Page 59 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Epithelial tissue (textus epithelialis) 41

Epithelial tissue is further characterised by the • surface epithelium,
VetBooks.ir following. • glandular epithelium or

• sensory or neuroepithelium.
• It rests upon a basal lamina.
• It consists of cells that are closely apposed, held
together by specialised intercellular junctions. Surface epithelium (epithelium superficiale)
• Features of particular sections of the cell membrane Surface epithelium is located on the surface of tissues
give rise to morphological and functional cellular and organs and forms the sheet-like covering of the outer
polarity. and inner surfaces of the body. The structure of these
epithelia is largely determined by the demands of their
In descriptions derived from electron microscopy, the environment (Table 2.1). The outer layers of the skin (epi-
layer upon which the basal surface of epithelial cells rests dermis) are subjected not only to mechanical forces, but
is referred to as the basal lamina. Using special stains (e.g. also to radiation and changes in temperature and humidity.
PAS), this acellular layer, composed of proteins and poly- The internal lining of the respiratory tract is exposed to
saccharides, can be visualised with the light microscope. humidified air, while the organs responsible for conveying
In this context it is termed the basement membrane (see urine, including the bladder and urethra, are frequently
Chapter 1, ‘The cell’). in contact with strongly hypertonic fluid. In the gut, the
In a small number of locations, the epithelia have lost intestinal epithelium encounters a wide range of nutrients.
their association with a free surface and with the basal lam- Reflecting these varied conditions, surface epithelia exhibit
ina. At these sites they are referred to as epithelioid tissue a range of specialisations.
and are generally associated with endocrine glands (e.g. Surface epithelia are classified according to the number
interstitial Leydig cells in the testes, luteal cells in the ovary, of cell layers present and the shape of the epithelial cells
islets of Langerhans in the pancreas, the parenchyma of (Figures 2.2 to 2.21). They may consist of one or more cell
the adrenal gland and the hypophysis). Epithelioid cells are layers and are thus divided into:
also present in the thymus.
• single-layered (simple and pseudostratified) epithe-
Histogenesis lium and
Epithelia can be distinguished based on their embryonic • multi-layered (stratified) epithelium.
origin. They arise from all three primary germ layers.
Tissues arising from the ectoderm include the skin
(epidermis) and its derivatives (e.g. sebaceous glands and Single-layered epithelium (epithelium simplex)
sweat glands), epithelial components of the oral and nasal Single-layered epithelium is found primarily on surfaces
cavities, and parts of the visual, olfactory, gustatory and involved in absorption or secretion. It offers little resis-
auditory systems. tance to mechanical forces yet can undergo distension.
The mesoderm gives rise to simple (single-layered) The shape of the epithelial cell varies with function and
epithelia that line body cavities (thoracic, abdominal and may be:
pelvic cavities) and cover the external surface of internal
organs as serosae. Epithelium of mesodermal origin also • squamous,
forms the internal lining of the circulatory and lymphatic • cuboidal or
systems (endothelium). • columnar.
The endoderm differentiates into a multitude of epi-
thelia that primarily line the interior of hollow organs or
become part of parenchymatous organs. These endoder- SIMPLE SQUAMOUS EPITHELIUM (EPITHELIUM
mal cells subsequently determine organ function. Epithelia SIMPLEX SQUAMOSUM)
of endodermal origin are found, for example, throughout In simple squamous epithelia, the cytoplasm of the epithe-
much of the gastrointestinal and respiratory tracts, in the lial cells is spread out (i.e. the cell is flat) (Figure 2.2). The
bladder and accessory sex glands and in the middle ear typically flattened nuclei may bulge above the free surface
(internal lining). The epithelium of exocrine glands (e.g. of the cell. This type of epithelium facilitates passive dif-
pancreas) and endocrine glands (e.g. thyroid gland and fusion across internal bodily surfaces. It forms the internal
parathyroid gland) is also derived from endoderm. lining of vessels, where it is referred to as endothelium.
Due to its conformability, it can accommodate changes in
Classification intraluminal volume. The presence of pores also allows
Based on the structure and function of individual cells, epi- for increased permeability and associated acceleration of
thelium can be classified as: metabolic processes.

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