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Uterus
uterus: The uterus is the same organ that is often known by its
old English name of womb. By either name it is the place in a
woman's body where an embryo settles after fertilization of an
egg; it is the place where the embryo grows into a fetus and
the fetus grows into a baby that is then born.
Size and location The uterus is normally a pear-shaped and
pear-sized organ located in the middle of a woman's body at
the top of the birth canal and just above the bladder. During
pregnancy the uterus expands considerably to accommodate the
growing fetus and its accompanying placenta and amniotic fluid.
Much of the bulge that appears during pregnancy is the uterus
itself. The location above the bladder causes pregnant women
to need to urinate more frequently than usual.
Role When an egg, or ovum, is fertilized, it attaches itself
to the lining of the uterus, which is a special tissue called
the endometrium (en-doh-MEE-tree-uhm).
Conditions that affect the uterus Of course the most dramatic
condition that affects the uterus is pregnancy. During women's
reproductive years (roughly from about age 12 to age 50,
although there is considerable individual variation), most
women shed the lining of the uterus in a process called
menstruation during the months that they are not pregnant.
The uterus is subject to several kinds of tumor. Fibroids are
benign tumors that seldom threaten life. Larger or painful
fibroids may be removed by a surgical procedure called
dilatation and curettage (D&C), in which the lining of the
uterus is scraped. If no further pregnancies are desired, the
uterus may be removed. Uterine cancer is second only to
cervical cancer in frequency for cancer of the reproductive
system in women. It is more common in older women who have not
had children. Because uterine cancer grows slowly, it is less
dangerous than many other cancers. Bleeding after menopause
is a common early warning sign. Rarely a tumor may grow in the
placenta during pregnancy, interfering with fetal development.
A common disorder that often requires no treatment is
inflammation or irritation of the lining of the uterus. This
condition, called endometriosis, may be treated with D&C.
Infection of the uterus, which often spreads to other
reproductive organs, is called pelvic inflammatory disease (PID).
The muscles that hold the uterus and vagina in place sometimes
become weakened by childbirth or by age. If the uterus begins
to sag, it may bulge into the wall of the vagina, a condition
called prolapse or fallen womb.
embryo: The embryo is a very early stage in the development of
a new individual, but people from different medical and
scientific specialties use the word to mean slightly different
stages.
Although many people would say that an individual is conceived
as soon as an ovum (egg) is fertilized by a sperm cell, the
fertilized egg is called a zygote (ZY-goht), not an embryo. When
a zygote divides into a pair of joined cells, the organism
begins to be called an embryo by workers in the field of in vitro
fertilization (also called "test tube fertilization"), but not
by other physicians or most biologists. During in vitro
fertilization or storage before implantation, a bundle of cells
that has grown to a number of cells (32, usually) that would
make it a candidate for implantation in a prospective mother
continues to be called an embryo. At this early stage, however,
the small group of cells is called a blastocyst (the cells are
called blastomeres) by most biologists.
When a mammalian blastocyst implants itself in the uterine wall,
it is called an embryo by biologists and all physicians. After
implantation in the uterus, the growing cell mass continues to be
known as an embryo for several weeks. Some writers wait until
cells begin to differentiate into recognizable tissues to use the
term embryo. After the shape of the new individual begins to
emerge and organs begin to develop, the term fetus is introduced.
The developing baby is then known as a fetus until birth.
Size and location An embryo is barely visible as a speck at first,
when it has just lodged in the uterine wall. There it grows for
a month or two to become an inch or two long. At that time, when
the embryo has become a fetus, the uterus has swollen, making it
possible to feel a slight bulge through the abdominal wall of the
mother-to-be.
Role Much has been learned in recent years about the roles of
specific genes and proteins in early development, although most of
the work has been with fruit flies and nematodes instead of with
humans. The first stage of development occurs in the zygote, when
some internal separation of cell parts and material occurs before
the first division into two cells. As a result, the two cells in
the first stage of the blastocyst are different from each other.
These differences cause further cells also to be individualized in
their structure and operation, leading eventually to the specialized
tissues of the fetus.
Conditions that affect the embryo Because the entire plan of
development depends on correct differentiation of cells in early
stages, an error in embryonic development usually leads to natural
abortion, commonly called miscarriage. Miscarriage is common; one
estimate is that it occurs in one out of ten pregnancies. The
defective embryo is expelled and pregnancy is terminated, usually
with no harm to the pregnant woman. However, complications can occur
if part of the embryo or the placenta, the organ that develops to
connect the embryo to its mother, remains in the uterus. Save solid
materials expelled during a miscarriage for a physician to examine.
Sometimes the embryo fails to implant in the uterine wall but
continues to develop in a different location, most often one of the
tubes that connects the ovaries to the uterus. This situation is
called an ectopic pregnancy. No signs of pregnancy appear, but
abdominal pain continues hour after hour. Abdominal pain in a woman
of childbearing age requires immediate medical assistance. If the
problem is an ectopic pregnancy, surgery will be required.
placenta: The developing fetus is nourished by the only temporary
organ in the body, the placenta (pluh-SEN-tuh) (from the Greek word
for "cake," so called because it is broad and flat like a cake).
Size and location The placenta occupies the uterus along with the
fetus. It is attached to the uterus by the umbilical cord, which
brings nutrients and oxygen from the placenta to the fetus and
carries wastes from the fetus to the placenta.
Role The placenta has many functions, including the production of
hormones, so it is an endocrine gland. It is formed from tissue from
both the embryo and the uterus. The placenta is rich in blood vessels.
The wall of the uterus is also filled with many blood vessels. The
capillaries are not connected between the two systems, but lie close
enough together that oxygen, glucose, and other necessities can flow
from the mother's bloodstream into the bloodstream of the placenta.
Similarly, wastes from the fetus, such as carbon dioxide and urea,
are transferred from the placenta to the mother's blood.
When the baby is fully formed, the cervix opens and the baby passes
through the vagina. The placenta also passes through the vagina as
afterbirth and is discarded.
Conditions that affect the placenta Sometimes a tumor, called a
trophoblastic (trof-uh-BLAS-tic) tumor,tumor develops on the placenta.
This may be a kind of benign tumor called a hydatid (HY-duh-tid) mole
or it may be cancer,Cancers. Whether benign or cancerous, such a tumor
must be removed and the pregnancy terminated. In the case of a hydatid
mole, spontaneous abortion may be the first sign of the problem.
Consult a physician without delay for irregular vaginal bleeding or
excess nausea and vomiting during pregnancy.
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