Histology is the study of tissues. Tissues are groups of cells that are similar in structure and function. There are four tissue types associated with animals such as humans. They are:

epithelial_tissue,

connective_tissue (CT),

muscle_tissue, and

nerve_tissue

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.

Epithelial cells can also form glands. There are two types of glands in the human body: endocrine and exocrine.

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).

NOTE:

(1) There are two important types of epithelia that are less easily categorized. They are (1) pseudostratified and (2) transitional.

(2) Pseudostratified is a special form of columnar epithelium in which the cells vary in height as well as the placement of their nuclei. Because their heights are asymmetrical and their nuclei are located at various levels, they appear to be "stacked". In fact, each cell in the so-called "stack" does touch the basement membrane and therefore there is no stack, just an irregular single (simple epithelium) layer of cells.

(3) Pseudostratified epithelium tends to be ciliated.

(4) Transitional epithelium consists of modified stratified squamous epithelial cells that are distensible and can slide over one another. These cells can stretch (flatten out) to accommodate a change in volume, and then return to their original (plump) shape. They are located in the kidneys, ureters and the urethra.