Chapter 4
Tissue: The Living Fabric
Epithelium has a high regenerative capacity. Some epithelia are
exposed to friction and their surface cells rub off. Others are
damaged by hostile substances in the external environment
(bacteria, acids, smoke). If and when their apical-basal polar-
ity and lateral contacts are destroyed, epithelial cells begin to
reproduce themselves rapidly. As long as epithelial cells receive
adequate nutrition, they can replace lost cells by cell division.
Check Your Understanding
Epithelial tissue is the only tissue type that has polarity, that
is, an apical and a basal surface. Why is this important?
Which of the following properties apply to epithelial tissue?
Has blood vessels, can repair itself (regenerate), cells joined
by lateral contacts.
For answers, see Appendix H.
Classification of Epithelia
Each epithelium has two names. Te first name indicates the
number of cell layers present, and the second describes the
shape of its cells. Based on the number of cell layers, there are
simple and stratified epithelia
(Figure 4.2a)
Simple epithelia
consist of a single cell layer. Tey are typi-
cally found where absorption, secretion, and filtration occur
and a thin epithelial barrier is desirable.
Stratified epithelia
, composed of two or more cell layers
stacked on top of each other, are common in high-abrasion
areas where protection is important, such as the skin surface
and the lining of the mouth.
In cross section, all epithelial cells have six (somewhat irregular)
sides, and an apical surface view of an epithelial sheet looks like a
honeycomb. Tis polyhedral shape allows the cells to be closely
packed. However, epithelial cells vary in height, and on that basis,
there are three common shapes of epithelial cells (Figure 4.2b):
Squamous cells
mus) are flattened and scalelike
Cuboidal cells
dahl) are boxlike, approximately as
tall as they are wide.
Columnar cells
nar) are tall and column shaped.
In each case, the shape of the nucleus conforms to that of the cell.
Te nucleus of a squamous cell is a flattened disc; that of a cuboidal
cell is spherical; and a columnar cell nucleus is elongated from top
to bottom and usually located closer to the cell base. Keep nuclear
shape in mind when you attempt to identify epithelial types.
Simple epithelia are easy to classify by cell shape because the
cells usually have the same shape. In stratified epithelia, how-
ever, cell shape differs in the different layers. ±o avoid ambiguity,
stratified epithelia are named according to the shape of the cells
in the
layer. Tis naming system will become clearer as we
explore the specific epithelial types.
As you read about the epithelial classes, study
Figure 4.3
. ±ry
to pick out the individual cells within each epithelium. Tis is
not always easy, because the boundaries between epithelial cells
o²en are indistinct. Furthermore, the nucleus of a particular cell
may or may not be visible, depending on the plane of the cut
made to prepare the tissue slides.
Simple Epithelia
Te simple epithelia are most concerned with absorption, secre-
tion, and filtration. Because they consist of a single cell layer and
are usually very thin, protection is not one of their specialties.
Apical surface
Basal surface
Apical surface
Basal surface
(a) Classification based on number of cell layers.
(b) Classification based on cell shape.
Figure 4.2
Classification of epithelia.
Note that cell shape
influences the shape of the nucleus.
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