30
UNIT 2
Basic Tissues
A
Mesothelium
Mesothelial cells
Lumen
Simple Squamous Epithelium
Figure 3-2A.
Simple squamous epithelium, oviduct.
H&E,
3
155; inset
3
310
Mesothelium
is a term for the epithelial layer of the
serous membranes (
peritoneum
,
pleura
, and
pericardium
)
that line the body cavities and cover the organs that
project into the cavities. The other component of these
membranes is the loose connective tissue layer beneath
the mesothelium. The membrane that lines the cavity
wall constitutes the
parietal layer
, and the
visceral layer
(
serosa
) covers the organs located within the cavity.
The mesothelium secretes a slippery
lubricating
fl uid
that allows organs to easily slide against and over
each other or against the cavity wall (e.g., beating heart,
expanding or contracting lungs, peristaltic activity of the
intestine) without damage to the mesothelium.
D. Cui
Mesothelial cell
Basement membrane
Connective tissue
B
Figure 3-2B.
A representation of a simple squamous
epithelium lining a body cavity.
Mesothelial
cells
are fl attened, generally pentagonal in
shape, and form irregular borders with one another. The
basement membrane
is immediately adjacent to the meso-
thelium, which is barely visible with the light microscope.
The basement membrane includes the
basal lamina
and an
additional layer, the
reticular lamina
. The term “basal lam-
ina” is used at the electron microscope level and includes
the
lamina densa
and
lamina lucida
.
Connective tissue
is
found beneath the basement membrane. The loose con-
nective tissue beneath the basement membranes of meso-
thelia is the second component of the serous membranes
lining the
peritoneal
,
pleural
, and
pericardial cavities
.
CLINICAL CORRELATION
Figure 3-2C.
Mesothelioma.
Mesothelioma
(cancer of the mesothelium) is a
neoplasm that arises from surfaces of the
pleural
and
peritoneal
cavities. It is only occasionally found in the
pericardial
mesothelium. This type of tumor is more
commonly seen in individuals who either were exposed
to asbestos or who smoke. Mesothelioma cells develop
long, slender, and curved microvilli. These tumors may
invade nearby tissues and organs. In general, cancer
cells are those that can
metastasize
(spread) from their
original site to distant parts of the body, and mesothe-
liomas are no exception. The lymphatic system is a
common route through which metastasis may occur.
D. Cui
Basement membrane
Normal mesothelial cell
Connective tissue
Fragmented
basement membrane
Tumor
cells
C
Excessive fl uid in a mesothelial-lined cavity is called an
“effusion,” for example, a
pleural effusion
. Infl amma-
tion of the pleura is called
pleurisy
; of the peritoneum, it
is called
peritonitis
; and of the pericardium,
pericarditis
.
These may exist in conjunction with a variety of clinical
conditions or diseases.
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