882
UNIT SIX
PRACTICE
6
What is cleavage?
7
How does a blastocyst attach to the endometrium?
8
How does the endometrium respond to the activities of the
blastocyst?
(microvilli) that grow into the endometrium. This nestling
of the blastocyst into the uterine lining is called
implanta-
tion
(im
-plan-ta
shun). It begins near the end of the F
rst
week and is completed during the second week of develop-
ment
(f
g. 23.7)
.
The trophoblast secretes the hormone
human cho-
rionic gonadotropin
(hCG), which maintains the corpus
luteum during the early stages of pregnancy and keeps the
immune system from rejecting the blastocyst. This hormone
also stimulates synthesis of other hormones from the devel-
oping placenta. The
placenta
(plah-sen
tah) is a vascular
structure, formed by the cells surrounding the embryo and
cells of the endometrium, that attaches the embryo to the
uterine wall and exchanges nutrients, gases, and wastes
between the maternal blood and the embryo’s blood.
If an embryo implants in tissues outside the uterus, such as those of a
uterine tube, an ovary, the cervix, or an organ in the abdominal cav-
ity, the result is an
ectopic pregnancy.
A fertilized egg implanted in the
uterine tube is a
tubal pregnancy.
The tube usually ruptures as the
embryo enlarges and causes severe pain and heavy vaginal bleed-
ing, threatening the pregnant woman and the embryo. Treatment is
prompt surgical removal of the embryo and repair or removal of the
damaged uterine tube.
FIGURE 23.6
About the sixth day of development, the blastocyst (
a
)
contacts the uterine wall and (
b
) begins to implant. The trophoblast,
which will help form the placenta, secretes hCG, a hormone that
maintains the pregnancy. (
c
) Light micrograph of a blastocyst from a
monkey in contact with the endometrium of the uterine wall (150×).
FIGURE 23.7
Light micrograph of a human cleavage embryo
(arrow) implanting in the endometrium (18×).
Trophoblast
Blastocyst
Inner cell
mass
Uterine
wall
Invading
trophoblast
(a)
(b)
(c)
Trophoblast
Inner cell mass
Endometrium
Lumen
Endometrium
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