Tissues
Mucous (MYOO-kus) membrane lines the
digestive, respiratory, urinary, and reproductive tracts. It
lines all body cavities that open to the outside. It is usually
cili-ated. Its most obvious function is to produce mucus,
but it also concentrates bile in the gallbladder. In the intes-
tine, it secretes enzymes for the digestion of food and nu-
trients before absorption. Mucous membrane protects,
absorbs nutrients, and secretes mucus, enzymes, and bile
salts.
Glandular epithelium forms glands. Glands are in-
volutions of epithelial cells specialized for synthesizing
special compounds. The body has two types of multi-
cellular glands. Exocrine (EKS-oh-krin) glands have ex-
cretory ducts that lead the secreted material from the gland
to the surface of a lumen (passageway) on the skin. There
are two types of exocrine glands. Simple exocrine
glands have single unbranching ducts. Some examples of
simple exocrine glands are the sweat glands, most of the
glands of the digestive tract, and the sebaceous glands. The
other type of exocrine gland is the compound exocrine
gland. These glands are made of several com-ponent
lobules each with ducts that join other ducts. Thus, the
ducts are branching. Examples of compound exocrine
glands are the mammary glands and the large salivary
glands. Endocrine glands are the second type of
multicellular glands in the body. They are ductless and
secrete hormones; examples are the thyroid and pitu-itary
glands. Goblet cells are unicellular glands that se-crete
mucus. They are interspersed among the epithelial cells that
make up mucous membranes.
Endothelium (en-doh-THEE-lee-um) is a special
name given to the epithelium that lines the circula-tory
system. This system is lined with a single layer of
squamous-type cells. Endothelium lines the blood ves-sels
and the lymphatic vessels. The endothelium that lines the
heart gets another special name and is called
endocardium. A blood capillary consists of only one
layer of endothelium. It is through this single layer of cells
that oxygen, carbon dioxide, nutrients, and waste are
transported by the blood cells to the various cells of our
bodies.
Our final type of epithelial tissue based on function is
mesothelium (mezo-THEE-lee-um). This tissue is also
called serous (SEER-us) tissue. It is the tissue that lines
the great cavities of the body that have no openings to the
outside. These membranes consist of a simple squa-mous
cell layer overlying a sheet of connective tissue. Special
names are associated with this type of epithelial tissue also.
The pleura (PLOO-rah) are the serous mem-brane or
mesothelial tissue that lines the thoracic cavity. The
pericardium is the serous membrane that covers the
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heart; the peritoneum (pair-ih-toh-NEE-um) is the se-
rous membrane lining the abdominal cavity. This tissue
protects, reduces friction between organs, and secretes
fluid. The term parietal refers to the walls of a cavity and
visceral refers to the covering on an organ.Connective Tissue.
The second major type of tissue is connective tissue.
This type of tissue allows movement and provides sup-port
for other types of tissue. In this tissue, unlike epi-thelial,
there is an abundance of intercellular material called
matrix (MAY-trikz). This matrix is variable in both type
and amount. It is one of the main sources of differ-ences
between the different types of connective tissue. There are
also fibers of collagen (KOL-ah-jen) and elastin (ee-
LASS-tin) embedded in this matrix. Sometimes the fibers
are very apparent under the microscope, as in a tendon,
whereas in other tissues the fibers are not very ap-parent as
in certain cartilage. We can classify connective tissue into
three subgroups: loose connective tissue, dense connective
tissue, and specialized connective tissue.Loose Connective Tissue
As the name implies, the fibers of loose connective tis-sue
are not tightly woven among themselves. There are three
types of loose connective tissue: areolar (ah-REE-oh-lah),
adipose (ADD-ih-pohz), and reticular (reh-TIK-you-
lar). Loose connective tissue fills spaces between and
penetrates into organs.
Areolar is the most widely distributed of the loose
connective tissue. It is easily stretched yet resists tear-ing.
This tissue has three main types of cells distrib-uted among
its delicate fibers: fibroblasts, histiocytes, and mast cells.
Fibroblasts (FYR-broh-blastz) are small flattened cells
with large nuclei and reduced cyto-plasm; they are also
somewhat irregular in shape. The term fibroblast (blast
meaning germinal or embryonic) refers to the ability of
these cells to form fibrils (small fibers). They are active in
the repair of injury. Histiocytes- (HISS-tee-oh-sightz) are
large, stationary phagocytic (fag-oh-SIH-tik) cells that
eat up (phago 5 to eat) -debris and microorganisms outside
the blood circu-latory system. They were originally
monocytes in the circulating blood. When they are motile
in tissue, they are called macrophages (MACK-roh-fay-
jez). A macro-phage of loose connective tissue is
specifically called a histiocyte. Histiocytes are stationary or
fixed in tissue. Mast cells are roundish or polygonal in
shape and are found close to small blood vessels. Mast cells
function