Chapter 28
Pregnancy and Human Development
1067
28
cells that form a berry-shaped cluster of 16 or more cells called
the
morula
(mor
9
u-lah; “little mulberry”) has been formed
(Figure 28.4c). All the while, transport of the embryo toward
the uterus continues.
By day 4 or 5 a±er fertilization, the embryo consists of about
100 cells, has begun accumulating fluid within an internal cav-
ity, and floats free in the uterus (Figure 28.4d). Te zona pel-
lucida now starts to break down and the inner structure, now
called a blastocyst, “hatches” from it. Te
blastocyst
(blas
9
to-
sist) is a fluid-filled hollow sphere composed of a single layer of
large, flattened cells called
trophoblast
cells
(tro
9
fo-blast) and
a small cluster of 20 to 30 rounded cells, called the
inner cell
mass
, located at one side (Figure 28.4e).
Soon a±er the blastocyst forms, trophoblast cells begin to dis-
play L-selectin adhesion molecules on their surface. Tey take
part in placenta formation, as suggested by the literal translation
of “trophoblast” (nourishment generator). Tey also secrete and
display several factors with immunosuppressive effects that pro-
tect the trophoblast (and the developing embryo) from attack
by the mother’s cells.
Te inner cell mass becomes the
embryonic disc
, which forms
the embryo proper, and three of the four extraembryonic mem-
branes. (Te fourth membrane, the chorion, is a trophoblast
derivative.)
Check Your Understanding
3.
Why is the multicellular blastocyst only slightly larger than
the single-celled zygote?
4.
What is the function of the trophoblast cells?
For answers, see Appendix H.
Implantation
Describe implantation.
While the blastocyst floats in the uterine cavity for two to three
days, it is nourished by the glycoprotein-rich uterine secretions,
which also contain steroids and various nutrients including iron
and fat-soluble vitamins. Ten, some six to seven days a±er ovu-
lation, given a properly prepared endometrium,
implantation
begins. Te receptivity of the endometrium to implantation—
the so-called
window of implantation
—is opened by the surging
levels of ovarian hormones (estrogens and progesterone) in the
blood. If the mucosa is properly prepared, integrin and selectin
proteins on the trophoblast cells bind respectively to the extra-
cellular matrix components (collagen, fibronectin, laminin, and
others) of the endometrial cells and to selectin-binding carbohy-
drates on the inner uterine wall, and the blastocyst implants high
in the uterus. If the endometrium is not yet optimally mature, the
blastocyst detaches and floats to a lower level, implanting when it
finds a site with the proper receptors and chemical signals.
As a mitotic spindle develops between them (Figure 28.3a
3
),
the pronuclei membranes rupture, releasing their chromosomes
together into the immediate vicinity of the newly formed spindle.
Te true moment of fertilization occurs as the maternal and
paternal chromosomes combine and produce the diploid
zy-
gote
, or fertilized egg (Figure 28.3a
4
). Some sources define
the term fertilization simply as the act of oocyte penetration
by the sperm. However, unless the chromosomes in the male
and female pronuclei are actually combined, the zygote is never
formed in humans. Almost as soon as the male and female pro-
nuclei come together, their chromosomes replicate. Te zygote,
the first cell of a new individual, is now ready to undergo the
first mitotic division of the conceptus.
Check Your Understanding
1.
What has to happen before ejaculated sperm can penetrate
an oocyte?
2.
What is the cortical reaction and what does it accomplish?
For answers, see Appendix H.
Events of Embryonic
Development: Zygote to
Blastocyst Implantation
Early embryonic development begins with fertilization and
continues as the embryo travels through the uterine tube, floats
free in the cavity of the uterus, and finally implants in the uter-
ine wall. Significant events of this early embryonic period are
cleavage,
which produces a structure called a blastocyst, and
implantation
of the blastocyst.
Cleavage and Blastocyst Formation
Describe the process and product of cleavage.
Cleavage
is a period of fairly rapid mitotic divisions of the zy-
gote without intervening growth (
Figure 28.4
, p. 1071). Te
goal of this first phase of development is to produce small cells
with a high surface-to-volume ratio, which enhances their up-
take of nutrients and oxygen and the disposal of wastes. It also
provides a large number of cells to serve as building blocks for
constructing the embryo. Consider, for a moment, the difficulty
of trying to construct a building from one huge block of granite.
If you now consider how much easier it would be if instead you
could use hundreds of bricks, you will quickly grasp the impor-
tance of cleavage.
Some 36 hours a±er fertilization, the first cleavage division of
the zygote has produced two identical cells called
blastomeres
.
Tese divide to produce four cells (Figure 28.4b), then eight,
and so on. By 72 hours a±er fertilization, a loose collection of
(Text continues on p. 1070.)
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