Hundreds of millions of sperm deposited in the vagina travel toward the oocyte, but
only a few hundred actually reach it. The number of sperm that reach the oocyte is
greatly reduced because of conditions within the female reproductive tract. Many sperm
are overcome by the acidity of the vagina, others are blocked by mucus in the cervix,
whereas others are attacked by phagocytic leukocytes in the uterus. Those sperm that do
survive undergo a change in response to those conditions. They go through the process of
capacitation, which improves their motility and alters the membrane surrounding the
acrosome, the cap-like structure in the head of a sperm that contains the digestive
enzymes needed for it to attach to and penetrate the oocyte.

The oocyte that is released
by ovulation is protected by a thick outer layer of granulosa cells known as the corona
radiata and by the zona pellucida, a thick glycoprotein membrane that lies just outside
the oocyte's plasma membrane. When capacitated sperm make contact with the oocyte, they
release the digestive enzymes in the acrosome (the acrosomal reaction) and are thus able
to attach to the oocyte and burrow through to the oocyte's zona pellucida. One of the
sperm will then break through to the oocyte's plasma membrane and release its haploid
nucleus into the oocyte. The oocyte's membrane structure changes in response (cortical
reaction), preventing any further penetration by another sperm and forming a
fertilization membrane. Fertilization is complete upon unification of the haploid nuclei
of the two gametes, producing a diploid zygote.

00:00 Fertilization
00:31 Transit of sperm
02:47 Contact between sperm and oocyte
07:34 The zygote
09:59 Everyday connections
13:52 Chapter review





Attribution: OpenStax, Anatomy & Physiology Book
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