Epithelium
:''This article discusses the epithelium, an animal anatomical structure. For the fungal anatomical structure of the same name, see
Pileipellis.
In
zootomy,
epithelium is a
tissue composed of a layer of
cells. In humans, it is one of four primary body tissues. Epithelium lines both the outside (
skin) and the inside cavities and
lumen of bodies. The outermost layer of our
skin is composed of dead stratified
squamous epithelial cells, as are the
mucous membranes lining the inside of mouths and body cavities. Other epithelial cells line the insides of the
lungs, the
gastrointestinal tract, the reproductive and urinary tracts, and make up the
exocrine and
endocrine glands.
Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability.
Endothelium (the inner lining of
blood vessels) is not related to epithelium except by name.
Epithelial cells:
* Sit on a
basal lamina (formerly called a basement membrane).
* Have a free surface (if a cell has all of the other properties, but no free surface, it is called an
epithelioid).
* Are avascular (contain no blood vessels).
* Have almost no extracellular space.
* Renew basally.
* Are derived from all three
germ layers.
* Are named for the most superficial live (nucleus-containing) layer.
Epithelial cells are classified by the following three factors:
* Shape
* Stratification (number of layers)
* Specialization
Shape
*
Squamous: Squamous cells are flat cells with an irregular flattened shape. The one-cell layer of simple squamous epithelium that forms the alveoli of the respiratory membrane, and the
endothelium of capillaries, and is a minimal barrier to
diffusion. Places where squamous cells can be found include the
alveoli of the
lungs, the filtration tubles of the
kidneys, and the major cavities of the body. These cells are relatively inactive metabolically, and are associated with the diffusion of water,
electrolytes, and other substances.
*
Cuboidal: As the name suggests, these cells have a shape similar to a cube, meaning its width is the same size as its height. The
nuclei of these cells are usually located in the center.
*
Columnar: These cells are taller than they are wide. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. The nucleus is also closer to the base of the cell. The
small intestine is a tubular
organ lined with this type of tissue. Unicellular glands called goblet cells are scattered throughout the simple columnar epithelial cells and secrete
mucus. The free surface of the columnar cell has tiny hairlike projections called
microvilli. They increase the surface area for absorption.
*
Transitional: This is a specialized type of epithelium found lining organs that can stretch, such as the
urothelium that lines the bladder and
ureter of mammals. Since the cells can slide over each other, the appearance of this epithelium depends on whether the organ is distended or contracted: if distended, it appears as if there are only a few layers; when contracted, it appears as if there are several layers.
Stratification
*
Simple: There is a single layer of cells.
*
Stratified: More than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the
basal lamina. Stratified cells can usually withstand large amounts of stress.
*
Pseudostratified with cilia: This is used mainly in one type of classification (pseudostratified columnar epithelium). There is only a single layer of cells, but the position of the nuclei gives the impression that it is stratified. If a specimen looks stratified, but you can identify cilia, the specimen is pseudostratified ciliated epithelium since stratified epithelium cannot have cilia.
Specializations
*
Keratinized cells contain
keratin (a
cytoskeletal protein). While keratinized epithelium occurs mainly in the skin, it is also found in the
mouth and
nose, providing a tough, impermeable barrier.
*
Ciliated cells have apical
plasma membrane extensions composed of
microtubules capable of beating rhythmically to move
mucus or other substances through a duct. Cilia are common in the
respiratory system and the lining of the
oviduct.
*
*
Simple squamous: Found in blood vessels &
lymph channels (called
endothelium) and body cavities (called
mesothelium)
*
Keratinized stratified squamous: Found in human skin (specifically, the dead superficial layer); also found in masticatory oral mucosa (attached gingiva, dorsum of tongue, hard palate, etc.)
*
Non-Keratinised Stratified squamous: Found in human
oesophagus (Oral Mucosa) specfically non-masticatory "movable" mucosa, and Vagina
*
Simple cuboidal: Found in
thyroid follicles
*
Stratified cuboidal: Exclusively found in sweat gland ducts
*
Ciliated simple columnar: Found in intestine and kidney (specifically,
proximal convoluted tubule)
*
Stratified columnar: Ducts of
submandibular glands
*
Transitional: Specialized to distend (stretch) as the
urinary bladder fills
As stated above, secretion is one major function of epithelial cells.
Glands are formed from the invagination / infolding of epithelial cells and subsequent growth in the underlying connective tissue. There are two major classification of glands:
endocrine glands and
exocrine glands.
All epithelia derive from either the
ectoderm or the
endoderm, two embryological structures.
*
Squamous cell*
Columnar epithelia*
Cuboidal epithelia*
Transitional epithelia*
Corneal epithelium*Molecular Biology of the CELL, 4th edition, Alberts et al., 2002
