level.Abbreviations r.m. ramus medialis - r.o.s. ramus ophthalmicus superficialis - r.s. ramus superior     

Bilateral asymmetry in bone measurements of the hand and lateral hand dominance

Two hundred thirty–five (235) normal male participants of the Baltimore Longitudinal Study were classified as right handed, left handed, and ambidextrous on the basis of their grip–strength performance. Their left and right hands were also radiographed and the measurements of the second metacarpal bones were evaluated on the basis of hand dominance. The results indicated that, as a rule, the right hand measurements are higher than those of the left hand, regardless of hand dominance. The bilateral differences in total width, length, total area and cortical area are significant among the right hand dominant and nonsignificant among the left hand dominant. Regardless off hand dominance the bilateral differences in medullary width are nonsignificant. These results suggest an inherent tendency of the right second metacarpal to have more bone than the left regardless of hand dominance. Differential stress due to hand dominance will increase the bilateral difference in the right handed and reduce it in the left handed.