UTERUS
Figure 17­25
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Holdings,
The uterus is a muscular organ that functions to provide a protective and nutritive environment for the developing
fetus. At term, contractions of the smooth muscle in its wall initiate passage of the fetus from the uterus into the
vaginal canal.
LLC.
Not
There are two components of the wall of the uterus. An inner endometrium undergoes cyclical changes in response
to
to pituitary and ovarian hormones and maintains the embryo and fetus during development. An outer myometrium
be
is composed of interlacing bundles of smooth muscle fibers that hypertrophy during pregnancy, allowing for expan-
redistributed
sion of the uterus to accommodate the growing fetus. Connective tissue cells and matrix, blood vessels, and nerve
fibers are located in both regions of the uterine wall.
or
The morphology of the endometrium changes during the menstrual cycle. It increases in height during the first part
modified
of the ovarian cycle, is maintained for a while, and then is reduced to a basal level if the ovum is not fertilized.
There are at least three stages of endometrial cycling. Following the cessation of menstruation, the endometrium
goes through a proliferative phase in which glands and blood vessels increase in size; this occurs under the influ-
in
any
ence of estrogen from the ovary. During the next phase, the secretory phase, the endometrium attains its greatest
way
height and is capable of supporting the fetus should fertilization and implantation occur. The maintenance of the
endometrium is promoted by progesterone from the corpus luteum. Should fertilization not occur, the endometrium
without
is sloughed off in the menstrual phase and is reduced to a small area of tissue adjacent to the myometrium.
permission.
This micrograph shows the endometrium and myometrium following the end of menstruation as the cells of the en-
dometrium proliferate under the influence of ovarian estrogen. The endometrium consists of two areas: a basal lay-
er, the decidua basalis, and a functional layer, the decidua functionalis. The functionalis is lost in the menstrual flow.
The right center of the image illustrates the elongated decidua functionalis of the endometrium. The region of the
endometrium adjacent to the myometrium is the decidua basalis (arrow). A portion of the myometrium is located to
the left of the arrow. The endometrium is lined by a simple columnar epithelial tissue that also lines the endometrial
glands. The epithelium will be more clearly distinguished in subsequent images. Spiral arteries also persist in the
decidua basalis and, during the proliferative phase, lengthen to accompany the glands.
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