HUMAN BIOLOGY

68 Chapter 4

basement membrane
Noncellular membrane
between an epithelium and
the underlying tissue.

endocrine gland Gland
that makes a hormone,
releasing it directly into the
fluid outside the gland.

epithelium Sheetlike tissue
that has one free surface.
Epithelia line body cavities,
ducts, and tubes or protect
an underlying tissue.

4.1 epithelium: the body’s Covering and Linings

there are two basic types of epithelium

Simple epithelium is the thinnest type of epithelium. It has

just one layer of cells. It lines the body’s cavities, ducts, and

tubes—for example, the chest cavity, tear ducts, and the

tubes in the kidneys where urine is formed (Figure 4.1B–D).

In general, the cells in a simple epithelium function in the

diffusion, secretion, absorption, or filtering of substances

across the layer.

Some single-layer epithelia look stratified in a side view

because the nuclei of neighboring cells don’t line up. Most

of the cells also have cilia. This type of simple epithelium

is termed pseudostratified (pseudo- means false). It lines the

throat, nasal passages, reproductive tract, and other sites

in the body where cilia sweep mucus or some other fluid

across the tissue’s surface.

Stratified epithelium has more than one layer of cells, and

it usually has a protective function. For example, this is the

n Epithelial tissues cover the body surface or line

its cavities and tubes.

n Link to the Cell cytoskeleton 3.9

A tissue is a group of similar cells

that perform a certain function. The

tissue called epithelium (plural: epi-

thelia) has a sheetlike structure, and

one of its surfaces faces an internal

body fluid or the outside environ-

ment (Figure 4.1A). The other surface

rests on a basement membrane

that is sandwiched between it and

the tissue below. A basement mem-

brane is densely packed with pro-

teins and polysaccharides. It does

not have cells.

Figure 4.1 Animated! All types of epithelium share basic characteristics. All epithelia have a free surface that faces either the

outside environment or an internal body fluid. A Squamous epithelium of skin consists of several layers of cells that flatten as they

near the free surface. The basement membrane is sandwiched between the lower epithelial surface and underlying connective

tissue. The diagram shows simple epithelium, a single layer of cells. B –D Examples of simple epithelium, showing the three basic

cell shapes in this type of tissue. (© Cengage Learning)

simple

squamous

epithelium

Flattened simple squamous epithelium

Squarish simple cuboidal epithelium

Tall simple columnar cells

basement

membrane

connective

Atissue

free surface

of epithelium

B

C

D

Ray Simmons/Science Source

Ed Reschke/Peter Arnold

Biophoto Associates/Science Source

Manfred Kage/Peter Arnold

© AYakovlev/Shutterstock.com

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).