Written and peer-reviewed by physicians—but use at your own risk. Read our disclaimer.

banner image


Trusted medical answers—in seconds.

Get access to 1,000+ medical articles with instant search
and clinical tools.

Try free for 5 days

General histology

Last updated: April 7, 2020


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 article. Connective, muscle, and nerve tissue are addressed in the respective articles. 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 article on cellular changes and adaptive responses.

For images on the histology of both normal and abnormal tissues see the articles 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


  • Simple epithelium: a single layer of epithelial cells with all cells attached to the basement membrane
  • Pseudostratified epithelium: a single layer of cells with the appearance of multiple layers as a result of the nuclei positioned at different levels
  • Stratified epithelium: two or more cell layers with the basal layer attached to the basement membrane

Cell shape

  • Explanation: The cell shape of the upper cell layer of a type of epithelium determines the name for the whole epithelium.
  • Characteristics
    • Flat (squamous epithelium)
      • Appearance: Upper cells have a squamous (flat) shape (width > height).
      • Examples: skin , esophagus , cervix , serous membrane (e.g., peritoneum)
    • Cube-shaped (cuboidal epithelium)
      • Appearance: Upper cells have a cube-like shape (width = height).
      • Examples: bile ducts , renal tubules
    • Column-shaped (columnar epithelium)
      • Appearance: Upper cells have a rectangular or cylindrical shape (width < height).
      • Examples: intestine , respiratory tract (respiratory epithelium)

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
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
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

Urothelium (transitional epithelium)

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:
  • 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

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):

  • Terminal ends
    • Definition: A closed glandular section that is connected to the excretory ducts and is the site of secretion production. There are numerous types of terminal ends (see shapes of the terminal ends of exocrine glands below).
    • Function: production and release of secretions into the lumen (the secretion drains from here into the excretory duct system)
  • Excretory duct system
    • Definition: ducts lined with epithelial cells that connect glandular cells (in the acinus) with the opening of the gland
    • Function: drainage of secretion; changes in the nature of the secretion (e.g., through the addition/removal of ions)
    • Shapes: Because extraepithelial glands are usually subdivided into lobules (or lobuli) by connective tissue septa, there are various forms of excretory ducts.
      1. Intralobular ducts: narrows ducts composed of a single layer of epithelium within the lobuli that drain secretions from the terminal ends into the larger interlobular ducts.
      2. Interlobular ducts: wide ducts with a single layer of cuboidal to columnar epithelium that extend between the lobuli in the broad connective tissue septa and drain into the excretory ducts
      3. Excretory ducts: thickest and last segment of the duct system with a single to stratified layer of cuboidal to columnar epithelium that collects the secretions of all terminal ends and diverts to the opening of the gland (rare in histological preparations)

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.

  • Eccrine secretion
  • Merocrine secretion
    • Definition: release of secretions rich in proteins through fusion of cytosolic vesicles with the cell membrane
    • Mechanism: exocytosis
    • Distribution: in almost all exocrine glandular cells
  • Apocrine secretion
    • Definition: release of secretions rich in lipids via budding off of a part of the apical cytoplasm
    • Mechanism: A portion of the cell membrane buds off with cytoplasm containing the secretory product (e.g., milk lipid droplets). As a result, the gland cell becomes smaller.
    • Distribution: found only in apocrine sweat glands and lactating mammary glands
  • Holocrine secretion
    • Definition: release of the entire cytoplasmic content through destruction of the gland cell
    • Mechanism: apoptosis of the glandular cell
    • Distribution: found only in sebaceous glands

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 articles virtual histology slide box and virtual histopathology slide box.


  1. Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences ; 2016