148
UNIT ONE
die. This “keratinization” produces a covering of dry, tough,
protective material that prevents water and other substances
from escaping from underlying tissues and blocks chemicals
and microorganisms from entering.
StratiF
ed squamous epithelium also lines the oral cavity,
esophagus, vagina, and anal canal. In these parts, the tissue
is not keratinized; it stays soft and moist, and the cells on its
free surfaces remain alive.
Stratif
ed Cuboidal Epithelium
Stratif
ed cuboidal epithelium
consists of two or three lay-
ers of cuboidal cells that form the lining of a lumen
(f g. 5.8)
.
The layering of the cells provides more protection than the
single layer affords.
StratiF ed cuboidal epithelium lines the larger ducts of
the mammary glands, sweat glands, salivary glands, and
pancreas. It also forms the lining of developing ovarian folli-
cles and seminiferous tubules, which are parts of the female
and male reproductive systems, respectively.
Nucleus
Basement
membrane
Microvilli
(free surface
of tissue)
Connective
tissue
Mucus
Cytoplasm
Goblet cell
(a)
(b)
FIGURE 5.4
Simple columnar epithelium consists of a single layer of elongated cells (400×).
FIGURE 5.5
A scanning electron micrograph of microvilli, which
fringe the free surfaces of some columnar epithelial cells (33,000×).
(a)
(b)
Cilia
(free surface
of tissue)
Goblet cell
Basement
membrane
Nucleus
Connective
tissue
Cytoplasm
FIGURE 5.6
±PseudostratiF
ed columnar epithelium appears stratiF
ed because the cell nuclei are located at di²
erent
levels (1,000×).
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