Very simple question about epithelium

[sameeralord]

wikipedia-In biology and medicine, an epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body.[1] Many glands are also formed from epithelial tissue.[2] It lies on top of connective tissue, and the two layers are separated by a basement membrane.[3]

When they say line the cavities and structures do the mean inside or outside. Does epithelium create a membrane outside(is it the outermost membrane) or is it inside the membrane like villi in the intestine. What does the term line in imply.

Thank you!! :smile:

[Andy Resnick]

"epithelia" as a tissue, are a boundary between inside and out: the epithelial lining of our mouths, throats, digestive tract, lungs, and nasal cavity are obvious examples. But many organs also have epithelia- kidneys, liver, pancreas, for example. The blood vessels are lined with endothelia, which are different. Epithelial tissue is distinguished from other tissues (neuronal, muscle, endothelial, etc) by virtue of *vectorial transport* of material from the outside to the inside (or vice versa).

Epithelial tissue is composed of a great many cell types and connective tissue (as you mention). Mucus is also a common component.

[Smachine]

Epithelium that lines something like intestine means that its the last layer of cells in case of intestine facing the lumen. The villi are actually all covered with epithelial cells, this image of small intestine villi might help to see how they line them.
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Epithelia/images/duo043he.jpg