CLASSIFICATION of EPITHELIUM
Type of Epithelium*                        Location in  Body                                                                                                                   
  Simple squamous           Type I pneumocytes of alveoli; prietal layer of Bowman's capsule;  endothelium of blood and
                                                            lymph vessels; mesothelium of body cavities;  corneal enclothelium
  Simple cuboidal               Lining of respiratory bronchioles; thyroid follicular cells; germinal epithelium of ovary;
                                                            lens of eye; pigment epithelium of retina;  epenclymal  cells of choroid plexus
  Simple columnar            Linings of: pulmonary bronchioles, gastrointestinal tract, anal canal above anal valves;
                                                            uterus and uterine tubes, large excretory ducts of glands
  Stratified squamous     Epidermis of skin; corneal epithelium; linings of: oral cavity and esophagus, anal canal below
                                                             anal valve, vagina, female urethra, fossa navicularis of the penile urethra
  Stratified columnar      Lining of prostatic, membranous, and penile urethra up to fossa navicularis
  Pseudostratified columnar   Lining of trachea and primary bronchii Lining of efferent ductules, epididymis, and
                                                            ductus deferens
  Transitional                       Lining of renal calyces, renal pelvis, ureters, and urinary bladder                                                     
                                                            * All epithelium is avascular and has a high repair (regeneration) capacity.

  POLARITY of an epithelial cell is evident by specializations that are found in various regions of the cell.

Apical region  
Microvilli contain a core of actin filaments that are anchored to the terminal web. The actin filaments are crosslinked by villin. Microvilli of intestinal epithelium are coated with a glycocalyx that consists of terminal oligosaccharides of integral membrane proteins. Enzymatic activity of the glycocalyx is involved in carbohydrate digestion.
 Stereocilia are long microvilli found on epididymal epithelium and hair cells of the inner ear.
 Cilia are motile cell processes that contain a core of microtubules called the axoneme. The axoneme consists of nine doublet microtubules uniformly spaced around two central microtubules (9+2 arrangement). Nexin connects the nine doublet microtubules. Each doublet has short arms consisting of dynein ATPase' which splits ATP to provide energy for cilia movement. At the base of each cilium is a basal body, which consists of nine triplet microtubules and zero central microtubules (9+0 arrangement).

Lateral region
 Zonula occludens is a tight junction that extends around the entire perimeter of the cell. The outer leaflets of the cell membrane of the two adjoining cells come into close proximity and fuse at various points.
 Zonula adherens is a junction that extends around the entire perimeter of the cell. The cell membranes of the two adjoining cells are separated by an intercellutar space filled with an amorphous material. A dense area on the cytoplasmic side of each cell consists of actin filaments that are linked by a-actinin and vinculin to a transmembrane protein called E,cadherin, or A-CAM (adherens cell adhesion molecule).
 Macula adherens, or desmosome, is a junction that occurs at small, discrete sites. The cell membranes of the two adjoining cells are separated by an intercellular space filled with a thin line of dense material. An attachment plaque on the cytoplasmic side of each cell anchors tonofilaments. Several protein components of the desmosome have been identified: desmoglein 1, desmocollin I and II, desmoplakin I and 11, and pakoglobin.
 Gap junction, or nexus, is a junction that occurs at small, discrete sites. The cell membranes of the two adjoining cells are separated by an intercellular space that is bridged by connexons.
a. Connexons contain central pores that allow passage of ions, cyclic adenosine monophosphate (cAMP), amino acids, and small molecules (< 1200 MW) between cells.
b. In addition to epithelium, gap junctions are located between osteocytes, astrocytes, cardiac muscle cells, smooth muscle cells, and endocrine cells.

Basal region
 Basal infoldings are invaginations of the cell membrane. They contain ion pumps (Na ' K ' - ATPase) in close association with mitochondria, which provide substrate ATP. Basal infoldings are found in cells of the proximal and distal convoluted tubules of the kidney and in ducts of salivary glands.
 Hemidesmosomes are junctions that anchor epithelial cells to the underlying basal lamina via a transmembrane protein called integrin. As a result, hemidesmosomes provide a connection between the cytoskeleton of the epithelial cell and the extra, cellular matrix.
 Basal lamina is a supporting layer between epithelium and the underlying connective tissue that consists of a lamina lucida and a lamina densa. The principal constituents are heparan sulfate, type IV collagen, laminin, and fibronectin (which binds to integrin of the hemidesmosome). Anchoring fibrils composed of type VII collagen loop around collagen fibers in the subjacent connective tissue and terminate in anchoring plaques, which consist of type IV collagen.