120
UNIT 1
Organization of the Body
4
Te spherical nuclei stain darkly, causing the cell layer to look like
a string of beads when viewed microscopically. Important func-
tions of simple cuboidal epithelium are secretion and absorption.
Tis epithelium forms the walls of the smallest ducts of glands and
of many kidney tubules.
Simple Columnar Epithelium
Simple columnar epithelium
is a single layer of tall, closely packed cells, aligned like sol-
diers in a row (Figure 4.3c). It lines the digestive tract from
the stomach through the rectum. Columnar cells are mostly
associated with absorption and secretion, and the digestive
tract lining has two distinct modifications that make it ideal
for that dual function:
Dense microvilli on the apical surface of absorptive cells
±ubular glands made primarily of cells that secrete mucus-
containing intestinal juice
Additionally, some simple columnar epithelia display cilia on
their free surfaces, which help move substances or cells through
an internal passageway.
Pseudostratified Columnar Epithelium
Te cells of
pseu-
dostratified columnar epithelium
(soo
0
do-stră
9
tĭ-fīd) vary in
height (Figure 4.3d). All of its cells rest on the basement mem-
brane, but only the tallest reach the free surface of the epithelium.
Because the cell nuclei lie at different levels above the basement
membrane, the tissue gives the false (pseudo) impression that
Simple Squamous Epithelium
Te cells of a
simple squamous
epithelium
are flattened laterally, and their cytoplasm is sparse
(Figure 4.3a). In a surface view, the close-fitting cells resemble
a tiled floor. When the cells are cut perpendicular to their free
surface, they resemble fried eggs seen from the side, with their
cytoplasm wisping out from the slightly bulging nucleus.
Tin and o²en permeable, simple squamous epithelium is
found where filtration or the exchange of substances by rapid
diffusion is a priority. In the kidneys, it forms part of the filtra-
tion membrane. In the lungs, it forms the walls of the air sacs
across which gas exchange occurs (Figure 4.3a).
±wo simple squamous epithelia in the body have special
names that reflect their location.
Endothelium
(en
0
do-the
9
le-um; “inner covering”) provides
a slick, friction-reducing lining in lymphatic vessels and in
all hollow organs of the cardiovascular system—blood ves-
sels and the heart. Capillaries consist exclusively of endothe-
lium, and its exceptional thinness encourages the efficient
exchange of nutrients and wastes between the bloodstream
and surrounding tissue cells.
Mesothelium
(mez
0
o-the
9
le-um; “middle covering”) is the
epithelium found in serous membranes, the membranes lin-
ing the ventral body cavity and covering its organs.
Simple Cuboidal Epithelium
Simple cuboidal epithelium
con-
sists of a single layer of cells as tall as they are wide (Figure 4.3b).
(a)
Simple squamous epithelium
Description:
Single layer of flattened cells
with disc-shaped central nuclei and sparse
cytoplasm; the simplest of the epithelia.
Function:
Allows materials to pass by
diffusion and filtration in sites where protection
is not important; secretes lubricating
substances in serosae.
Location:
Kidney glomeruli; air sacs of lungs;
lining of heart, blood vessels, and lymphatic
vessels; lining of ventral body cavity (serosae).
Photomicrograph:
Simple squamous epithelium
forming part of the alveolar (air sac) walls (140
m
).
Air sacs of
lung tissue
Nuclei
of squamous
epithelial
cells
Figure 4.3
Epithelial tissues.
(a)
Simple epithelium. (For related images, see
A Brief Atlas of
the Human Body
, Plates 1 and 2.)
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