Chapter 28
Pregnancy and Human Development
1065
28
Before fertilization can occur, sperm must reach the ovulated
secondary oocyte. Te oocyte is viable for 12 to 24 hours aFer it
is cast out of the ovary. Te chance of pregnancy drops to almost
zero the next day. Most sperm retain their fertilizing power for
24 to 48 hours aFer ejaculation. Consequently, for successful
fertilization to occur, coitus must occur no more than two days
before ovulation and no later than 24 hours aFer. At this point
the oocyte is approximately one-third of the way down the
length of the uterine tube.
Accomplishing Fertilization
Fertilization
occurs when a sperm’s chromosomes combine
with those of an egg (actually a secondary oocyte) to form a
fertilized egg, or
zygote
(zi
9
gōt; “yoked together”), the first
cell of the new individual. Let’s look at the events leading to
fertilization.
Sperm Transport and Capacitation
During copulation, a man expels millions of sperm with consid-
erable force into his partner’s vaginal canal. Despite this “head
start,” most sperm don’t reach the oocyte, even though it is only
about 12 cm (5 inches) away. Millions of sperm leak from the
vagina almost immediately aFer being deposited there. Of those
Let’s get started by defining some terms. Te term
preg-
nancy
refers to events that occur from the time of fertilization
(conception) until the infant is born. Te pregnant woman’s de-
veloping offspring is called the
conceptus
(kon-sep
9
tus; “that
which is conceived”). Development occurs during the
gestation
period
(
gestare
5
to carry), which extends by convention from
the last menstrual period (a date the woman is likely to remem-
ber) until birth, approximately 280 days. So, at the moment of
fertilization, the mother is officially (but illogically) two weeks
pregnant!
±rom fertilization through week 8, the
embryonic period
, the
conceptus is called an
embryo
, and from week 9 through birth,
the
fetal period
, the conceptus is called a
fetus
(“the young in
the womb”). At birth, it is an infant.
Figure 28.1
shows the
changing size and shape of the conceptus as it progresses from
fertilization to the early fetal stage.
From Egg to Zygote
Describe the importance of sperm capacitation.
Explain the mechanism of the slow block to polyspermy.
Define fertilization.
Fertilization
1-week
conceptus
3-week
embryo
(3 mm)
Embryo
12-week fetus
(90 mm)
8-week embryo
(22 mm)
5-week embryo
(10 mm)
Figure 28.1
Diagrams showing the approximate size of a human conceptus from
fertilization to the early fetal stage.
The embryonic stage is from fertilization through
week 8; the fetal stage begins in week 9. (Measurements are crown to rump length.)
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