Chapter 28
Pregnancy and Human Development
1071
28
Check Your Understanding
5.
Which portion of the trophoblast accomplishes implantation?
6.
Marie, wondering if she is pregnant, buys an over-the-
counter pregnancy test to assess this possibility. How will the
blastocyst, if present, “make itself known”?
For answers, see Appendix H.
Placentation
Describe placenta formation, and list placental functions.
Placentation
(plas
0
en-ta
9
shun) refers to the formation of a
placenta
(“flat cake”), a temporary organ that originates from both embry-
onic and maternal (endometrial) tissues. Cells from the original in-
ner cell mass give rise to a layer of extraembryonic mesoderm that
of the second month. Ten, blood levels decline sharply to
reach a low value by 4 months, a situation that persists for
the remainder of gestation
(Figure 28.6)
. Between the second
and third month, the placenta (which we describe next) as-
sumes the role of progesterone and estrogen production for
the remainder of the pregnancy. Te corpus luteum then de-
generates and the ovaries remain inactive until aFer birth. All
pregnancy tests used today are antibody tests that detect hCG
in a woman’s blood or urine.
Initially, the implanted embryo obtains its nutrition by di-
gesting the endometrial cells, but by the second month, the
placenta is providing nutrients and oxygen to the embryo and
carrying away embryonic metabolic wastes. Since placenta for-
mation is a continuation of the events of implantation, we will
consider it next, although we will be getting a little ahead of our-
selves as far as embryonic development is concerned.
(a) Zygote
(fertilized egg)
(b) 4-cell stage
2 days
(c) Morula
(a solid ball
of blastomeres).
3 days
(d) Early blastocyst
(Morula hollows out,
fills with fluid, and
“hatches” from the
zona pellucida).
4 days
Blastocyst
cavity
Degenerating
zona
pellucida
Zona
pellucida
Inner cell
mass
Blastocyst
cavity
Trophoblast
(e) Implanting blastocyst
(Consists of a sphere
of trophoblast cells and
an eccentric cell cluster
called the inner cell
mass). 7 days
Cavity of
uterus
Uterus
Endometrium
Ovulation
Ovary
Fertilization
(sperm
meets and
enters egg)
Uterine
tube
Oocyte
(egg)
Sperm
Figure 28.4
Cleavage: From zygote
to blastocyst.
The zygote begins to divide
about 24 hours after fertilization, and
continues the more rapid mitotic divisions
of cleavage as it travels down the uterine
tube. Three to four days after ovulation, the
embryo reaches the uterus and floats freely
for two to three days, nourished by secretions
of the endometrial glands. Because there
is little time for growth between successive
cleavage divisions, the resulting blastocyst is
only slightly larger than the zygote. At the
late blastocyst stage, the embryo implants
into the endometrium; this begins at about
day 7 after ovulation.
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