Tissue: The Living Fabric
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Tissues are collections of structurally similar cells with related func-
tions. ±e four primary tissues are epithelial, connective, nervous,
and muscle tissues.
Preparing Human Tissue for Microscopy
Preparation of tissues for microscopic examination involves
cutting thin sections of the tissue and using dyes to stain the
tissue. Minor distortions called artifacts can be introduced by the
tissue preparation process.
Epithelial tissue is the covering, lining, and glandular tissue of
the body. Its functions include protection, absorption, excretion,
ﬁltration, secretion, and sensory reception.
Special Characteristics of Epithelium
Epithelial tissues exhibit specialized contacts, polarity, avascularity,
support from connective tissue, and high regenerative capacity.
Classiﬁcation of Epithelia
Epithelium is classiﬁed by arrangement as simple (one layer) or
stratiﬁed (more than one layer) and by cell shape as squamous,
cuboidal, or columnar. ±e terms denoting cell shape and
arrangement are combined to describe the epithelium fully.
Simple squamous epithelium is a single layer of squamous cells.
Highly adapted for ﬁltration and exchange of substances, it forms
walls of air sacs of the lungs and lines blood vessels. It contributes
to serosae as mesothelium and lines all hollow circulatory system
organs as endothelium.
Simple cuboidal epithelium, commonly active in secretion and
absorption, is found in glands and in kidney tubules.
Simple columnar epithelium, specialized for secretion and
absorption, consists of a single layer of tall columnar cells that
exhibit microvilli and oFen mucus-producing cells. It lines most
of the digestive tract.
Pseudostratiﬁed columnar epithelium is a simple columnar
epithelium that appears stratiﬁed. Its ciliated variety, rich in mucus-
secreting cells, lines most of the upper respiratory passages.
Stratiﬁed squamous epithelium is multilayered; cells at the free
surface are squamous. It is adapted to resist abrasion. It lines
the esophagus and vagina; its keratinized variety forms the skin
Stratiﬁed cuboidal epithelia are rare in the body, and are found
chieﬂy in ducts of large glands. Stratiﬁed columnar epithelium
has a very limited distribution, found mainly in the male urethra
and at transition areas between other epithelia types.
Transitional epithelium is a modiﬁed stratiﬁed squamous
epithelium, adapted for responding to stretch. It lines hollow
urinary system organs.
A gland is one or more cells specialized to secrete a product.
On the basis of site of product release, glands are classiﬁed as
exocrine or endocrine. Glands are classiﬁed structurally as
multicellular or unicellular.
Unicellular glands, typiﬁed by goblet cells and mucous cells, are
mucus-secreting single-celled glands.
Multicellular exocrine glands are classiﬁed according to duct
structure as simple or compound, and according to the structure
of their secretory parts as tubular, alveolar, or tubuloalveolar.
Multicellular exocrine glands of humans are classiﬁed
functionally as merocrine or holocrine.
Connective tissue is the most abundant and widely distributed
tissue of the body. Its functions include binding and support,
protection, insulation, fat storage, and transportation (blood).
Common Characteristics of Connective Tissue
Connective tissues originate from embryonic mesenchyme and
have a matrix. Depending on type, a connective tissue may be
well vascularized (most), poorly vascularized (dense connective
tissue), or avascular (cartilage).
Structural Elements of Connective Tissue
±e structural elements of all connective tissues are extracellular
matrix and cells.
±e extracellular matrix consists of ground substance and ﬁbers
(collagen, elastic, and reticular). It may be ﬂuid, gel-like, or ﬁrm.
Each connective tissue type has a primary cell type that can exist
as a mitotic, matrix-secreting cell (blast) or as a mature cell (cyte)
responsible for maintaining the matrix. ±e undiﬀerentiated cell
type of connective tissue proper is the ﬁbroblast; that of cartilage is
the chondroblast; that of bone is the osteoblast; and that of blood-
forming tissue is the hematopoietic stem cell (see Chapter 17).
Types of Connective Tissue
Embryonic connective tissue is called mesenchyme.
Connective tissue proper consists of loose and dense varieties.
±e loose connective tissues are
Areolar: gel-like ground substance; all three ﬁber types loosely
interwoven; a variety of cells; forms the lamina propria and soF
packing around body organs; the prototype.
Adipose: consists largely of adipocytes; scant matrix; insulates
and protects body organs; provides reserve energy fuel. Brown
fat is more important for generating body heat.
Reticular: ﬁnely woven reticular ﬁbers in soF ground
substance; the stroma of lymphoid organs and bone marrow.
Dense connective tissue proper includes
Dense regular: dense parallel bundles of collagen ﬁbers; few cells,
little ground substance; high tensile strength; forms tendons,
ligaments, aponeuroses; in cases where this tissue also contains
numerous elastic ﬁbers it is called elastic connective tissue.
Dense irregular: like regular variety, but ﬁbers are arranged
in diﬀerent planes; resists tension exerted from many