All of the immature egg cells
(oocytes) a woman will ever produce are stored
in the ovaries by the time she is born. The average
age that girls begin to menstruate is 12 years
old. Each menstrual cycle occurs approximately
every 28 days.
During each cycle, hormonal messages
from the brain cause the ovaries to develop a
single mature egg cell for potential fertilization,
even as other hormones instruct the uterine lining
to thicken in preparation for nourishing the
fertilized egg cell. The cycle starts when a
follicle grows within one of the ovaries. A follicle
is composed of the developing egg cell and the
support cells that surround and nourish it.
On
day 1 of the cycle, a small structure in the
brain, the pituitary gland, releases two hormones,
FSH and LH, both of which cause the follicle
to begin growing. Over the next 13 days, the
growing follicle releases estrogen, a hormone
that prepares the lining of the uterus to receive
a fertilized egg cell. Meanwhile, the estrogen
in the bloodstream causes the brain to release
a surge of LH. In response to the LH surge, the
follicle enlarges rapidly.
On day 14, it ruptures
and releases the egg cell in a process known
as ovulation. The ruptured follicle begins secreting
the hormone progesterone, which also helps to
prepare the uterine lining for a fertilized egg
cell. The egg cell is swept into the fallopian
tube entrance by its waving structures called
fimbriae. Once the egg cell is within the fallopian
tube, it will either be fertilized by a sperm
cell, or fertilization will fail to take place.
If the egg cell is not fertilized within 24 hours
after its release from the ovary, it will stop
developing and will dissolve before reaching
the uterus.
The absence of a fertilized egg cell
gradually causes a woman’s body to stop
releasing the hormones that would otherwise prepare
the uterus for the developing egg cell. In response,
the uterus sheds its lining on days 24 through
28 during menstruation. If a sperm does fertilize
the egg cell, tiny hair-like cells called cilia
will transport it towards the uterus. The fertilized
egg now called a blastocyst, lodges in the uterine
wall in a process called implantation to receive
nourishment from the uterine lining. The remaining
cells of the ruptured follicle in the ovary produce
progesterone so that the uterine lining will
stay rich in blood vessels, and the fertilized
egg cell will survive.
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