Essential Cell Biology 5th edition

702 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
Figure 20–18 Cells can be packed
together in different ways to form an
epithelial sheet. Micrographs along
with drawings show four basic types
of epithelia. In each case, the cells are
sitting on a thin mat of extracellular
matrix, the basal lamina (yellow), as
discussed shortly. (From D.W. Fawcett,
A Textbook of Histology, 12th ed. 1994.
With permission from Taylor & Francis
Books UK.)
columnar
cuboidal
basal lamina
stratified
basal lamina
squamous
basal lamina
free surface
APICAL
tightly together. In some cases, the sheet is many cells thick, or stratified,
as in the epidermis (the outer layer of the skin); in other cases, it is a simple
epithelium, only one cell thick, as in the lining of the gut. The epithelial
cells, themselves, can also ECB5 take n20.100/20.19 many forms. They can be tall and columnar,
squat and cuboidal, or flat and squamous (Figure 20–18). Within a
given sheet, the cells may be all the same type or a mixture of different
types. Some epithelia, like the epidermis, act mainly as a protective
barrier; others have complex biochemical functions. Some secrete specialized
products such as hormones, milk, or tears; others, such as the
epithelium lining the gut, absorb nutrients; yet others detect signals, such
as light, sensed by the layer of photoreceptors in the retina of the eye, or
sound, sensed by the epithelium containing the auditory hair cells in the
ear (see Figure 12−28).
Despite these and many other variations, one can recognize a standard
set of features that virtually all animal epithelia share. Epithelia cover
the external surface of the body and line all its internal cavities, and they
must have been an early feature in the evolution of animals. Cells joined
together into an epithelial sheet create a barrier, which has the same
significance for the multicellular organism that the plasma membrane
has for a single cell. It keeps some molecules in, and others out; it takes
up nutrients and exports wastes; it contains receptors for environmental
signals; and it protects the interior of the organism from invading microorganisms
and fluid loss.
The arrangement of cells into epithelia is so commonplace that we sometimes
take it for granted. Yet, as we discuss in this section, establishing
epithelia requires a collection of specialized structures and molecular
devices, which are common to a wide variety of epithelial cell types.
connective tissue
basal lamina
BASAL
Figure 20–19 A sheet of epithelial
cells has an apical and a basal
surface. The basal surface sits on
a specialized sheet of extracellular
matrix called the basal lamina, while
the apical ECB5 surface e20.20-20.20
is free.
Epithelial Sheets Are Polarized and Rest on a Basal
Lamina
An epithelial sheet has two faces: the apical surface is free and exposed
to the air or to a bodily fluid; the basal surface is attached to a sheet
of connective tissue called the basal lamina (Figure 20–19). The basal
lamina consists of a thin, tough sheet of extracellular matrix, composed
mainly of a specialized type of collagen (type IV collagen) and a protein
called laminin (Figure 20–20). Laminin provides adhesive sites for
integrin molecules in the basal plasma membranes of epithelial cells, and
it thus serves a linking role like that of fibronectin in other connective
tissues.