Epithelial Tissue: Definition and General Characteristics
Epithelial tissue is a fundamental type of animal tissue that forms a continuous layer covering the body's external surfaces and lining its internal cavities, organs, and vessels. It also constitutes the secretory portion of glands. The word epithelium comes from the Greek words epi meaning 'upon' and thele meaning 'nipple'. This etymology reflects the initial observation of epithelial tissue on the surface of the nipple, although it is now known that epithelial tissues cover and line various surfaces throughout the body.
Epithelial cells:
Simple squamous epithelium: Forms thin linings specialised for diffusion and filtration. It can be found in the lining of blood vessels (endothelium) and body cavities (mesothelium).
Stratified squamous epithelium: Found in areas where protection is needed, such as the outer layer of the skin (epidermis).
Simple cuboidal epithelium: This type is commonly found in glands and ducts. For example, it lines the collecting ducts of the kidneys.
Stratified cuboidal epithelium: Not as common as other types, it lines the excretory ducts of salivary and sweat glands and is involved in secretion.
Simple columnar epithelium: Often involved in absorption and secretion. An example is the lining of the small intestine.
Stratified columnar epithelium: Found in the conjunctiva of the eye and some larger excretory ducts, where it offers both protection and mucus secretion.
Pseudostratified columnar epithelium: Frequently ciliated, this type lines the upper respiratory tract, where it plays a role in moving mucus. A notable example is the lining of the trachea.
Transitional epithelium, also called urothelium, is a stratified epithelium found in the urinary system. It lines organs that experience significant changes in volume, including the urinary bladder, ureters, and part of the urethra. The most superficial layer of transitional epithelium is characterised by large, dome-like cells known as umbrella cells.
Key features of transitional epithelium:
Stratified structure: Consists of multiple layers of cells, the number of which varies depending on the degree of stretching.
Variable cell shape: The cells in the basal layer are cuboidal or columnar when the organ is not distended. As the organ stretches, the cells at the apical surface become flattened, taking on a more squamous shape.
Stretch and recoil: It is highly distensible, enabling the urinary organs to stretch as they fill with urine. This ability to stretch and recoil helps prevent damage to the epithelium and underlying tissues.