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Welcome to The Visible Embryo, a comprehensive educational resource on human development from conception to birth.

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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 Nov 21, 2013


Male mouse pheromones, or sex attractants released in urine, attract female mice
and increase the male's chance of mating and producing offspring.
In that way, mouse pheromones act much like a male peacock's tail to attract mates.

Credit: Illustration by Sarah Bush, University of Utah.

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Promiscuous mouse moms bear sexier sons

Male mice make more pheromone if mama had access to many mates.

University of Utah biologists found that when mother mice compete socially for mates in a promiscuous environment, their sons play hard and die young: They attract more females by making more urinary pheromones, but smelling sexier shortens their lives.

"If your sons are particularly sexy, and mate more than they would otherwise, it's helping get your genes more efficiently into the next generation," says biology professor Wayne Potts, senior author of the new study.

"Only recently have we started to understand that environmental conditions experienced by parents can influence the characteristics of their offspring. This study is one of the first to show this kind of 'epigenetic' process working in a way that increases the mating success of sons."

Male mice whose parents freely competed for mates in seminatural "mouse barns" produced 31 percent more major urinary proteins or MUPs – sex attractants called pheromones – than male mice from caged monogamous parents.

The biologists results appeared online Nov. 18 in the journal Proceedings of the National Academy of Sciences. Pheromone output increased even though the male offspring never competed socially.

Yet the male mice that produced more pheromone had shorter lifespans in another recent study by Potts and Nelson.

Only 48 percent of them lived to the end of the experiment, compared with 80 percent of the male mice whose parents lived monogamously in cages.

That's likely because it takes so much energy to produce the sex attractants, which are secreted in urine and from certain glands.

"Production of pheromones is outrageously expensive," Potts says. "A single mouse's investment in pheromone production compares with the investment that 10 male peacocks make in the production of their tails, which also are used to attract females."

Female mice prefer scent marks saturated with pheromones produced in mouse urine and other glands, and they mate more often with males who produce such marks. Previous research found that male mice with promiscuous parents actually produce about one-third more progeny than sons of monogamous parents.

The new study illustrates that a mouse's health, lifespan and ability to attract mates depend not only on their parents' genes, but on epigenetics – how parents' environment modifies their offspring's genes to influence how much protein is produced.

"Pheromones are the language of mice," says the study's first author, former University of Utah doctoral student Adam C. Nelson. "When females mate in a socially competitive environment, they program their sons to have a head start by producing more pheromones."

The findings may help programs to breed endangered species in captivity. A more natural way of confining such animals – in social groups rather than lone pairs of mates – may increase their ability to reproduce when later released into the wild.

What does it mean for people? The impacts of social environments on human pheromone output and other traits haven't been studied. Potts: "Researchers just have started to scratch the surface of discovering traits that are influenced by parental experience. It is difficult to predict which and how many traits will be involved."

Conflicting Parental Interests: Dads Make Sons Less Sexy

Domesticated mice usually breed monogamously because they are caged only with one mate. "In nature, mice must seek out and choose their own mates – a process that is eliminated in standard lab breeding conditions," says Nelson, who performed the study for his doctoral thesis at the University of Utah.

Seminatural lab enclosures – mouse barns where many mice live together – reintroduce mice to social competition, allowing them access to many mates. "Social mice are like barn mice, which build up huge populations when food is in abundance," says Potts. "Domesticated mice are more like field mice, scraping a living out of small seeds, and hardly coming in contact with other individuals."

Mouse barns in the study were 22-foot-by-13.5-foot enclosures, divided by wire mesh fencing into six sections or territories. Mice could climb easily over the mesh. Within each of the six sections was a nest box, a feeding station and drinking water. Four of the six sections in each barn were more desirable territories and two were less desirable, so the mice had to compete for territory and mates.

Mice in the study descended from wild mice and were raised for 10 generations in domesticated conditions: in cages with assigned mates. Twenty-three male-female pairs were designated as nonsocial, monogamous mice and remained in those cages. Twenty males and 40 females assigned to social, promiscuous conditions were placed in mouse barns, where they competed for territories and mates.

To assess parents' roles in influencing their sons' attractiveness, researchers designed this experiment: prospective parents lived in either the promiscuous mouse barn or in monogamous cages. They were removed after eight weeks and bred in cages in four combinations:(1) mother and father from promiscuous environment; (2) both from monogamous environment; (3) mother from promiscuous environment and father from monogamous environment; and (4) vice versa.

Regardless of the dad's original environment (promiscuous or monogamous), sons of moms from a promiscuous, social environment produced more pheromones than sons of monogamous, domesticated moms. That highlights the mom's interest in passing her genes to the next generation.

"Sons are primed to respond to conditions that their moms experienced – such as exposure to multiple potential mates," Potts says.

Unlike mother mice, fathers from a promiscuous, socially competitive environment had a surprising negative influence on their sons' sexual attractiveness.

Those sons produced 5 percent fewer pheromones than sons of monogamous fathers.

"Fathers are competing with their sons and usually drive them out of the territory quickly, while they let daughters stay," says Potts. "If you're worried about your sons impinging on your own reproductive success, then why make them sexy?"

Inheritance beyond the Genetic Code

The impact of mom's social life on her sons' pheromone levels, which is independent of her genetic code, is an example of epigenetic inheritance – the influence of parents' environment on how their offspring's genes are activated or "expressed," meaning how much protein is produced under the genes' commands.

Thus, environmental challenges faced by parents can affect their offspring's well-being for better or worse – in this study, increased or reduced pheromone output – by influencing the production of proteins needed for survival and reproduction.

An example of epigenetic inheritance gone awry is suggested when a famine affecting parents leaves their offspring with a greater propensity to store calories – a trait that is good during famine but may lead to obesity when food is plentiful.

On the molecular level, epigenetic changes involve modifying genes, rather than mutating them. A common chemical modification of genes is called methylation, a change that reduces a gene's production of a protein.

In the new study, researchers looked at a pheromone gene named Mup11. They found that methylation of the Mup11 gene was twice as high in sons of monogamous, domesticated mice than in sons of promiscuous, social mice.

So the sons of the promiscuous mice were able to produce more pheromone.

Understanding the role of social environment on inheritance may improve the success of captive-breeding programs. When reintroduced to the wild, captive animals are often ill-equipped to compete with wild animals for mates and other resources.

"It's amazing how often reintroduction of captive-breed endangered species fails – it's estimated to be as high as 89 percent," says Potts. "Domestication stimulates epigenetic mechanisms that make animals less fit for nature."

Recreating social networks in captivity, as was done in the new study, may help prepare captive animals for life in the wild. Nelson says pheromone levels in social lab mice approach those of wild mice, which increases their reproductive success.

When wild-derived laboratory mice are reintroduced to socially competitive populations, they quickly adapt by producing attractive sons that otherwise have no fitness advantages, consistent with the sexy sons model of sexual selection. These attractive sons inherit up-regulated expression of several pheromones belonging to the major urinary protein (MUP) family. Up-regulation is controlled by maternal social experience, and is associated with epigenetic modifications of MUP promoters that could enhance transcription. Inheritance of up-regulated MUPs is likely adaptive because females have odor preferences for male scent marks with higher MUP concentration. These results represent one of only a few cases where parental social experience adaptively modifies progeny phenotype.

When brought into captivity, wild animals can adapt to domestication within 10 generations. Such adaptations may decrease fitness in natural conditions. Many selective pressures are disrupted in captivity, including social behavioral networks. Although lack of sociality in captivity appears to mediate domestication, the underlying mechanisms are not well understood. Additionally, determining the contribution of genetic inheritance vs. transgenerational effects during relaxed selection may provide insight into the flexibility of adaptation. When wild-derived mice kept under laboratory conditions for eight generations were reintroduced to sociality and promiscuity (free mate choice), they adapted within two generations. Fitness assessments between this promiscuous lineage and a monogamous laboratory lineage revealed male-specific effects. Promiscuous-line males had deficits in viability, but a striking advantage in attracting mates, and their scent marks were also more attractive to females. Here, we investigate mechanistic details underlying this olfactory signal and identify a role of major urinary protein (MUP) pheromones. Promiscuous-line males inherit higher MUP expression than monogamous-line males through transgenerational inheritance. Sociality-driven maternal and paternal effects reveal intriguing conflicts among parents and offspring over pheromone expression. MUP up-regulation is not driven by hormone-driven transduction pathways, but rather is associated with reduction in DNA methylation of a CpG dinucleotide in the promoter. This reduction in methylation could enhance transcription by promoting the binding of transcription factor USF1 (upstream stimulatory factor 1). Finally, we experimentally demonstrate that increased MUP expression is a female attractant. These results identify molecular mechanisms guiding domestication and adaptive responses to fluctuating sociality.

The study was funded by the National Science Foundation and the National Institutes of Health. Potts and Nelson, who is now a postdoctoral fellow at Harvard University, conducted the study with four University of Utah co-authors: oncology professor Bradley Cairns and former doctoral student Andrew Oler at the Huntsman Cancer Institute, and undergraduates Joseph Cauceglia and Seth Merkley. Other co-authors were Neil Youngson at the University of New South Wales in Sydney, Randy Nelson at Ohio State University and Emma Whitelaw of La Trobe University in Melbourne, Australia.