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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

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).

(2) The uppermost cells are keratinized and dead, but the cells at the bottom of the stack are alive and active. These live, active cells are called basal_cells.