Since I was an undergraduate student at the Veterinary School in Milan, and throughout the rest of my scientific career, I have been fascinated with the complexities of mammalian preimplantation development. That’s why the publication of our recent paper “Waves of early transcriptional activation and pluripotency program initiation during human preimplantation development” feels like the natural conclusion of a long journey that started with buckets of cow ovaries in Italy, and ended with a collective effort shared by our team in the Stem Cell Bank at the Center for Regenerative Medicine in Barcelona (CMR[B]), under the direction of Drs Izpisua Belmonte and Veiga.

In just hours, the newly formed mammalian embryo terminates the program of the two gametes that formed it, escapes apoptosis, remodels its chromatin to a functional state, starts dividing, and turns on its genome while using up the reserves of protein and RNA inherited from mum (and dad, a little). This last process, termed embryonic genome activation (EGA), represents one of the first signs of “independent life” of a new individual.

As much as the researchers at the CMR[B] are used to manipulating embryos and work with tiny amount of material, studying preimplantation processes in general, and EGA in particular, is a great challenge in our species, as embryos are very scarce, heterogeneous in quality, and RNA amplification methods almost invariably introduce a very significant bias in downstream data quality. An incredible opportunity to look into details of this process came as the CMR[B] established a collaboration with Herbert Auer of the University of Barcelona. His facility just recently validated a method to amplify very small amount of RNA without bias, and we proceeded to apply this “pico-profiling” to a very detailed time course of single human embryos.

Among the many complex interactions that our study has unveiled, one certainly stands out: the human embryo starts EGA one full day before we thought since seminal works were published more than 30 years ago (Braude et al, 1988). Using a combination of extremely reliable transcriptional profiling and de novo transcription inhibition by amanitin treatment, we have been able to show that the human embryo transcribes from its own genome as early as the 2-cell stage (about 30hr after fertilization).

In order to make our data genome-wide expression data easily available to the scientific community at large we have prepared a free online database, HuMER (Human Embryo Resource; http://intranet.cmrb.eu/Human_embryos/home.html). It is our hope and desire that this resource would help us improve our ability to draw interdisciplinary connections between biological events and, in the process, increase our understanding of preimplantation development.

Written by first author of the paper, Dr. Rita Vassena.

Vassena, R., Boue, S., Gonzalez-Roca, E., Aran, B., Auer, H., Veiga, A., & Belmonte, J. (2011). Waves of early transcriptional activation and pluripotency program initiation during human preimplantation development Development, 138 (17), 3699-3709 DOI: 10.1242/dev.064741

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Here are the highlights from the current issue of Development:

Human embryos make an early transcriptional start

Human preimplantation development is a highly dynamic process that lasts about 6 days. During this time, the embryo must complete a complex program that includes activation of embryonic genome transcription and initiation of the pluripotency program. Here, Juan Carlos Izpisua Belmonte and co-workers use pico-profiling (an accurate transcriptome amplification method) to reveal the timing of sequential waves of transcriptional activation in single human oocytes and embryos (see p. 3699). The researchers (who have developed HumER, a free, searchable database of their gene expression data) report that initiation of transcriptional activity in human embryos starts at the 2-cell stage rather than at the 4- to 8-cell stage as previously reported. They also identify distinct patterns of activation of pluripotency-associated genes and show that many of these genes are expressed around the time of embryonic genome activation. These results link human embryonic genome activation with the initiation of the pluripotency program and pave the way for the identification of factors to improve epigenetic somatic cell reprogramming.

See the post written by the first author of this paper for more information

Worming into organ regeneration

Planarian flatworms have amazing regenerative abilities. Tissue fragments from almost anywhere in their anatomically complex bodies can regenerate into complete, perfectly proportioned animals, a feat that makes planarians ideal for the study of regenerative organogenesis. Now, on p. 3769, Alejandro Sánchez Alvarado and colleagues provide the first detailed description of the excretory system of Schmidtea mediterranea, which consists of internal protonephridial tubules composed of specialised epithelial cells. Using α-tubulin antibodies to stain cilia in the planarian’s excretory system and screens of gene expression patterns in whole mounts, the researchers show that protonephridial tubules form a complex branching structure that has a stereotyped succession of cell types along its length. Organ regeneration originates from a precursor structure that undergoes extensive branching morphogenesis, they report. Moreover, in an RNAi screen of signalling molecules, they identify EGF signalling as a crucial regulator of branching morphogenesis. Overall, these results establish the planarian protonephridia as a model system in which to study the regeneration and evolution of epithelial organs.

Axons lead, lymphatics follow

Given the similar anatomies of vertebrate nerves, blood vessels and lymphatics, it is not surprising that guidance cues such as the netrins, which were discovered as molecules involved in axon pathfinding, also guide vessels. But do nerves and vessels share patterning mechanisms or do axons provide guidance for vessels? The laboratories of Dean Li, Chi-Bin Chien and Brant Weinstein now report that zebrafish motoneurons are essential for vascular pathfinding (see p. 3847). Netrin 1a is required for the development of the parachordal chain (PAC), a string of endothelial cells that are precursors of the main zebrafish lymphatic vessel. Here, the researchers identify muscle pioneers at the horizontal myoseptum (HMS) as the source of Netrin 1a for PAC formation. netrin 1a and dcc (which encodes the Netrin receptor) are required for the sprouting of the rostral primary axons and neighbouring axons along the HMS, they report, and genetic removal or laser ablation of these motoneurons prevents PAC formation. Together, these results reveal a direct requirement for axons in vascular guidance.

Satellite cells: stem cells for regenerating muscle?

Adult vertebrate skeletal muscle has a remarkable capacity for regeneration after injury and for hypertrophy and regrowth after atrophy. In 1961, Alexander Mauro suggested that satellite cells, which lie between the sarcolemma and basement membrane of myofibres, could be adult skeletal muscle stem cells. Subsequent cell transplantation and lineage-tracing studies have shown that satellite cells, which express the Pax7 transcription factor, can repair damaged muscle tissue, but are these cells essential for muscle regeneration and other aspects of muscle adaptability? In this issue, four papers investigate this long-standing question.

On p. 3639, Chen-Ming Fan and colleagues report that genetic ablation of Pax7⁺ cells in mice completely blocks regenerative myogenesis after cardiotoxin-induced muscle injury and after transplantation of ablated muscle into a normal muscle bed. Because Pax7 is specifically expressed in satellite cells, the researchers conclude that satellite cells are essential for acute injury-induced muscle regeneration but note that other stem cells might be involved in muscle regeneration in other pathological conditions.

Anne Galy, Shahragim Tajbakhsh and colleagues reach a similar conclusion on p. 3647. They report that local depletion of satellite cells in a different mouse model leads to marked loss of muscle tissue and failure to regenerate skeletal muscle after myotoxin- or exercise-induced muscle injury. Other endogenous cell types do not compensate for the loss of Pax7⁺ cells, they report, but muscle regeneration can be rescued by transplantation of adult Pax7⁺ satellite cells alone, which suggests that Pax7⁺ cells are the only endogenous adult muscle stem cells that act autonomously.

On p. 3625, Gabrielle Kardon and colleagues confirm the essential role of satellite cells in muscle regeneration in yet another mouse model. They show that satellite cell ablation results in complete loss of regenerated muscle, misregulation of fibroblasts and a large increase in connective tissue after injury. In addition, they report that ablation of muscle connective tissue (MCT) fibroblasts leads to premature satellite cell differentiation, satellite cell depletion and smaller regenerated myofibres after injury. Thus, they conclude, MCT fibroblasts are a vital component of the satellite cell niche.

Finally, on p. 3657, Charlotte Peterson and colleagues investigate satellite cell involvement in muscle hypertrophy. By removing the gastrocnemius and soleus muscles in the lower limb of mice, the researchers expose the plantaris muscle to mechanical overload, which induces muscle hypertrophy. After two weeks of overload, muscles genetically depleted of satellite cells show the same increase in muscle mass and similar hypertrophic fibre cross-sectional areas as non-depleted muscles but reduced new fibre formation and fibre regeneration. Thus, muscle fibres can mount a robust hypertrophic response to mechanical overload that is not dependent on satellite cells.

Together, these studies suggest that satellite cells could be a source of stem cells for the treatment of muscular dystrophies but also highlight the potential importance of fibroblasts in such therapies. Importantly, the finding that muscle regeneration and hypertrophy are distinct processes suggests that muscle growth-promoting exercise regimens should aim to minimise muscle damage and maximise intracellular anabolic processes, particularly in populations such as the elderly where satellite activity is compromised.

Plus…

Notch signaling: simplicity in design, versatility in function.

The evolutionarily conserved Notch signalling pathway operates in numerous cell types and at various developmental stages. Here, Andersson, Sandberg and Lendhal review recent insights into how versatility in Notch signalling output is generated and modulated.

See the Review article on p.3593

Evolution of nervous system patterning: insights from sea urchin development

Recent studies have elucidated the mechanisms that pattern the nervous system of sea urchin embryos. Angerer and colleagues review these conserved nervous system patterning signals and consider how the relationships between them might have changed during evolution.

See the Review article on p. 3613

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For the past 24 years, the Mouse Molecular Genetics meeting has been a leading forum for researchers who apply the methods of genetics and genomics to fundamental problems in mammalian biology, including stem cell biology, early development, and models of human disease. In particular, the meeting showcases the latest technical developments in genetics and engineering of the mouse genome, and this year will feature a session devoted to imaging. The Mouse Molecular Genetics meeting assembles leaders in the field to present unpublished research findings, encourages junior investigators to participate in oral and poster presentations, and provides a stimulating environment for the exchange of ideas and information.

Sessions:
Epigenetics
Genetics and Genomics
Imaging
Models of Human Disease
Organogenesis
Patterning
Stem Cells and Germ Cells
Technology

Scientific Programme Committee:
Allan Bradley, Wellcome Trust Sanger Institute, UK
Kat Hadjantonakis, Sloan-Kettering Institute, USA
Haruhiko Koseki, RIKEN Research Center for Allergy and Immunology, Japan
Michael Shen, Columbia University Medical Center, USA

Keynote speakers:
Kathryn Anderson, Sloan Kettering Institute, USA
William Skarnes, Wellcome Trust Sanger Institute, UK

Speakers include:
David Adams, Wellcome Trust Sanger Institute, UK
Shinichi Aizawa, RIKEN CDB, Japan
Phil Avner, Institut Pasteur, France
Yann Barrandon, EPFL, Switzerland
David Beier, Harvard Medical School, USA
Richard Behringer, MD Anderson Cancer Center, USA
Shuomo Bhattacharya, University of Oxford, UK
Neal Copeland, Institute of Molecular and Cell Biology, Singapore
Xiaoxia Cui, Sigma-Aldrich Corp, USA
Elizabeth Fisher, UCL , UK
Scott Fraser, Beckman Institute, USA
Matthias Merkenschlager, MRC Clinical Sciences Centre, UK
Olivier Pourquie, Institute of Genetics and Molecular and Cellular Biology, France
James Sharpe, Centre for Genomic Regulation, Spain
Ludovic Vallier, Cambridge University, UK
Magda Zernicka-Goetz, The Gurdon Institute, UK

Conference Programme Information:
The conference will start on Tuesday, 20 September with registration at 14.00.
The conference will finish after lunch on Friday, 23 September 2011 at approximately 13.30.

https://registration.hinxton.wellcome.ac.uk/display_info.asp?id=258

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I’ve just added some 2012 conferences to the events calendar, and thought I’d give a quick reminder on how to add events here. If you have an account on the Node, you will have received these instructions when your account was approved. (And if you don’t have a Node account, you can get one here.)

When logged in, click “add/edit events” in the sidebar of the Node admin panel.

You’ll now get to the page where you can add an event. In the following picture, the numbers correspond to the list below:

1. Add name of event
2. Select both boxes (skip the first if there is no website)
3. Add start and end date of event
4. Add location
5. This is set to “conference” by default, but can be changed to “workshop” or “course”
6. Add website address

Don’t forget to save. Your event now appears on the events calendar!

If you want to share more detailed information about conferences (eg. if you’re organising one) you can write a post about it, but don’t forget to also add it to the calendar!

(Get these instructions as a pdf)

(NB – events that are not relevant to the developmental biology community, or commercial/sales events (such as product demos) will be removed from the calendar. If you’re not sure whether an event is suitable, you can contact us or leave a comment below.)

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ISDN2012 Neurodevelopment and Neurological diseases in Mumbai India.

http://www.isdn-conference.elsevier.com/

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(This interview originally appeared in Development.)

Magdalena Götz is the Director of the Institute for Stem Cell Research at the Helmholtz Center and Professor at the Ludwig-Maximilians-University in Munich, Germany. Her developmental work in neurogenesis has identified radial glial cells as the source of neurons in the developing brain. Magdalena joined Development as Editor in 2010, and she agreed to be interviewed about her scientific inspirations and about finding a place for adult stem and progenitor cells within developmental biology.

When did you first become interested in science?

I have always loved biology, and in school I was truly inspired by my biology teacher. In our rather non-innovative school system, we had a young American biology teacher who made us actually think and do things, and I was simply fascinated.

What was your PhD about and how did it inform your subsequent career choices?

My PhD was on development of the cerebral cortex and investigated how specific cell types develop and form their specific connections. This work laid the basis for many research questions, which I continued to pursue into much later stages. For example, it led to the isolation of specific progenitor subtypes in order to understand stem cell and progenitor heterogeneity, and the molecular specification of these subtypes. The new questions that arose from my PhD project also determined how I chose my postdoc lab, and many of the basic questions from this time still keep us busy now.

Did you have a mentor or someone who inspired you in your early career?

After my inspiring biology teacher in school, my PhD supervisor, Jürgen Bolz, was also key in shaping my way. His readiness to discuss science at any time was certainly very important to further fuel my enthusiasm for understanding how the cerebral cortex develops. My interest in developmental biology was originally inspired by a course at the Max-Planck Institute for Developmental Biology in Tübingen and by the fascinating questions of axon growth and regeneration studied by Friedrich Bonhoeffer and Claudia Stürmer.

Typically, I have always been inspired by people we call `Querdenker’ in German – i.e. people whose thoughts and ideas are contrary to common beliefs and who follow their own ideas entirely independent of the field. Therefore, people like Nils Birbaumer in Tübingen and Rüdiger Wehner in Zürich were important for me to see that following your own way and ideas is the way to go.

(more…)

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The winner of the fourth round of Development cover images, collecting more than half of the total votes, was this squid embryo image, taken by Amber O’Connor from the University of Alabama at Birmingham.

Runners-up in this round of images (all taken by participants of the Woods Hole Embryology course in 2010) were a fly image by Sylvia Bonilla (Purdue University) and Mazdak Lachidan (Samuel Lunenfeld Research Institute, Toronto), a mouse image by Elsa Denker (Sars International Centre for Marine Molecular Biology, Bergen) and a Ciona image by Qinwen Liu (University of Maryland, College Park) and Xinwei Cao (St. Jude’s Children’s Research Hospital).

Amber’s image will be on the cover of Development some time in the coming months. Meanwhile, you can download another squid as your August desktop:

The desktop calendar for August, is now up, featuring the runner-up from the first Development cover image voting round of images taken at the 2010 Woods Hole Embryology course. It shows a squid embryo with DAPI staining in blue and phalloidin in red. The image was taken by Jennifer Hohagen of Georg-August-Universitaet in Göttingen.

Visit the calendar page to select the resolution you need for your screen. The page will be updated at the end of each month with a new image, and all images are chosen from either the intersection image contest or from the images we’ve featured from the Woods Hole Embryology 2010 course.

(5 votes)

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I currently have an opening in my research group for a post-doc to investigate the development of the vertebrate skeleton.  Our lab studies the development of the neural crest derived skeleton in a comparative manner in chicken and fish embryos (zebrafish and Mexican tetra).   This position will focus on the signals involved in the patterning of skeletal elements in one or more of these animals and the interactions between neural crest and mesodermal tissues.  Applicants who have recently completed a PhD, have experience in molecular biology, developmental and cell biology are strongly encouraged to apply.  

MSVU is an undergraduate university on the East Coast of Canada in beautiful Nova Scotia.  Although a small university, I have a large research group.  This position offers opportunities to interact with a growing research group of undergraduates and graduate students in a well equipped CFI funded lab.  In addition, opportunities to teach, train undergraduates/graduate students and to help manage my lab will be available.  

If you want to hone your teaching and management skills, as well as engage in exciting research then this position might be for you!  Please email a CV, a one page statement of your research experience and interests, and the contact information for three referees to:

Dr Tamara Franz-Odendaal    Tamara.franz-odendaal@msvu.caBiology Dept, Mount Saint Vincent University,

166 Bedford Highway, Halifax, Nova Scotia, B3M 3E4,CANADA

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Stem cells have often been imaged live in culture, but very few stem cell systems are conducive to live imaging within their native tissues.  An essential property of adult stem cells that they are maintained at specific anatomical locations called niches.  The interactions between stem cells and their niche are crucial, but are often disrupted when stem cells are studied in vitro.  In our recent Development paper, we use live imaging to directly watch male Drosophila germline stem cells (GSCs) in action within their native niche by ex-vivo imaging of intact testes.  We find that while wild type GSC divisions usually result in the production of a stem cell and daughter cell, they occasionally produce two stem cells or two daughter cells.  Our findings highlight a flexibility in stem cell output that is modulated during regeneration similar to mouse spermatogonial stem cells (Nakagawa et al., 2007, Nakagawa et al., 2010).

Niches are usually composed of stromal cells that activate developmental signaling pathways in nearby stem cells, thus maintaining them in an undifferentiated state.  The cluster of non-dividing stromal cells to which germline and somatic stem cells in the Drosophila testis apex are attached is appropriately called the “hub” and secretes cytokines required for the maintenance of both stem cell populations.  Cells adjacent to the hub have activated Jak-STAT signaling, and remain undifferentiated.  Cells further away from the hub initiate differentiation and eventually form sperm.

After running into similar issues as what Lucy Morris described in her previous post, we developed a live imaging protocol using either spinning-disk or 2-photon microscopy that enabled us to image stem cells within intact testes for up to 12 hours – about half of the GSC cell cycle.  By manually tracking individual stem cells and their daughter cells, we saw the most GSCs divide with a stereotypical spindle orientation perpendicular to the hub, thus displacing daughter cells out of the niche.  This phenomenon had been seen many times previously in immunostained, fixed testes and in testes studied by short-term live imaging, and had been the only observed mode of GSC divisions (Yamashita et al., 2003).  Surprisingly, in wild type young testes, we saw a few cases where GSCs, after first displacing their daughter cells away from hub, then swiveled such that the daughter cell gained and maintained contact with the hub until the end of imaging.  This suggested that GSCs can switch from their normal mode of division where one GSC and one daughter cell re produced, to one where two GSCs are produced.  We also saw the converse, where GSCs-daughter pairs lost contact with the hub and appeared to directly differentiate into a pair of spermatogonia.  Thus, our live imaging captured three different modes of division – asymmetric division, symmetric renewal, and symmetric differentiation – that occur simultaneously within a stem cell niche to balance its output during steady-state (click here to see a movie of this!).

By using the same imaging technique, we were able to look at cells undergoing the process of dedifferentiation, where a more differentiated cell reverts into a less differentiated cell or stem cell.  Our lab previously demonstrated that spermatogonia (transit-amplifying stem cell daughters) are able to revert into stem cells when they encounter a niche depleted of stem cells (Brawley and Matunis, 2004).  While examining dedifferentiation, we stumbled upon a scenario where we thought that the normally immobile spermatogonia were able to move towards the hub in order to gain physical contact with the hub before reverting into stem cells.  Whether the cells were gaining a migratory morphology during the process was unknown.  While we never attained a satisfactory answer to whether Drosophila spermatogonia gain intrinsic migratory properties, we saw many examples of spermatogonia initially not in contact with the hub gain (click here to see a movie of this).  The caveat is that we were not able to image the somatic cells within the niche, and these somatic cells could easily be shuttling the dedifferentiating spermatogonia in a manner similar to that of escort stem cells in the Drosophila ovary (Morris and Spradling, 2011).  We also noticed that the modes of GSC divisions changed greatly from that seen during steady-state (movie of this too!).  The balance of the different modes of stem cell division was now tipped to that of producing more GSCs in an effort to replenish the niche.

Our findings that stem cells are frequently replaced and regenerated in the Drosophila testis are in line with recent studies in mammalian tissues using lineage tracing and mathematical modeling of clone behavior.  These studies suggest that the classical stem cell definition of a long-lived, slow cycling cell may no longer be accurate.  While this view has yet to be challenged in the mammalian hematopoietic system, it has been demonstrated recently that mammalian gut, interfollicular epidermis, and testes all contain stem cells that not only undergo turnover on the scale of weeks, but also divide frequently (reviewed in Klein and Simmons, 2011).  I believe that more and more stem cell systems will be demonstrated to have these properties, and that diseases such as cancer may eventually be similarly modeled.

Sheng, X., & Matunis, E. (2011). Live imaging of the Drosophila spermatogonial stem cell niche reveals novel mechanisms regulating germline stem cell output Development, 138 (16), 3367-3376 DOI: 10.1242/dev.065797

(7 votes)

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Because Woods Hole, MA is home to both an Oceanographic Institute and the Marine Biological Laboratory, most of the people in this small town have some connection to the scientific community. As a result, the fourth of July festival in Woods Hole, MA is a celebration of uninhibited science-geekery.

Some of the highlights included the neurobiology students:

Microbial diversity, with their giant squid:

And my personal favorite, Mendel with his peas:

Bringing up the rear was our own Embryology course.  We have a time-honored tradition of performing gastrulation through interpretive dance, which is quite possibly the best way to show gastrulation:

If that wasn’t self explanatory, let me help.

The three different colors of our shirts represent the three germ layers: blue for ectoderm, red for mesoderm, yellow for endoderm.  In this particular display, we are performing gastrulation as it occurs in the sea urchin (that was obvious, right?) We started out as a blastula (a hollow ball of cells), and then invaginated to create the three layers of tissue. Finally, some of the mesoderm cells start to form spicules, which create the skeleton of the urchin.

Sea urchins were only the beginning. We also performed gastrulation as it occurs in frogs, nematode worms, fruit flies, as well as chaotic cleavage (like you find in some sea anemones and jellyfish) and chicken neurulation for good measure.  The parade lasted less than an hour, and only traveled a few blocks, but we got as many gastrulations as we could in there.

If you’re ever in the area during the fourth of July, I highly recommend you check out the Woods Hole parade.  Their were a lot of people, and a lot of energy (including an epic water gun fight).  You can get a more detailed explanation of gastrulation and some more pictures over at the blog BioBlueprints.

(4 votes)

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