Epithelial tissues

Epithelial tissues

Epithelial tissues differ by their structure, functional role, and origin. However, all the epithelial tissues are marked by a number of features, which allow considering them as a single tissue group.

General morphological signs of epithelia

• they are found at the boundaries between the internal and external environment thereby separating them
• they form strata of epithelial cells where cells are found in close contact with one another while there is no intercellular substance
• they rest on the basement membrane
• they have no blood and lymphatic vessels; their nutrition is ensured by the underlying loose connective tissue
• they exhibit a high physiological and reparative regenerative potential

Epithelial cells

They are remarkable for polarity that is due to the borderline role of the epithelial strata

• apical pole faces the external environment; it can contain microvilli, cilia, and stereocilia
• lateral surface ensures epithelial cell interaction via tight, adhesive, gap junctions, and interdigitations
• basal pole faces the internal environment; it is attached to the basement membrane by hemidesmosomes

Intermediate cytoskeletal filaments are formed by such proteins as cytokeratins, which are epithelial cell markers. There are 20 isoforms of these proteins; this diversity is due to a varied level of cell specialization and localization within an epithelial stratum..

Basement membrane

• is a portion of extracellular matrix between the epithelium and adjacent connective tissue
• is constituted by a number of proteins, glycoproteins, and proteoglycans, which are produced by cells of these tissues
• tightly binds the epithelium to the connective tissue, defines the architecture of its epithelial stratum, ensures selective transportation of substances

Types of epithelia

• surface epithelia cover the outer body surface, line internal body cavities and inner surfaces of hollow organs
• glandular epithelia form glands inside the walls of hollow organs or outside them

Sensory epithelia represent a kind of surface epithelium. They perceive some specific signals (reception) being a part of certain sensory organs (this is studied in the relevant section of special histology course)

Morphofunctional classification of epithelial tissues