which is later expelled. Meiosis is completed. The approach-
ing nuclei from the two sex cells are called pronuclei, until
they meet and merge. Next, the pronuclei unite. Their
nuclear membranes disassemble, and their chromosomes
mingle, completing the process of fertilization.
Each sex cell provides 23 chromosomes, so the prod-
uct of fertilization is a cell with 46 chromosomes—the usual
number in a human body cell. This cell, called a
¯t), is the F
rst cell of the future offspring.
What factors aid the movements of the secondary oocyte and
sperm cells through the female reproductive tract?
Where in the female reproductive system does fertilization
normally take place?
List the events of fertilization.
onception requires the meeting and
merging of sperm cell and egg cell, which
naturally occurs in the woman’s uter-
ine tube. Abnormal gametes or blockages that
impede this meeting of cells can result in infer-
tility (see Clinical Applications 22.2 and 22.3,
pp. 842–843, 861). Assisted reproductive technolo-
gies (ART) can help couples conceive. The proce-
dures usually involve a laboratory technique and
sometimes participation of a third individual. These
techniques are often costly and may take several
attempts, and some have very low success rates.
Most ARTs were developed in nonhuman
animals. For example, the ±
rst intrauterine insem-
inations were performed in dogs in 1782, and the
first successful
in vitro
fertilization was accom-
plished in 1959, in a rabbit. ARTs are now com-
monplace. In the United States about 1% of the 4
million or so births each year used an ART. Here is
a look at some of the procedures.
Donated Sperm—Intrauterine
In intrauterine (“arti±
cial”) insemination, a doctor
places donated sperm in a woman’s reproductive
tract. A woman might seek intrauterine insemi-
nation if her partner is infertile or carries a gene
for an inherited illness that could a²
ect a child’s
health, or if she desires to be a single parent.
Millions of babies have been born worldwide as
a result of this procedure. The ±
rst human intra-
uterine inseminations by donor were done in the
1890s. For many years, physicians donated sperm,
and this became a way for male medical students
to earn a few extra dollars. By 1953, sperm could
be frozen and stored for later use. Today, sperm
banks freeze and store donated sperm and then
provide it to physicians who perform the proce-
dure. Since 1983, sperm banks have asked donors
if they wished to be contacted by their children
years later. The first such meetings occurred in
2002, and thousands of others have followed.
Today several websites offer DNA tests that
enable people to ±
nd their sperm donor fathers.
A woman or couple choosing intrauter-
ine insemination can select sperm from a cata-
log that lists the personal characteristics of the
donors, including blood type; hair, skin, and eye
color; build; and even educational level and inter-
ests. Not all of these traits are inherited. Rarely,
intrauterine insemination has led to dilemmas
(table 23A).
In Vitro Fertilization
in vitro
fertilization (IVF), which means “fertiliza-
tion in glass,” sperm cell meets egg cell outside
the woman’s body. The fertilized egg cell divides
two or three times and is then introduced into
Assisted Reproductive Technologies
Assisted Reproductive
1. A physician in California used his own
sperm for intrauterine insemination
of ±
fteen patients and told them that
he had used sperm from anonymous
2. A plane crash killed the wealthy
parents of two early embryos stored
at –320°F (–195°C) in a hospital in
Melbourne, Australia. Adult children of
the couple were asked to share their
estate with two eight-celled balls.
3. Several couples in Chicago planning
to marry discovered that they were
half-siblings. Their mothers had been
inseminated with sperm from the
same donor.
4. Two Rhode Island couples sued a
fertility clinic for misplacing embryos.
5. A man sued his ex-wife for possession
of their frozen embryos as part of the
divorce settlement.
6. A man who donated sperm when he
was healthy later developed a late-
onset genetic disease, cerebellar ataxia.
Each of the 18 children conceived
using his sperm faces a 1 in 2 chance of
having inherited the condition.
A couple expecting a child can estimate the approximate time of
fertilization (conception) by adding fourteen days to the date of the
onset of the last menstrual period. They can predict the time of birth
by adding 266 days to the fertilization date. Most babies are born
within ten to ±
fteen days of this calculated time.
Tracking a pregnancy’s progress can be confusing, because some
health-care providers measure 40 weeks from the last menstrual
period, rather than the more accurate 38 weeks from fertilization.
Obstetricians can, however, estimate the date of conception by scan-
ning the embryo with ultrasound and comparing the crown-to-rump
length to known values that are the average for each day of gesta-
tion. This approach is inaccurate if an embryo is smaller or larger than
usual due to a medical problem.
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