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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 in 1993 as a first generation internet teaching tool consolidating human embryology teaching for first year medical students.

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 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
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December 13, 2012--------News Archive Return to: News Alerts


This zebrafish embryo developed a limb looking more like a leg than a fin, after being engineered to produce the HoxD13 protein in the cells at the tip of its' developing fin.

Changes in HoxD13 production likely contributed to the transition from fin to leg development, during animal evolution.

Credit: Freitas et al., Developmental Cell







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From Fish to Man: Research Reveals How Fins Became Legs

Vertebrates' transition to living on land represented a major event in the history of life on earth – and reveals the role of the gene Hoxd13 in the determination of body parts

Researchers reporting in the December issue of the Cell Press journal Developmental Cell provide new evidence that the development of hands and feet occurred through the gain of new DNA elements that activate particular genes.


"First, and foremost, this finding helps us to
understand the power modification of gene
expression has on shaping our bodies.

Second, many genetic diseases are associated
with a 'misshaping' of organs during development.

In the case of genes involved in limb formation,
their abnormal function is associated with
synpolydactyly and hand-foot-genital syndrome."

Dr. José Luis Gómez-Skarmeta
CSIC-Universidad Pablo de Olavide-Junta de Andalucía
Seville, Spain


In order to understand how fins may have evolved into limbs, researchers led by Dr. Gómez-Skarmeta and his colleague Dr. Fernando Casares, also of the CSIC-Universidad Pablo de Olavide-Junta de Andalucía, introduced extra copies of the gene Hoxd13, known to play a role in distinguishing body parts, to the tip of a zebrafish embryo's fin.

Surprisingly, this led to the generation of new cartilage tissue and reduction in fin tissue—changes that strikingly recapitulate key aspects of land-animal limb development.

The researchers wondered whether novel Hoxd13 control elements may have increased Hoxd13 gene expression in the past to cause similar effects during limb evolution. They turned to a DNA control element that is known to regulate the activation of Hoxd13 in mouse embryonic limbs and that is absent in fish.


"We found that in the zebrafish and mouse the
Hoxd13 control element was capable of driving
gene expression in the distal fin rudiment."

Dr. Fernando Casares
CSIC-Universidad Pablo de Olavide-Junta de Andalucía


"This result indicates that molecular machinery capable of activating this control element was also present in the last common ancestor of finned and legged animals and is proven by its remnants in zebrafish," says Dr. Casares.

Freitas et al.: "Hoxd13 contribution to the evolution of vertebrate appendages."

Original article: http://www.cell.com/developmental-cell/abstract/S1534-5807(12)00478-9