Women Not Born With Lifetime Supply of Eggs?
Animal Study Shows Mammals Have a Reserve of Egg-Producing Follicles
March 10, 2004 -- A surprising finding about female fertility
in mice is challenging the notion that women are born with a lifetime's supply
Researchers have long believed that female mammals, including
mice and humans, were born with a fixed supply of egg-producing follicles in
their ovaries. In contrast, most male mammals continuously generate new sperm
cells that allow them to reproduce throughout their adult life.
But a new study published in the March 11 issue of
Nature shows that female mice continue to produce eggs to replace
damaged ones after birth.
Researchers say the discovery indicates that females may share
the ability to replenish reproductive cells during life, and if the same
process occurs in humans it may also help explain why female fertility declines
rapidly after age 30.
Finding Challenges Basic Reproductive Biology Beliefs
Researchers say that the theory that female mammals are born
with a fixed supply of eggs stems from studies that showed females develop a
finite number of egg-producing follicles in their ovaries during fetal life, as
opposed to males who continue to generate sperm-producing cells throughout
"Although this dogma has persisted for more than 50 years,
the present study provides evidence that challenges the validity of this
belief, which represents one of the most basic underpinnings of reproductive
biology," write researcher Joshua Johnson of Harvard Medical School and
Rather than having a fixed supply of egg-producing follicles,
the study shows that female mice have a reserve supply of cells that replenish
the follicle pool during adolescence as damaged follicles die.
In the study, researchers carefully measured follicle numbers
at birth and then tracked their subsequent loss in female mice. They found that
adolescent mice had about 2,500-5,000 healthy follicles but the number of dying
follicles increased rapidly to up to 1,200 per ovary after adolescence.
Dying follicles degenerate or disappear within a few days,
which means that the total number of healthy follicles should have dropped
rapidly during this time period. Instead the study showed that in female mice
the ovaries contain a population of cells that are required to maintain overall
number of follicles for adult life.
Researchers found that the number of healthy follicles actually
decreased relatively slowly despite this rapid loss of follicles. That finding
shows that healthy egg-producing follicles must be produced somewhere in young
To test this theory, researchers treated the young mice with a
chemical that kills egg cells and instead found that the mice still produced
viable eggs in adulthood.
Researchers say the results show that a reserve of stem cells
that form the building blocks for reproductive cells must exist in female mice
as they do in male mammals. But more research is needed to determine how they
function and what causes them to decline after adolescence.