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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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The World Health Organization (WHO) has created a new Web site to help researchers, doctors and
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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 Oct 6, 2014

RED telomeres on chromosome tips function as caps which stabilize chromosomes.
A telomere sequence is made up of over 1000 repeats of the DNA chromosome letters: TTAGGG.
Telomere shortening occurs with every cell division, which in human cells is about 30-50
cell division cycles. Telomere shortening eventually destabilizes chromosomes completely.
But, excessive oxidative stress can also increases telomere shortening.

 






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Discovered, an On/Off switch for aging cells

Scientists at the Salk Institute have discovered an on-and-off “switch” in cells that may hold the key to healthy aging. This switch may give us a way to encourage healthy cells to keep dividing and regenerating, such as creating new lung tissue even through old age.

In our bodies, newly divided cells constantly replenish lungs, skin, liver and other organs. However, most human cells do not divide indefinitely.

With each cell division, a timekeeper at the end of each chromosome shortens. When this timekeeper, called a telomere, becomes too short, cells no longer divide, causing organs and tissues to fail. But there may be a way around this countdown. Some cells produce an enzyme called telomerase, which rebuilds telomeres allowing indefinite cell division.

In a new study published September 19 in the journal Genes and Development, scientists at the Salk Institute have discovered that telomerase, even when present, can be turned off.


“Previous studies had suggested that once assembled, telomerase is available whenever it is needed. We were surprised to discover instead that telomerase has what is in essence an ‘off’ switch, whereby it disassembles.”

Vicki Lundblad, senior author, professor and holder of Salk’s Ralph S. and Becky O'Connor Chair.


Understanding how this “off” switch can be manipulated to slow down the telomere shortening process, could lead to treatments for diseases of aging and regeneration of vital organs even late in life.

Lundblad and first author and graduate student Timothy Tucey conducted their studies in the yeast Saccharomyces cerevisiae, critical to the production of wine and bread. Previously, Lundblad’s group used this simple single-celled organism to reveal numerous insights about telomerase laying the groundwork for similar findings in human cells.

“We wanted to be able to study each component of the telomerase complex but that turned out to not be a simple task,” explained Tucey. So he developed a technique that allowed him to microscopically observe each component of cell growth and division. His ability to observe every detail of active cell division lead to a set of discoveries on how and when the telomere "machine" is assembled and disassembled.


Every time a cell divides, its entire gene content — or genome, must be duplicated. While this duplication is going on, Tucey discovered that telomerase sits poised in a “preassembled” complex mode — but still missing a critical molecular subunit.

When the genome is finished fully duplicating itself, the missing subunit appears and joins the “preassembled” complex to form a fully active telomere. Now the telomere can replenish the ends of eroding chromosomes and ensure another healthy cell division.

However, Tucey and Lundblad also found that immediately following a full telomere assembly, the telomere rapidly disassembles again into an inactive complex — effectively flipping into an “off” position.

This disassembly process may provide a means of keeping telomerase at exceptionally low levels inside the cell to avoid the unending cell divisions of cancer.


While eroding telomeres in normal cells can contribute to aging — cancer cells rely on elevated telomerase levels to ensure unregulated cell growth. The “off” switch discovered by Tucey and Lundblad may help keep telomerase activity below the threshold for "turning on" the continuous cell divisions of cancers.

Extract
The idea that chromosomes have special terminal structures first arose as a consequence of experiments conducted by Muller, who found that treatment of Drosophila with X-rays rarely resulted in terminal deletions or inversions of the chromosomes (Muller 1938). Complementary experiments in maize by McClintock expanded upon the idea that telomeres, the physical ends of chromosomes, are required for chromosome stability, by contrasting the breakage-fusion-bridge cycle resulting from broken dicentric chromosomes with the stability of chromosomes with intact termini (McClintock 1941, 1942). With the dawn of molecular biology, telomeres in most eukaryotes are now known to be composed of short repeated G-rich sequences complexed with proteins to form a special heterochromatin-like structure. More recent experimental manipulation of chromosome termini and of the proteins that bind them have confirmed the early observations of Muller and McClintock, showing that a primary role of telomeres is to insulate the ends of chromosomes both from fusion with other ends and from nucleolytic digestion (Counter et al. 1992; Sandell and Zakian 1993; Garvik et al. 1995; van Steensel et al. 1998).

This research was supported by the National Institutes of Health, the Fritz B. Burns Foundation and a Rose Hills Foundation Fellowship.

About the Salk Institute for Biological Studies:
The Salk Institute for Biological Studies is one of the world's preeminent basic research institutions, where internationally renowned faculty probes fundamental life science questions in a unique, collaborative, and creative environment. Focused both on discovery and on mentoring future generations of researchers, Salk scientists make groundbreaking contributions to our understanding of cancer, aging, Alzheimer's, diabetes and infectious diseases by studying neuroscience, genetics, cell and plant biology, and related disciplines.
Faculty achievements have been recognized with numerous honors, including Nobel Prizes and memberships in the National Academy of Sciences. Founded in 1960 by polio vaccine pioneer Jonas Salk, MD, the Institute is an independent nonprofit organization and architectural landmark.


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