ovaries also secrete estrogen.
The twoFallopian tubesare narrow passages that open off the uterus. Each tube reaches
one of the ovaries. However, the tubes are not attached to the ovaries. Notice inFigure
22.3that the end of each Fallopian tube by the ovary has “fingers.” They sweep an egg into
the Fallopian tube. Then the egg passes through the Fallopian tube to the uterus.
Eggs and Egg Production
When a baby girl is born, her ovaries contain all the eggs they will ever produce. However,
the eggs are not fully developed. They develop only after she starts having menstrual periods
at about age 12 or 13. Just one egg develops each month. This usually continues until a
woman is in her 40s.
Eggs
Eggs are very big cells. In fact, they are the biggest cells in the human body. An egg is
about 30 times as wide as a sperm cell. It is large enough to see without a microscope. Like
a sperm cell, the egg contains a nucleus with half the number of chromosomes as other body
cells. Unlike a sperm cell, the egg contains a lot of cytoplasm, which is why it is so big. The
egg also does not have a tail.
Egg Production
Egg production takes place in the ovaries. The process occurs in several steps. It begins
before a girl is born. Before birth, special cells in the ovaries undergo mitosis. The daughter
cells then start to divide by meiosis. However, they only go through the first of the two
cell divisions of meiosis at that time. They remain in this state until the female has gone
through puberty.
In a mature female, an egg develops in an ovary about once a month. The drawing inFigure
22.4shows how this happens. As you can see from the figure, the egg is enclosed in a nest of
cells called afollicle. The follicle and egg grow larger and go through other changes. After
a couple of weeks, the egg bursts out of the follicle and through the wall of the ovary. This
is calledovulation. The moving fingers of the nearby Fallopian tube sweep the egg into the
tube. The empty follicle then changes into a structure called acorpus luteum.
KEY:
- Undeveloped eggs
- Egg and follicle developing
- Egg and follicle developing