HUMAN BIOLOGY

74 Chapter 4

Stem cell therapy is still in its infancy. To date,

we have seen genuine progress with only a few

diseases and disorders. Still, some hopeful patients

ill with a crippling or possibly fatal disease travel

to countries where clinics hold out the promise of

a “stem cell cure.” Using the Web, check out what

the American Medical Association (AMA) says

about such “stem cell tourism.” Is it risky? What

advice does the AMA provide for those who may be

considering it?

ThiNk ouTSiDE ThE Book

What do cell Junctions do?

• Cell junctions hold cells together in tissues or allow
  communication between cells.


• Tight junctions help stop leaks in a tissue.


• Adhering junctions cement cells together in a tissue.


• Gap junctions are channels that allow ions and small
  molecules to cross between cells.

taKe-hoMe Message

adhering junction Weldlike
junction between cells that
keeps cells tightly attached
to one another.

gap junction Channel that
connects the cytoplasm of
neighboring cells.

tight junction Strand of
protein that helps stop leaks
between cells by forming a
gasketlike seal.

Cell Junctions: holding tissues together

n    Junctions between the cells in a tissue knit the

cells firmly together, stop leaks, and serve as

communication channels.

n Links to Plasma membrane 3.4, Cytoskeleton 3.9

Our tissues and organs would fall into disarray if there

were not some way for individual cells to “stick together”

and to communicate. Cell junctions meet these needs.

Junctions are most common where

substances must not leak from one

body compartment to another.

Figure 4.7 shows some examples

of cell junctions. Tight junctions

(Figure 4.7A) are strands of protein

that help stop leaks across a tissue.

The strands form gasketlike seals that

prevent molecules from moving eas-

ily across the junction. In epithelium

tight junctions allow the epithelial

cells to control what enters the body.

For instance, while food is being digested, various types of

nutrient molecules can diffuse into epithelial cells or enter

them selectively by active transport, but tight junctions keep

those needed molecules from slipping between cells. Tight

junctions also prevent the highly acidic gastric fluid in your

stomach from leaking out and digesting proteins of your

own body instead of those you consume in food.

Adhering junctions (Figure 4.7B) cement cells together.

One type, sometimes called desmosomes, is like a spot weld

at the plasma membranes of two adjacent cells. They are

anchored to the cytoskeleton in each cell and help hold cells

together in tissues that often stretch, such as epithelium of

the skin, the lungs, beating heart muscle, and the stomach.

Gap junctions (Figure 4.7C) are channels that con-

nect the cytoplasm of neighboring cells. They help cells

communicate because ions and small molecules can pass

through them from cell to cell. Smooth muscle and cardiac

muscle have the most gap junctions. As you will read in

Chapter 6, ions moving through them from muscle cell

to muscle cell play an important role in the contraction of

whole muscles.

Figure 4.7 Animated! Junctions knit cells together in tissues.

(© Cengage Learning)

channel

basement

membrane

cell

cytoskeleton

filaments

plasma

membrane

of one cell

A B C

Tight Junction Adhering Junction Gap Junction

4.6

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