All epithelial cells are separated from the underlying tissue by the basement membrane.
What is the basement membrane?
Most epithelial tissues are essentially large sheets of cells. A mixture of glycoproteins and collagen which is known as basal lamina provides an attachment site for the epithelium. Thus, it separates it from underlying connective tissue. The basal lamina attaches to a reticular lamina is secreted by the underlying connective tissue. This forms a basement membrane that helps hold it all together.
The basement membrane is a thin, fibrous, extracellular matrix of tissue that separates the lining of an internal or external body surface from underlying connective tissue in metazoans.