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.
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.
ed Cuboidal Epithelium
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.
Simple columnar epithelium consists of a single layer of elongated cells (400×).
A scanning electron micrograph of microvilli, which
fringe the free surfaces of some columnar epithelial cells (33,000×).
ed columnar epithelium appears stratiF
ed because the cell nuclei are located at di²