Epithelial Tissue

Epithelial tissue is the basic tissue that covers and lines surfaces of body parts, makes up the secretory units of endocrine and exocrine glands, and forms the sensory epithelium of sense organs. Epithelial tissues are avascular tissues composed of sheets or clusters of contiguous cells with little extracellular matrix, except a basal lamina beneath the basal surface of the epithelial cells.

Other Names

Synonyms

• Epithelium

Types

• Surface epithelium
  • Simple epithelium
    • Simple squamous epithelium
    • Simple cuboidal epithelium
    • Simple columnar epithelium
    • Pseudostratified epithelium
  • Stratified epithelium
    • Keratinized stratified squamous epithelium
    • Non-keratinized stratified squamous epithelium
    • Stratified cuboidal epithelium
    • Stratified columnar epithelium
    • Transitional epithelium
• Glandular epithelium
  • Exocrine glandular epithelium
  • Endocrine glandular epithelium
  • Amphicrine glandular epithelium
• Sensory epithelium

Description

Epithelial tissues are composed of sheets or clusters of epithelial cells. Epithelial cells are "polarized", that is, they have distinct apical and basal surfaces. The lateral surfaces form intercellular junctions between epithelial cells. Apical surfaces may display projections - cilia or microvilli. The basal surfaces form cell-to-matrix junctions with the basal lamina, which the epithelial cells helped to construct. Otherwise, epithelial tissues lack extracellular matrix. They are also avascular; epithelial tissues depend on the vascular supply of adjacent connective tissues.

Surface epithelium

Surface epithelia are composed of sheets of epithelial cells. Some subtypes are named by their thickness and the shape of the most superficial cells. Squamous cells are flat, columnar cells are tall and narrow, and cuboidal cells are intermediate.

• A simple epithelium is a sheet that is one cell thick.
• A stratified epithelium is a sheet that is two or more cells thick.

Thicker epithelia provide more protection to underlying tissues and are generally more of a barrier to fluid movement through the epithelium. Keratinized stratified squamous epithelium and transitional epithelium have modifications that make them more water-resistant than other epithelia.

Glandular epithelium

Glandular epithelia are composed of epithelial cells that synthesize and release secretory products.

• Exocrine glandular epithelium is composed of a sheet of secretory epithelial cells folded to form a hollow sphere or tube. Exocrine cells secrete through their apical surfaces into the lumen of the gland.
• Endocrine glandular epithelium is composed of cords or clusters of secretory cells that release their secretory products through their basal surface. The thyroid gland is an exception to this description as its glandular epithelium forms follicles.
• Amphicrine glandular epithelium (tissues that are both exocrine and endocrine) forms the parenchyma of the liver. Hepatocytes are arranged in branching plates. The basal surfaces of the hepatocytes make up the surfaces of the plates, and their apical surfaces are located between cells within the plate. The spaces between apposing apical surfaces form channels that serve as bile canaliculi.

Sensory epithelium

Sensory epithelia of sense organs are specialized surface epithelia. They contain receptor cells that transduce specific sensory stimuli into neural activity, supporting cells, and, in some cases, stem cells. The receptor cells of taste buds and the inner ear are considered non-neuronal because they lack axons and are contacted by peripheral processes of primary sensory neurons. The receptor cells of olfactory epithelium have axons that make up the fibers of the olfactory nerves (cranial nerve I).

Role of Epithelial Tissue in the Body

Epithelial tissues are widely distributed, and play numerous roles:

• The epidermis (a keratinized stratified squamous epithelium) forms the superficial layer of the cutaneous covering of the body.
• The epithelium of mucosae lines the tubular and cavernous organs of the alimentary, respiratory, urinary, and genital systems. Mucosal epithelium may contain intraepithelial glandular epithelial cells (e.g., goblet cells and neuroendocrine cells)
• The mesothelium (a simple squamous epithelium) forms the superficial layer of the serous membranes that line the body cavities (i.e., the abdominopelvic, thoracic, and pericardial cavities).
• The endothelium (a simple squamous epithelium) lines the lumen of blood and lymphatic vessels.
• Surface epithelium also lines the ducts of exocrine glands and genital systems, and constitutes the tubular structures of the kidney.
• Glandular epithelium forms the secretory tissues of exocrine glands and endocrine glands, and the amphicrine tissues of liver.
• Sensory epithelium contains the receptor cells in the inner ear, olfactory epithelium and taste buds.

History

In Anatomie générale (1801), Bichat's set of 21 tissus included several that are roughly equivalent to types of epithelial tissues:

• Tissu muqueux (mucous membranes)
• Tissu séreux (serous membranes)
• Tissu glandulaire (glandular tissues)
• Tissu épidermique (epidermis)
• Tissu pileux (hair follicles)
• Tissu des vaisseaux exhalants (sweat glands might be considered Bichat's "external exhalant vessels")[[Category:|Category:]][[Category:|Category:]]