Epithelium

Epithelium is a tissue of closely packed cells that covers body surfaces, lines internal cavities and organs, or forms glands. It typically has little extracellular matrix and rests on a basement membrane.

Basement membrane: A thin extracellular structure that supports epithelial cells and separates them from underlying tissues. It anchors epithelial sheets and marks their boundary with connective tissue.
Epithelial transport: Movement of substances across epithelial cells or between them. Polarized epithelia absorb, secrete, or selectively pass substances between compartments.
Simple squamous epithelium: A single layer of thin, flattened epithelial cells. Its minimal thickness supports rapid exchange in alveoli and blood vessels.
Epithelial–mesenchymal transition: A cellular program in which epithelial cells lose epithelial traits and acquire mesenchymal characteristics. This transition lets epithelial-derived cells migrate during development, repair, and cancer progression.
Cell polarity: The organization of a cell into distinct regions with different structures and functions. Epithelial cells orient distinct apical and basal surfaces toward different environments.
Transcytosis: Transport of material across a cell in vesicles from one surface to another. Some epithelia move large molecules across the sheet without releasing them between cells.
Pseudostratified columnar epithelium: A single layer of cells of unequal height that appears multilayered, often bearing cilia. It lines much of the respiratory tract and helps move mucus.
Mesenchyme: An embryonic connective tissue composed of loosely arranged, migratory cells. It contrasts with cohesive epithelium and is linked to it through developmental transitions.
Tight junction: A cell junction that seals neighboring cells and regulates passage between them. It helps epithelial sheets control movement through the spaces between cells.
Exocytosis: The release of cellular contents when vesicles fuse with the plasma membrane. Glandular epithelial cells use it to release secretions onto surfaces or into ducts.

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