Reproductive System
The head portion of one sperm cell enters the second-
ary oocyte, leaving the mitochondria-rich middle section
and tail outside. This action triggers lysosome-like vesicles
just beneath the oocyte cell membrane to release enzymes
that harden the zona pellucida. This reduces the chance that
other sperm cells will penetrate.
The sperm nucleus enters the secondary oocyte’s cyto-
plasm and swells. The secondary oocyte then divides
unequally to form a large cell, whose nucleus contains the
female’s genetic contribution, and a tiny second polar body,
Sperm movement is inefF cient. Even though as many as
200 million to 600 million sperm cells may be deposited in the
vagina by a single ejaculation, only a few hundred ever reach
a secondary oocyte. The journey to the upper part of the uter-
ine tube takes less than an hour following sexual intercourse.
Many sperm cells may reach a secondary oocyte, but usually
only one sperm cell fertilizes it
(f g. 23.2)
. If a second sperm
were to enter, the fertilized ovum would have three sets of
chromosomes and be very unlikely to develop. About one in
a million births produces a severely deformed child who has
inherited three sets of chromosomes.
A secondary oocyte may survive for only twelve to
twenty-four hours following ovulation. In contrast, sperm
cells may survive up to six days, but only have the power to
fertilize an oocyte within twenty-four to forty-eight hours after
entering the female reproductive tract. Consequently, sexual
intercourse probably should occur not earlier than forty-eight
hours before ovulation, or within twenty-four hours follow-
ing ovulation if fertilization is to take place. ±rom Science to
Technology 23.1 describes assisted routes to conception.
When a sperm reaches a secondary oocyte, it invades the
follicular cells that adhere to the oocyte’s surface (corona
radiata) and binds to the
zona pellucida
that surrounds
the oocyte’s cell membrane. The acrosome of a sperm cell
releases enzymes (including hyaluronidase) that aid pen-
etration of the sperm head by digesting proteins in the zona
(f g. 23.3)
Scanning electron micrograph of sperm cells on the
surface of an egg cell (1,200×). For normal development to proceed,
only one sperm cell can fertilize the egg cell.
Egg cell
Path of egg cell
Sperm cells
Semen deposited in vagina
during sexual intercourse
Body of uterus
Path of
sperm cells
The paths of the egg
and sperm cells through the female
reproductive tract.
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