Ovulation is a part of the menstrual cycle
when the ovary releases a ripe egg, or ovum. Inside the ovary are hundreds
of thousands of follicles. Each follicle is a hollow ball of cells
with an immature egg in the center. The typical 28-day
menstrual cycle begins on the first day
of menstrual bleeding. During the first
7 days of the cycle, a few follicles begin
to grow at the same time. These maturing follicles secrete
estrogen hormone into the blood stream to prepare the lining
of the uterus for pregnancy. Around Day 7, all of the
follicles stop growing, and begin to degenerate,
except for one. This dominant follicle
continues to grow and nourishes the
developing egg inside it. Around Day 12, the follicle secretes a large
amount of estrogen into the blood stream. When the estrogen reaches the hypothalamus
and the pituitary gland in the brain, the anterior, or front part,
of the pituitary gland releases a huge surge
of luteinizing hormone into the blood stream. Around day 14,
luteinizing hormone causes the follicle
to undergo a sudden growth spurt. Right before ovulation, the egg detaches from
the inside of the follicle. The bulging follicle
releases chemicals, causing one of the
two fallopian tubes to move in closer
and surround the follicle. The follicle swells until
it bursts open, ejecting the egg and
fluid from the follicle into the abdominal cavity. In response, the fimbriae,
tiny projections at the end of
the fallopian tube, sweep across the ovulation site
and pick up the egg. Microscopic cilia
on the fimbriae’s surface transport the egg to
the entrance of the fallopian tube. Inside the walls of
the fallopian tube, muscular contractions gently push
the egg toward the uterus. After ovulation, the egg
lives for 12-24 hours, so it must be fertilized
by a sperm from the male during this time for a
woman to become pregnant. If it is not fertilized,
the egg dissolves away and is shed along with the
uterine lining during menstruation.