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In situ hybridization enables visualization of mRNA localization, and immunohistochemistry enables visualization of protein localization within a tissue or organism. Both techniques have been extensively utilized in zebrafish (Thisse et al., Development 119:1203-1215, 1993; Dutton et al., Development 128:4113-4125, 2001; Gilmour et al., Neuron 34:577-588, 2002; Lyons et al., Curr Biol 15:513-524, 2005) including for visualization of mRNA localization in Schwann cells (Lyons et al., Curr Biol 15:513-524, 2005; Monk et al., Science 325:1402-1405, 2009). For in situ hybridization, here, we outline how to generate RNA probes, conduct whole mount in situ hybridization for larvae, and list RNA probes that label different stages of Schwann cell development in zebrafish. For immunohistochemistry, the protocol we outline can be used to mark Schwann cells of sensory and motor nerves to examine properties such as developmental stage, morphology, proliferation, and apoptosis.

Whole-mount in situ hybridization (WMISH) is a common approach that is used to visualize spatial and temporal gene expression in embryos. In this process, digoxygenin-labeled antisense RNA is hybridized to the complementary transcript of interest and RNA hybrids are immunohistochemically detected using an alkaline phosphatase-conjugated antibody against digoxigenin. During

In situ hybridization enables visualization of mRNA localization, and immunohistochemistry enables visualization of protein localization within a tissue or organism. Both techniques have been extensively utilized in zebrafish (Thisse et al., Development 119:1203-1215, 1993; Dutton et al., Development 128:4113-4125, 2001; Gilmour et al., Neuron 34:577-588, 2002; Lyons et al., Curr Biol 15:513-524, 2005) including for visualization of mRNA localization in Schwann cells (Lyons et al., Curr Biol 15:513-524, 2005; Monk et al., Science 325:1402-1405, 2009). For in situ hybridization, here, we outline how to generate RNA probes, conduct whole mount in situ hybridization for larvae, and list RNA probes that label different stages of Schwann cell development in zebrafish. For immunohistochemistry, the protocol we outline can be used to mark Schwann cells of sensory and motor nerves to examine properties such as developmental stage, morphology, proliferation, and apoptosis.

Rabbits are valuable experimental animals used to evaluate the teratogenicity of chemical entities. If teratogenicity is recognized, it is very important to investigate the pathogenesis to assess the potential risk in humans. For this investigation, whole-mount in situ hybridization (WISH) is highly useful for visualizing spatial and temporal changes in the expression patterns of the targeted genes and or proteins. In this chapter, the WISH method in rabbit embryos, using a digoxigenin-labeled RNA probe, and the morphometric analysis of the expression pattern is described.

The photostability and narrow emission spectra of nanometer-scale semiconductor crystallites (QDs) make them desirable candidates for whole-mount fluorescentin situhybridization to detect mRNA transcripts in morphologically preserved intact embryos. We describe a method for direct QD labeling of modified oligonucleotide probes through streptavidin-biotin and antibody-mediated interactions (anti-FITC and anti-digoxigenin). To overcome permeability issues and allow QD conjugate penetration, embryos were treated with proteinase K. The use of QDs dramatically increased sensitivity of whole-mountin situhybridization (WISH) in comparison with organic fluorophores and enabled fluorescent detection of specific transcripts within cells without the use of enzymatic amplification. Therefore, this method offers significant advantages both in terms of sensitivity, as well as resolution. Specifically, the use of QDs alleviates issues of photostability and limited brightness plaguing organic fluorophores and allows fluorescent imaging of cleared embryos. It also offers new imaging possibilities, including intracellular localization of mRNAs, simultaneous multiple-transcript detection, and visualization of mRNA expression patterns in 3D.

Determining the precise expression pattern of a gene of interest at various stages of development is essential to understanding its biological function in embryology. This protocol describes a sensitive method for whole-mount in situ hybridization (WISH) to mouse embryos, using cRNA probes. Adaptations are provided that allow the protocol to be applied to embryonic stages ranging from blastocysts to postimplantation stage embryos, and to embryoid bodies. We also describe an in situ method for differential detection of two probes. Probe labeling and dissection and preparation of the embryos can be performed in 2 d. The actual WISH procedure can be completed in another 3 d.

Historically, techniques to analyze the localized distribution of mRNAs during development were performed on sectioned embryos using radioactively labeled riboprobes. The processing of the tissues and the use of emulsion autoradiography were laborious and time-consuming, leading to the development of more direct approaches. The nonradioactive whole-mount in situ hybridization method was first introduced in

MicroRNAs (miRNAs) loaded on argonaute proteins guide RNA-induced silencing complexes to target mRNAs. An excellent method to decipher the spatiotemporal expression patterns of miRNAs is whole-mount in situ hybridization (WISH), which has been successfully used in vertebrate embryos but still remains unavailable for many animal species. Here, we describe a WISH method for miRNA detection in Caenorhabditis elegans at both embryonic and post-embryonic stages. Strategies devised for detection include fixation of animals with carbodiimide at a high temperature and subsequent partial digestion of the fixed animals with an extremely high concentration of proteinase. WISH signals are visualized by staining with a chromogenic substrate or a fluorescent dye.

MicroRNAs (miRNAs) loaded on argonaute proteins guide RNA-induced silencing complexes to target mRNAs. An excellent method to decipher the spatiotemporal expression patterns of miRNAs is whole-mount in situ hybridization (WISH), which has been successfully used in vertebrate embryos but still remains unavailable for many animal species. Here, we describe a WISH method for miRNA detection in Caenorhabditis elegans at both embryonic and post-embryonic stages. Strategies devised for detection include fixation of animals with carbodiimide at a high temperature and subsequent partial digestion of the fixed animals with an extremely high concentration of proteinase. WISH signals are visualized by staining with a chromogenic substrate or a fluorescent dye.

Nucleic acids of intact biological tissues are rich in biological information. Whole-mount in situ hybridization is a powerful technique to mine the wealth of data contained in DNAs or RNAs, especially mRNAs. However, there are no simple, rapid approaches to precisely locate mRNAs in whole-mount tissues such as intact brains. By combining the penetration procedures of iDISCO with the signal amplification approach termed hybridization chain reaction, we herein developed a method for whole-brain in situ hybridization at cellular resolution. Based on fluorescence tomography instead of tissue clearing, this method provides a simple, rapid way to precisely locate mRNAs in the whole brain with cytoarchitectonic landmarks. As a proof of principle, we investigated the exact distribution of Cre mRNA in a Thy1-Cre mouse brain. We found high levels of Cre mRNA in most regions of the subcortical nuclei and the brain stem but comparatively low levels in major areas of the cerebral cortex. This method may have broad applications in studies of RNA function and its relations with diseases.

Techniques for studying RNA-protein interactions have lagged behind those for DNA-protein interactions as a consequence of the complexities associated with working with RNA. This unit describes a method for the adaptation of the In Situ Hybridization-Proximity Ligation Assay (ISH-PLA) to the study of RNA regulation (rISH-PLA). The rISH-PLA assay allows the identification of a given RNA-protein complex at subcellular and single-cell resolution, thus avoiding the lack of spatial resolution and sensitivity associated with assaying heterogeneous cell populations from which conventional RNA-protein interaction detection techniques suffer. This technique will be particularly usefully for studying the activity of RNA binding proteins (RBPs) in complex mixtures of cells, for example tissue sections or whole embryos. © 2017 by John Wiley & Sons, Inc.

Whole mount in situ hybridization (WISH) is a popular technique that is used to study temporal-spatial gene expression at mRNA level in model systems. Sensitive and high-resolution WISH is important for every laboratory that uses this technique to study expression pattern of genes in wildtype and perturbed embryos. Based on the commonly used protocols in fish community, our lab optimizes them to get more sensitive and specific results. Here, we briefly summarize the history of this technology using zebrafish as an example, describe a detailed experimental protocol, and provide troubleshooting for imperfect results and their solution.

In this study, we cloned the myeloperoxidase (mpo) gene of zebrafish and prepared a digoxigenin-labeled mpo RNA probe to investigate mpo gene expression in zebrafish during embryonic development by whole-mount in situ hybridization (WISH). The earliest mpo expression was detected in cells of the intermediate cell mass (ICM) at 18 h post-fertilization (hpf). It was detected 1 to 2 h later in cells in the rostral blood island (RBI) and strong signals were observed in the anterior ICM. Then, it spread over the yolk sac. By 72 hpf these mpo-expressing cells were in the circulation and distributed throughout the embryo. We identified that the level of mpo expression detected by WISH at an early stage was consistent with the data of cytological analyses of adult fish. The use of this method enabled us to track the gene changes that took place before morphological phenotypes were detected, as well to as investigate the hematopoietic cell fate in mutational or transgenic models in vivo. In this study, we modified several steps of WISH. The improved hybridization results demonstrated high specificity, distinct coloration and low background figures.

In situ hybridization is widely used to visualize transcribed sequences in embryos, tissues, and cells. For whole mount detection of mRNAs in embryos, hybridization with an antisense RNA probe is followed by visual or fluorescence detection of target mRNAs. A limitation of this approach is that a cDNA template of the target RNA must be obtained in order to generate the antisense RNA probe. Here we investigate the use of short (12–24 nucleotides) locked nucleic acid (LNA) containing DNA probes for whole mount in situ hybridization detection of mRNAs. Following extensive protocol optimization, we show that LNA probes can be used to localize several mRNAs of varying abundances in chicken embryos. LNA probes also detected alternatively spliced exons that are processed in a tissue specific manner. The use of LNA probes for whole mount in situ detection of mRNAs will enable in silico design and chemical synthesis and will expand the general use of in situ hybridization for studies of transcriptional regulation and alternative splicing.

A procedure for whole-mount in situ hybridization developed for detecting gene expression of Ccn2 in developing calcified tissues of mouse embryos is presented. In this method, embryos are hybridized with Dig-labeled riboprobes, and the riboprobes are detected by use of the alkaline-phosphatase reaction in the presence of a 4-nitro-blue tetrazolium chloride and 5-bromo-4-chloro-3-indolyl-phosphate (NBT + BCIP) mixture. Obvious detection of positive signals for Ccn2 in the cartilage of developing phalanges indicates that this method can be applied to gene expression analysis of other Ccn genes in developing calcified tissues.

A procedure for whole-mount in situ hybridization developed for detecting gene expression of Ccn2 in developing calcified tissues of mouse embryos is presented. In this method, embryos are hybridized with Dig-labeled riboprobes, and the riboprobes are detected by use of the alkaline-phosphatase reaction in the presence of a 4-nitro-blue tetrazolium chloride and 5-bromo-4-chloro-3-indolyl-phosphate (NBT + BCIP) mixture. Obvious detection of positive signals for Ccn2 in the cartilage of developing phalanges indicates that this method can be applied to gene expression analysis of other Ccn genes in developing calcified tissues.

Whole mount in situ hybridization (WMISH) is a technique that allows for the spatial resolution of nucleic acid molecules (often mRNAs) within a 'whole mount' tissue preparation, or developmental stage (such as an embryo or larva) of interest. WMISH is extremely powerful because it can significantly contribute to the functional characterization of complex metazoan genomes, a challenge that is becoming more of a bottleneck with the deluge of next generation sequence data. Despite the conceptual simplicity of the technique much time is often needed to optimize the various parameters inherent to WMISH experiments for novel model systems; subtle differences in the cellular and biochemical properties between tissue types and developmental stages mean that a single WMISH method may not be appropriate for all situations. We have developed a set of WMISH methods for the re-emerging gastropod model Lymnaea stagnalis that generate consistent and clear WMISH signals for a range of genes, and across all developmental stages. These methods include the assignment of larvae of unknown chronological age to an ontogenetic window, the efficient removal of embryos and larvae from their egg capsules, the application of an appropriate Proteinase-K treatment for each ontogenetic window, and hybridization, post-hybridization and immunodetection steps. These methods provide a foundation from which the resulting signal for a given RNA transcript can be further refined with probe specific adjustments (primarily probe concentration and hybridization temperature).

Whole mount in situ hybridization (WMISH) is a technique that allows for the spatial resolution of nucleic acid molecules (often mRNAs) within a 'whole mount' tissue preparation, or developmental stage (such as an embryo or larva) of interest. WMISH is extremely powerful because it can significantly contribute to the functional characterization of complex metazoan genomes, a challenge that is becoming more of a bottleneck with the deluge of next generation sequence data. Despite the conceptual simplicity of the technique much time is often needed to optimize the various parameters inherent to WMISH experiments for novel model systems; subtle differences in the cellular and biochemical properties between tissue types and developmental stages mean that a single WMISH method may not be appropriate for all situations. We have developed a set of WMISH methods for the re-emerging gastropod model Lymnaea stagnalis that generate consistent and clear WMISH signals for a range of genes, and across all developmental stages. These methods include the assignment of larvae of unknown chronological age to an ontogenetic window, the efficient removal of embryos and larvae from their egg capsules, the application of an appropriate Proteinase-K treatment for each ontogenetic window, and hybridization, post-hybridization and immunodetection steps. These methods provide a foundation from which the resulting signal for a given RNA transcript can be further refined with probe specific adjustments (primarily probe concentration and hybridization temperature).

To analyze gene regulatory networks active during embryonic development and organogenesis it is essential to precisely define how the different genes are expressed in spatial relation to each other in situ. Multi-target chromogenic whole-mount in situ hybridization (MC-WISH) greatly facilitates the instant comparison of gene expression patterns, as it allows distinctive visualization of different mRNA species in contrasting colors in the same sample specimen. This provides the possibility to relate gene expression domains topographically to each other with high accuracy and to define unique and overlapping expression sites. In the presented protocol, we describe a MC-WISH procedure for comparing mRNA expression patterns of different genes in Drosophila embryos. Up to three RNA probes, each specific for another gene and labeled by a different hapten, are simultaneously hybridized to the embryo samples and subsequently detected by alkaline phosphatase-based colorimetric immunohistochemistry. The described procedure is detailed here for Drosophila, but works equally well with zebrafish embryos.

In situ hybridization is a powerful technique used for locating specific nucleic acid targets within morphologically preserved tissues and cell preparations. A labeled RNA or DNA probe hybridizes to its complementary mRNA or DNA sequence within a sample. Here, we describe RNA in situ hybridization protocol for whole-mount zebrafish embryos.

There is cumulating evidence that microRNAs (miRNAs) are important regulators of pluripotency and differentiation and, hence, of early lineage segregation in embryo development. To unravel the function of specific miRNAs, it is important not only to analyze miRNA expression in the entire blastocyst but also to determine the site and level of expression in the inner cell mass (ICM) versus trophectoderm (TE). A new strategy has been developed for miRNA expression analysis in ICM and TE using two complementary techniques. By whole mount in situ hybridization (WISH), it was visualized that bta-miR-155 is mainly expressed in the ICM. However, WISH does not provide quantitative data on expression differences between the two cell types. By reverse transcription quantitative polymerase chain reaction (RT–qPCR) on ICM and TE isolates taken from single blastocysts with laser capture microdissection (LCM), it was quantified that bta-miR-155 was 50-fold higher expressed in ICM than in TE. The possibility to quantify both miRNAs and messenger RNAs (mRNAs) in LCM samples offers the opportunity to analyze the expression of both miRNAs and potential targets in one sample. This article shows that a combination of WISH with LCM and subsequent RT–qPCR is a robust strategy to qualitatively and quantitatively analyze differential miRNA expression in discrete cell types of a single blastocyst.

We have developed "b" and "c" isoform-specific chicken fibroblast growth factor (FGF) receptor 1-3 probes for in situ hybridization. We rigorously demonstrate the specificity of these probes by using both dot blot hybridization and whole-mount in situ hybridization during neurulation and early postneurulation stages, and we compare expression patterns of each of the three isoform-specific probes to one another and to generic probes to each of the three (non-isoform-specific) FGF receptors. We show that the expression pattern of each receptor is represented by the collective expression of each of its two isoforms, with the expression of each FGF receptor being most similar to that of its "c" isoform at two of the three stages studied, and that tissue and stage differences exist in the patterns of expression of the six isoforms. We demonstrate the usefulness of these probes for defining the differential tissue expression of FGF receptor 1-3 isoforms.

Techniques for studying RNA-protein interactions have lagged behind those for DNA-protein complexes as a consequence of the complexities associated with working with RNA. Here we present a method for the modification of the existing In Situ Hybridisation-Proximity Ligation Assay (ISH-PLA) protocol to adapt it to the study of RNA regulation (rISH-PLA). As proof of principle we used the well-characterised interaction of the Xenopus laevis Staufen RNA binding protein with Vg1 mRNA, the complex of which co-localises to the vegetal pole of Xenopus oocytes. The applicability of both the Stau1 antibody and the Locked Nucleic Acid probe (LNA) recognising Vg1 mRNA were independently validated by whole-mount Immunohistochemistry and whole-mount in situ hybridisation assays respectively prior to combining them in the rISH-PLA assay. The rISH-PLA assay allows the identification of a given RNA-protein complex at subcellular and single cell resolution, thus avoiding the lack of spatial resolution and sensitivity associated with assaying heterogenous cell populations from which conventional RNA-protein interaction detection techniques suffer. This technique will be particularly usefully for studying the activity of RNA binding proteins (RBPs) in complex mixtures of cells, for example tissue sections or whole embryos.

MicroRNAs (miRNAs) affect fundamental processes of development. In plants miRNAs regulate organ development, transition to flowering, and responses to abiotic/biotic stresses. To understand the biological role of miRNAs, in addition to identifying their targeted transcripts, it is necessary to characterize the spatiotemporal regulation of their expression. Many methods have been used to define the set of organ-specific miRNAs by tissue dissection and miRNA profiling but none of them can describe their tissue and cellular distribution at the high resolution provided by in situ hybridization (ISH). This article describes the setup and optimization of a whole-mount ISH protocol to target endogenous miRNAs on intact Arabidopsis seedlings using DIG-labeled Zip Nucleic Acid (ZNA) oligonucleotide probes. Automation of the main steps of the procedure by robotized liquid handling has also been implemented in the protocol for best reproducibility of results, enabling running of ISH experiments at high throughput.

Small regulatory RNAs are pivotal regulators of gene expression and play important roles in many plant processes. Although our knowledge of their biogenesis and mode of action has significantly progressed, we comparatively still know little about their biological functions. In particular, knowledge about their spatiotemporal patterns of expression rely on either indirect detection by use of reporter constructs or labor-intensive direct detection by in situ hybridization on sectioned material. None of the current approaches allows for a systematic investigation of small RNAs expression patterns.Here, we present a method for the sensitive in situ detection of micro- and siRNAs in intact plant tissues that utilizes both double-labelled probes and a specific cross linker. We determined the expression patterns of several small RNAs in plant roots and embryos.

Summary Small RNAs, such as microRNAs (miRNAs), regulate gene expression and play important roles in many plant processes. Although our knowledge of their biogenesis and mode of action has significantly progressed, we still have comparatively little information about their biological functions. In particular, knowledge about their spatio-temporal expression patterns rely on either indirect detection by use of reporter constructs or labor-intensive direct detection by in situ hybridization on sectioned material. None of the current approaches allows a systematic investigation of small RNA expression patterns. Here, we present a sensitive method for in situ detection of miRNAs and siRNAs in intact plant tissues that utilizes both double-labeled probes and a specific cross-linker. We determined the expression patterns of several small RNAs in diverse plant tissues.

In situ hybridization is a powerful technique used for locating specific nucleic acid targets within morphologically preserved tissues and cell preparations. A labeled RNA or DNA probe hybridizes to its complementary mRNA or DNA sequence within a sample. Here, we describe RNA in situ hybridization protocol for whole-mount zebrafish embryos.

Whole-mount immunofluorescence technique provides a way to reveal integrated expression patterns of biological molecules in individuals. Well-documented morphological preservation ability in biology makes aldehydes the fixative of choice. Cross-linking among biocomponents and aldehydes is the key for maintaining morphology but masks the biological molecules for immunodetection. This study performs an easily accessible method by applying heat-induced retrieval, which can rescue the antigenicity of the proteins and also enhance the labeling sensitivity of the fluorescence dye in overfixed zebrafish embryos. The results show that the immunoreactivities of antibodies to myosin in the muscles, green fluorescent protein in the blood vessels and the nuclei in the cells can be recovered significantly, and the morphology of the zebrafish embryos, even the fragile mutants, is at the same time well maintained. Therefore, we provide a choice for antigen retrieval, which is effective for whole-mount immunofluorescence microscopy.

In order to broaden the comparative scope of evolutionary developmental biology and to refine our picture of animal macroevolution, it is necessary to establish new model organisms, especially from previously underrepresented groups, like the Lophotrochozoa. We have established the culture and protocols for molecular developmental biology in the rotifer species Brachionus plicatilis Müller (Rotifera, Monogononta). Rotifers are nonsegmented animals with enigmatic basal position within the lophotrochozoans and marked by several evolutionary novelties like the wheel organ (corona), the median eye, and the nonpaired posterior foot. The expression of Bp-Pax-6 is shown using whole-mount in situ hybridization. The inexpensive easy culture and experimental tractability of Brachionus as well as the range of interesting questions to which it holds the key make it a promising addition to the “zoo” of evo-devo model organisms. Electronic supplementary material The online version of this article (doi:10.1007/s00427-008-0234-z) contains supplementary material, which is available to authorized users.

In situ hybridization is a powerful technique used for locating specific nucleic acid targets within morphologically preserved tissues and cell preparations. A labeled RNA or DNA probe hybridizes to its complementary mRNA or DNA sequence within a sample. Here, we describe RNA in situ hybridization protocol for whole-mount zebrafish embryos.

Gene expression profiling is an important component of functional genomics. We present a time and cost efficient high-throughput whole-mount in situ technique to perform a large-scale gene expression analysis in medaka fish (Oryzias latipes) embryos. Medaka is a model system ideally suited for the study of molecular genetics of vertebrate development. Random cDNA clones from an arrayed stage 20 medaka plasmid library were analyzed by whole-mount in situ hybridization on embryos of three representative stages of medaka development. cDNA inserts were colony PCR amplified in a 384-format. The PCR products were used to generate over 2000 antisense RNA digoxigenin probes in a high-throughput process. Whole-mount in situ hybridization was carried out in a robot and a broad range of expression patterns was observed. Partial cDNA sequences and expression patterns were documented with BLAST results, cluster analysis, images and descriptions, respectively; collectively this information was entered into a web-based database, "MEPD" (http://www.embl-heidelberg.de/mepd/), that is publicly accessible.

Beard worms (Siboglinidae, Polychaeta) lack a mouth and a digestive tract and harbour chaemosynthetic bacteria in the bacteriocytes of the trophosome. Since beard worms depend on the organic compounds produced by the bacteria for nourishment, the bacteriocytes should be efficient in exchanging various substances with body fluids. For this reason, it is important to determine how the bacteriocytes are organized in the trophosome. As the first step of the present study, the appearance of bacteriocytes was examined in routinely stained paraffin sections. Second, visualization of the actual distribution of the bacteriocytes was attempted using whole-mount in situ hybridization with a probe of the 16S rRNA nucleotide sequence of the bacterium. After routine haematoxylin & eosin staining, the bacteriocytes appeared to be aligned in cell cords accompanied with nutrient-deposit cells that extended from both sides of the trophosome toward the dorsal side and folded up in the coelomic spaces. In whole-mount preparations, however, bacteriocytes with intense signals of 16S rRNA were seen three-dimensionally as many irregular leaves arranged from both sides of the ventral vessel toward the dorsal vessel. We will discuss the physiological significance of this characteristic distribution of the bacteriocytes in the present species.

金沢大学環日本海域環境研究センター生物多様性研究部門, 金沢大学理学部, Beard worms (Siboglinidae, Polychaeta) lack a mouth and a digestive tract and harbour chaemosynthetic bacteria in the bacteriocytes of the trophosome. Since beard worms depend on the organic compounds produced by the bacteria for nourishment, the bacteriocytes should be efficient in exchanging various substances with body fluids. For this reason, it is important to determine how the bacteriocytes are organized in the trophosome. As the first step of the present study, the appearance of bacteriocytes was examined in routinely stained paraffin sections. Second, visualization of the actual distribution of the bacteriocytes was attempted using whole-mount in situ hybridization with a probe of the 16S rRNA nucleotide sequence of the bacterium. After routine haematoxylin & eosin staining, the bacteriocytes appeared to be aligned in cell cords accompanied with nutrient-deposit cells that extended from both sides of the trophosome toward the dorsal side and folded up in the coelomic spaces. In whole-mount preparations, however, bacteriocytes with intense signals of 16S rRNA were seen three-dimensionally as many irregular leaves arranged from both sides of the ventral vessel toward the dorsal vessel. We will discuss the physiological significance of this characteristic distribution of the bacteriocytes in the present species. © 2007 The Authors Journal compilation © 2007 The Royal Swedish Academy of Sciences.

Early-life stage fish can be more sensitive to toxicants than adults, so delineating mechanisms of perturbation of biological pathways by chemicals during this life stage is crucial. Whole-mount in situ hybridization (WISH) paired with quantitative real-time polymerase chain reaction (QPCR) assays can enhance pathway-based analyses through determination of specific tissues where changes in gene expression are occurring. While WISH has frequently been used in zebrafish (Danio rerio), this technology has not previously been applied to fathead minnows (Pimephales promelas), another well-established small fish model species. The objective of the present study was to adapt WISH to fathead minnow embryos and larvae, and use the approach to evaluate the effects of estrone, an environmentally-relevant estrogen receptor (ER) agonist. Embryos were exposed via the water to 0, 18 or 1800 ng estrone/L (0, 0.067 and 6.7nM) for 3 or 6 days in a solvent-free, flow-through test system. Relative transcript abundance of three estrogen-responsive genes, estrogen receptor-α (esr1), cytochrome P450-aromatase B (cyp19b), and vitellogenin (vtg) was examined in pooled whole embryos using QPCR, and the spatial distribution of up-regulated gene transcripts was examined in individual fish using WISH. After 3 days of exposure to 1800 ng estrone/L, esr1 and cyp19b were significantly up-regulated, while vtg mRNA expression was not affected. After 6 days of exposure to 1800 ng estrone/L, transcripts for all three genes were significantly up-regulated. Corresponding WISH assays revealed spatial distribution of esr1 and vtg in the liver region, an observation consistent with activation of the hepatic ER. This study clearly demonstrates the potential utility of WISH, in conjunction with QPCR, to examine the mechanistic basis of the effects of toxicants on early-life stage fathead minnows.