microvilli or cilia at their apical surface for secretion
and absorption
Transitional—Change shape from flat to cuboidal and
back, These cells are found in organs that can stretch
(for example, the urinary bladder).
Epithelial tissue is also classified according to its cell layers.
Simple epithelium has one layer of cells. Pseudostrati-
fied epithelium is a single layer of cells that looks like
it has many layers because the cells’ nuclei are at many
levels. Finally, stratified epithelium consists of multiple
layers of cells. Cell shape and layering characteristics are
combined in the classification of covering and lining epi-
thelium; examples include simple squamous epithelium,
simple cuboidal epithelium, pseudostratified columnar epi-
thelium, and stratified columnar epithelium (Figure 3.22
on next page).
G lan d u lar e p ith e liu m Glandular epithelial cells
work together to secrete various substances (enzymes,
hormones, perspiration, milk, saliva, and so on). Glan-
dular epithelium forms both exocrine glands and en-
docrine glands. Exocrine glands (EK-so-krin) secrete
substances through tubes or ducts; examples include
sweat glands, salivary glands, and mammary glands. In
contrast, endocrine glands (EN-do-krin) secrete sub-
stances (mainly hormones) into interstitial fluid and
then the blood; examples include the thyroid gland,
pituitary gland,
adrenal gland.
glands contain both endocrine and exocrine glandular
epithelium; examples include the pancreas, ovaries,
and testes.
The next major type of tissue we will discuss, connec-
tive tissue, protects and holds together tissues and organs
such as the pancreas.
Classifications of covering and lining epithelium
Table 3.2
Simple epithelium
Sim p le squam ous
Diffusion, o sm o sis, filtration,
Blood ve sse ls, lining of the heart, lym ph ve sse ls, lungs (air sacs),
glom erulus of the kidneys, serous m em branes (SIR -us), peritoneum
Sim p le cuboidal
Secretion, absorption
Kidney tubules, sm all ducts of m any glan ds (for exam ple, thyroid,
pancreas), ovary lin in gs, eye surfaces
Sim p le colum nar
N onciliated
Secretion, absorption
Lining of digestive tract, gallbladder, and ducts of m any glands
M oves m ucus and other su b -
Lining of upper respiratory tract, uterine tubes, uterus, nasal sin u ses,
central canal of spinal cord
Pseudostratified colum nar
N onciliated
Absorption, protection
Lining of larger ducts of m any glan ds, epididym is, m ale urethra
Secretion, m oves m ucus
Upper respiratory tract
Stratified epithelium
Stratified squam ous
Skin (upper layers)
Lining of wet surfaces ( for exam ple, m outh, esop h agu s, pharynx,
vagin a, tongue)
Stratified cuboidal
Protection, secretion, absorption
Lining of ducts of sweat glands, esophageal glands, part of male urethra
Stratified colum nar
Protection, secretion
Lining of urethra, large excretory ducts of som e glan ds (for exam ple,
esophageal), anus, eye, conjunctiva
Lining of urinary bladder, parts of ureters, and urethra
Cells Specialize into Various Tissues 73
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