Structural Characteristics: Closely aggregated, tightly adherent cells with very little substance between them. Epithelial cells are often arranged in broad sheets or tube-like structures. Epithelium is commonly found on the surfaces of the body and organs, and the lining of body cavities, tubes, and ducts - the boundary between the body's internal and external environments.

• Epithelial tissues are physically separated from underlying connective tissues by a basement membrane (also called the basal lamina). The portion of an epithelial cell attached to the basement membrane is called its basal surface. The opposite side - facing the external environment, or lumen of a body cavity, is its apical surface. Basement membranes are composed of a special type of collagen and a substance called laminin (see below). The basement membrane helps epithelial cells orient themselves in relation to other tissues. After epithelial injury (e.g., an abrasion), the basement membrane serves as a scaffolding upon which new cells attach themselves during healing.

• The plasma membranes of adjacent epithelial cells frequently contain transmembrane proteins called junctional complexes (also called intercellular junctions) that link cells together. Several types of junctional complexes exist:
  1. Anchoring junctions (desmosomes, adherens junctions, hemidesmosomes) - Proteins embedded in, and spanning, the plasma membranes of adjacent cells allowing epithelia to adhere to one another forming structural units of cells - i.e., allowing individual cells to function as a solid sheet rather than as isolated cells. Fibers attached to anchoring junctions within cells help them to resist stretching forces. This is particularly important in those areas of the body that are subject to mechanical stresses (e.g., epidermis of the skin).

  2. Tight junctions - These are junctional complexes that serve as barriers to the free diffusion of molecules across epithelial surfaces. By fusing the plasma membranes on their lateral surfaces, tight junctions prevent molecules from moving between the cells. This means that any substance absorbed across a layer of epithelial cells has to pass through the cells - not around them. This provides a mechanism by which epithelial tissue can control absorption (selective permeability), and keep tissue compartments with different chemical compositions separate from one another.

  3. Gap junctions - These junctional complexes contain channels which permit ion flow and the passage of small molecules between contiguous cells. Cells with gap junctions are able to communicate and coordinate their activity.

  
  

  

  
  

• Epithelial tissues contain no blood vessels. Cells receive nourishment by diffusion from a highly vascular area of loose connective tissue just below the basement membrane called the lamina propria.

Germ Cell Origin: Epithelial tissues are derived from all three primary germ cell layers.

• Ectoderm: The epithelial cells of the skin and oral cavity (epidermis) are derived from ectoderm. Epithelial cells covering the cornea and lens, as well as sensory receptors of the eyes, ears, and nose, are also ectodermal in origin.

• Mesoderm: The epithelial lining of blood vessels (endothelium) is derived from mesoderm. The epithelial lining of the pleural and peritoneal cavities (mesothelium) also originate from mesodermal cells.

• Endoderm: The epithelial lining of the respiratory system and digestive tracts - as well as the functional cells (parenchyma) of the liver, pancreas, gallbladder, thyroid, and parathyroid, are derived from endoderm.

Functions and distribution:

• Barrier: Epithelial tissue commonly functions as a covering or lining for organs and other tissues (e.g., skin, mucous membranes, intestinal tract, pleural cavity, etc.). In this way, epithelial cells serve as selective barriers between the environment and the internal structures of the body. They protect underlying tissues from drying, and from mechanical and chemical injury. Tight junctions between individual cells play an important role in the barrier function of epithelium. Some barrier epithelial cells have motile cilia that propel fluid or particulate matter over tissue surfaces (e.g., cells lining the bronchi).

  
  

• Absorption: Epithelial cells are found in those organs (e.g., small intestine) which are involved in absorption of substances important for life. These cells often have microscopic projections on the apical surface of their plasma membranes called microvilli which increase cell surface area in order to facilitate absorption.

  
  

  

  
  Groups of epithelial cells lining the small intestine are organized into larger finger-like structures called villi which project into the lumen of the gut to further increase its surface area and aid the process of nutrient absorption.

• Secretion: The secretory cells of endocrine and exocrine glands are epithelia. (see below)

  
  

  

  
  

  
  

  

  
  

• Sensory: Many of the more complex sensory receptors of the nervous system are derived from specialized epithelia called neuroepithelia (e.g., the rods and cones of the retina, olfactory receptors of the nose, taste receptors on the tongue, etc.). Sensory receptors function by converting mechanical, chemical, or electromagnetic signals from the environment into nerve impulses which can be processed by the nervous system.

• Contractility: Some very specialized epithelial cells (myoepithelia) contain the contractile proteins myosin and actin similar to muscle. Myoepithelia are associated with the ducts of sweat, salivary, lacrimal, amd mammary glands and assist in the secretory process.

Types of Epithelial Tissues: The nomenclature of epithelial tissues classifies them according to the shape of their component cells, whether they are organized in sheets of single-cells or layers of many cells, and by their general functions.

• Covering epithelia

  1. Simple epithelium - Cells arranged in a single layer.
     • Simple squamous epithelium (squamous = scale-like) - Tissues composed of irregular, thin, flat cells with elongated and elliptical nuclei. Squamous cells are wider than they are tall.
       • Endothelium - the special term applied to the simple squamous epithelium lining blood vessels.

       • Mesothelium - the special term applied to the simple squamous epithelium lining the thoracic, abdominal, and pericardial cavities.

     • Simple cuboidal epithelium - Regularly shaped block-like cells with spherical nuclei. Found on the surface of ovaries and thyroid. The width and height of cuboidal cells are approximately equal.

     • Simple columnar epithelium - Tall, elongated, column-like cells. Found in the lining of the stomach, intestine, gallbladder, and uterine tubes. Columnar cells are taller than they are wide.

     
     

     

     
     

  2. Stratified epithelium - Cells arranged in two or more layers. Found primarily on the "wear and tear" surfaces of the body. In stratified epithelium, the height and width of cells vary from one layer to the next. Only the shape of the cells of the surface layer are used to classify stratified epithelium as squamous, cuboidal, or columnar.
     • Squamous keratinized stratified epithelium - Cells that are located near the basement membrane are block-like in shape, but gradually become flat and irregular as they migrate to the surface. Found primarily as covering on the dry surfaces of the body - e.g., the epidermis of the skin. The most superficial layers of the epidermis contain dead squamous cells composed mostly of the protein keratin. These cells are continually being lost from the surface of the body and replaced by new cells produced in the deeper (basal) layers.

     • Squamous nonkeratinized stratified epithelium - Stratified squamous cells that cover many of the moist surfaces of the body such as the mouth, vagina, and anal canal. Surface cells are not heavily keratinized.

     • Cuboidal stratified epithelium - Several layers of cuboidal cells. Found in the ducts of sweat and sebaceous glands, and ovarian follicles.

     • Transitional epithelium - Also called uroepithelium. Stratified epithelial tissue with unique dome-shaped surface cells giving it a cobblestoned appearance. Found exclusively in the urinary system (lining of bladder and ureter). They serve as a barrier preventing the exposure of underlying tissues to urine. They are also able to stretch in response to bladder filling.

     • Columnar stratified epithelium - Not very common. Found in the bulbar conjunctiva (membrane covering whites of eyes).

  3. Pseudostratified epithelium - Intermediary between simple and stratified epithelium. Consists of one layer of irregularly shaped and sized cells attached to a basement membrane. Because these cells vary in height, their nuclei are at different levels, giving the appearance (under the microscope) of more than one layer of cells. Found in lining of the trachea, bronchi, pharynx, nasal cavity, and urethra.

• Glandular Epithelia - Multicellular epithelial structures that specialize in synthesizing and secreting complex molecules. Glands are commonly classified by the mechanism they use to secrete their products:
  
  1. Exocrine glands - Glandular tissue that have ducts that open onto a body surface or into a body cavity. Examples include sebaceous, sweat, and mammary glands. Salivary glands and certain glandular structures of the pancreas that secrete digestive enzymes are examples of exocrine glands secreting into a body cavity. The secretions of exocrine glands can be mucous (viscous), or serous (thin, watery).

     The ducts of exocrine glands may be simple straight tubes, or can be arranged as more complex branching tubes. Some of the more complex branching duct systems may terminate in multiple sac-like structures called acini giving them a "bunches of grapes" appearance.

     

     Exocrine glands can also be classified by their secretion mechanism.

     • Merocrine glands - Glands that secrete substances by the process of exocytosis (fusion of cytoplasmic vesicles with the plasma membrane resulting in the release of their contents into the extracellular space without compromising the integrity of the cell membrane). Sweat glands (eccrine sweat glands) and salivary glands are merocrine in nature. The lining of the respiratory and digestive tracts contain "goblet cells" - glandular epithelia that synthesize and secrete a complex glycoprotein called mucus. Mucus provides a protective function as well as serving as a lubricant.

     • Apocrine glands - Glands that secrete by shedding the apical portion of their cytoplasm into a duct (e.g., mammary glands). Sweat glands in the axillae, perianal region, and external genitalia are also apocrine in nature.

     • Holocrine glands - Glands that secrete by shedding entire cells from the lining of a duct (e.g., sebaceous glands).

  2. Endocrine glands - Glandular tissues that have no ducts opening onto a surface or into a cavity. Most endocrine glands secrete their products (hormones) across basement membranes into connective tissues where they are absorbed by nearby blood vessels and transported to target organs (e.g., pituitary, and adrenal glands).

  3. Unicellular Glands - These are the simplest forms of glands which consist of single cells called goblet cells commonly found in the lining of the digestive and respiratory tracts. The secretory product of unicellular glands is mucous.