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MGH study finds female mammals produce egg cells into adulthood

ts taken during the early stages of life found a steady, low level of dying follicles, but as the mice reached adulthood the number of dying follicles increased markedly. In young adult animals, the researchers measured 1,200 dying follicles per ovary, compared with about 3,000 healthy follicles remaining. Similarly elevated levels of dying follicles were measured well into maturity. Although such a surprisingly high rate of follicle loss would be expected to completely deplete a fixed population of oocytes within a matter of days or weeks, female mice retain healthy egg cells well past one year of age.

To make sure they were accurately measuring the rate at which follicles were dying, the researchers evaluated whether dead and dying follicles were being cleared from the ovaries. Their results confirmed that the dying follicles were being cleared within three days of death and thus represented a continuing level of cell death, not an accumulation of "cellular corpses."

"Finding such a high level of follicle degeneration without a corresponding reduction in the number of healthy follicles brought us up against the dogma of a fixed supply of oocytes," Tilly says. "The only probable interpretation was that the postnatal ovary had to be retaining the ability to make new oocytes and follicles." To investigate such a potentially revolutionary possibility, the researchers ran several additional experiments.

  • Careful examination of ovaries of young and mature mice identified cells on the organs' outer surface that resembled germ cells, which are the source of oocytes that develop in fetal animals. These cells were found to express a gene known to be present only in germ cells and were shown to maintain the ability to undergo cell division in juvenile and adult ovaries.

  • A key stage in the development of any germ cell is meiosis, which results in egg or sperm cells with a single set of chromosomes instead of the paired sets f
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  • Contact: Sue McGreevey
    smcgreevey@partners.org
    617-724-2764
    Massachusetts General Hospital
    10-Mar-2004


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