Epithelial cells

Epithelial cells form continuous sheets that cover body surfaces and line cavities and organs. They create selective barriers and perform functions including absorption, secretion, and sensation.

Apical–basal polarity: The organization of a cell into distinct apical and basal domains with different structures and functions. Epithelial cells direct transport and secretion by keeping their two surfaces compositionally distinct.
Transcytosis: The vesicular transport of material across a cell, from one membrane surface to another. It moves selected substances through epithelial cells without opening the spaces between them.
Intestinal epithelium: The epithelial lining of the intestine, specialized for nutrient absorption, secretion, and barrier defense. Its villi and transport systems show how epithelial cells maximize absorption.
Connective tissue: A tissue type in which cells are embedded in abundant extracellular matrix that supports, connects, or protects other tissues. Unlike epithelial sheets, connective tissues typically contain more extracellular matrix than closely packed cells.
Tight junction: A cell junction that seals the space between neighboring cells and helps maintain apical–basal polarity. It limits passage between epithelial cells and preserves distinct membrane domains.
Exocytosis: The release of cellular contents when a vesicle fuses with the plasma membrane. Epithelial cells use it to deliver secretions to a surface or lumen.
Alveolar epithelium: The thin cellular lining of lung alveoli, where gases pass between air and blood. Its flattened cells create a short diffusion path for oxygen and carbon dioxide.
Endothelium: A specialized simple squamous epithelium lining blood and lymphatic vessels and the heart. It is an epithelial subtype adapted to regulate exchange between vessels and surrounding tissues.
Adherens junction: A cell junction that links actin cytoskeletons across neighboring cells through cadherin proteins. It couples epithelial cells into mechanically connected sheets.
Endocytosis: The uptake of substances into a cell through membrane invagination and vesicle formation. It enables epithelial cells to absorb and sort material from their exposed surface.

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