By Becca Yeamans
There are many ways by which a woman can be infertile, however, most of the time, the problems center around the formation and maturation of the egg (oogenesis). Age at pregnancy is one of those factors that are often cited when discussing fertility rates among women, however, it still is debated exactly how maternal age plays into the fertility story and what can and does go wrong when a woman gets to be of a certain age while pregnant.
There are many ways by which a woman can be infertile, however, most of the time, the problems center around the formation and maturation of the egg (oogenesis). Age at pregnancy is one of those factors that are often cited when discussing fertility rates among women, however, it still is debated exactly how maternal age plays into the fertility story and what can and does go wrong when a woman gets to be of a certain age while pregnant.
In addition to age, external stimuli, such as estrogen-like
compounds like BPA and other EDCs found in the environment as a result of the
various plastics and processed materials in our lives which are designed to
make life “easier”.
In 2008, two researchers from Washington State University in
Pullman, WA put together a very comprehensive review paper illustrating the
different ways in which human female infertility can occur, specifically
focusing on the egg itself and the processes it must go through between
formation and maturation that can run into problems and cause infertility. This paper provides excellent framework for
the mechanisms behind female infertility related to problems with egg
development, which I will summarize briefly for you in this post and draw from
periodically when making possible connections to endocrine disruptors in future
posts.
On average, the age at which women in the United States
experience their first pregnancy is 3.6 years later than it has been in
previous decades, a trend which is seen in many other parts of the world as
well. It is also known that pregnancies
occurring in older women tend to experience higher risks, including genetic
disorders caused by age-related problems with the “egg development machinery”
such as Trisomy disorders and increased infertility related to chromosomal abnormalities
of the egg.
The review paper by researchers at Washington State
University found three different times during the development of the egg that
are solidly linked to increased trisomies or other chromosomal abnormalities,
including the stage of meiotic prophase, formation of the follicle, and egg
growth and final maturation. These times
include: during the recombination stage of meiotic prophase in the developing
fetus, follicle formation in the developing fetus, and the maturation stage of
oogenesis (i.e. egg development) in the adult ovaries.
Fetal Meiotic
Prophase
During the prophase segment of meiosis, the chromosomes
experience a variety of changes, most importantly the breaking and recombining
of chromosomes to create a completely unique individual egg that is unlike any
other egg with the ultimate goal of spreading genetic diversity among the
population. After this stage, the
developing egg goes into a resting phase where it sits and waits patiently for
the next step in development. It is also
during this phase where the egg goes through a checkpoint—the body’s way of
making sure there were no errors in recombination and that it’s safe to move
forward with the development.
Development of the
Follicle in the Fetus
Simply put, the follicle is a “community” of cells,
including the oocytes (developing eggs), and granulosa cells. It is within the follicle that the developing
egg undergoes intensive hormone-driven growth and the place which provides a
safe environment for the egg to continue on its maturation path. The follicle itself forms in the fetus during
the second trimester of pregnancy, which is triggered by what some researchers
think is a sudden drop in estrogen levels, though the consensus is not all
uniform.
In order to form a follicle, you need the breakdown of
connections between lumped together immature oocytes, as well as the encasing
of the now-individual oocytes in somatic cells (granulosa cell precursors). If errors occur and the connections between
lumped together immature oocytes are not broken down, then there is a significantly
increased risk of “multi-oocyte follicles”, which is basically a death sentence
for the embryo. If a woman continually
has this problem of multi-oocyte follicles, then her fertile egg pool will be
significantly diminished and she will likely experience an early end to her
reproductive potential and also possibility an overall decrease in the quality
of eggs produced, potentially leading to miscarriage or health problems for the
offspring.
Oocyte Growth and
Maturation in the Adult Female
Many fertility issues can be linked to how the oocytes
develop in the fetus, though there are also some fertility problems that can
arise in a woman’s prime reproductive years.
Specifically, when meiosis starts up again in the adult after lying
dormant for many years, the oocyte continues to grow until ovulation occurs (85
days in humans). In order for the
follicles and oocytes to grow, several hormones play roles in developing a
healthy egg that is viable and fertilization-worthy. These hormones include pituitary
gonadotropin, estrogen, and hormones in the TGF-β superfamily.
As a woman ages, problems in the meiotic maturation of her
remaining oocytes can continue to increase until she is no longer fertile. Though the mechanism behind which is not
currently agreed upon, FSH hormone levels are shown to rise with age and
possibly by environmental factors.
Researchers have also found that increased FSH levels in women may be
related to increase reproductive failure.
Similarly along those lines, with age come more potential difficulties
or errors in spindle and microtubule formation during meisos, which could lead
to errors in chromosome division and allocation into individual developing egg
cells. In other words, as a woman ages,
and as she is exposed to certain environmental toxins, it becomes much more
difficult to produce and healthy, viable egg, thus leading to trisomies, other
chromosomal disorders, miscarriage, or early menopause.
Summary
This review paper, pulled together by researchers from Washington
State University in Pullman, WA, illustrated just when and where problems can
occur during the maturation of the eggs not only as an adult, but also when in
utero as well. It is important to understand
these mechanisms, as it will give us an opportunity to understand just exactly
how some of these situations where you have a “good egg gone bad” have
connections to environment toxins, such as BPA or other endocrine disruptors.
What does this information tell us about endocrine disruptor
function and fertility in women? Stayed
tuned for a follow up to this post delving into this side of things!
Source:
The most common causes of female infertility include problems with ovulation, damage to Fallopian tubes or uterus etc. You can find the Best Urologist at jerseyurology.
ReplyDeleteThere is no doubt that it is the most important thing for a woman to maintain her health to get prepared for the fertility. So she ought to be more careful in this stage. The best thing she can do is get natural gender selection process.
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