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
consist of a single cell layer. Tey are typi-
cally found where absorption, secretion, and ﬁltration occur
and a thin epithelial barrier is desirable.
, 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):
mus) are ﬂattened and scalelike
dahl) are boxlike, approximately as
tall as they are wide.
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
layer. Tis naming system will become clearer as we
explore the speciﬁc epithelial types.
As you read about the epithelial classes, study
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.
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.
(a) Classification based on number of cell layers.
(b) Classification based on cell shape.
Classiﬁcation of epithelia.
Note that cell shape
inﬂuences the shape of the nucleus.