General histology


Tissues are formed by cells and can be differentiated into four different types: connective tissue, muscle tissue, nerve tissue, and epithelium (epithelial tissue). Epithelium is a component of almost every organ in the body. The subtypes and functions of epithelium will be discussed in this learning card. Connective, muscle, and nerve tissue are addressed in the respective learning cards. Epithelium covers all inner and outer surfaces of the body (except joint cavities), namely the skin (epidermis) and all mucous membranes. Based on its predominant function, epithelium can be differentiated into protective surface epithelium and secretory glandular epithelium (exocrine glands). Surface epithelium is named and classified according to various criteria (layers, cell shape, surface characteristics). Glandular tissue can be classified according to its location, shape, secretory mechanism, and the type of secretion associated with it (serous or mucinous). The basement membrane anchors the epithelium to connective tissue and creates a diffusion barrier. The basement membrane is composed of a band-like layer of fibers and large proteins and is tightly bound to the cytoskeleton of cells.

For more information about cellular adaptive responses including apoptosis, necrosis, see the learning card on cellular changes and adaptive responses.

For images on the histology of both normal and abnormal tissues see the learning cards virtual histology slide box and virtual histopathology slide box.

Overview of the four basic tissue types

There are four basic types of tissues in the body: epithelium, connective tissue, nervous tissue, and muscle tissue.

  1. Connective tissue: There are four subtypes:
  2. Nervous tissue:
  3. Muscle tissue: Muscle tissue is divided into two types based on structure and distribution.
  4. Epithelium (epithelial tissue)


Surface epithelium

  • Definition: A specialized layer of tissue formed by closely aggregated cells that line the outer surface of organs, blood vessels, the skin, and the inner surface of body cavities. Divided into squamous, cuboidal, and columnar types.
  • Functions
    • Protects against mechanical/chemical effects and harmful radiation
    • Resorption: e.g., nutrients in the gastrointestinal tract
  • Classification: Surface epithelia are classified according to the number of layers, cell shape, and surface differentiation.

Criteria for classification


Cell shape

Epithelial specialization

Keratinization (horny layer)

Cell projections

Microvilli Stereocilia (stereovilli) Kinocilia
  • Finger-shaped, non-active motile projections on the apical surface of epithelial cells
  • Eyelash-shaped, active motile projections on the apical surface of epithelial cells
  • 1–2 μm in length and 0.1 μm diameter
  • 10 μm in length and 0.2 μm diameter
  • 5 μm in length and 0.25 μm diameter
  • Inner ear: hair cells of the organ of Corti (hearing) and the vestibular organ (sense of balance)
  • Epithelium of the epididymal duct and vas deferens
Active and passive motion
  • No active motion possible
  • Inflexible (rigid)
  • No active motion possible
  • Flexible (passive motion) in the inner ear
  • Inflexible (rigid) in the epididymal duct and vas deferens
  • Increases the absorption surface area of the epithelia
  • Inner ear
    • Hearing
    • Sense of balance
  • Epididymal duct and vas deferens: secretion and absorption
Special types -
Associated disorders

Disorders associated with cell projections

Microvilli and stereocilia are comprised of actin filaments, while kinocilia are composed of microtubules!

Special surface epithelia

Some surface epithelia possess more than one of the criteria mentioned above or occur especially often in a number of different organs.

Keratinized stratified squamous epithelium

Keratinized stratified squamous epithelium is only found in the epidermis. (see Skin and skin appendage).

Nonkeratinized squamous epithelium

  • Definition: epithelium that does not show keratinization and is composed of multiple cell layers upon a basement membrane
  • Structure: There are three different cell layers:
    • Basal layer: a layer of cuboidal to columnar cells that lie on the basement membrane
    • Intermediate layer: multiple layers of polygonal cells
    • Surface layer: multiple layers of squamous cells
  • Properties: especially resistant to mechanical stress (e.g., through food passage)
  • Distribution

Urothelium (transitional epithelium)

  • Definition: stratified epithelium with cells of variable height and shape
  • Structure: There are three cell layers (stratum basale, stratum intermedium, superficial layer) between the basement membrane and the surface.
    • Empty bladder: 5–7 cell layers; cells in the superficial layer (multinucleated umbrella cells) are hemispherical
    • Full bladder: 3–4 cell layers; umbrella cells are flattened
  • Properties: adapts to variations in pressure/volumetric load; permeability barrier for urine and substances dissolved in urine
  • Distribution

Respiratory epithelium

Glandular epithelium

  • Definition: A single cell or group of cells that produce and secrete specific products (e.g., mucin). Glandular epithelium commonly invaginates from surface epithelium into other tissue (e.g., connective tissue), but it is separated from the tissue by the basal lamina. Glandular epithelium can be endocrine (e.g., secrete products in the bloodstream) or exocrine (e.g., secrete products onto a surface).
  • Function: Depending on the type of gland, it secretes:
    • Digestive juices containing enzymes (saliva, gastric and pancreatic juices)
    • Sweat
    • Sebum
    • Breast milk
    • Mucous (mucins)
  • Classification: Glandular tissue can be classified according to its location, shape, secretory mechanism, and the type of secretion associated with it.

Classification criteria

Location of glandular tissue

Based on the location of glandular cells in relation to the surface epithelium, glands can be classified as either intraepithelial (located in the surface epithelium) or extraepithelial (located beneath the surface epithelium).

Intraepithelial glands

  • Definition: glandular cells that are located within the surface epithelium
  • Shapes
    • Goblet cells
      • Definition: single intraepithelial glandular cells with a vacuole that is characteristically filled with mucus (goblet form)
      • Function: secrete mucus (mucins)
      • Distribution
    • Secretory epithelium
      • Definition: conglomerates of columnar epithelial cells with a bright cytoplasm (filled with mucus) and no vacuole
      • Distribution: e.g., the surface epithelium of the stomach and cervical canal is entirely composed of secretory epithelium

Extraepithelial glands

Typical extraepithelial gland structure

Since extraepithelial glands migrate during embryogenesis from the surface epithelium into the underlying connective tissue, they have an elaborate structure composed of excretory ducts and terminal ends. These components are named below in the sequence corresponding to the path of the secretion (from formation to the opening):

Intercalated ducts are only present in the three large salivary glands and in the exocrine pancreas! Striated ducts are only present in the three large salivary glands!

Shapes of the terminal ends of exocrine glands

Secretory mechanisms of exocrine glands

Exocrine glands are intraepithelial glands or extraepithelial glands that release their secretions on the inner (e.g., intestinal lumen) or outer (e.g., skin) surfaces of the body.

Type of secretion

Depending on the type of secretion, glands can be categorized as serous, mucous, or seromucous.

Myoepithelial cells

Basement membrane

The basal lamina and lamina reticularis form the basement membrane! The basement membrane is usually identified with light microscopy, whereas the basal lamina is identified with electron microscopy!


Microscopy of cells and tissues

  • Light microscope: the most commonly used method of examining individual cells (cytology) as well as normal (histology) and pathological tissue (histopathology) for diagnostic and teaching purposes
    • Visualization of structures using stains (e.g., H&E stain)
  • Electron microscope: method primarily used for research purposes and in the diagnosis of certain kidney, muscle, and CNS diseases, with a higher resolution than light microscopy
    • Visualization of the structures through compounds of heavy metals (e.g., osmium tetroxide)

For images on the histology of both normal and abnormal tissues see the learning cards virtual histology slide box and virtual histopathology slide box.

last updated 10/22/2018
{{uncollapseSections(['joc_bW0', 'PocWXW0', '4oc3XW0', 'kocmXW0', 'OocIXW0', 'locvXW0', 'Noc-XW0'])}}