(2) Epithelial tissues. The tissues on the outer surfaces of the body are
faced with epithelial cells. The deep surfaces of the skin are layered with connective
tissue which strengthens membranes. There are two types of epithelium tissue:
mucous membrane epithelial tissues (covering and lining epithelium) and serous
membrane epithelial tissues (glandular epithelium). Both types of epithelial tissue
consist of tightly packed cells with very little intercellular material between the cells. The
cells are arranged in continuous sheets with either one layer or several layers. It is
possible for nerves to extend through the sheets but blood vessels do not. The
underlying connective tissue holds the epithelial tissue in place and prevents it from
being torn. A thin extra cellular cellar called the basement membrane glues the
epithelium and the connective tissue together. All epithelial cells face a certain amount
of wear, tear, and injury. To replace themselves, epithelial cells divide and produce new
cells.
(a) Mucous membrane epithelial tissues. These epithelial cells line the
tubes and cavities that open to the exterior of the body; for example, the mouth, nose,
intestinal tract and urinary and reproductive tracts are lined with mucous membrane
epithelial cells. Simple epithelial cells (tissue cells arranged in a single layer) line the air
sacs of the lungs where oxygen is exchanged with carbon dioxide. This type of lining is
present in the part of the kidney that filters the blood. The inner surfaces of interior
parts of the ear are lined with epithelial cells. This lining, as evidenced by these
examples, is found in body parts that have very little wear and tear. Another function of
mucous membrane epithelial cells is to secrete mucus that serves as protection against
the entry of foreign particles into the body. Some epithelial cells are ciliated. Cells with
hair-like processes called cilia are found in some parts of the respiratory tract. These
cilia wave in unison and move mucus plus trapped foreign particles toward the throat
where the substance can either be swallowed or coughed out. This is the process of
filtering air before it enters the lungs.
(b) Serous membrane epithelial tissues. These tissues are better
known as glands or glandular epithelium. Serous membrane epithelial cells may be one
cell or a group of specialized epithelial cells whose function is to secrete substances
into ducts, onto a surface, or into the blood. Glandular cells work to produce
substances and expend energy in that effort. Glands that secrete substances into ducts
(tubes) that empty at the surface of covering and lining epithelium or directly onto a
surface are classified as exocrine glands. Products secreted by exocrine glands include
mucus, perspiration, oil, wax, and digestive enzymes. Those glands that have no ducts
and secrete their substances directly into the blood are classified as endocrine glands.
Endocrine glands secrete hormones. Examples of endocrine glands include the
pituitary, thyroid, and adrenal glands. Serous membrane epithelial tissues also cover
some organs of the body: the pleura enclose the lungs; the pericardium covers the
heart; and the peritoneum lines the abdominal cavity. These membranes secrete a thin
fluid that prevents friction when organs are in contact with one another.
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