Essential Cell Biology 5th edition

Meiosis and Fertilization
663
gamete at fertilization, the resulting embryo contains three copies of
Chromosome 21 instead of two. This chromosome imbalance produces
an extra dose of the proteins encoded by Chromosome 21 and thereby
interferes with the proper development of the embryo and normal functions
in the adult.
The frequency of chromosome mis-segregation during the production
of human gametes is remarkably high, particularly in females: nondisjunction
occurs in about 10% of the meioses in human oocytes, giving
rise to eggs that contain the wrong number of chromosomes (a condition
called aneuploidy). Aneuploidy occurs less often in human sperm,
perhaps because sperm development is subjected to more stringent quality
control than egg development. If meiosis goes wrong in male cells,
a cell-cycle checkpoint mechanism is activated, arresting meiosis and
leading to cell death by apoptosis. Regardless of whether the segregation
error occurs in the sperm or the egg, nondisjunction is thought to be one
reason for the high rate of miscarriages—spontaneous early pregnancy
losses—in humans.
Fertilization Reconstitutes a Complete Diploid Genome
Having seen how chromosomes are parceled out during meiosis to
form haploid germ cells, we now briefly consider how they are reunited
in the process of fertilization to form a new cell with a diploid set of
chromosomes.
Of the 300 million human sperm ejaculated during sexual intercourse,
only about 200 reach the site of fertilization in the oviduct. Sperm are
attracted to an ovulated egg by chemical signals released by both the
egg and the supporting cells that surround it. Once a sperm finds the
egg, it must migrate through a protective layer of cells and then bind to,
and tunnel through, the egg coat, called the zona pellucida. Finally, the
sperm must bind to and fuse with the underlying egg plasma membrane
(Figure 19−17). Although fertilization normally occurs by this process
of sperm–egg fusion, it can also be achieved artificially by injecting the
sperm directly into the egg cytoplasm; this is often done in infertility clinics
when there is a problem with natural sperm–egg fusion.
Although many sperm may bind to an egg (see Figure 19−2), only one
normally fuses with the egg plasma membrane and introduces its DNA
into the egg cytoplasm. The control of this step is especially important
because it ensures that the fertilized egg—also called a zygote—will
contain two, and only two, sets of chromosomes. Several mechanisms
prevent multiple sperm from entering an egg. In one mechanism, the first
successful sperm triggers the release of a wave of Ca 2+ ions in the egg
cytoplasm. This flood of Ca 2+ in turn triggers the secretion of enzymes
that cause a “hardening” of the zona pellucida, which prevents “runner
up” sperm from penetrating the egg. The Ca 2+ wave also helps trigger the
development of the egg. To watch a fertilization-induced calcium wave,
see Movie 19.2.
The process of fertilization is not complete, however, until the two
haploid nuclei (called pronuclei) come together and combine their chromosomes
into a single diploid nucleus. Soon after the pronuclei fuse,
the diploid cell begins to divide, forming a ball of cells that—through
repeated rounds of cell division and differentiation—will give rise to
an embryo and, eventually, an adult organism. Fertilization marks the
beginning of one of the most remarkable phenomena in all of biology—
the process by which a single-celled zygote initiates the developmental
program that directs the formation of a new individual.
5 µm
Figure 19−17 A sperm binds to the plasma
membrane of an egg. Shown here is a
scanning electron micrograph of a human
sperm coming in contact with a hamster
egg. The egg has been stripped of its zona
pellucida, exposing the plasma membrane,
which is covered in fingerlike microvilli. Such
uncoated hamster eggs were sometimes
used in infertility clinics to assess whether a
man’s sperm were capable of penetrating
an egg. The zygotes resulting from this test
do not develop. ECB5 e19.17/19.17
(David M. Phillips/
The Population Council/Science Source.)