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Welcome to The Visible Embryo, a comprehensive educational resource on human development from conception to birth.

The Visible Embryo provides visual references for changes in fetal development throughout pregnancy and can be navigated via fetal development or maternal changes.

The National Institutes of Child Health and Human Development awarded Phase I and Phase II Small Business Innovative Research Grants to develop The Visible Embryo. Initally designed to evaluate the internet as a teaching tool for first year medical students, The Visible Embryo is linked to over 600 educational institutions and is viewed by more than one million visitors each month.

Today, The Visible Embryo is linked to over 600 educational institutions and is viewed by more than 1 million visitors each month. The field of early embryology has grown to include the identification of the stem cell as not only critical to organogenesis in the embryo, but equally critical to organ function and repair in the adult human. The identification and understanding of genetic malfunction, inflammatory responses, and the progression in chronic disease, begins with a grounding in primary cellular and systemic functions manifested in the study of the early embryo.

WHO International Clinical Trials Registry Platform

The World Health Organization (WHO) has created a new Web site to help researchers, doctors and
patients obtain reliable information on high-quality clinical trials. Now you can go to one website and search all registers to identify clinical trial research underway around the world!




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Pregnancy Timeline by SemestersFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresFemale Reproductive SystemEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterSecond TrimesterFirst TrimesterFertilizationDevelopmental Timeline
CLICK ON weeks 0 - 40 and follow along every 2 weeks of fetal development
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Home | Pregnancy Timeline | News Alerts |News Archive Nov 27, 2013


Nerve pathways in a 24hour old embryonic Zebrafish. Brain and spinal cord
revealed by Monoclonal Anti-Acetylated Tubulin.

Image Credit: Sigma Aldrich.

WHO Child Growth Charts




Clue to how the circulatory system is wired

A new mechanism that regulates the way blood vessels grow and connect to each other has been discovered . The knowledge might open up new opportunities for future cancer therapy.

The study by an international team of researchers at Karolinska Institutet in Sweden, and Heinrich Heine University Düsseldorf, Germany is published in the scientific journal PNAS.

If stretched out, the blood vessels in a human body would reach more than twice around the earth. The complex circulatory system nourishes every cell of our body and proper development of new blood vessels is crucial for embryonic development.

In the current study, the scientists demonstrated for the first time that the enzyme glutaredoxin 2 has an essential role during cardiovascular development.

Glutaredoxin 2 belongs to a family of enzymes that convey specific signals within cells. In previous studies, the same researchers have shown that glutaredoxin 2 is indispensable for nerve cell survival during embryonic brain development.

To reduce the number of laboratory mice used, the team was running most of their experiments in zebrafish that were genetically modified so that the circulatory system glowed in a green fluorescent colour. As the young zebrafish is completely transparent, the scientists could follow the growth of the fluorescent blood vessels directly under the microscope.
When levels of glutaredoxins were reduced, the blood vessels of the zebrafish embryos were growing randomly without establishing a proper circulatory system.

The researchers found that glutaredoxin 2 controls a chemical switch in another protein, sirtuin 1, and that this simple modification of a single amino acid is vital for the circulatory system to develop normally.

This knowledge is not only essential to better understand development of our circulatory system in general. Growth of new blood vessels, a process called angiogenesis, also plays a crucial role in the pathology of many diseases, including cancer. The ability to promote angiogenesis is a hallmark of cancer, since growing tumours and metastasis are dependent on vessel formation.

"The understanding how blood vessels develop and how this process can be modulated, can provide a new way to fight cancer in the future," says first study author Lars Bräutigam, at the Department of Medical Biochemistry and Biophysics of Karolinska Institutet and also affiliated with the Science for Life Laboratory (SciLifeLab) in Stockholm, Sweden.

Embryonic development is one of the most amazing miracles in nature. The proteins and signaling events driving this highly complex process are far from being elucidated completely. For a long time, an important role of protein reduction and oxidation during development has been assumed. Here, we demonstrate the essential role of such a regulation during cardiovascular development: The modification of a single cysteine in the protein sirtuin 1 by the vertebrate-specific oxidoreductase glutaredoxin 2 is required for vessel formation and guidance. Our data indicate that this redox-signaling pathway based on glutaredoxin-dependent reversible S-glutathionylation may be also important for diseases of the cardiovascular system and pathological situations connected to angiogenesis, e.g., malignancies.

Embryonic development depends on complex and precisely orchestrated signaling pathways including specific reduction/oxidation cascades. Oxidoreductases of the thioredoxin family are key players conveying redox signals through reversible posttranslational modifications of protein thiols. The importance of this protein family during embryogenesis has recently been exemplified for glutaredoxin 2, a vertebrate-specific glutathione–disulfide oxidoreductase with a critical role for embryonic brain development. Here, we discovered an essential function of glutaredoxin 2 during vascular development. Confocal microscopy and time-lapse studies based on two-photon microscopy revealed that morpholino-based knockdown of glutaredoxin 2 in zebrafish, a model organism to study vertebrate embryogenesis, resulted in a delayed and disordered blood vessel network. We were able to show that formation of a functional vascular system requires glutaredoxin 2-dependent reversible S-glutathionylation of the NAD+-dependent protein deacetylase sirtuin 1. Using mass spectrometry, we identified a cysteine residue in the conserved catalytic region of sirtuin 1 as target for glutaredoxin 2-specific deglutathionylation. Thereby, glutaredoxin 2-mediated redox regulation controls enzymatic activity of sirtuin 1, a mechanism we found to be conserved between zebrafish and humans. These results link S-glutathionylation to vertebrate development and successful embryonic angiogenesis.

The work was performed mainly at the Department of Medical Biochemistry and Biophysics, the Department of Tumour and Cell Biology at Karolinska Institutet, Sweden, and Heinrich Heine University Düsseldorf, Germany. Researchers at Linköping University, Sweden also participated in the work. The study was supported by funding from Karolinska Institutet, the research commission of the Medical Faculty of Heinrich Heine University, the Knut and Alice Wallenberg foundation, the Swedish Cancer Society and the Swedish Research Council.

Publication: 'Glutaredoxin regulates vascular development by reversible glutathionylation of sirtuin,' Lars Bräutigam, Lasse Dahl Ejby Jensen, Gereon Poschmann, Staffan Nyströma, Sarah Bannenberg, Kristian Dreij, Klaudia Lepka, Timour Prozorovsk, Sergio J. Montano, Orhan Aktas, Per Uhlén, Kai Stühler, Yihai Cao, ArneHolmgren, and Carsten Berndt, Proceedings of the National Academy of Sciences (PNAS), online 25-29 November 2013.