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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 June 26, 2014

The newt NotophthalmusViridescens
Salamander muscle cells are teaching us about regeneration and reprogramming.


WHO Child Growth Charts




Limb regeneration: Do salamanders hold the key?

The secret of how salamanders successfully regrow body parts is being unravelled. For the first time, researchers have found that a protein path called the 'ERK pathway' must be constantly active in order for salamander cells to reprogram and regenerate body parts.

The ERK pathway exists within a cell and is made up of a chain of proteins. It serves as a communication link between the cell surface and the DNA in the nucleus of that cell. A molecule binding to a receptor on a cell's surface begins the communication process by adding phosphate groups to each protein in the chain. These phosphates act as "on" or "off" switches from one protein to the next until the DNA in the nucleus initiates some kind of change in the cell — such as cell division.

The team has identified a key difference between the activity of the 'ERK pathway' in salamanders and mammals which helps explain why humans can't regrow limbs. The ERK pathway is not fully active in mammalian cells. But, it can be forced to be constantly active and give cells more potential for reprogramming and regeneration.

The study is published in the journal Stem Cell Reports.

"While humans have limited regenerative abilities, other organisms, such as the salamander, are able to regenerate an impressive repertoire of complex structures including parts of their hearts, eyes, spinal cord and tails. They are the only adult vertebrates able to regenerate full limbs.

We're thrilled to have found a critical molecular pathway, the ERK pathway, that determines whether an adult cell is able to be reprogrammed and help the regeneration processes. Manipulating this mechanism could contribute to therapies directed at enhancing regenerative potential of human cells."

Maximina Yun, PhD, lead researcher, the Institute of Structural and Molecular Biology, UCL

Further research will focus on understanding how this very important pathway is regulated during limb regeneration, and which molecules are involved in the process.

•Sustained ERK activation is required for serum reprogramming of salamander cells
•Only transient ERK activation is observed in their mammalian counterparts
•Constant ERK activation promotes expression of S phase genes in mammalian myotubes
•The extent of ERK activation could underlie differences in regenerative competence

In regeneration-competent vertebrates, such as salamanders, regeneration depends on the ability of various differentiated adult cell types to undergo natural reprogramming. This ability is rarely observed in regeneration-incompetent species such as mammals, providing an explanation for their poor regenerative potential. To date, little is known about the molecular mechanisms mediating natural reprogramming during regeneration. Here, we have identified the extent of extracellular signal-regulated kinase (ERK) activation as a key component of such mechanisms. We show that sustained ERK activation following serum induction is required for re-entry into the cell cycle of postmitotic salamander muscle cells, partially by promoting the downregulation of p53 activity. Moreover, ERK activation induces epigenetic modifications and downregulation of muscle-specific genes such as Sox6. Remarkably, while long-term ERK activation is found in salamander myotubes, only transient activation is seen in their mammalian counterparts, suggesting that the extent of ERK activation could underlie differences in regenerative competence between species.

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