HUMAN BIOLOGY

426 Chapter 22

n As genes switch on and off, they determine when and how

fast the cell will grow and divide, when it will stop dividing,

and even when it will die. Cancer can result when controls

over cell division are lost.

n Links to Cell structure 3.2, Cell differentiation 17.1,

Mutations in dNA 21.2

Some tumors are cancer, others are not

If cells in a tissue overgrow—an abnormal enlargement

called hyperplasia—the result is a defined mass of tissue

called a tumor. Technically, a tumor is a neoplasm, which

means “new growth.”

A tumor may not be “cancer.” As Figure 22.1A shows, the

cells of a benign tumor are often enclosed by a capsule of

connective tissue, and inside the capsule they are organized

in an orderly way. They also tend to grow slowly and to be

well differentiated (structurally

specialized for a particular func-

tion), much like normal cells of

the same tissue (Section 17.1).

Benign tumors usually stay put

in the body, push aside but don’t

invade surrounding tissue, and

generally can be easily removed

by surgery. Benign tumors can

threaten health, as when they

occur in the brain. Nearly every-

one has at least several of the

benign tumors we call moles.

Most of us also have or have

had some other type of benign

neoplasm, such as a cyst. A malignant growth, by contrast, is

potentially harmful. Table 22.1 compares the main features

of malignant and benign tumors, and Figure 22.2 shows the

outward differences between a harmless mole and a malig-

nant melanoma, the most dangerous skin cancer.

Dysplasia (“bad form”) is an abnormal change in the

sizes, shapes, and organization of cells in a tissue. Such

change is often an early step toward cancer. Under the

microscope, the edges of a cancerous tumor usually look

ragged (Figure 22.1B), and its cells form a disorganized

clump. Most cancer cells also have characteristics that

enable them to behave differently from normal cells.

the Characteristics of Cancer

Figure 22.1 Cancer cells are abnormal in their growth and

appearance. A Sketch of a benign neoplasm. Cells appear

nearly normal, and connective tissue encapsulates the tumor.

B A cancerous neoplasm. Due to the abnormal growth of

cancer cells, the tumor is a disorganized heap of cells. Some

of the cells may break off and invade surrounding tissues, a

process called metastasis. (© Cengage Learning)

A

B

capsule

22.1

Figure 22.2 Normal moles are examples of benign growths.

A Harmless moles, like this one, are all one color, symmetrical, and

have a smooth edge. B Malignant melanomas are asymmetrical

(they look blobby), have a ragged edge, and often have differently

colored areas. A “mole” with these characteristics is suspicious

and should be evaluated right away by a doctor. (A: Science Source;

B: James Stevenson/Science Source)

A Benign mole B Melanoma

Table 22.1

Main Features of Malignant and

Benign Tumors

Malignant Tumor Benign Tumor

Rate of growth Rapid Slow

Nature of growth Invades surrounding

tissue

Expands in the same

tissue

Spread Metastasizes via the

bloodstream and the

lymphatic system

Does not spread

Cell differentiation Usually poor Nearly normal

cancer Disease state in

which cells divide in an

uncontrolled manner and

develop other abnormal bio-

logical features.

dysplasia Abnormal

change in the sizes, shapes,

and organization of cells in

a tissue; often a step toward

cancer.

hyperplasia Overgrowth of

cells in a tissue.

tumor A defined mass of

tissue formed as cells of the

tissue overgrow.

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