628
Chapter 17
acrosomal enzymes are now exposed to the outside—that is,
to the zona pellucida. The enzymes digest a path through the
zona pellucida as the sperm, using its tail, advances through
this coating. The fi rst sperm to penetrate the entire zona pel-
lucida and reach the egg’s plasma membrane fuses with this
membrane. The head of the sperm then slowly passes into the
egg’s cytoplasm.
Viability of the newly fertilized egg, now called a
zygote,
depends upon preventing the entry of additional sperm. A spe-
cifi c mechanism mediates this
block to polyspermy.
The ini-
tial fusion of the sperm and egg plasma membranes triggers
a reaction that changes membrane potential, preventing addi-
tional sperm from binding. Subsequently, during the
cortical
reaction,
cytosolic secretory vesicles located around the egg’s
periphery release their contents, by exocytosis, into the narrow
space between the egg plasma membrane and the zona pellu-
cida. Some of these molecules are enzymes that enter the zona
pellucida and cause both inactivation of its sperm-binding
sites and hardening of the entire zona pellucida. This prevents
additional sperm from binding to the zona pellucida and those
sperm already advancing through it from continuing.
The fertilized egg completes its second meiotic division
over the next few hours, and the one daughter cell with practi-
cally no cytoplasm—the second polar body—is extruded and
disintegrates. The two sets of chromosomes—23 from the
egg and 23 from the sperm, which are surrounded by distinct
membranes and are known as pronuclei—migrate to the cen-
ter of the cell. During this period of a few hours, the DNA
of the chromosomes in both pronuclei is replicated, the pro-
nuclear membranes break down, the cell is ready to undergo
a mitotic division, and fertilization is complete. Fertilization
also triggers activation of the egg enzymes required for the
ensuing cell divisions and embryogenesis. The major events of
fertilization are summarized in
Figure 17–24
. If fertilization
had not occurred, the egg would have slowly disintegrated and
been phagocytized by cells lining the uterus.
Rarely, a fertilized egg remains in a fallopian tube and
embeds itself in the tube wall. Even more rarely, a fertilized
egg may move backwards out of the fallopian tube into the
abdominal cavity, where implantation can occur. Both kinds
of
ectopic pregnancies
cannot succeed, and surgery is neces-
sary to end the pregnancy (unless there is a spontaneous abor-
tion) because of the risk of maternal hemorrhage.
Early Development, Implantation,
and Placentation
The events from the LH surge to the rescue of the corpus
luteum are summarized in
Table 17–9
. The
conceptus
—a col-
lective term for everything ultimately derived from the original
zygote (fertilized egg) throughout the pregnancy—remains in
the fallopian tube for three to four days. The major reason is
that estrogen maintains the contraction of the smooth muscle
near where the fallopian tube enters the wall of the uterus. As
plasma progesterone levels rise, this smooth muscle relaxes and
allows the conceptus to pass. During its stay in the fallopian
tube, the conceptus undergoes a number of mitotic cell divi-
sions, a process known as
cleavage.
These divisions, however,
are unusual in that no cell growth occurs before each division.
Zygote begins
embryogenesis
Nuclei of sperm
and egg unite
Egg enzymes
are activated
Block to
polyspermy
occurs
Egg completes
2nd meiotic
division
Enzymes enter
zona pellucida
Sperm is drawn
into egg
Egg releases
contents of
secretory vesicles
One sperm binds to egg
plasma membrane
Sperm move through
zona pellucida
Many sperm bind to receptors
on the zona pellucida and
undergo the acrosome reaction
Begin
Figure 17–24
Events leading to fertilization, block to polyspermy, and the
beginning of embryogenesis.
Table 17–9
Summary of Events from Ovulation
and Fertilization to Implantation
Days After
LH Peak
Event
Location
1O
v
u
l
a
t
i
o
n
O
v
a
r
y
2
Fertilization
Fallopian tube
2–4
Cell division to ~32 cells
Fallopian tube
5
Blastocyst enters the uterine
cavity
Uterus
6–7
Implantation
Uterus
9–10
Human chorionic
gonadotropin (hCG)
from implanted blastocyst
(trophoblast cells) rescues
corpus luteum
(see Figure 17–29)
Trophoblast
maternal ovary
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