The authors would like to thank Dr. Louis Reichardt for the TrkAtaulacZ mice and Dr. Annette Markus for insightful discussion and suggestions. This work was supported by startup funds from the Burke Foundation as well as Whitehall Foundation research grant 2010-08-61, a research grant from Wings for Life Foundation (WFL-US-028/14), grant ZB1-1102-1 from the Christopher &#38; Dana Reeve Foundation, and grants 1R01EY022409 and 3R01EY022409-01S1 from the National Eye Institute, to JZ. KJO is a Goldsmith fellow.

NOTE: All procedures comply with the NIH Guide for the Use and Care of Laboratory Animals. The animal protocol was approved by the IACUC at Weill Cornell Medical College.
1. Tissue Preparation
<ol>
	<li>Euthanize timed-pregnancy females by cervical dislocation15. Dissect embryonic E16 - E18 embryos from timed-pregnancy females and place embryos individually in the wells of a 6-well dish, filled with cold phosphate buffered saline (PBS).</li>
	<li>Rinse embryos in cold PBS.</li>
	<li>Remove a small portion of the amniotic membrane of the embryo and place in a pre-prepared proteinase K solution for subsequent genotyping. Alternatively, if the amniotic membrane is lost, remove a small portion of the tip of the embryo tail and place it in proteinase K solution</li>
	<li>Poke holes into the skin all around the embryo using insect pins (at least 100 per side).</li>
	<li>Remove PBS and slowly add fresh 2% paraformaldehyde (PFA), pH 7.4, to the well.</li>
	<li>Gently shake for 2 hr at 4 &#176;C.</li>
	<li>Rinse embryos twice in PBS and store in PBS at 4 &#176;C until staining.</li>
</ol>
2. Genotyping
<ol>
	<li>Immerse amniotic membranes or embryo tails in proteinase K mixture according to the manufacturer&#8217;s instructions.</li>
	<li>Incubate at 56 &#176;C for 10 min.</li>
	<li>Extract DNA using a commercially available DNA purification kit.</li>
	<li>Take 0.5 &#181;l of the total 200 &#181;l purified genomic DNA solution, combine it with 0.5 &#181;l of each of the gene-specific sense and antisense primers (10 &#181;M), 2 &#181;l dNTP Mix (2.5 mM of dATP, dCTP, dGTP and dTTP), 0.2 &#181;l Taq polymerase (5 U/&#181;l) and 2 &#181;l PCR buffer (10x), add H2O to a total volume of 20 &#181;l. 
	NOTE: Primer sequences are as follows (Table 1): 
	TrkA: TrkA_F: GCG GGC GCC GCC GCG ATG, TrkA_R: GAA GCC GCC TGC GCG GCT CTG CCA GGG TG; PCR fragment length: 400 basepairs (bp). 
	Bax: In5R: GTT GAC CAG AGT GGC GTA GG, Ex5F: TGA TCA GAA CCA TCA TG, NeoR: CCG CTT CCA TTG CTC AGC GG; PCR fragment lengths: wild type, 304 bp; Bax null, 507 bp. 
	TauLacZ: lacZup: ACA ACG TCG TGA GTG GGA AAA, lacZdown: ATC AAC ATT AAA TGT GAG CGA G; PCR fragment length: 360 bp.</li>
	<li>Run a PCR with the following program for TrkA: step 1: 1 min at 95 &#176;C, step 2: 10 sec at 95 &#176;C, step 3: 20 sec at 68 &#176;C, step 4: 30 sec at 72 &#176;C. Repeat steps 2 - 4 for 40 cycles.
	<ol>
		<li>For LacZ and Bax, run a PCR with the following program: step 1: 1 min at 95 &#176;C, step 2: 10 sec at 95 &#176;C, step 3: 1 min at 54 &#176;C, step 4: 1 min at 72 &#176;C, repeat steps 2 - 4 for 35 cycles.</li>
	</ol>
	</li>
	<li>Run PCR samples on a 2% agarose gel at 100 V in 1x Tris-Acetate-EDTA buffer for 20 min with a lane of DNA ladder to assess fragment lengths.</li>
	<li>Analyze embryos for the presence of wild type TrkA, TrkA-tauLacZ and Bax null alleles. The genotypes of experimental embryos are TrkAtaulacZ/WT : Bax -/- and TrkAtaulacZ/taulacZ : Bax-/-, depending on the experimental questions. Embryos lacking the tauLacZ reporter will be negative for axonal staining and can therefore serve as negative controls to calibrate the staining process.</li>
</ol>
3. Embryo Staining
<ol>
	<li>Use a commercial kit for lacZ staining with some modifications as described below in sections 3.2 - 3.8. Here, use the Millipore kit.</li>
	<li>Immerse embryos in Buffer A for at least 1 hr, shaking gently at room temperature.</li>
	<li>Directly immerse embryos in Buffer B for at least 15 min, shaking gently at room temperature.</li>
	<li>While embryos are in Buffer A/B, pre-warm Tissue Base solution at 37 &#176;C. Use 5 ml of Tissue Base solution per embryo.</li>
	<li>While embryos are in Buffer A/B, prepare fresh 5-bromo-&#8203;4-&#8203;chloro-&#8203;3-&#8203;indolyl &#945;-D-galactopyranoside (X-gal) at a concentration of 40 mg/ml in 100% dimethyl sulfoxide (DMSO).</li>
	<li>Dilute X-gal into the pre-warmed Tissue Base solution at a concentration of 1 mg/ml and add 5 ml of X-gal/Tissue Base mixture to a glass scintillation vial.</li>
	<li>Transfer embryos to a glass scintillation vial containing X-gal/Tissue Base mixture. Seal lid with Parafilm and incubate at 37 &#176;C overnight, checking for presence of blue stain.</li>
	<li>Postfix embryos with fresh 4% PFA, pH 7.4. Gently shake for 4 hr at 4 &#176;C.</li>
	<li>Rinse embryos twice in cold PBS.</li>
</ol>
4. Tissue Dehydration and Clearing
<ol>
	<li>Place embryos in 50% methanol/ 50% PBS mixture for 30 min at room temperature, shaking in the glass vials.</li>
	<li>Continue to dehydrate in 75% methanol/ 25% PBS for 30 min at room temperature, shaking in glass vials.</li>
	<li>Continue to dehydrate in 90% methanol/ 10% PBS for 30 min at room temperature, shaking in glass vials.</li>
	<li>Continue to dehydrate in 100% methanol for 30 min (2x) at room temperature, shaking in glass vials.</li>
	<li>While embryos are dehydrating, prepare a 1:1 (v:v) mixture of benzoyl alcohol and benzoyl benzoate. 
	NOTE: Benzoyl alcohol and benzoyl benzoate is toxic.</li>
	<li>Immerse embryos in the 1:1 (v:v) mixture of 100% methanol: 100% BABB for 15 min. Only use glass because the BABB will dissolve plastic.</li>
	<li>Immerse embryos in 100% BABB to completely clear the tissue and to visualize X-gal staining in the cleared embryo. 
	NOTE: Image as soon as possible because the X-gal stain will fade over time.</li>
</ol>
5. Whole Mount Imaging
<ol>
	<li>Stabilize the embryo, immersed in 100% BABB, at the appropriate angles for photography using metal forceps. Illuminate using a combination of up-light and under-light light sources.</li>
	<li>Acquire images with a dissecting microscope outfitted with a digital camera. Take multiple bright field exposures at five successive focal planes 0.5 mm apart along the Z axis. Exposure times and f-stops may vary according to illumination and camera specs and need to be optimized for each setup.</li>
	<li>Use standard image processing software to process overlay images and generate photomontages.</li>
</ol>

<ol>
	<li>Verze, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cutaneous+innervation+in+hereditary+sensory+and+autonomic+neuropathy+type+IV.">Cutaneous innervation in hereditary sensory and autonomic neuropathy type IV.</a> Neurology. 55, 126-128 (2000).</li><li>Sethna, N. F., Meier, P. M., Zurakowski, D., Berde, C. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cutaneous+sensory+abnormalities+in+children+and+adolescents+with+complex+regional+pain+syndromes.">Cutaneous sensory abnormalities in children and adolescents with complex regional pain syndromes.</a> Pain. 131, 153-161 (2007).</li><li>Uceyler, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Small+fibers+in+Fabry+disease:+baseline+and+follow-up+data+under+enzyme+replacement+therapy.">Small fibers in Fabry disease: baseline and follow-up data under enzyme replacement therapy.</a> J Peripher Nerv Syst. 16, 304-314 (2011).</li><li>Reichardt, L. F., Mobley, W. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Going+the+distance,+or+not,+with+neurotrophin+signals.">Going the distance, or not, with neurotrophin signals.</a> Cell. 118, 141-143 (2004).</li><li>White, F. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synchronous+onset+of+NGF+and+TrkA+survival+dependence+in+developing+dorsal+root+ganglia.">Synchronous onset of NGF and TrkA survival dependence in developing dorsal root ganglia.</a> J Neurosci. 16, 4662-4672 (1996).</li><li>Farinas, I., Wilkinson, G. A., Backus, C., Reichardt, L. F., Patapoutian, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Characterization+of+neurotrophin+and+Trk+receptor+functions+in+developing+sensory+ganglia:+direct+NT-3+activation+of+TrkB+neurons+in+vivo.">Characterization of neurotrophin and Trk receptor functions in developing sensory ganglia: direct NT-3 activation of TrkB neurons in vivo.</a> Neuron. 21, 325-334 (1998).</li><li>Markus, A., Zhong, J., Snider, W. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Raf+and+akt+mediate+distinct+aspects+of+sensory+axon+growth.">Raf and akt mediate distinct aspects of sensory axon growth.</a> Neuron. 35, 65-76 (2002).</li><li>Kuruvilla, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+neurotrophin+signaling+cascade+coordinates+sympathetic+neuron+development+through+differential+control+of+TrkA+trafficking+and+retrograde+signaling.">A neurotrophin signaling cascade coordinates sympathetic neuron development through differential control of TrkA trafficking and retrograde signaling.</a> Cell. 118, 243-255 (2004).</li><li>Zhong, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Raf+kinase+signaling+functions+in+sensory+neuron+differentiation+and+axon+growth+in+vivo.">Raf kinase signaling functions in sensory neuron differentiation and axon growth in vivo.</a> Nature. 10, 598-607 (2007).</li><li>Bulfone, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+olfactory+sensory+map+develops+in+the+absence+of+normal+projection+neurons+or+GABAergic+interneurons.">An olfactory sensory map develops in the absence of normal projection neurons or GABAergic interneurons.</a> Neuron. 21, 1273-1282 (1998).</li><li>Moqrich, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Expressing+TrkC+from+the+TrkA+locus+causes+a+subset+of+dorsal+root+ganglia+neurons+to+switch+fate.">Expressing TrkC from the TrkA locus causes a subset of dorsal root ganglia neurons to switch fate.</a> Nature. 7, 812-818 (2004).</li><li>Knudson, C. M., Tung, K. S., Tourtellotte, W. G., Brown, G. A., Korsmeyer, S. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bax-deficient+mice+with+lymphoid+hyperplasia+and+male+germ+cell+death.">Bax-deficient mice with lymphoid hyperplasia and male germ cell death.</a> Science. 270 (5233), 96-99 (1995).</li><li>Lentz, S. I., Knudson, C. M., Korsmeyer, S. J., Snider, W. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neurotrophins+support+the+development+of+diverse+sensory+axon+morphologies.">Neurotrophins support the development of diverse sensory axon morphologies.</a> J. Neurosci. 19, 1038-1048 (1999).</li><li>Patel, T. D., Jackman, A., Rice, F. L., Kucera, J., Snider, W. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+sensory+neurons+in+the+absence+of+NGF/TrkA+signaling+in+vivo.">Development of sensory neurons in the absence of NGF/TrkA signaling in vivo.</a> Neuron. 25, 345-357 (2000).</li><li>Donovan, K. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=B-RAF+kinase+drives+developmental+axon+growth+and+promotes+axon+regeneration+in+the+injured+mature+CNS.">B-RAF kinase drives developmental axon growth and promotes axon regeneration in the injured mature CNS.</a> The Journal of experimental medicine. 211, 801-814 (2014).</li><li>Mercer, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Expression+of+endogenous+oncogenic+V600EB-raf+induces+proliferation+and+developmental+defects+in+mice+and+transformation+of+primary+fibroblasts.">Expression of endogenous oncogenic V600EB-raf induces proliferation and developmental defects in mice and transformation of primary fibroblasts.</a> Cancer research. 65, 11493-11500 (2005).</li><li>Tronche, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Disruption+of+the+glucocorticoid+receptor+gene+in+the+nervous+system+results+in+reduced+anxiety.">Disruption of the glucocorticoid receptor gene in the nervous system results in reduced anxiety.</a> Nature genetics. 23, 99-103 (1999).</li><li>Madisen, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+toolbox+of+Cre-dependent+optogenetic+transgenic+mice+for+light-induced+activation+and+silencing.">A toolbox of Cre-dependent optogenetic transgenic mice for light-induced activation and silencing.</a> Nat Neurosci. 15, 793-802 (2012).</li><li>Feng, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+Neuronal+Subsets+in+Transgenic+Mice+Expressing+Multiple+Spectral+Variants+of+GFP.">Imaging Neuronal Subsets in Transgenic Mice Expressing Multiple Spectral Variants of GFP.</a> Neuron. 28, 41-51 (2000).</li><li>Schmidt, H., Rathjen, F. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=DiI-labeling+of+DRG+neurons+to+study+axonal+branching+in+a+whole+mount+preparation+of+mouse+embryonic+spinal+cord.">DiI-labeling of DRG neurons to study axonal branching in a whole mount preparation of mouse embryonic spinal cord.</a> J Vis Exp. , (2011).</li></ol>

The authors declare that there are no competing interests.

The above-described X-gal staining procedure of embryonic TrkAtaulacZ mice allows for the rapid and detailed visualization of long distance axon projections in the intact fixed embryo. Because of the Bax null background these mice allow for the probing of signaling mechanisms that may contribute to both axon growth and neuronal survival. Mating with transgenic or knockout mice of interest allows for comprehensive assessment of axonal phenotypes and can serve as useful guide for futur...

Establishment of precise neuronal networks is a complex developmental process essential for the functionality of the nervous system. Disturbance in this process leads to neuronal dysfunction which has been implicated in human neurological diseases1-3. To study the underlying molecular mechanisms of axon growth and target innervation in mammals, we have developed a protocol to visualize the axonal trajectories of TrkA-expressing sensory neurons using a combination of two genetically modified mouse lines.
TrkA is a receptor for nerve growth factor NGF and is a functional marker of nociceptive sensory neurons4. TrkA is highly expressed in nociceptive neurons during early development and mediates NGF-dependent neuron survival, axon growth, arborization and target innervation5-9. In TrkAtaulacZ mice, the wild type TrkA gene is replaced by a taulacZ expression cassette10, such that the axonal morphology of putative TrkA-positive neurons can be visualized by &#946;-gal (X-gal) staining11. Using a heterozygous TrkAtaulacZ/WT line, we can examine factors that may regulate or interfere with the development of sensory afferent projections in vivo.
Moreover, TrkA expression is absent in homozygous TrkAtaulacZ/taulacZ mice, which can therefore be used to assess the axon growth promoting mechanisms in the absence of NGF/TrkA signaling. Since nociceptive neurons depend on NGF/TrkA signaling not only for axon growth, but also for survival, we employ another mouse line, lacking the pro-apoptotic Bax gene, to inhibit apoptosis in embryonic DRG neurons, rescuing them from cell death that is otherwise observed in the absence of TrkA signaling. The Bax-/- background12 thus allows for the molecular dissection of signaling pathways that specifically affect axon growth7-9,13-15. In TrkA-/- : Bax-/- mice, DRG neurons survive, but sensory afferent innervation in the skin is completely abolished14,15. We can selectively activate signaling pathways to determine their respective contributions to the development of axon projections. The utility of this method is that it allows the assessment of changes in axonal growth phenotypes when different genetic modifications are bred onto the TrkAtaulacZ/taulacZ : Bax-/- or TrkAtaulacZ/WT : Bax-/- backgrounds.

<table><tbody><tr><td>PFA</td><td>Sigma-Aldrich</td><td>P6418</td><td></td></tr><tr><td>PBS</td><td>Life Tech</td><td>10010-023</td><td></td></tr><tr><td>Tissue Rinse Solution A</td><td>Millipore</td><td>BG-6-B</td><td></td></tr><tr><td>Tissue Rinse Solution B</td><td>Millipore</td><td>BG-7-B</td><td></td></tr><tr><td>Tissue Stain Base Solution</td><td>Millipore</td><td>BG-8-C</td><td></td></tr><tr><td>X-gal</td><td>Sigma-Aldrich</td><td>B4252</td><td></td></tr><tr><td>Glass scintiallation vial</td><td>Kimble Chase</td><td>74500-20</td><td></td></tr><tr><td>Incubator</td><td>Labline</td><td>Model 120</td><td></td></tr><tr><td>Insect pins</td><td>FST</td><td>26000-30</td><td></td></tr><tr><td>DMSO</td><td>Sigma-Aldrich</td><td>D8418</td><td></td></tr><tr><td>6-well dish</td><td>USA Scientific</td><td>CC7672-7506</td><td></td></tr><tr><td>Primers</td><td>IDT</td><td>custom DNA primers</td><td></td></tr><tr><td>Takara dNTP mixture</td><td>Takara</td><td>4030</td><td></td></tr><tr><td>Takara LA buffer</td><td>Takara</td><td>RR002A</td><td></td></tr><tr><td>Takara LA Taq</td><td>Takara</td><td>RR002A</td><td></td></tr><tr><td>PCR machine</td><td>Bio-Rad</td><td>DNA Engine Dyad</td><td></td></tr><tr><td>Benzyl alcohol</td><td>Sigma-Aldrich</td><td>B-1042</td><td></td></tr><tr><td>Benzyl benzoate</td><td>Sigma-Aldrich</td><td>B-6630</td><td></td></tr><tr><td>Dissecting microscope</td><td>Leica</td><td>M205A</td><td></td></tr><tr><td>Camera</td><td>Leica</td><td>DFC310FX</td><td></td></tr><tr><td>Ring light</td><td>Leica&amp;nbsp;</td><td>MEB110</td><td></td></tr><tr><td>Photoshop</td><td>Adobe</td><td>Photoshop 4.0</td><td></td></tr></tbody></table>

whole-mount imaging of mouse embryo sensory axon projections

The visualization of full-length neuronal projections in embryos is essential to gain an understanding of how mammalian neuronal networks develop. Here we describe a method to label in situ a subset of dorsal root ganglion (DRG) axon projections to assess their phenotypic characteristics using several genetically manipulated mouse lines. The TrkA-positive neurons are nociceptor neurons, dedicated to the transmission of pain signals. We utilize a TrkAtaulacZ mouse line to label the trajectories of all TrkA-positive peripheral axons in the intact mouse embryo. We further breed the TrkAtaulacZ line onto a Bax null background, which essentially abolishes neuronal apoptosis, in order to assess growth-related questions independently of possible effects of genetic manipulations on neuronal survival. Subsequently, genetically modified mice of interest are bred with the TrkAtaulacZ/Bax null line and are then ready for study using the techniques described herein. This presentation includes detailed information on mouse breeding plans, genotyping at the time of dissection, tissue preparation, staining and clearing to allow for visualization of full-length axonal trajectories in whole-mount preparation.

The genotypes of TrkAWT/taulacZ : Bax-/- and TrkAtaulacZ/taulacZ : Bax-/- embryos can be unambiguously determined by standard PCR genotyping (Figure 1). X-gal staining displays detailed peripheral axonal arbors subcutaneously in conventionally stained embryos (Figures 2, 3a), and throughout the embryo after tissue clearing (Figures 3b, 4).
We have bred the TrkAWT/taulacZ : Bax-/-</sup...

Watch this Scientific Journal Video about Whole-mount Imaging of Mouse Embryo Sensory Axon Projections at JoVE.com

Whole-mount Imaging of Mouse Embryo Sensory Axon Projections

We present here an optimized protocol to genotype, stain and prepare fetal mice for the imaging of peripheral nociceptor axon projections in the whole animal, as an effective method to assess sensory axon growth phenotypes in developing genetically engineered mice.

The visualization of full-length neuronal projections in embryos is essential to gain an understanding of how mammalian neuronal networks develop. Here we ...

whole-mount-imaging-of-mouse-embryo-sensory-axon-projections

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Neuroscience

10.0K Views.  Weill Medical College of Cornell University. We present here an optimized protocol to genotype, stain and prepare fetal mice for the imaging of peripheral nociceptor axon projections in the whole animal, as an effective method to assess sensory axon growth phenotypes in developing genetically engineered mice.

Video: Whole-mount Imaging of Mouse Embryo Sensory Axon Projections

Organotypic Slice Culture of GFP-expressing Mouse Embryos for Real-time Imaging of Peripheral Nerve Outgrowth

For many purposes, the cultivation of mouse embryos ex vivo as organotypic slices is desirable. For example, we employ a transgenic mouse line (tauGFP) in which the enhanced version of the green fluorescent protein (EGFP) is exclusively expressed in all neurons of the developing central and peripheral nervous system1, allowing the possibility to both film the innervation of the forelimb and to manipulate this process with pharmacological and genetic techniques2. The most critical parameter in the successful cultivation of such slice cultures is the method by which the slices are prepared. After extensive testing of a variety of methods, we have found that a vibratome is the best possible device to slice the embryos such that they routinely result in a culture that demonstrates viability over a period of several days, and most importantly, develops in an age-specific manner. For mid-gestation embryos, this includes the normal outgrowth of spinal nerves from the spinal cord and the dorsal root ganglia to their targets in the periphery and the proper determination of skeletal and muscle tissue. 
In this work, we present a method for processing whole embryos of embryonic day (E) E10 to E12 into 300 - 400 micrometer slices for cultivation in a standard tissue culture incubator, which can be studied for up to two days after slice preparation. Critical for the success of this approach is the use of a vibratome to slice each agarose-embedded embryo. This is followed by the cultivation of the slices upon Millicell culture membrane inserts placed upon a small volume of medium, resulting in an interface culture technique. One litter with an average of 7 embryos routinely produces at least 14 slices (2-3 slices of the forelimb region per embryo), which varies slightly due to the age of the embryos as well as to the thickness of the slices. About 80% of the cultured slices show nerve outgrowth, which can be measured througout the culturing period2. Representative results using the tauGFP mouse line are demonstrated.

We present a method to prepare organotypic slices of mid-gestation mouse embryos for the cultivation and time-lapse imaging of peripheral nerve outgrowth.

For many purposes, the cultivation of mouse embryos ex vivo as organotypic slices is desirable. For example, we employ a transgenic mouse line (tauGFP) in ...

Micron-scale Resolution Optical Tomography of Entire Mouse Brains with Confocal Light Sheet Microscopy

Understanding the architecture of mammalian brain at single-cell resolution is one of the key issues of neuroscience. However, mapping neuronal soma and projections throughout the whole brain is still challenging for imaging and data management technologies. Indeed, macroscopic volumes need to be reconstructed with high resolution and contrast in a reasonable time, producing datasets in the TeraByte range. We recently demonstrated an optical method (confocal light sheet microscopy, CLSM) capable of obtaining micron-scale reconstruction of entire mouse brains labeled with enhanced green fluorescent protein (EGFP). Combining light sheet illumination and confocal detection, CLSM allows deep imaging inside macroscopic cleared specimens with high contrast and speed. Here we describe the complete experimental pipeline to obtain comprehensive and human-readable images of entire mouse brains labeled with fluorescent proteins. The clearing and the mounting procedures are described, together with the steps to perform an optical tomography on its whole volume by acquiring many parallel adjacent stacks. We showed the usage of open-source custom-made software tools enabling stitching of the multiple stacks and multi-resolution data navigation. Finally, we illustrated some example of brain maps: the cerebellum from an L7-GFP transgenic mouse, in which all Purkinje cells are selectively labeled, and the whole brain from a thy1-GFP-M mouse, characterized by a random sparse neuronal labeling.

In this article we describe the full experimental procedure to reconstruct, with high resolution, the fine brain anatomy of fluorescently labeled mouse brains. The described protocol includes sample preparation and clearing, specimen mounting for imaging, data post-processing and multi-scale visualization.

Understanding the architecture of mammalian  brain at single-cell resolution is one of the key issues of neuroscience.  However, mapping neuronal soma and ...

Live Imaging of Mitosis in the Developing Mouse Embryonic Cortex

Although of short duration, mitosis is a complex and dynamic multi-step process fundamental for development of organs including the brain. In the developing cerebral cortex, abnormal mitosis of neural progenitors can cause defects in brain size and function. Hence, there is a critical need for tools to understand the mechanisms of neural progenitor mitosis. Cortical development in rodents is an outstanding model for studying this process. Neural progenitor mitosis is commonly examined in fixed brain sections. This protocol will describe in detail an approach for live imaging of mitosis in ex vivo embryonic brain slices. We will describe the critical steps for this procedure, which include: brain extraction, brain embedding, vibratome sectioning of brain slices, staining and culturing of slices, and time-lapse imaging. We will then demonstrate and describe in detail how to perform post-acquisition analysis of mitosis. We include representative results from this assay using the vital dye Syto11, transgenic mice (histone H2B-EGFP and centrin-EGFP), and in utero electroporation (mCherry-&#945;-tubulin). We will discuss how this procedure can be best optimized and how it can be modified for study of genetic regulation of mitosis. Live imaging of mitosis in brain slices is a flexible approach to assess the impact of age, anatomy, and genetic perturbation in a controlled environment, and to generate a large amount of data with high temporal and spatial resolution. Hence this protocol will complement existing tools for analysis of neural progenitor mitosis.

Neural progenitor mitosis is a critical parameter of neurogenesis. Much of our understanding of neural progenitor mitosis is based on analysis of fixed tissue. Live imaging in embryonic brain slices is a versatile technique to assess mitosis with high temporal and spatial resolution in a controlled environment.

Although of short duration, mitosis is a complex and dynamic multi-step process fundamental for development of organs including the brain. In the ...

Microinjection of DNA into Eyebuds in Xenopus laevis Embryos and Imaging of GFP Expressing Optic Axonal Arbors in Intact, Living Xenopus Tadpoles

The primary visual projection of tadpoles of the aquatic frog Xenopus laevis serves as an excellent model system for studying mechanisms that regulate the development of neuronal connectivity. During establishment of the retino-tectal projection, optic axons extend from the eye and navigate through distinct regions of the brain to reach their target tissue, the optic tectum. Once optic axons enter the tectum, they elaborate terminal arbors that function to increase the number of synaptic connections they can make with target interneurons in the tectum. Here, we describe a method to express DNA encoding green fluorescent protein (GFP), and gain- and loss-of-function gene constructs, in optic neurons (retinal ganglion cells) in Xenopus embryos. We explain how to microinject a combined DNA/lipofection reagent into eyebuds of one day old embryos such that exogenous genes are expressed in single or small numbers of optic neurons. By tagging genes with GFP or co-injecting with a GFP plasmid, terminal axonal arbors of individual optic neurons with altered molecular signaling can be imaged directly in brains of intact, living Xenopus tadpoles several days later, and their morphology can be quantified. This protocol allows for determination of cell-autonomous molecular mechanisms that underlie the development of optic axon arborization in vivo.

Microinjection of DNA into Eyebuds in Xenopus laevis Embryos and Imaging of GFP Expressing Optic Axonal Arbors in Intact, Living Xenopus Tadpoles

This protocol aims to demonstrate how to microinject a DNA/DOTAP mixture into eyebuds of one day old Xenopus laevis embryos, and how to image and reconstruct individual green fluorescent protein (GFP) expressing optic axonal arbors in tectal midbrains of intact, living Xenopus tadpoles.

The primary visual projection of tadpoles of the aquatic frog Xenopus laevis serves as an excellent model system for studying mechanisms that regulate the ...

Developmental Biology

Flat Mount Imaging of Mouse Skin and Its Application to the Analysis of Hair Follicle Patterning and Sensory Axon Morphology

Skin is a highly heterogeneous tissue. Intra-dermal structures include hair follicles, arrector pili muscles, epidermal specializations (such as Merkel cell clusters), sebaceous glands, nerves and nerve endings, and capillaries. The spatial arrangement of these structures is tightly controlled on a microscopic scale - as seen, for example, in the orderly arrangement of cell types within a single hair follicle - and on a macroscopic scale - as seen by the nearly identical orientations of thousands of hair follicles within a local region of skin. Visualizing these structures without physically sectioning the skin is possible because of the 2-dimensional geometry of this organ. In this protocol, we show that mouse skin can be dissected, fixed, permeabilized, stained, and clarified as an intact two dimensional object, a flat mount. The protocol allows for easy visualization of skin structures in their entirety through the full thickness of large areas of skin by optical sectioning and reconstruction. Images of these structures can also be integrated with information about position and orientation relative to the body axes.

Mammalian skin contains a diverse array of structures - such as hair follicles and nerve endings - that exhibit distinctive patterns of spatial organization. Analyzing skin as a flat mount takes advantage of the 2-dimensional geometry of this tissue to produce full-thickness high-resolution images of skin structures.

Skin is a highly heterogeneous tissue. Intra-dermal structures include hair follicles, arrector pili muscles, epidermal specializations (such as Merkel ...

Three and Four-Dimensional Visualization and Analysis Approaches to Study Vertebrate Axial Elongation and Segmentation

Somitogenesis is a hallmark of vertebrate embryonic development. For years, researchers have been studying this process in a variety of organisms using a wide range of techniques encompassing ex vivo and in vitro approaches. However, most studies still rely on the analysis of two-dimensional (2D) imaging data, which limits proper evaluation of a developmental process like axial extension and somitogenesis involving highly dynamic interactions in a complex 3D space. Here we describe techniques that allow mouse live imaging acquisition, dataset processing, visualization and analysis in 3D and 4D to study the cells (e.g., neuromesodermal progenitors) involved in these developmental processes. We also provide a step-by-step protocol for optical projection tomography and whole-mount immunofluorescence microscopy in mouse embryos (from sample preparation to image acquisition) and show a pipeline that we developed to process and visualize 3D image data. We extend the use of some of these techniques and highlight specific features of different available software (e.g., Fiji/ImageJ, Drishti, Amira and Imaris) that can be used to improve our current understanding of axial extension and somite formation (e.g., 3D reconstructions). Altogether, the techniques here described emphasize the importance of 3D data visualization and analysis in developmental biology, and might help other researchers to better address 3D and 4D image data in the context of vertebrate axial extension and segmentation. Finally, the work also employs novel tools to facilitate teaching vertebrate embryonic development.

Here, we describe computational tools and methods that allow visualization and analysis of three and four-dimensional image data of mouse embryos in the context of axial elongation and segmentation, obtained by in toto optical projection tomography, and by live imaging and whole-mount immunofluorescence staining using multiphoton microscopy.

Somitogenesis is a hallmark of vertebrate embryonic development. For years, researchers have been studying this process in a variety of organisms using a ...

Visualization of the Axonal Projection Pattern of Embryonic Motor Neurons in Drosophila

The establishment of functional neuromuscular circuits relies on precise connections between developing motor axons and target muscles. Motor neurons extend growth cones to navigate along specific pathways by responding to a large number of axon guidance cues that emanate from the surrounding extracellular environment. Growth cone target recognition also plays a critical role in neuromuscular specificity. This work presents a standard immunohistochemistry protocol to visualize motor neuron projections of late stage-16 Drosophila melanogaster embryos. This protocol includes a few key steps, including a genotyping procedure, to sort the desired mutant embryos; an immunostaining procedure, to tag embryos with fasciclin II (FasII) antibody; and a dissection procedure, to generate filleted preparations from fixed embryos. Motor axon projections and muscle patterns in the periphery are much better visualized in flat preparations of filleted embryos than in whole-mount embryos. Therefore, the filleted preparation of fixed embryos stained with FasII antibody provides a powerful tool to characterize the genes required for motor axon pathfinding and target recognition, and it can also be applied to both loss-of-function and gain-of-function genetic screens.

Visualization of the Axonal Projection Pattern of Embryonic Motor Neurons in Drosophila

This work details a standard immunohistochemistry method to visualize motor neuron projections of late stage-16 Drosophila melanogaster embryos. The filleted preparation of fixed embryos stained with FasII antibody provides a powerful tool to characterize the genes required for motor axon pathfinding and target recognition during neural development.

The establishment of functional neuromuscular circuits relies on precise connections between developing motor axons and target muscles. Motor neurons ...

Visualization of Motor Axon Navigation and Quantification of Axon Arborization In Mouse Embryos Using Light Sheet Fluorescence Microscopy

Spinal motor neurons (MNs) extend their axons to communicate with their innervating targets, thereby controlling movement and complex tasks in vertebrates. Thus, it is critical to uncover the molecular mechanisms of how motor axons navigate to, arborize, and innervate their peripheral muscle targets during development and degeneration. Although transgenic Hb9::GFP mouse lines have long served to visualize motor axon trajectories during embryonic development, detailed descriptions of the full spectrum of axon terminal arborization remain incomplete due to the pattern complexity and limitations of current optical microscopy. Here, we describe an improved protocol that combines light sheet fluorescence microscopy (LSFM) and robust image analysis to qualitatively and quantitatively visualize developing motor axons. This system can be easily adopted to cross genetic mutants or MN disease models with Hb9::GFP lines, revealing novel molecular mechanisms that lead to defects in motor axon navigation and arborization.

Here, we describe a protocol for visualizing motor neuron projection and axon arborization in transgenic Hb9::GFP mouse embryos. After immunostaining for motor neurons, we used light sheet fluorescence microscopy to image embryos for subsequent quantitative analysis. This protocol is applicable to other neuron navigation processes in the central nervous system.

Spinal motor neurons (MNs) extend their axons to communicate with their innervating targets, thereby controlling movement and complex tasks in ...

Large-scale Three-dimensional Imaging of Cellular Organization in the Mouse Neocortex

The mammalian neocortex is composed of many types of excitatory and inhibitory neurons, each with specific electrophysiological and biochemical properties, synaptic connections, and in vivo functions, but their basic functional and anatomical organization from cellular to network scale is poorly understood. Here we describe a method for the three-dimensional imaging of fluorescently-labeled neurons across large areas of the brain for the investigation of the cortical cellular organization. Specific types of neurons are labeled by the injection of fluorescent retrograde neuronal tracers or expression of fluorescent proteins in transgenic mice. Block brain samples, e.g., a hemisphere, are prepared after fixation, made transparent with tissue clearing methods, and subjected to fluorescent immunolabeling of the specific cell types. Large areas are scanned using confocal or two-photon microscopes equipped with large working distance objectives and motorized stages. This method can resolve the periodic organization of the cell type-specific microcolumn functional modules in the mouse neocortex. The procedure can be useful for the study of three-dimensional cellular architecture in the diverse brain areas and other complex tissues.

Here we describe a procedure for tissue clearing, fluorescent labeling, and large-scale imaging of mouse brain tissue which, thereby, enables visualization of the three-dimensional organization of cell types in the neocortex.

The mammalian neocortex is composed of many types of excitatory and inhibitory neurons, each with specific electrophysiological and biochemical ...

Visualizing Neuronal and Glial Cell Dynamics in the Developing Mouse Brain

Time-Lapse Imaging of Migrating Neurons and Glial Progenitors in Embryonic Mouse Brain Slices

During the development of the cerebral cortex, neurons and glial cells originate in the ventricular zone lining the ventricle and migrate toward the brain surface. This process is crucial for proper brain function, and its dysregulation can result in neurodevelopmental and psychiatric disorders after birth. In fact, many genes responsible for these diseases have been found to be involved in this process, and therefore, revealing how these mutations affect cellular dynamics is important for understanding the pathogenesis of these diseases. This protocol introduces a technique for time-lapse imaging of migrating neurons and glial progenitors in brain slices obtained from mouse embryos. Cells are labeled with fluorescent proteins using in utero electroporation, which visualizes individual cells migrating from the ventricular zone with a high signal-to-noise ratio. Moreover, this in vivo gene transfer system enables us to easily perform gain-of-function or loss-of-function experiments on the given genes by co-electroporation of their expression or knockdown/knockout vectors. Using this protocol, the migratory behavior and migration speed of individual cells, information that is never obtained from fixed brains, can be analyzed.

Author Spotlight: Exploring Cell Migration and Gene Roles in the Developing Brain

During the development of the cerebral cortex, neurons and glial cells originate in the ventricular zone lining the ventricle and migrate toward the brain surface. Many genes are involved in this process. This protocol introduces the technique for the time-lapse imaging of migrating neurons and glial progenitors.

During the development of the cerebral cortex, neurons and glial cells originate in the ventricular zone lining the ventricle and migrate toward the brain ...

Whole-mount Imaging of Mouse Embryo Sensory Axon Projections

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