1066
UNIT 5
Continuity
28
involves the release of acrosomal enzymes (hyaluronidase, acro-
sin, proteases, and others) that digest holes through the zona
pellucida. Hundreds of acrosomes must undergo exocytosis to
digest holes in the zona pellucida. Tis is one case that does not
bear out the adage, “Te early bird catches the worm.” A sperm
that comes along later, aFer hundreds of sperm have undergone
acrosomal reactions to expose the oocyte membrane, is in the
best position to be
the
fertilizing sperm.
Once a path is cleared, the sperm’s whiplike tail gyrates, forc-
ing the sperm’s head to rock and move toward the oocyte mem-
brane. Tere the sperm’s postacrosomal “collar,” the rear portion
of the acrosomal membrane, binds to the oocyte’s plasma mem-
brane receptors (±igure 28.2
3
). Tis binding event has two
consequences. (a) It causes the oocyte to form microvilli that
surround the sperm head, and the sperm and oocyte mem-
branes to fuse (±igure 28.2
4
), and then (b) like a snake
crawling out of its skin, the cytoplasmic contents of the sperm
enter the oocyte, leaving the sperm’s plasma membrane behind
(±igure 28.2
5
). Te gametes fuse together with such perfect
contact that the contents of both cells are combined within a
single membrane—all without spilling a drop.
Block to Polyspermy
Polyspermy
(entry of several sperm into an egg) occurs in some
animals, but in humans only one sperm is allowed to penetrate
the oocyte, ensuring
monospermy
, the one-sperm-per-oocyte
condition. Once the sperm head has entered the oocyte, waves
of Ca
2
1
are released by the oocyte’s endoplasmic reticulum into
its cytoplasm, which activates the oocyte to prepare for the
second meiotic division. Tese calcium surges also cause the
cortical reaction
(±igure 28.2
5
), in which granules located
just inside the plasma membrane spill their enzymes into the
extracellular space beneath the zona pellucida. Tese enzymes,
called
zonal inhibiting proteins
(
ZIPs
), destroy the sperm recep-
tors, preventing any more sperm from entering.
Additionally, the spilled material binds water, and as
the material swells and hardens, it detaches all sperm still
bound to receptors on the oocyte membrane, completing
the so-called
slow block to polyspermy
. In the rare cases of
polyspermy that do occur, the embryos contain too much ge-
netic material and die.
Completion of Meiosis II and Fertilization
As the sperm’s cytoplasmic contents enter the oocyte, the sperm
loses its plasma membrane. Te centrosome from its midpiece
elaborates microtubules which the sperm uses to locomote its
DNA-rich nucleus toward the oocyte nucleus. On the way, its
nucleus swells to about five times its normal size to form the
male pronucleus
(pro-nu
9
kle-us;
pro
5
before). Meanwhile
the secondary oocyte, activated from its semidormant state by the
calcium surges, completes meiosis II, forming the ovum nucleus
and the second polar body (
Figure 28.3a
1
and
2
, p. 1070).
Tis accomplished, the ovum nucleus swells, becoming the
female pronucleus
, and the two pronuclei approach each other.
remaining, millions more are destroyed by the vagina’s acidic
environment. Millions more fail to make it through the cervix,
unless the thick “curtain” of cervical mucus has been made fluid
by estrogens.
Sperm that do reach the uterus, propelled by their whiplike
tail movements, are then subjected to forceful uterine contrac-
tions that act in a washing machine–like manner to disperse
them throughout the uterine cavity, where thousands more are
destroyed by resident phagocytes. Only a few thousand (and
sometimes fewer than 100) sperm, out of the millions in the
male ejaculate, are conducted by reverse peristalsis into the
uterine tube, where the oocyte may be moving leisurely toward
the uterus.
Tese difficulties aside, there is still another hurdle to over-
come. Sperm freshly deposited in the vagina are incapable of
penetrating an oocyte. Tey must first be
capacitated
over
the next 2 to 10 hours. Specifically, their motility must be
enhanced and their membranes must become fragile so that
the hydrolytic enzymes in their acrosomes can be released.
As sperm swim through the cervical mucus, uterus, and uter-
ine tubes, secretions of the female tract remove some of their
protective membrane proteins, and the cholesterol that keeps
their acrosomal membranes “tough” and stable is depleted.
Even though the sperm may reach the oocyte within a few
minutes, they must “wait around” (so to speak) for capacita-
tion to occur.
Tis elaborate mechanism prevents the spilling of acrosomal
enzymes. But consider the alternative. ±ragile acrosomal mem-
branes could rupture prematurely in the male reproductive
tract, causing some degree of autolysis (self-digestion) of the
male reproductive organs.
How do sperm navigate to find a released oocyte in the
uterine tube? Tis question is an area of active research. It now
appears that they “sniff” their way to the oocyte. Sperm bear
proteins called
olfactory receptors
that respond to chemical
stimuli. It is presumed that the oocyte or its surrounding cells
release signaling molecules that direct the sperm.
Acrosomal Reaction and Sperm Penetration
Te ovulated oocyte is encapsulated by the corona radiata and by
the deeper zona pellucida, a transparent layer of glycoprotein-
rich extracellular matrix secreted by the oocyte. Both must be
breached before the oocyte can be penetrated. Once in the im-
mediate vicinity of the oocyte, a sperm weaves its way through
the cells of the corona radiata. Tis journey is assisted by a cell-
surface hyaluronidase on the sperm that digests the intercellular
cement between the granulosa cells in the local area, causing
them to fall away from the oocyte (
Focus on Sperm Penetration
and the Cortical Reaction
,
Figure 28.2
1
, p. 1068).
AFer breaching the corona, the sperm head binds to a ZP3
glycoprotein of the zona pellucida, which functions as a sperm
receptor. Tis binding opens calcium channels, which leads to a
rise in Ca
2
1
inside the sperm that triggers the acrosomal (ak
0
ro-
so
9
mal) reaction (±igure 28.2
2
). Te
acrosomal reaction
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