Evidence That Mice Produce Egg Cells After Birth
13 april 2009--Scientists in Shanghai have challenged the orthodox medical view that a woman is born with egg cells to last a lifetime and will never generate any new ones. Overthrow of this view could hold major implications for treatment of infertility.
Similar challenges have been made before, but none have been sustained. Earlier this month, however, the same medical doctrine with respect to heart muscle cells — that you die with the same cells you are born with — was shown by a Swedish scientist, Jonas Frisen, to be incorrect: the muscle cells do get replaced, though very slowly, at the rate of 1 percent or less per year.
The Chinese team, led by Kang Zou and Ji Wu of Shanghai Jiao Tong University, worked only with mice, but because of the similarity of all mammalian physiology, any proof that mice could produce eggs after birth would set off a race to prove that people could too.
In essence, the Shanghai researchers say they have detected, in both young and old mice, the germ-line cells that produce unfertilized eggs, or oocytes.
The researchers report in the current issue of Nature Cell Biology that they scanned a mouse’s ovaries for cells producing a protein called vasa homolog that is found only in the germ-line cells. During the embryo’s formation, these cells generate all the oocytes that will be needed over the female’s lifetime.
The researchers detected vasa-producing cells in the mouse ovaries, fished them out and grew them in laboratory glassware. There the cells were injected with a gene that makes green fluorescent protein, a standard way of marking cells.
The researchers then injected the germ-line cells into the ovaries of another batch of mice whose own eggs had been killed. When the mice were mated, some of their offspring were green, indicating that they originated from eggs produced by the injected germ-line cells.
David F. Albertini, an expert on reproduction at the University of Kansas Medical Center, said the result was “a pretty exciting observation” but added that the experiment was difficult to interpret. He said that perhaps the authors had fished out a few oocytes, despite their efforts to exclude them, along with the germ-line cells, and that these oocytes could have been the origin of the infant mice produced later.
Until this and other issues have been sorted out, the observation is “not relevant clinically,” Dr. Albertini said, given the physiological differences between mice and people.
Dr. Albertini also said that in publishing the Shanghai paper and earlier claims of oocytes being produced after birth, Nature had neglected to seek the advice of a network of ovarian experts, including himself.
Dr. Frisen, the Swedish researcher who proved that heart muscle cells were generated throughout life, said he had not yet been able to apply his method to oocytes. His approach is to measure radioactive carbon-14, which was generated by aboveground nuclear tests in the 1960s and for years could be found in the DNA of cells throughout the world. The amount of carbon-14 in each cell type indicates its birth date.Dr. Frisen said that there were not enough oocytes in a person’s body to give a reliable signal at present but added that he hoped to study oocytes’ birth date when he had improved the sensitivity of his technique.
SOURCE : NATURE CELL BIOLOGY