organs, epithelial tissue comes in more varieties than any other tissue.
Epithelial tissues, which generally are arranged in sheets or tubes of tightly-packed cells,
always have a free, or apical, surface that can be exposed to the air or to fluid. That free surface
also can be covered by additional layers of epithelial tissue. But whether it’s layered or
not, each epithelial cell has polarity, and all but one side of the cell is
tucked snugly against neighboring cells. The apical side sometimes has cytoplasmic projections
such as cilia, hair-like growths that can move material over the cell’s surface, or
microvilli, finger-like projections that increase the cell’s surface area for absorption. Opposite
the apical side is the basal side (think basement), which typically attaches to some kind of
connective tissue.
Epithelial tissue serves several key functions, including the following:
Protection: Skin protects vulnerable structures or tissues deeper in the body.
Barrier: Epithelial tissues prevent foreign materials from getting inside the body.
Sensation: Sensory nerve endings embedded in epithelial tissue connect the
body with outside stimuli.
Secretion: Epithelial tissue in glands can be specialized to secrete enzymes, hormones,
and fluids.
Single-layer epithelial tissue is classified as simple. Tissue with more than one layer is
called stratified. Epithelial tissues also can be classified according to shape: Squamous
is a thin, flat cell; cuboidal is, as the name implies, equal in height and width and
shaped like a cube; and columnar cells are taller than they are wide.
No comments:
Post a Comment