Human Physiology, 14th edition (2016)

Reproduction 735

be released by exocytosis. These enzymes, including a protein-

digesting enzyme and hyaluronidase (which digests hyaluronic

acid, a constituent of the extracellular matrix), allow the sperm

to digest a path through the zona pellucida to the oocyte.

Fertilization, acting through a second messenger (inositol

triphosphate), stimulates the endoplasmic reticulum of the oocyte

to release its stored Ca^2 1. This causes a rise in cytoplasmic Ca^2 1

that spreads from the point of sperm entry to the opposite pole of

in sperm pH also activates unique Ca^2 1 channels located in the
flagellum’s principal piece (see fig. 20.18 ). These complex chan-
nels, known as CatSper (cationic channel of sperm), are gated by
the rise in pH to allow Ca^2 1 to enter the sperm. This rise in sperm
Ca^2 1 then causes hyperactivation: the flagellum beats in higher-
amplitude, asymmetrical, more whiplike motions to produce more
forceful swimming. The capacitated sperm are guided in their pas-
sage up the oviduct toward the ovum by chemotaxis (attraction
toward particular chemicals) and thermotaxis (attraction toward
the warmer temperatures higher in the oviduct).
A woman usually ovulates only one ovum a month, for a
total of less than 450 ova during her reproductive years. Each
ovulation releases a secondary oocyte arrested at metaphase of
the second meiotic division. The secondary oocyte, as previously
described, enters the uterine tube surrounded by its zona pellu-
cida (a thin transparent layer of protein and polysaccharides) and
corona radiata of granulosa cells ( fig. 20.37 ).

Fertilization

Fertilization normally occurs in the uterine tubes and begins when
the sperm binds to specific carbohydrates in the glycoproteins of
the zona pellucida. In this process, the sperm are exposed to pro-
gesterone, which is secreted by the cells of the corona radiata that
surround the ovum. Progesterone rapidly activates the CatSper
channels (previously discussed) in the sperm head, permitting the
entry of Ca^2 1. This action of progesterone appears to be too rapid
for the usual genomic action of steroids (chapter 11, section 11.2),
and may involve binding of progesterone to the CatSper chan-
nels or associated molecules in the membrane. Entry of Ca^2 1 is
required for the acrosomal reaction, discussed next.
Each sperm contains a large, enzyme-filled vesicle above
its nucleus known as an acrosome ( fig.  20.38 ). Binding of the
sperm with the zona pellucida triggers the entry of Ca^2 1 and
the acrosome reaction. This involves the progressive fusion
of the acrosomal membrane with the plasma membrane of the
sperm, creating pores through which the acrosomal enzymes can

Sperm cell nucleus

inside ovum

Acrosome

containing

enzymes

Zona pellucida

Nucleus

containing

chromosomes

Corona

radiata

Ovum

Figure 20.37 The process of fertilization. As the

head of the sperm cell encounters the gelatinous corona radiata

of the secondary oocyte, the acrosomal vesicle ruptures and the

sperm cell digests a path for itself by the action of the enzymes

released from the acrosome. When the plasma membrane of the

sperm cell contacts the plasma membrane of the ovum, they

become continuous, and the nucleus of the sperm cell moves

into the cytoplasm of the ovum.

Figure 20.38 The

acrosome reaction. ( a ) Prior to

activation, the acro some is a large,

enzyme-containing vesicle over

the sperm nucleus. ( b ) After the

sperm binds to particular proteins

in the zona pellucida surrounding

the egg, the acrosomal membrane

fuses with the plasma membrane in

many locations, creating openings

through which the acrosomal

contents can be released by

exocytosis. ( c ) When the process

is complete, the inner acrosomal

membrane has become continuous

with the plasma membrane.

Before

activation

Binding to zona

pellucida stimulates

fusion and exocytosis

After acrosomal

reaction

Plasma
membrane

Plasma

membrane

Outer
acrosomal
membrane
Acrosome
Inner
acrosomal
membrane

Inner

acrosomal

membrane

Nucleus

Fusion

Acrosomal

contents

(a) (b) (c)