Page 57 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

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Epithelial tissue

(textus epithelialis)

Epithelial tissue consists of sheets of closely apposed cells.
With few exceptions, it forms a boundary between the (endocrine) (secretory epithelium), e.g. (respec-
internal or external surfaces of the body and the under- tively) the pancreas and the thyroid gland,
lying connective tissue. As such, epithelium coats the • serves to eliminate harmful substances (excretory
external body surfaces and lines internal cavities (including epithelia), e.g. in the kidney (Figure 2.1),
the luminal surface of vessels). It also lines the tracts that • lines the internal surface of vessels and body cavi-
connect the internal organs with the exterior (gastrointes- ties, enabling transport of substances and fluids,
tinal, respiratory and urogenital tracts) and constitutes the e.g. endothelium and mesothelium (transport
secretory component of glands. epithelium),
In addition, epithelial tissues serve as receptors in organ • fulfils a sensory role (sensory epithelium), e.g. taste
systems associated with sensation (taste, touch, hearing, buds in the oral cavity and
balance and vision). • performs a specialised role in the differentiation and
The epithelium thus represents a boundary between maturation of male and female gametes (germinal
biologically distinct compartments. Its specific functions epithelium).
are correspondingly numerous and varied, and can be used
as the basis for dividing epithelia into tissue that: As well as forming a selective barrier between the con-
nective tissue compartment and other environments (the
• facilitates transport of substances along cellular sur- exterior, internal body cavities, the lumen of blood and
faces by means of cilia (transport epithelia), e.g. in lymph vessels), epithelia covering internal and external
the respiratory mucosa, surfaces are vital for limiting the loss of fluid from the
• secretes various chemical substances into ducts that body.
lead to the exterior (exocrine) or into the blood

2.1 Scanning electron micrograph of the low columnar epithelium of a renal tubule in a dog (x5500).

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