Epithelium, one of the four primary tissues of the body, is characterized by

• A free edge or surface and
• A basal lamina which anchors it to the underlying structure.

Unlike other tissue types, epithelia consist mostly of cells, with little intervening space between the adjacent cells.

• The classification of an epithelial layer is based on the shape of cells that comprise it,
  • Particularly the cells on its free or exposed surface, and
• The number of cellular layers.
• These structural features also reflect the principal activities of the epithelium.

Learning Objectives:

• Understand how epithelia are classified into simple, stratified, and pseudostratified types and the usual functional significance of each type.
• Recognize the (extracellular) basement membrane of epithelia, apical specializations such as the terminal web, and ultrastructural features of the various types of intercellular junctions.
• Understand that epithelium forms barriers and compartments by covering and lining the body’s outer and inner surfaces.
• Understand that epithelium is specialized for protection (or as a barrier), transport, absorption, and secretion.

Let's start with simple squamous epithelium.