HUMAN BIOLOGY

tIssUes, orGans, and orGan systeMs 69

What are epithelial tissues?

• Epithelial tissues serve as linings or provide protection inside
  the body or at its surface.


• Simple epithelium functions in the diffusion, secretion,
  absorption, or filtering of substances. Stratified epithelium
  protects underlying tissues.


• Glands develop from epithelium. They make and secrete various
  types of substances.


• Exocrine glands release substances through ducts or tubes.
  Endocrine glands release hormones directly into body fluids.

taKe-hoMe Message

exocrine gland Gland that

releases the substance it

makes through a duct or

tube.

gland Structure built of one

or more cells that makes

and releases products such

as saliva, milk, mucus, or oil.

tissue Group of similar

cells that perform a specific

function.

Figure 4.2 Glands make and release substances. This illustration shows examples of an exocrine and an endocrine gland. Chapter 15
discusses the endocrine system, which includes a variety of glands and cells that release hormones.

parotid gland
(secretes saliva)

parotid duct (delivers
saliva to mouth)

thyroid gland (secretes

hormones into blood)

blood

vessel

cell that secretes

hormone

Endocrine gland

Exocrine gland

tissue at the surface of your skin, which is exposed to nicks,
bumps, scrapes, and so forth.
The two basic types of epithelium are categorized by the
shape of cells at the tissue’s free surface (Table 4.1). The cells
of squamous epithelium are flattened, while they are cube-
shaped in cuboidal epithelium and elongated in columnar
epithelium. Each shape correlates with a given function. For
instance, oxygen and carbon dioxide easily diffuse across
the thin simple squamous epithelium that makes up the
walls of fine blood vessels (Figure 4.1B). Cells of cuboidal
and columnar epithelia may secrete or absorb substances.

glands develop from epithelium

A gland makes and releases products such as saliva or
mucus. Some glands consist of a single cell, while others
are more complex. All glands develop from epithelial tis-
sue and often stay connected to it. Mucus-secreting goblet
cells, for instance, are embedded in epithelium that lines

the trachea (your windpipe) and

other tubes leading to the lungs. The

stomach’s epithelial lining contains

gland cells that release mucus and

digestive juices.

Glands may be classified by how

their products reach the place where

they are used (Figure 4.2). Exocrine

glands release substances through

ducts or tubes (Figure 4.2A). Mucus,

saliva, oil, earwax, milk, and diges-

tive enzymes are in this group.

Many exocrine glands simply release the substance they

make; salivary glands and most sweat glands are like this.

In other cases, a gland’s secretions include bits of the gland

cells. For instance, milk from a nursing mother’s mammary

glands contains bits of the glandular epithelial tissue. In

still other cases, such as sebaceous (oil) glands in your skin,

whole cells full of material are shed into the duct, where

they burst and their contents spill out.

Endocrine glands do not release substances through

tubes or ducts. They make hormones that are released

directly into the fluid bathing the glands (Figure 4.2B).

© © iStockphoto.com/Flashon Studio

A

B © Cengage Learning

Type Shape Typical Locations

Simple Squamous Linings of blood vessels,

air sacs of lungs (alveoli)

Cuboidal Glands and their ducts, ovary

surfaces, iris of eye

Columnar Stomach, intestines, uterus

Pseudostratified Columnar Throat, nasal passages,

sinuses, trachea, male

genital ducts

Stratified Squamous Skin, mouth, throat, vagina

Cuboidal Ducts of sweat glands

Columnar Male urethra, salivary

gland ducts

Table 4.1 Major Types of Epithelium

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