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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.

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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 Aug 13, 2013


gene expression

Gene Expression

DNA strands are tightly wound around histones
in order to fit within the cell.

Image credit: The Liggins Institute, University of Aukland, NZ

WHO Child Growth Charts




Rethinking 'The Code'

Rules governing developmental genes in mouse embryonic stem cells do not simply tell a gene whether to be "on" or "off."

A decade ago, gene expression seemed so straightforward: genes were either switched on or off. Then in 2006, a blockbuster finding reported that developmental genes in mouse embryo stem cells can have markers for both activation (on) and repression (off), and that such genes—referred to as "bivalently marked genes"—can committ one way or another during development and differentiation.

This paradox—akin to figuring out how to navigate between a red and green traffic signal—has since undergone intense examination by labs worldwide. Out of much investigation arose the idea that control regions ("promoter regions") of some genes critical to development in undifferentiated cells, stay "poised" or plastic, ready to communicate with both activating and repressing histones in a state biologists call "bivalency."

A recent study from the Stowers Institute for Medical Research now revisits this idea. In this week's advance online edition of the journal Nature Structural and Molecular Biology, a team led by investigator Ali Shilatifard, Ph.D., identified a protein complex activating a histone marker "poised" in mouse embryonic stem (ES) cells—but found it had little effect on gene activation. This finding suggests there is more to learn about interpreting histone modification in embryonic cells, or even in cancer cells.

"There has been a lot of excitement over the idea that promoters of developmentally regulated genes exhibit both stop and go signals. Such work promotes the idea that histone modification could constitute a code regulating gene expression. However, we argue that the code is not absolute and is context dependent."

Ali Shilatifard, Ph.D.

Shilatifard has a long interest in gene regulation governing development and cancer. In 2001, his laboratory was the first to describe a complex of yeast proteins called COMPASS, which enzymatically change histones to favor gene expression. Later, he discovered that mammals have six COMPASS look-alikes—two SET proteins (1A and 1B), and four Mixed-Lineage Leukemia ( MLL) proteins—named because they are only mutated in some leukemias. His group has since focused on understanding functional differences between thsee COMPASS methylases. The role of mouse Mll2 in establishing bivalency was the topic of his latest paper.

There are three potential methylation states for histone H3: active transcription of the chromosome; a silenced region of the chromosome; or poised for activation in an undifferentiated state.

The team wanted to identify which COMPASS family member is involved in these processes, and came up with Mll2. Mll2-deficient cells show H3K4me3 loss, but only at the promoter regions of developmentally regulated genes—such as Hox genes. The revelation came when the researchers found Mll2-deficient mouse embryo stem cells continued to display the defining property of a stem cell—the ability to "self-renew."

Genes that permit stem cell versatility were undisturbed by Mll2 loss. But remarkably, when cultured with a factor that induces maturation, Mll2-deficient mouse embryonic stem (ES) cells showed no apparent abnormalities. In fact, expression ("turning on") of Hox genes that normally exhibit bivalent histone markers was the same in Mll2-deficient cells as it was in non-mutant cells.

"This means that Mll2-deficient mouse embryonic stem (ES) cells, after receiving a differentiation signal, can still activate genes required for maturation, even after losing the H3K4me3 mark at bivalent regions. This work paves the way for understanding what the real function of bivalency is in pluripotent cells and on development." says Deqing Hu, Ph.D., postdoctoral fellow and study leader.

The study may potentially impact cancer research (oncogenesis) as tumor-initiating "cancer stem cells" exhibit bivalent histone marks at some genes.

"Cancer stem cells are resistant to chemotherapy, making them difficult to eradicate. Our work could shed light on how cancer stem cells form a tumor or suggest a way to shut these genes down."

Deqing Hu, Ph.D., postdoctoral fellow and study leader

Other Stowers contributors from the Shilatifard lab were Alexander S. Garruss, Xin Gao, Marc A Morgan, Ph.D., Malcolm Cook, and Edwin R Smith, Ph.D.

Funding for the study came from the Stowers Institute for Medical Research and the United States National Cancer Institute.

About the Stowers Institute for Medical Research

The Stowers Institute for Medical Research is a non-profit, basic biomedical research organization dedicated to improving human health by studying the fundamental processes of life. Jim Stowers, founder of American Century Investments, and his wife Virginia opened the Institute in 2000. Since then, the Institute has spent over 900 million dollars in pursuit of its mission.

Currently the Institute is home to over 550 researchers and support personnel; over 20 independent research programs; and more than a dozen technology development and core facilities. Learn more about the Institute at http://www.stowers.org.

Original press release: http://www.stowers.org/media/news/aug-12-2013