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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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August 2, 2012--------News Archive Return to: News Alerts


All information is specific to a mouse model

(a) Following primordial germ cell (PGC) expansion start at embryonic day 7.5 (e7.5),
proliferation of female germ cells (oogonia; pink) ceases at e 13.5
concomitant with a 5-day period of germ cell meiotic commitment
that drives formation of oocytes (blue);
as all oocytes produced during this time are of equivalent “depth”,
the production-line hypothesis of postnatal oocyte maturation cannot
logically explain increasing oocyte depth as females age.

(b) If continued proliferation of OSCs (red) and their subsequent differentiation
into oocytes (blue) during postnatal life is superimposed on the
production-line hypothesis, the emerging picture is consistent with
a progressive increase in oocyte depth in females as they age.


doi:10.1371/journal.pgen.1002848.g001.

WHO Child Growth Charts

       

Do Ovaries Continue to Produce Eggs During Adulthood?

A compelling new genetic study tracing the origins of immature egg cells, or 'oocytes', from the embryonic period throughout adulthood adds new information to a growing controversy

A compelling new genetic study traces the origins of immature egg cells, or 'oocytes', from the embryonic period through adulthood adding new information to a growing controversy. The notion of a "biological clock" in women arises from the fact that oocytes progressively decline in number as females get older. This idea is combined with a decades-old dogmatic view that oocytes cannot be renewed in mammals after birth.

After careful assessment of data from the recent study, the results were published in PLoS Genetics. Scientists from Massachusetts General Hospital and the University of Edinburgh argue that the findings support formation of new eggs during adult life - a topic with a long history of controversial debate.

Eggs form from progenitor germ cells that exit the mitotic cycle, ending their cell division, to enter meiosis, a process unique to forming eggs and sperm. Meiosis removes one half of the genetic material from each sex cell prior to fertilization.


While traditional thinking has held that female mammals
are born with all of the eggs they will ever have,
newer research has demonstrated that adult mouse
and human ovaries contain a rare population of
progenitor germ cells called oogonial stem cells
capable of dividing and generating new oocytes.


Using a powerful new genetic tool that traces the number of divisions a cell undergoes with age (its 'depth') Shapiro and colleagues counted the number of times progenitor germ cells divided before becoming oocytes. Their study was published in PLoS Genetics in February 2012.

In traditional thinking, all oocyte divisions occur prior to birth, thus all oocytes should exhibit the same depth regardless of age of the female mouse. However, the opposite was found – eggs showed a progressive increase in depth as the female mice grew older.

In their assessment of the work by Shapiro and colleagues – published recently in a PLoS Genetics Perspective article – reproductive biologists Dori Woods, Evelyn Telfer and Jonathan Tilly conclude that the most plausible explanation for these findings is that progenitor germ cells in ovaries continue to divide throughout reproductive life, resulting in production of new oocytes with greater depth as animals age.


Although these investigations were performed in mice,
there is emerging evidence that oogonial stem cells
are also present in the ovaries of reproductive-age women,
and these cells possess the capacity, like their mouse
counterparts, to generate new oocytes under certain
experimental conditions.


While more work is needed to settle the debate over the significance of oocyte renewal in adult mammals, Woods and colleagues emphasize that "the recent work of Shapiro and colleagues is one of the first reports to offer experimental data consistent with a role for postnatal oocyte renewal in contributing to the reserve of ovarian follicles available for use in adult females as they age."

Everything published by PLoS Genetics is open access, allowing anyone to download, reuse, reprint, modify, distribute, and/or copy articles, so long as the original authors and source are cited. Please mention PLoS Genetics in your report and use the link(s) below to take readers straight to the online articles. Thank you.

FINANCIAL DISCLOSURE: This work was supported by a Method to Extend Research in Time (MERIT) Award from the National Institute on Aging (NIH R37-AG012279), the Henry and Vivian Rosenberg Philanthropic Fund, and Vincent Memorial Hospital Research Funds. The funders had no role in the preparation of the article.

COMPETING INTERESTS: DCW declares interest as a scientific consultant for OvaScience, Inc. (Cambridge, MA); JLT declares interest in intellectual property described in US Patent 7,955,846 and is a co-founder of OvaScience, Inc.; EET declares no competing interests.

CITATION: Woods DC, Telfer EE, Tilly JL (2012) Oocyte Family Trees: Old Branches or New Stems? PLOS Genet 8(7): e1002848. doi:10.1371/journal.pgen.1002848

Original article: http://www.eurekalert.org/pub_releases/2012-07/plos-doc072312.php