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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.

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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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May 11, 2012--------News Archive Return to: News Alerts


One giant mouse weighs more than six 'mini-mice' of the same age. The biggest mice in
the world evolved through targeted breeding over many generations. Scientists can use
these animals to identify the genes responsible for body growth.

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Parallel Selection Tweaks Many of the Same Genes to Make Big and Heavy Mice

Organisms adapt to their environment through individual characteristics, such as body size and body weight. Such complex traits are usually controlled by many genes. As a result, individuals show tremendous variations

Researchers from the Max Planck Institute for Evolutionary Biology in Plön have now investigated how evolution alters such traits through selection.

To do this, they examined the genomes of mouse lines that were selected independently of each other for extreme body size. They discovered that a number of gene regions, or loci, have undergone changes that underlie these complex characteristics. They also discovered many new genes that play a role in the regulation of body weight, and obesity.

The Plön-based researchers obtained mouse lines that have been specifically selected for extreme body weight for 25 years. The mice, which have been bred for over 150 generations, belong to seven different strains and now weigh two to four times more than mice of normal weight.

The Max Planck scientists were able to identify a total of 67 loci on the genome that had changed in the heavy mice. The different strains have become so similar in these regions as a result of the extreme artificial selection pressure, that the genomes of the heavier but unrelated animals were more similar at these loci than with their closely related sibling mouse strains of those with normal weight.

This clearly indicates that these loci are involved in the regulation of body weight.

The discovered loci regulate, among other things, energy balance, metabolic processes and growth. The Gpr133 gene, which is expressed in the adrenal gland, is a novel gene and presumably controls body weight through the release of hormones. The second identified gene, Gpr10, which is active in the hypothalamus in the brain, was found to influence appetite and metabolic rates.

Accordingly, the team has also identified genes for the regulation of fat cells and for taste and olfactory perception that can affect body weight. Moreover, many of the regions discovered coincide with loci on the human genome that influence body weight.

"These genes probably also determine body weight in humans, because size and body weight are such tightly linked processes. This evolutionary connection serves as a nice confirmation," says Frank Chan from the Max Planck Institute for Evolutionary Biology.

Interestingly, the genome of mouse populations living in the wild on remote islands, shaped by natural selection, have also changed in similar ways to the animals bred in the laboratory.

For example, on the Faroe Islands and St Kilda off the coast of Scotland, mice populations have evolved to be among the largest mice in the world. The researchers have found that island mice retained little variation specifically at the same genomic loci that changed in the heavy laboratory-bred animal strains.

These telltale signs suggest that artificial selection in the laboratory changes the same loci in the genome as natural selection.


When complex characteristics adapt to altered environmental conditions, the selection of those characteristics affects many genes simultaneously. These genes then change in parallel and contribute to the organism's capacity for adaptation.


In this way, the genetic basis of complex traits can be decoded through parallel selections.

Article: Chan, Y. F. et al. Parallel selection mapping using artificially selected mice reveals body weight control loci. Published in Current Biology: Volume 22, Issue 9, 8 May 2012, Pages 794 doi:10.1016/j.cub.2012.03.011

Original article: http://www.eurekalert.org/pub_releases/2012-05/m-rap050812.php