HUMAN BIOLOGY

reprodUCtIVe systeMs 315

fertilization Fusion of the

nuclei of an egg and a

sperm.

ovum A mature egg.

zygote The first cell of a

new individual.

What is the role of fertiliZation in

reproduction?

• At fertilization, a sperm’s nucleus fuses with the nucleus of
  an egg cell. The result is a zygote, a single cell with a full set of
  chromosomes—half from the mother and half from the father.

taKe-hoMe Message

Fusion with a sperm prompts

the completion of the cell division

process that began when the oocyte

was being formed in an ovary (Sec-

tion 16.2). The result is a mature egg,

or ovum (plural: ova), plus another

polar body. (Recall from Section 16.2

that one or two polar bodies are

produced in the cell division step that gives rise to the

secondary oocyte. Often three tiny polar bodies eventually

are packaged with the ovum.) The nuclei of the sperm and

ovum swell up, then fuse.

Each sperm and oocyte has 23 chromosomes, half the

number in other body cells. Fertilization combines them

into a full set of 46 chromosomes. Thus a zygote has all the

DNA required to guide development of the embryo.

n    Fertilization combines the genetic material in a male’s

sperm with that in a female’s egg.

Fertilization is the fusion of an egg cell’s nucleus and a

sperm’s nucleus. It begins when a sperm enters a secondary

oocyte. After several steps, fertilization produces a zygote

(zye-goat, “yoked together”), the first cell of the new indi-

vidual. Figure 16.10 shows these steps.

As sperm swim through the cervix and uterus and into

the oviducts, capacitation occurs. In this process, chemical

changes weaken the membrane over the sperm’s acrosome.

Only a sperm that is capacitated (“made able”) can ferti lize

an oocyte. Of the millions of sperm in the vagina after an

ejaculation, just several hundred reach the upper part of an

oviduct, where fertilization usually occurs. Contractions

of smooth muscle in the uterus help move sperm toward

the oviducts.

When a capacitated sperm contacts an oocyte, enzymes

are released from the now-weakened membrane over the

acrosome. These enzymes clear a path through the zona

pellucida. Many sperm can reach and bind to the oocyte.

Usually, however, only one sperm fuses with the oocyte.

Rapid chemical changes in the oocyte’s cell membrane pre-

vent more sperm from entering.

Fertilization

egg and

sperm nuclei

zona

pellucida

follicle

cell

oocyte

nucleus

zona

pellucida

Fertilization

Ovulation

oviduct

ovary

uterus

opening

of cervix

vagina

A Fertilization most often

occurs in the oviduct. Many

sperm swim through the

vagina into oviducts

(blue arrows).

Inside an oviduct, the sperm

surround a secondary oocyte

that was released by ovulation.

B Enzymes released from the cap of each sperm

clear a path through the zona pellucida. Penetration

of the secondary oocyte by a sperm causes the

oocyte to release substances that prevent other

sperm from binding.

C The oocyte completes meiosis II.

The sperm’s tail and other organelles

degenerate.

Later, the egg and sperm nuclei will

fuse and the zygote will form.

Figure 16.10 Animated! Fertilization unites a sperm and oocyte. (A, B: © Cengage Learning; C: Courtesy of Elizabeth Sanders, Women’s Specialty, Jackson, MS)

16.6

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