Your skin’s outer sheet has no blood vessels of its own
Epithelium is a thin cellular blanket with almost no extracellular matrix—think epidermis on skin, or the linings of organs and vessels. It is one of four basic animal tissues, yet it is avascular: nourishment must diffuse upward through a basement membrane from connective tissue below.
Cell shapes come in three main catalogues—squamous (scaly), cuboidal, and columnar—stacked as a single simple layer or as stratified sheets two or more cells deep. Pseudostratified columnar tissue fools the eye: nuclei sit at staggered heights so a monolayer looks multilayered. Transitional epithelium in stretchy organs flips between squamous and cuboidal as tension changes. Everywhere, cells pack tightly with abundant junctions, leaving little space between neighbors and forming continuous sheets atop a fibrous basement membrane.
Simple epithelium favors absorption and filtration; its thinness is the point. Stratified sheets take abrasion instead: outer layers can scrape away without exposing deeper tissue, which is why mouth, esophagus, and vagina rely on nonkeratinized stratified squamous lining while skin’s surface keratinizes and dies in place. Endothelium lines blood and lymph channels; mesothelium coats pericardium, pleura, and peritoneum—though pathologists often refuse to call those mesoderm-derived sheets true epithelium because their cancers behave as sarcomas, not carcinomas, unlike classic epithelial tumors.
Turnover is among the body’s fastest. Crowding triggers extrusion by an actin-myosin purse-string that squeezes cells out without opening gaps; too few cells and stretch signals division. Failures of that mechanical census can open paths for aggressive tumors. Glands themselves are epithelial infoldings—endocrine products slip into blood, exocrine products ride ducts as tears, saliva, sebum, or sweat—while cilia on respiratory sheets sense chemistry, temperature, and flow in the extracellular world.
Source: Epithelium
