Welcome to The Visible Embryo

Home- - -History-- -Bibliography- -Pregnancy Timeline- --Prescription Drugs in Pregnancy- -- Pregnancy Calculator- --Female Reproductive System- News Alerts -Contact

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 ' million visitors each month.


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!



Home

History

Bibliography

Pregnancy Timeline

Prescription Drug Effects on Pregnancy

Pregnancy Calculator

Female Reproductive System

Contact The Visible Embryo

News Alerts Archive

Disclaimer: The Visible Embryo web site is provided for your general information only. The information contained on this site should not be treated as a substitute for medical, legal or other professional advice. Neither is The Visible Embryo responsible or liable for the contents of any websites of third parties which are listed on this site.
Content protected under a Creative Commons License.

No dirivative works may be made or used for commercial purposes.

Return To Top Of Page
Pregnancy Timeline by SemestersFemale Reproductive SystemFertilizationThe Appearance of SomitesFirst TrimesterSecond TrimesterThird TrimesterFetal 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 HemispheresEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterDevelopmental Timeline
Click weeks 0 - 40 and follow fetal growth
Search artcles published since 2007

June 28, 2012--------News Archive Return to: News Alerts

New Role for RNA - RNAi


Multigenerational inheritance and piRNAs converge on same nuclear silencing pathway
HRDE1/WAGO-9 and chromatin factors required for inheritance of piRNA silencing
piRNAs can induce multigenerational silencing for more than 20 generations.
Long-term memory independent of piRNA is triggered,
but remains dependent on nuclear pathway



WHO Child Growth Charts

       

Research Discovers New Role for RNAi

Epigenetic memory may pass RNA silencing from one generation to the next

Organisms employ a fascinating array of strategies to identify and restrain invasion of foreign DNA, such as those introduced by viruses. For example, many viruses produce double-stranded (ds)RNA during their life cycle. The RNA interference (RNAi) mechanism is believed to recognize dsRNA and begin turning off (silencing) foreign DNA.

Now, UMass Medical School researchers have identified a mechanism related to RNAi that scans for intruders not by recognizing dsRNA or some other odd feature of the foreign sequence, but by comparing the foreign sequence to a memory of previously expressed native RNA.


Once identified, an "epigenetic memory" of the foreign DNA fragments is created and can be passed on from one generation to the next, permanently silencing the gene.


A remarkable feature of this RNAi-related phenomenon (referred to as RNA-induced epigenetic silencing, or RNAe), is that we carry a memory of previous gene expression. The memory of active genes serves as an "anti-silencing" signal, protecting native genes from RNAe and - in some circumstances - adopts foreign genes as self.

These findings, described in three studies (including one by Eric Miska and colleagues of the Gurdon Institute, University of Cambridge and Wellcome Trust, UK) published online to appear in the July 6 issue of Cell, provide new insights into how identical organisms can have the same DNA sequence - but opposite patterns of gene expression and dramatically different obervable traits (phenotypes).

"If a worm modulates gene expression by carrying a memory of the genes it expressed in previous generations, perhaps other organisms (including humans) can as well. If so, mechanisms of this type could have an important impact on evolution," said Craig C. Mello, PhD, Howard Hughes Medical Institute Investigator, Blais University Chair in Molecular Medicine and distinguished professor of molecular medicine and cell biology.

"The RNAe mechanism could accelerate evolutionary change by increasing heritable phenotypic variation (without the need for DNA mutations). There is growing evidence that many organisms can track and respond epigenetically to gene expression patterns. Our findings provide insight into a whole new level of sophistication in the recognition and memory of gene expression programs."

Dr. Mello and colleagues knew that when a foreign piece of DNA encoding the green fluorescent protein, or GFP, was inserted into the small roundworm C. elegans, some of the worms would silence the newly introduced DNA while others would express the GFP gene. They then explored a role for RNAi in the decision to silence or express GFP. RNAi is a process whereby cells modify the activity of their genes.

In RNAi-related phenomena, Argonaute proteins interact with and use small RNAs as little genetic guides to recognize target nucleic acids.

Based on their findings, Mello and colleagues pose that up to three separate Argonaute systems work together to:
(1) scan
(2) identify and
(3) silence foreign DNA,
while protecting the expression of normal genes.

In this system, Argonaute PRG-1 (Piwi) is bound to piwi-interacting RNA (piRNA) and begins:
(1) scanning molecules of RNA as they leave the nucleus of the cell and
(2) determining if they are indigenous to the organism or foreign; if PRG-1 and its piRNA cofactors identify a foreign sequence
(3) initiate (or activate) the second Argonaute system, known as WAGO, which turns the genetic material off so it can't be expressed.


Once the DNA is identified as foreign and silenced, an epigenetic memory is created that silences the foreign gene from one generation to the next.


While the inheritance of this memory requires more research, the authors showed that successive generations of C. elegans are unable to express foreign DNA even if the corresponding piRNA is missing.

"It appears that piRNAs are responsible for the initial scanning and identification of foreign nucleic acids," said Darryl Conte Jr., PhD, research assistant professor of molecular medicine and one of the co-authors on the Cell papers. "Because the foreign DNA in successive generations is being silenced, even in worms that don't have the piRNA, the information necessary for silencing is being passed on epigenetically and independently of the initial scanning done by the piRNA complex in the previous generations."

Originating from clustered regions of the genome, piRNA are diverse and abundant small non-coding RNA molecules in animals, numbering in the millions in mammals. For the most part, piRNAs in worms— and many piRNAs in mammals—lack obvious complementary targets and their function is not clearly understood.


It's possible that piRNAs act as a genetic security system,
using imperfect base pairs to help
identify foreign nucleic acids
Darryl Conte


So what prevents piRNAs from recognizing and permanently silencing a gene that the worm identifies as its own? Remarkably, the authors found that such "self" transcripts are somehow protected from entering the WAGO system and that some active genes can actually turn on silent genes.

Because the self transcripts are associated with a third Argonaute known as CSR-1, the authors propose that CSR-1 provides an anti-silencing or protective function, which licenses the expression of genes that the worm recognizes as its own.

"This is one of the truly unique findings of these studies," said Conte. "Before, we knew that the RNAi process could be used to regulate genes or to turn them off completely. In this case, what we see is an RNAi mechanism that appears to prevent a gene from being silenced by the piRNA pathway. It works almost as a form of protection that allows the gene to be expressed."

"Taken together, these studies posit a surprisingly complex role for small-RNA systems in epigenetic programming," said Mello. "It shows how piRNAs continuously scan all the genes expressed in the germline, constantly comparing each sequence to a memory of previous gene expression. When foreign genes are recognized and silenced, this new epigenetic knowledge can be passed down to successive generations. On the other hand, occasionally new genes are expressed, apparently stochastically, and this active state too can be passed on as a stable epigenetic memory, thus the organism effectively adopts the foreign gene as self."

The University of Massachusetts Medical School, one of the fastest growing academic health centers in the country, has built a reputation as a world-class research institution, consistently producing noteworthy advances in clinical and basic research. The Medical School attracts more than $270 million in research funding annually, 80 percent of which comes from federal funding sources. The mission of the Medical School is to advance the health and well-being of the people of the commonwealth and the world through pioneering education, research, public service and health care delivery with its clinical partner, UMass Memorial Health Care. For more information, visit www.umassmed.edu.

Original article: http://www.eurekalert.org/pub_releases/2012-06/uomm-ums062612.php