An epithelium is an organized and polarized sheet of cells (the two sides are different). The apical surface of an epithelial sheet faces the so-called outside; the basal surface is bordered by an extracellular matrix called the basal lamina. Epithelial folding is microtubule-dependent.
Epithelial cells are bound by tight junctions immediately beneath the apical surface. Tight junctions prevent leakages and influxes from creeping between cells. Immediately basal to the tight junctions are adherens junctions, which form a ring around the apical end of the cell and which are directly linked to the actin cytoskeleton via E-cadherin. The "glue" of an adherens junction, E-cadherin is a homophilic transmembrane cell adhesion molecule which binds to β-catenin (which then associates with F-actin).
Any cell not organized into an epithelium is mesenchyme. Mesenchyme is a loose arrangement of cells -- sometimes these cells are individually mobile -- surrounded by a matrix of extracellular material such as glycoprotein and/or proteoglycan.
An epithelium is an organized sheet of cells that has the characteristic of being polarized. The basal side is bordered by a basement membrane (discussed below). The apical side faces a lumen (like the cavity of the gut) or the outside surface of the organism. Cells in an epithelium are bonded together by tight junctions, discussed in a previous lecture, that prevent the leakage or influx of fluids or molecules between cells. Just basal to the tight junction is the adherens junction, discussed below. The adherens junction is directly liked to the cytoskeleton and thus, as we will see, plays an important role in cell shape change.