It has long been believed that female mammals – including humans – are born with
all the eggs they need to reproduce during their lifetimes. But some claim that
the female mammal’s egg supply is renewed over her adult lifetime and that the
source of these eggs is stem cells that originate in the bone
marrow.
Now, Weizmann Institute scientists have reached scientific
conclusions about the claims by using an original method for reconstructing
lineage trees for cells. Their work was published recently in PLoS
Genetics.
The method, developed over several years in the lab of Prof.
Ehud Shapiro of the Rehovot institute’s departments of biological chemistry,
computer science and applied mathematics, uses mutations in specific genetic
markers to determine which cells are most closely related and how far back they
share a common parent cell, to create a sort of “family tree” for
cells.
Shapiro and members of his lab, including Drs. Shalev Itzkovitz
and Rivka Adar, together with Prof. Nava Dekel and research student Yitzhak
Reizel of the biological regulation department, used their method to see if ova
could be descended from bone-marrow stem cells. Their findings, reached in
cooperation with colleagues from Tel Aviv University and the Technion-Israel
Institute of Technology in Haifa, indicated that any relationship between the
two types was too distant for one to be an ancestor of the other.
These
scientists also found, surprisingly, that the ova of older mice had undergone
more cell divisions than those of younger mice. This could be the result of
replenishment during adulthood, but an alternate theory holds that all eggs are
created before birth, and those that undergo fewer divisions are simply selected
earlier on for ovulation. Further experimentation, says Shapiro, will resolve
the issue.
Cell “lineage trees” are similar to modern evolutionary and
taxonomic “trees” based on genome comparisons between organisms.
Shapiro
and his team used mutations in cells that are passed on to daughter cells over
an organism’s lifetime (though not on to the next generation). By comparing a
number of genetic sequences called microsatellites – areas where mutations occur
like clockwork – they can place cells on trees to reveal their developmental
history.
A number of papers published in recent months by Shapiro, his
team and collaborators have demonstrated the power and versatility of this
method. One study, for instance, lent support to the notion that the adult stem
cells residing in tiny crypts in the lining of the colon do not harbor, as
thought, “immortal DNA strands.” Immortal strands may be retained by dividing
stem cells if they always relegate the newly-synthesized DNA to the
differentiating daughter cell and keep the original stand in the one that
remains a stem cell. A second study addressed an open question about developing
muscle cells. Here they found that two kinds of progenitor cell – myogenic
cells, which eventually give rise to muscle fiber, and non-myogenic cells found
within the same muscle – are more closely related than similar cells in
different muscles.
One immediate advantage of the cell lineage analysis
method developed by Shapiro’s team is that it is non-invasive and retrospective
and thus can be applied to the study of human cell lineages. Most other studies
of development rely on genetically engineered lab animals in which the stem
cells are tagged with fluorescent markers. Not only does it provide a powerful
new research method that doesn’t rely on such markers, but Shapiro believes it
could one day be used as a diagnostic tool that might, for instance, reveal the
history of an individual’s cancer and help doctors determine the best course of
treatment.
ANCIENT WRITINGS TO STUDY CLIMATE CHANGE
Reconstructing
climates from the past provides historical comparison to modern weather events
and valuable context for climate change. In the natural world, trees, ice cores
and coral provide evidence of past weather, but from human sources scientists
are limited by the historical information available. Until now, researchers have
relied on official records detailing weather patterns, including air force
reports from World War II and 18th-century ships’ logs. Now, ancient manuscripts
written by Arabic scholars are believed to provide valuable meteorological
information to help modern scientists reconstruct the climate of the past, a new
study published in the Weather journal has found. Spanish scientists from the
University of Extremadura have analyzed the writings of scholars, historians and
diarists in Iraq from the Islamic Golden Age between 816 and 1009 CE for
evidence of abnormal weather patterns.
Arabic documentary sources –
historians and “political commentators” – from the ninth and 10th centuries
focus on the social and religious events of the time but do refer to abnormal
weather events.
“Climate information recovered from these ancient sources
mainly refers to extreme events which impacted wider society, such as droughts
and floods,” said lead author Dr. Fernando Domínguez-Castro. “However, they also
document conditions that were rarely experienced in ancient Baghdad, such as
hailstorms, the freezing of rivers or even cases of snow.”
Baghdad was a
center for trade, commerce and science in the ancient Islamic world. In 891 CE,
Berber geographer al-Ya’qubi wrote that the city had no rival in the world, with
hot summers and cold winters, climatic conditions that favored strong
agriculture.
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