Patterning of body segments of the zebrafish embryo

Single Cell Transcriptomic Analysis of Spinal Dmrt3 Neurons in Zebrafish and Mouse Identifies Distinct Subtypes and Reveal Novel Subpopulations Within the dI6 Domain

The spinal locomotor network is frequently used for studies into how neuronal circuits are formed and how cellular activity shape behavioral patterns. A population of dI6 interneurons, marked by the Doublesex and mab-3 related transcription factor 3 (Dmrt3), has been shown to participate in the coordination of locomotion and gaits in horses, mice and zebrafish. Analyses of Dmrt3 neurons based on morphology, functionality and the expression of transcription factors have identified different subtypes. Here we analyzed the transcriptomes of individual cells belonging to the Dmrt3 lineage from zebrafish and mice to unravel the molecular code that underlies their subfunctionalization. Indeed, clustering of Dmrt3 neurons based on their gene expression verified known subtypes and revealed novel populations expressing unique markers. Differences in birth order, differential expression of axon guidance genes, neurotransmitters, and their receptors, as well as genes affecting electrophysiological properties, were identified as factors likely underlying diversity. In addition, the comparison between fish and mice populations offers insights into the evolutionary driven subspecialization concomitant with the emergence of limbed locomotion.

Nucleoporin 62-Like Protein is Required for the Development of Pharyngeal Arches through Regulation of Wnt/β-Catenin Signaling and Apoptotic Homeostasis in Zebrafish

We have previously observed the predominant expression of nucleoporin 62-like (Nup62l) mRNA in the pharyngeal region of zebrafish, which raises the question whether Nup62l has important implications in governing the morphogenesis of pharyngeal arches (PA) in zebrafish. Herein, we explored the functions of Nup62l in PA development. The disruption of Nup62l with a CRISPR/Cas9-dependent gene knockout approach led to defective PA, which was characterized by a thinned and shortened pharyngeal region and a significant loss of pharyngeal cartilages. During pharyngeal cartilage formation, prechondrogenic condensation and chondrogenic differentiation were disrupted in homozygous nup62l-mutants, while the specification and migration of cranial neural crest cells (CNCCs) were unaffected. Mechanistically, the impaired PA region of nup62l-mutants underwent extensive apoptosis, which was mainly dependent on activation of p53-dependent apoptotic pathway. Moreover, aberrant activation of a series of apoptotic pathways in nup62l-mutants is closely associated with the inactivation of Wnt/β-catenin signaling. Thus, these findings suggest that the regulation of Wnt/β-catenin activity by Nup62l is crucial for PA formation in zebrafish.

Analysis of novel domain-specific mutations in the zebrafish ndr2/cyclops gene generated using CRISPR-Cas9 RNPs

Nodal-related protein (ndr2) is amember of the transforming growth factor type β superfamily of factors and is required for ventral midline patterning of the embryonic central nervous system in zebrafish. In humans, mutations in the gene encoding nodal cause holoprosencephaly and heterotaxy. Mutations in the ndr2 gene in the zebrafish (Danio rerio) lead to similar phenotypes, including loss of the medial floor plate, severe deficits in ventral forebrain development and cyclopia. Alleles of the ndr2 gene have been useful in studying patterning of ventral structures of the central nervous system. Fifteen different ndr2 alleles have been reported in zebrafish, of which eight were generated using chemical mutagenesis, four were radiation-induced and the remaining alleles were obtained via random insertion, gene targeting (TALEN) or unknown methods. Therefore, most mutation sites were random and could not be predicted a priori. Using the CRISPR-Cas9 system from Streptococcus pyogenes, we targeted distinct regions in all three exons of zebrafish ndr2 and observed cyclopia in the injected (G₀) embryos.We show that the use of sgRNA-Cas9 ribonucleoprotein (RNP) complexes can cause penetrant cyclopic phenotypes in injected (G₀) embryos. Targeted polymerase chain reaction amplicon analysis using Sanger sequencing showed that most of the alleles had small indels resulting in frameshifts. The sequence information correlates with the loss of ndr2 activity. In this study, we validate multiple CRISPR targets using an in vitro nuclease assay and in vivo analysis using embryos. We describe one specific mutant allele resulting in the loss of conserved terminal cysteine-coding sequences. This study is another demonstration of the utility of the CRISPR-Cas9 system in generating domain-specific mutations and provides further insights into the structure-function of the ndr2 gene.

Cell proliferation in the developing and adult hindbrain and midbrain of trout and medaka (teleosts): a segmental approach

We report here the histogenesis of the brainstem of the trout (Salmo trutta fario) and the medaka (Oryzias latipes) chosen as examples of teleosts with slow and fast growth, respectively. Our results reveal that the sequence of formation of brain structures is rather similar in the teleosts species examined so far, but some interspecific differences do exist in terms of brainstem maturation at particular developmental stages, such as the end of the gastrulation and hatching periods. This sequence includes the subdivision of the brainstem in different transverse segments and longitudinal zones, where morphologically discernible boundaries are observed along the caudorostral and ventrodorsal axis. The boundary formation and subsequent subdivision of the trout and medaka brainstems, together with the proliferation pattern observed by immunohistochemistry with an antibody against the proliferating cell nuclear antigen (PCNA), support a segmental model throughout the brainstem. The spatiotemporal pattern of PCNA immunoreactivity is similar in the mesencephalon and rhombencephalon of the two teleosts species studied, although proliferation centers are less clearly defined in the medaka. Moreover, the segmental appearance of the brainstem, as revealed by PCNA immunohistochemistry, is blurred earlier in the medaka than in the trout. Thus, the trout brain appears a suitable model for morphogenetic studies because it allows more gradual survey of the changes throughout development.

Study on the common teratogenic pathway elicited by the fungicides triazole-derivatives

Triazole-derivatives alter the pharyngeal apparatus morphogenesis of rodent embryos cultured in vitro. The hindbrain segmentation and the rhombencephalic neural crest cell (NCCs) migration are altered by Fluconazole exposure in vitro. The aim of the present work is to identify if a common pathogenic pathway is detectable also for other molecules of this class of compounds. 9.5 days post coitum (d.p.c.) old rat embryos were exposed in vitro to the teratogenic concentrations of Flusilazole, Triadimefon and Triadimenol and cultured for 24, 48 or 60 h. The expression and localisation of Hox-b1 and Krox-20 proteins (used as markers for hindbrain segmentation) were evaluated after 24 h of culture. The localisation and distribution of NCC was evaluated after 24, 30 and 48 h of culture. The morphology of the embryos was analysed after 48 h, while the branchial nerve structures were evaluated after 60 h of culture. Hindbrain segmentation and NCC migration alteration as well as pharyngeal arch and cranial nerve abnormalities were detected after exposure of the tested molecules. A common severe teratogenic intrinsic property for the tested molecules of this chemical class has been found, acting through alteration of the normal hindbrain developmental pattern.

Pathogenic pathways in fluconazole-induced branchial arch malformations

BACKGROUND

A widely-used antimycotic agent, bis-triazole fluconazole (FLUCO), is able to produce abnormalities to the branchial apparatus (hypoplasia, agenesis, and fusion) in postimplantation rodent embryos cultured in vitro. The branchial apparatus is a complex and transient structure in vertebrate embryos and is essential for the development of the face skeleton. Branchial arch mesenchyme is formed by two different cellular populations: paraxial mesenchyme and ectomesenchyme, which originate from rhombencephalic neural crest cell (NCC) migration. We investigated the possible pathogenic pathways involved in FLUCO-related branchial arch abnormalities. Perturbations in physiological apoptosis, cell proliferation, NCC migration and branchial mesenchyme induction have been considered.

METHODS

Rat embryos (9.5-day postcoitum; 1-3 somites) were exposed in vitro to 0 or 500 microM FLUCO. After 24, 36, or 48 hr of culture, embryos were examined for apoptosis (acridine orange method) and cell proliferation (BrdU incorporation and detection method). Rhombencephalic NCC migration was analyzed using immunostaining of NCC (using anti-CRABP antibodies) and the extracellular matrix (using anti-fibronectin antibodies). The differentiative capability of the branchial mesenchymes was investigated using anti-endothelin and anti-endothelin-receptor antibodies.

RESULTS

During the whole culture period, no alterations in physiological apoptosis, cell proliferation, and mesenchymal cell induction were observed in FLUCO-exposed embryos in comparison to controls. On the contrary, severe alterations in NCC migration pathways were observed in FLUCO-exposed embryos.

CONCLUSIONS

The findings suggest that FLUCO produces teratogenic effects by interfering with the cellular and molecular mechanisms that control NCC migration.

Development of the locomotor network in zebrafish

The zebrafish is a leading model for studies of vertebrate development and genetics. Its embryonic motor behaviors are easy to assess (e.g. for mutagenic screens), the embryos develop rapidly (hatching as larvae at 2 days) and are transparent, permitting calcium imaging and patch clamp recording in vivo. We review primarily the recent advances in understanding the cellular basis for the development of motor activities in the developing zebrafish. The motor activities are generated largely in the spinal cord and hindbrain. In the embryo these segmented structures possess a relatively small number of repeating sets of identifiable neurons. Many types of neurons as well as the two types of muscle cells have been classified based on their morphologies. Some of the molecular signals for cellular differentiation have been identified recently and mutations affecting cell development have been isolated. Embryonic motor behaviors appear in sequence and consist of an early period of transient spontaneous coiling contractions, followed by the emergence of twitching responses to touch, and later by the ability to swim. Coiling contractions are generated by an electrically coupled network of a subset of spinal neurons whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming. Swimming becomes sustained in larvae once the neuromodulatory serotonergic system develops. These results indicate many similarities between developing zebrafish and other vertebrates in the properties of the synaptic drive underlying locomotion. Therefore, the zebrafish is a useful preparation for gaining new insights into the development of the neural control of vertebrate locomotion. As the types of neurons, transmitters, receptors and channels used in the locomotor network are being defined, this opens the possibility of combining cellular neurophysiology with forward and reverse molecular genetics to understand the principles of locomotor network assembly and function.

Control of her1 expression during zebrafish somitogenesis by a Delta-dependent oscillator and an independent wave-front activity

Somitogenesis has been linked both to a molecular clock that controls the oscillation of gene expression in the presomitic mesoderm (PSM) and to Notch pathway signaling. The oscillator, or clock, is thought to create a prepattern of stripes of gene expression that regulates the activity of the Notch pathway that subsequently directs somite border formation. Here, we report that the zebrafish gene after eight (aei) that is required for both somitogenesis and neurogenesis encodes the Notch ligand DeltaD. Additional analysis revealed that stripes of her1 expression oscillate within the PSM and that aei/DeltaDsignaling is required for this oscillation.aei/DeltaD expression does not oscillate, indicating that the activity of the Notch pathway upstream ofher1 may function within the oscillator itself. Moreover, we found that her1 stripes are expressed in the anlage of consecutive somites, indicating that its expression pattern is not pair-rule. Analysis of her1 expression inaei/DeltaD, fused somites (fss), and aei;fss embryos uncovered a wave-front activity that is capable of continually inducing her1 expression de novo in the anterior PSM in the absence of the oscillation of her1. The wave-front activity, in reference to the clock and wave-front model, is defined as such because it interacts with the oscillator-derived pattern in the anterior PSM and is required for somite morphogenesis. This wave-front activity is blocked in embryos mutant for fssbut not aei/DeltaD. Thus, our analysis indicates that the smooth sequence of formation, refinement, and fading ofher1 stripes in the PSM is governed by two separate activities.

In vitro teratogenic potential of two antifungal triazoles: triadimefon and triadimenol

The teratogenic potential of two antifungal triazoles (Triadimefon and Triadimenol) has been investigated in vitro by the rat postimplantation whole embryo culture method. Rat embryos 9.5 d old were cultured for 48 h in rat serum with Triadimefon (12.5-250 microM) or Triadimenol (6.25-125 microM) and then examined. Some embryos exposed to Triadimenol (6.25-125 microM) were cultured for 12 extra hours in control serum to improve their developmental degree and then immunostain cranial nerves and ganglia. The exposure to the highest doses of triazoles only moderately reduced some morphometrical developmental parameters. By contrast, 25-250 microM Triadimefon and 25-125 microM Triadimenol induced specific concentration-related teratogenic effects at the level of first and second branchial arches. After immunostaining, embryos exposed to 12.5-125 microM Triadimenol showed specific cranial nerve and ganglia abnormalities. The possible implication of neural crest cell alterations on triazole-related abnormalities is discussed.

Somitogenesis in zebrafish

Both genetic and embryological studies in the zebrafish, Danio rerio, have contributed to our general understanding of how somites form and differentiate. In the zebrafish, mutants have been isolated that have specific effects on virtually every aspect of somite development. The fss-type mutants, defining 5 genes, affect somite segmentation and epithelialization. The you-type mutants, comprising 7 genes, and mutants in another 13 genes defective in notochord formation, have somites with abnormal pattern and morphology. Eighteen genes have been identified that are required for the differentiation and maintenance of the somitic musculature, and 2 genes have been identified that are involved in the development of motoneurons that innervate the somitic musculature. The true utility of the zebrafish lies in the ability to combine genetic analysis with embryological experimentation. Such analysis of somite segmentation suggests that homologues of both the Drosophila pair-rule and segment polarity genes, her1 and Sonic hedge-hog, respectively, are involved generating periodicity during somitogenesis. The Sonic hedge-hog protein secreted from the notochord also induces the formation of specific muscle types including the slow muscle fibers which are initially induced in the medial somite and undergo a series of morphological transitions including migration through the somite to the lateral surface where they complete their differentiation. The role of the notochord in patterning the somite is also demonstrated by its involvement in regulating the permissiveness of the somite to the extension of axons of primary motoneurons.

Muscle growth and development in normal-sex-ratio and all-female diploid and triploid Atlantic salmon

Muscle development and growth were investigated in diploid populations of normal-sex-ratio and all-female Atlantic salmon (Salmo salar L.) and their triploid counterparts produced by high-pressure treatment. Somites were formed at the rate of 6 h-1 in both diploids and triploids at 6 degrees C. The rostral-to-caudal development of myotubes, myofibrils and acetylcholinesterase staining at the myosepta was slightly more advanced in triploid than in diploid fish, although the differences were smaller than among individual families. The c-met receptor tyrosine kinase was used as a molecular marker for the satellite cells involved in postembryonic muscle growth. Satellite cell nuclei comprised 17.5 % of total myonuclei in smolts and they were 24 % more abundant in diploid than in triploid fish. Cells expressing the myogenic regulatory factor myf-6, a marker of satellite cells committed to differentiation, represented 14.8 % of total myonuclei in diploids and 12.5 % in triploids. At ambient temperatures, the number of white muscle fibres in normal-sex-ratio fish increased more than 30-fold between the alevin and smolt stages, and approximately 3.5-fold further during the first year of seawater growth. The rate of muscle fibre recruitment in seawater stages was significantly greater in diploid than in triploid fish, reaching 1162 fibres day-1 and 608 fibres day-1, respectively, in all-female groups 800 days post-hatching. For 42 cm fork-length fish, there were approximately one-third more muscle fibres per myotome in diploid than in triploid groups, 649 878 and 413 619, respectively, for all-female fish. The probability density function of muscle fibre diameters in each fish was estimated using non-parametric smoothing techniques, and the mean densities for diploids (fD) and triploids (fT) were calculated. The peak fibre diameter was approximately 20 (micro)m in all age classes, irrespective of ploidy. Distinct bimodal distributions of muscle fibre diameter were evident in all groups 775 days and 839 days post-hatching, reflecting seasonal cycles of fibre recruitment. fD and fT were compared using a non-parametric bootstrap technique and the reference band representing the null-hypothesis indicated that there was no difference with ploidy. Reference bands for normal-sex-ratio fish at 315 days and 470 days indicated that diploids had a higher percentage of smaller-diameter fibres and that triploid distributions had a thicker right-hand tail. Similar differences in fD and fT of muscle fibre diameters were found for all-female fish, although the statistical evidence was less strong. Reference bands indicated differences in the middle range of the distributions of muscle fibre diameter in fish 620–775 days post-hatch, with triploids having a thicker right-hand tail. Thus, a lower density of satellite cells was associated with reduced rates of fibre recruitment but a compensatory increase in muscle fibre hypertrophy in triploid compared with diploid fish.

Heat shock produces periodic somitic disturbances in the zebrafish embryo

Environmental influences are known to produce segmental defects in a variety of organisms. In this paper we report upon segmental aberrations produced by brief heat shocks delivered to developing zebrafish embryos. The initial defects in the segmental pattern of somitic boundaries and motoneuron axon outgrowth were usually observed five somites caudal to the somite which was forming at the time of heat shock application. Segmental defects in zebrafish embryos exposed to a single heat shock treatment can occur in a periodic pattern similar to the multiple disturbances observed to occur in chick embryos. These data are discussed with regard to models involving cell cycle synchrony or 'clock and wavefront' schemes in the process of somitogenesis.

Temperature and neural development of the Atlantic herring (Clupea harengus L.)

Embryos of Atlantic herring (Clupea harengus L.) from the Buchan (Northern North Sea) stock were incubated from fertilisation until hatching at temperatures of 5, 8, 12, and 15 degrees C. The relative timing of development of the Kolmer-Agduhr (KA) neurons, the posterior lateral line nerve, the motor neurons, and myotubes were determined with respect to somite stage of the embryo. Development of the KA neurons, the lateral line nerve, and the myotubes was similar at all temperatures. In contrast, timing of outgrowth of the motor neuron axons with respect to somite stage was earlier at higher (> or = 12 degrees C) than at lower temperatures (< or = 8 degrees C) although it reached a similar point at all temperatures by the 58-somite stage. Our hypothesis to explain these observations is that delayed motor axon outgrowth in the lower temperature groups is probably due to a delay in a signalling interaction between motor neurons and the somite.

Musculoskeletal patterning in the pharyngeal segments of the zebrafish embryo

The head skeleton and muscles of the zebrafish develop in a stereotyped pattern in the embryo, including seven pharyngeal arches and a basicranium underlying the brain and sense organs. To investigate how individual cartilages and muscles are specified and organized within each head segment, we have examined their early differentiation using Alcian labeling of cartilage and expression of several molecular markers of muscle cells. Zebrafish larvae begin feeding by four days after fertilization, but cartilage and muscle precursors develop in the pharyngeal arches up to 2 days earlier. These chondroblasts and myoblasts lie close together within each segment and differentiate in synchrony, perhaps reflecting the interdependent nature of their patterning. Initially, cells within a segment condense and gradually become subdivided into individual dorsal and ventral structures of the differentiated arch. Cartilages or muscles in one segment show similar patterns of condensation and differentiation as their homologues in another, but vary in size and shape in the most anterior (mandibular and hyoid) and posterior (tooth-bearing) arches, possibly as a consequence of changes in the timing of their development. Our results reveal a segmental scaffold of early cartilage and muscle precursors and suggest that interactions between them coordinate their patterning in the embryo. These data provide a descriptive basis for genetic analyses of craniofacial patterning.

The zebrafish Fgf-3 gene: cDNA sequence, transcript structure and genomic organization

We report the isolation and characterization of genomic and cDNA clones encoding zebrafish fibroblast growth factor 3 (FGF3). An initial cDNA clone was generated by PCR amplification using degenerate oligo primers corresponding to a conserved region of protein found in the mouse and human homologues. Screening a cDNA library made from 30-33-h-old zebrafish embryos with this PCR product led to the isolation of two cDNA clones. Sequence analysis of the longest cDNA insert (1810 bp) revealed a 256-amino-acid (aa) orf. The central region, composed of approx. 155 aa, shares 78% identity with the analogous region of Xenopus laevis FGF3 and 72% identity with the product of the more distantly related human gene. However, the N-and C-terminal domains of zebrafish FGF3 are very different from those of other known homologues. The cDNA was used as a probe on genomic DNA to create a physical map of the locus and to isolate a genomic clone encompassing the entire coding region and 5' sequences. DNA sequencing and RNase protection analyses indicate that zebrafish Fgf-3 (ZFgf-3) is structurally analogous to the mouse gene and regulated through two different promoters. The transcription start point of the proximal promoter aligns to that of mouse promoter P3 and lies within a conserved region of sequence.

Caudalization by the amphibian organizer: brachyury, convergent extension and retinoic acid

Caudalization, which is proposed to be one of two functions of the amphibian organizer, initiates posterior pathways of neural development in the dorsalized ectoderm. In the absence of caudalization, dorsalized ectoderm only expresses the most anterior (archencephalic) differentiation. In the presence of caudalization, dorsalized ectorderm develops various levels of posterior neural tissues, depending on the extent of caudalization. A series of induction experiments have shown that caudalization is mediated by convergent extension: cell motility that is based on directed cell intercalation, and is essential for the morphogenesis of posterior axial tissues. During amphibian development, convergent extension is first expressed all-over the mesoderm and, after mesoderm involution, it becomes localized to the posterior mid-dorsal mesoderm, which produces notochord. This expression pattern of specific down regulation of convergent extension is also followed by the expression of the brachyury homolog. Furthermore, mouse brachyury has been implicated in the regulation of tissue elongation on the one hand, and in the control of posterior differentiation on the other. These observations suggest that protein encoded by the brachyury homolog controls the expression of convergent extension in the mesoderm. The idea is fully corroborated by a genetic study of mouse brachyury, which demonstrates that the gene product produces elongation of the posterior embryonic axis. However, there exists evidence for the induction of posterior dorsal mesodermal tissues, if brachyury homolog protein is expressed in the ectoderm. In both cases the brachyury homolog contributes to caudalization. A number of other genes appear to be involved in caudalization. The most important of these is pintavallis, which contains a fork-head DNA binding domain. It is first expressed in the marginal zone. After mesoderm involution, it is present not only in the presumptive notochord, but also in the floor plate. This is in contrast to the brachyury homolog, whose expression is restricted to mesoderm. The morphogenetic effects of exogenous RA on anteroposterior specification during amphibian embryogenesis are reviewed. The agent inhibits archencephalic differentiation and enhances differentiation of deuterencephalic and trunk levels. Thus the effect of exogenous RA on morphogenesis of CNS is very similar to that of caudalization, which is proposed to occur through the normal action of the organizer. According to a detailed analysis of the effect of lithium on morphogenesis induced by the Cynops organizer, lithium has a caudalizing effect closely comparable with that of RA. Furthermore, lithium induces convergent extension in the prechordal plate, which normally does not show cell motility.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular mechanisms of segmental patterning in the vertebrate hindbrain and neural crest

Recent work has shown that segmentation underlies the patterning of the vertebrate hindbrain and its neural crest derivatives. Several genes have been identified with segment-restricted expression, and evidence is now emerging regarding their function and regulatory relationships. The expression patterns of Hox genes and the phenotype of null mutants indicate roles in specifying segment identity. A zinc finger gene Krox-20 is a segment-specific regulator of Hox expression, and it seems probable that retinoic acid receptors also regulate Hox genes in the hindbrain. The receptor tyrosine kinase gene Sek may mediate cell-cell interactions that lead to segmentation. These studies provide a starting point for understanding the molecular basis of segmental patterning in the hindbrain.

Molecular regulation of neural crest development

The neural crest is a transient embryonic structure that gives rise to a multitude of different cell types in the vertebrate. As such, it is an ideal model to study the processes of vertebrate differentiation and development. This review focuses on two major questions related to neural crest development. The first question concerns the degree and time of commitment of the neural crest cells to different cell lineages and the emerging role of the homeobox containing genes in regulating this process. Evidence from the cephalic crest suggests that the commitment process does start before the neural crest cells migrate away from the neural tube and gene ablation experiments suggest that different homeobox genes are required for the development of neural and mesenchymal tissue derivatives. However, clonal analysis of neural crest cells before migration suggests that many of the cells remain multi-potential indicating that the final determinative steps occur progressively during migration and in association with environmental influences. The second question concerns the nature of the environmental factors that determine the differentiation of neural crest cells into discrete lineages. Evidence is provided, mainly from in vitro experiments, that purified growth factors selectively promote the differentiation of neural crest cells down either sympathetic, adrenal, sensory, or melanocytic cell lineages.

Segmental arrangement of reticulospinal neurons in the goldfish hindbrain

The hindbrain is evolutionarily conserved among diverse vertebrate phyla. In vertebrate embryos, the hindbrain is segmentally organized as a series of overt swellings known as rhombomeres. In the larval zebrafish Brachydanio rerio, conspicuous and identifiable reticulospinal neurons are positioned in the center of rhombomeres. Segmentally homologous reticulospinal neurons that share a range of morphological, developmental, and biochemical features occupy adjacent rhombomeres. We have recently shown that reticulospinal neurons of the zebrafish survive ontogeny without considerable morphological modification and we suggested that homologous neurons may share similar functions at different stages of development (Lee and Eaton: Journal of Comparative Neurology 304:34-52, 1991). The goldfish Carassius auratus, a related cyprinid, is especially suited for neurophysiological and behavioral studies. However, it is not yet known if the various reticulospinal neurons of zebrafish are generalizable to other species such as the goldfish. Therefore, we sought to examine the extent to which reticulospinal neurons of the zebrafish are also present in the adult goldfish. Analysis of 45 brains retrogradely labeled with horseradish peroxidase (HRP) from the spinal cord showed that reticulospinal neurons are arranged as a series of seven segments within the hindbrain; a regular interval of approximately 200 microns separates adjacent segments. Although the goldfish reticulospinal system has more neurons than the zebrafish, many reticulospinal neuron types continue to be identifiable. Moreover, comparisons of dendritic arborizations and axon paths between the two species showed that the morphology between various neuron types is virtually identical. The cross-taxonomic similarities between the reticulospinal systems of these related cyprinids make it possible to pursue functional considerations of segmentally homologous neurons in the goldfish hindbrain.

Cloning of the zebrafish krox-20 gene (krx-20) and its expression during hindbrain development

To begin to examine the function of genes that control early development in the hindbrain, we have screened an embryonic zebrafish cDNA library with a murine krox-20 gene probe that contained the conserved zinc finger regions. We have isolated two overlapping cDNAs, zf187 and zf201 which are homologues of the murine krox-20 gene. The N-terminal of the longest cDNA (zf201) contains two acidic regions identical to those of the murine krox-20. This indicates that the functional organisation of these proteins is probably conserved. Northern Blot analysis identified a single transcript of 2.0 kb. Wholemount in situ hybridisation established that expression of the zebrafish gene (krx-20) first appears at 100% epiboly as a single anterior domain of the prospective neuroepithelium, followed very soon after by a second more posterior domain. The alternating pattern of expression of this gene in rhombomeres(r) r3 and r5 is apparent by 12 hr post-fertilisation, that is prior to the morphological appearance of the rhombomeres. Around 14 hr neural crest migration begins from the dorsal surface of r5, moving caudally into r6 and then ventrally towards the pharyngeal arches. Crest migration is not apparent at or after 16 hr. No neural crest migration was observed from r3. Expression of krx-20 is down regulated firstly in r3 around 26 hr and later in r5 around 30 hr.

Somite pattern regulation in the avian segmental plate mesoderm

Previous experimental evidence suggested that the avian segmental pattern is already specified in the apparently unsegmented paraxial (segmental plate) mesoderm, but is susceptible to modification and reconstitution. We explored capacities of embryos to alter the specified pattern and restore it after disruption. In control experiments, right segmental plates of chicken or Japanese quail embryos were removed after about 48 hours of incubation and immediately replaced. Hensen's node and the primitive streak were removed to halt further segmental plate formation and the embryos were cultured for about 18 hours more. Somite numbers on the operated and unoperated sides were nearly identical (r = 0.904, n = 31, P &lt; 0.001); no species differences were noted. Right segmental plates of chicken hosts were then replaced with right segmental plates from quail donors. The numbers of somites formed by donors and grafts were not significantly correlated (r = 0.305, n = 30, P &lt; 0.1), but the correlation between the graft and the host's unoperated side was significant (r = 0.666, n = 30, P &lt; 0.001). The host is therefore able to alter the number of somites formed by the graft to one more compatible with the host's pattern. From orthostereoscopic reconstructions, it appeared that the location and size of somites could also be adjusted by the host. Similar results were obtained for tandem grafts of anterior halves of segmental plates and for grafts of minced segmental plates, though in the latter case contact with tissues near the midline was necessary for somite formation.