Page 158 - Fundamentals of anatomy physiology

Classification of Epithelia Chapter 4 The Tissue Level of Organization 145

Epithelial cells have three basic shapes: squamous, cuboidal, and Simple epithelia are also characteristic of regions in which
columnar. For classification purposes, we look at the superficial secretion or absorption occurs. Examples are the lining of the
cells in a section perpendicular to both the apical surface and intestines and the gas-exchange surfaces of the lungs. In these
the basement membrane. In sectional view, squamous cells
appear thin and flat, cuboidal cells look like little boxes, and 4places, thinness is an advantage. It reduces the time required
columnar cells are tall and relatively slender rectangles.
for materials to cross the epithelial barrier.
Once you have determined whether the superficial cells In a stratified epithelium, several layers of cells cover
are squamous, cuboidal, or columnar, then look at the number
of cell layers. There are only two options: simple or stratified. the basement membrane. Stratified epithelia are generally
located in areas that are exposed to mechanical or chemical
If only one layer of cells covers the basement membrane, stresses. The surface of the skin and the lining of the mouth are
that layer is a simple epithelium. Simple epithelia are neces- examples.
sarily thin. All the cells have the same polarity, so the distance
from the nucleus to the basement membrane does not change &T i p s T r i c k s
from one cell to the next. To help you remember the meanings of the terms squamous
and stratified, associate the word “squamous” with “scaly,” and
Because they are so thin, simple epithelia are fragile. A sin- the word “stratified” with “stratosphere,” an upper layer of
gle layer of cells cannot provide much mechanical protection, Earth’s atmosphere.
so simple epithelia are located only in protected areas inside
the body. They line internal compartments and passageways, Squamous Epithelia
such as the pleural, pericardial, and peritoneal cavities, the
heart chambers, and blood vessels. The cells in a squamous epithelium (SKWA■ -mus; squama,
plate or scale) are thin, flat, and somewhat irregular in shape,
like pieces of a jigsaw puzzle (Figure 4–3). From the surface, the

Figure 4–3 Squamous Epithelia. Cytoplasm
Nucleus
Simple Squamous Epithelium
LOCATIONS: Mesothelia Connective tissue LM × 238
lining pleural, pericardial, a Lining of peritoneal cavity LM × 310
and peritoneal cavities;
endothelia lining heart Squamous
and blood vessels; portions superficial cells
of kidney tubules (thin
sections of nephron loops); Stem cells
inner lining of cornea; Basement
alveoli of lungs membrane
FUNCTIONS: Reduces Connective
friction; controls vessel
permeability; performs tissue
absorption and secretion

Stratified Squamous Epithelium
LOCATIONS: Surface of
skin; lining of mouth, throat,
esophagus, rectum, anus,
and vagina
FUNCTIONS: Provides physical
protection against abrasion,
pathogens, and chemical attack

b Surface of tongue