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【bio-news】Gene Studies Tell Placenta's Tale
By Elizabeth Pennisi
ScienceNOW Daily News
18 April 2008
When it comes to pregnancy, mice and elephants couldn't be more different: A mouse mom births a dozen pups in less than a month, whereas it takes an elephant about 2 years to produce a single calf. A well-adapted placenta makes this diversity possible. Now, a new genetic study is shedding light on how this important organ evolved--and how it became fine-tuned to meet the specific reproductive needs of mice, elephants, and humans.
Every embryo needs resources to grow. In most birds and reptiles, a simple membrane attached to the inside of the egg shell transfers oxygen--and sometimes nutrients--to the embryo. The mammalian placenta is far more complex. Tissue from both the mother and the fetus come together to form a hollow structure that surrounds the entire fetus. The placenta transfers oxygen and nutrients from the mother and removes wastes and carbon dioxide from the developing child--all while protecting the fetus from the mother's immune system.
To understand how this complexity arose, Stanford University developmental biologist Julie Baker and graduate student Kirstin Knox reconstructed evolutionary history from the genes that make and operate placentas. They first examined placentas from mice at various stages of pregnancy, isolating RNA from both the maternal and fetal components of this lifeline. Genes activated early on in placental development date back to the first multicellular organisms, as they have counterparts in many species. Thus it appears that placental mammals rely on the same genetic program to get their placentas started as birds, reptiles, and other organisms do to set up membranes in their eggs.
By the time the mouse placenta matured, about half of its active genes were unique to rodents. When Knox and Baker analyzed genes active in a fully developed human placenta, collected after birth, they found that more than a quarter of those were primate-specific, suggesting that the placentas of different species evolved new sets of genes to deal with unique reproductive requirements. The mouse heart and uterus did not utilize a preponderance of rodent-specific genes but instead had many that were vertebrate-specific, Baker and Knox report, indicating the importance of recently evolved genes in the placenta.
These rodent- and primate-specific genes included families of related genes with roles in pregnancy. For example, mice, with their big litters, need a higher-performing placenta than humans do--one that has a larger number of hormones and enzymes involved in embryonic nutrition, Knox and Baker report in the May issue of Genome Research.
"It's the first global look at gene expression in the placenta," says evolutionary biologist David Haig of Harvard University. As such, adds Tom Moore, a veterinarian at University College Cork in Ireland, it "adds to a very neglected area. Any systematic approach to placental development has to be welcomed." [标签:content1][标签:content2]
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作者:admin@医学,生命科学 2011-01-03 17:53
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