Epithelial TissueCommon characteristics●Cellularity: composed entirely of cells, bound close together by different types ofintercellular junctions. Minimal amount of extracellular matrix separated the cells.●Polarity: Epithelium has an apical surface (free or top surface), basal surface (fixed orbottom surface, and two lateral surfaces. Apical can have modifications such as cilia ormicrovilli and lateral can have intercellular junctions.●Attachment to a basement membrane: At the basal surface, epithelial is bound to a thinbasement membrane. A molecular structure made by the epithelium and underlyingconnective tissue●Avascularity: Epithelial tissue lack blood cells so nutrients are obtained directly from theapical surface or by diffusion across the basal surface.●Innervation: Richly innervated to detect changes in the environment at a particular bodyor organ surface region.●High regeneration capacity: the apical surface is exposed to the environment, whichmakes damages common. Damages are replaced quickly through the mitotic divisions ofthe deepest epithelial cells (stem cells). Found within the epithelium near basementmembraneFunctions●Physical protection: protects exposed and internal surfaces from dehydration, abrasion,and destruction by physical, chemical, or biological agents●Selective permeability: Epithelial cells act as “gatekeepers” for substances that enter oror leave the body.●Secretion: Some cells are specialized to produce secretions, with large groups of cellsforming a gland.●Sensation: Innervated by nerve endings that detect changes in the external environmentat their surface. Nerve endings supply information to the nervous system regardingtouch, pressure, temperature, and pain. Some specialized epithelium, calledneuroepithelium house cells responsible for senses of taste, smell, vision, hearing, andequilibriumSpecialized structuresBasement membrane: found between the epithelium and the underlying connective tissue.Consists of three layers (lamina lucida, lamina densa, and reticular lamina) composed ofproteins and carbohydrates. First two lamina are secreted by epithelial cells and the reticularlamina is secreted by connective tissue.●Provides physical support for epithelium●Anchors the epithelium to connective tissue●Acts as barrier to regulate movement of large molecules between epithelium andconnective tissue.
Intercellular JunctionsEpithelial cells are bound together by specialized connections in the plasma membrane of theirlateral surfaces●Tight Junctions: zonula occludens (“occluding belt”). Encircles epithelial cells near theirapical surface and completely attaches each cell to its neighbor. Plasma membraneproteins are fused so the apical surface of cells are tightly connected. This preventssubstances from passing between epithelial cells. Ensures all materials move through,rather than between, the epithelial cells to cross the epithelium. Epithelia controlwhatever leaves and enters the body●