Human Physiology, 14th edition (2016)

736 Chapter 20

the oocyte, creating a Ca 2 1 wave. The egg cell is so large (about
0.1 mm in diameter) that this Ca^2 1 wave takes about 2 seconds
to spread from one side of the oocyte to the other. The Ca^2 1 wave
activates the fertilized egg cell, causing numerous structural and
metabolic changes. Some of these changes prevent other sperm
from fertilizing the same oocyte. Polyspermy (the fertilization of
an oocyte by many sperm) is thereby prevented; only one sperm
can fertilize an egg cell.
When the secondary oocyte was released from the ovary at
ovulation, its cell cycle was arrested at metaphase II. The Ca^2 1
wave initiated at fertilization activates proteins that allow the
cell cycle to continue past this arrested stage. As a result, the
secondary oocyte is stimulated to complete its second meiotic
division ( fig.  20.39 ). Like the first meiotic division, the sec-
ond produces one cell that contains all of the cytoplasm—the
mature ovum—and one polar body. The second polar body,
like the first, ultimately fragments and disappears.
Within 12 hours after fertilization the nuclear membrane in
the ovum disappears and the haploid number of chromosomes
(23) in the ovum is joined by the haploid number of chromo-
somes from the sperm cell. A fertilized egg, or zygote, contain-
ing the diploid number of chromosomes (46) is thereby formed
( fig.  20.39 ). Monozygotic twins (identical twins) are derived
from a single zygote that splits and becomes two embryos,
whereas dizygotic twins (fraternal twins) are derived from two
different zygotes produced by two ovulated oocytes fertilized
by two different sperm.
It should be noted that the sperm cell contributes more than
the paternal set of chromosomes to the zygote. The sperm, not
the oocyte, contributes the centrosome, which is needed for the

Figure 20.39 Changes in the oocyte following fertilization. A secondary oocyte, arrested at metaphase II of meiosis,
is released at ovulation. If this cell is fertilized, it will complete its second meiotic division and produce a second polar body. The
chromosomes of the two gametes are joined in the zygote.

Primary oocyte Secondary oocyte Secondary oocyte

at metaphase II

Degenerate

First polar

body

First polar

body

First

meiotic

division

Ovulation Fertilization

Spindle

apparatus

First polar

body

Second polar

body

Zygote

Chromosomes

Nuclear

membrane

disappearing

Sperm cell

nucleus

CLINICAL APPLICATION

In vitro fertilization, a technique pioneered by a British sci-

entist who was awarded the 2010 Nobel Prize in Physiology

or Medicine, has helped millions of otherwise infertile cou-

ples to reproduce. In this technique, a woman undergoes

ovarian hyperstimulation in which she receives injections

of gonadotropin (usually FSH) over the course of days to

stimulate the development of many ovarian follicles. Other

treatments (with GnRH analogues) prevent normal ovula-

tion from occurring so that she will have a number of fol-

licles that develop at the same time for the harvesting of her

egg cells. Tubes that are guided by vaginal ultrasound then

aspirate the follicular fluid and the oocytes from her ovary.

Fertilization may be produced by intracytoplasmic sperm

injection ( ICSI ), a technique in which a single, capacitated

sperm is injected through the zona pellucida and into the

cytoplasm of an isolated secondary oocyte ( fig.  20.40 ). A

number of embryos are produced in this way and grown for

three days (to the eight-cell stage) or for five days (to the

blastocyst stage) before they are transferred to the uterus

using a small tube. Three or more of these embryos are

usually transferred at the same time, and any remaining

embryos are preserved frozen in liquid nitrogen.

organization of microtubules into the spindle apparatus (chapter 3,

section 3.5) that separates duplicated chromosomes during mito-

sis. The midpiece of the sperm brings some mitochondria into

the oocyte during fertilization, but these do not contribute to the