Page 125 - Atlas of Histology with Functional Correlations

morphology or structure of the surface cells. A basement membrane is a thin,

noncellular region that separates the epithelium from the underlying connective
tissue and is seen with a light microscope. An epithelium with a single layer of
cells is called simple and that with numerous cell layers is called stratified. A
pseudostratified epithelium consists of a single layer of cells that attaches to a
basement membrane, but not all cells reach the surface. An epithelium that

exhibits flat cells is called squamous. When the surface cells are round, or as tall
as they are wide, the epithelium is cuboidal. When the cells are taller than they
are wide, the epithelium is called columnar.

SPECIAL SURFACE MODIFICATIONS

AND JUNCTIONAL COMPLEXES IN

EPITHELIAL CELLS

Epithelial cells in different organs exhibit special cell membrane modifications
on their apical (upper) surfaces. These modifications are cilia, stereocilia, or
microvilli. Cilia are motile structures found on certain cells in the uterine tubes,

uterus, efferent ducts in the testes, and conducting tubes of the respiratory
system. Microvilli are small, nonmotile projections that cover the surfaces of all
absorptive cells in the small intestine and the proximal convoluted tubules in the
kidney. Their function in these organs is the absorption of fluid and nutrients
during the digestive processes.

In addition to microvilli, certain cells exhibit long apical processes that
extend from their surfaces into the lumina. These structures are the nonmotile
stereocilia. They are longer than the microvilli and are found in the epididymis,
vas deferens, and in the sensory organ of the inner ear. In the epididymis, the

stereocilia increase the cell surfaces and facilitate absorption of testicular fluid
produced in the seminiferous tubules of the testes. In the inner ear, stereocilia are
located in the auditory and vestibular systems where they perform sensory
functions that respond to sound and balance.

Various specialized structures in the epithelium link individual cells into a

functional unit that provides strong adhesion to and rapid communication
between neighboring cells. The apical zonulae occludentes (singular, zonula
occludens), or tight junctions, form a seal that prevents the entrance of material

between the epithelial cells. The zonulae adherens (adhering junctions)

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