Each of these tissue categories has distinctive cell shapes and functions.
Epithelial tissues are avascular and they are supported by a structure known as the basement_membrane. The basement membrane is amorphous, and consists of a combination of secretions from epithelial cells as well as CT cells. The portion of the membrane that is composed of epithelial secretions is termed the basal_lamina. The portion of the membrane composed of CT secretions is called the reticular_lamina. Epithelial tissues have multiple functions, including covering and lining, secretion, absorption, protection, filtration, excretion, and sensory reception.
Endocrine glands are "ductless glands". In other words, they lack an exit path from their internal structure to the epithelial surface. The endocrine glands of the human produce and release regulatory chemicals, called hormones. Exocrine glands do have ducts, and they release their products directly to the epithelial surface. Glands such as sweat and oil glands are examples of exocrine glands.
We classify the covering and lining epithelia based upon (1) cell shape and (2) the number of layers. There are three basic epithelial shapes, plus several specializations. The three basic shapes of epithelial cells are squamous, cuboidal, and columnar. All three types of cells look the same in whole mount: six-sided. In order to distinguish among the three cell types you must view them in cross-section or longitudinal section. Epithelial cells may form single layers (simple_epithelium), multiple layers (stratified_epithelium), or appear to be multiple layers when, in fact, they are only a single layer (pseudostratified_epithelium).
(1) Simple squamous epithelium is a single layer of flattened cells with elongated nuclei.
(2) Simple squamous epithelium is the most basic type of epithelium.