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 images on the histology of both normal and abnormal tissues see the articles and .
- Connective tissue: There are four subtypes:
- Neuroglia (neuroglial cells)
- Melanocytes (in the skin and retina; produce melanin)
- Odontoblasts (in teeth; produce dentin)
- Chromaffin cells (in the adrenal medulla)
- Muscle tissue: Muscle tissue is divided into two types based on structure and distribution.
- Epithelium (epithelial tissue)
- Definition: : The epithelium is one of four basic types of tissues and is present in almost all organs of the human body as lining and glandular tissue.
- Classification according to function
- Distribution: covers all inner and outer surfaces of the body (skin and mucous membranes)
- Formation: Different types of epithelia are formed from the three germ layers.
- 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.
- 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
- Explanation: The cell shape of the upper cell layer of a type of epithelium determines the name for the whole epithelium.
- Flat ( )
Cube-shaped ( )
- Appearance: Upper cells have a cube-like shape (width = height).
- Examples: bile ducts , renal tubules
Column-shaped ( )
- Appearance: Upper cells have a rectangular or cylindrical shape (width < height).
- Examples: intestine , respiratory tract ( )
Keratinization (horny layer)
- Specialization on the surface of epithelial cells can be differentiated into the horny layer (keratinization) and cell projections (microvilli, stereocilia, kinocilia).
- Keratinized epithelium is only found on the skin (stratum corneum). The epidermis is a keratinized stratified squamous epithelium.
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Disorders associated with cell projections
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
Nonkeratinized squamous epithelium
- Definition: epithelium that does not show and is composed of multiple cell layers upon a basement membrane
Structure: There are three different cell layers:
- Basal layer: a layer of to cells that lie on the
- 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)
- Definition: stratified epithelium with cells of variable height and shape
- Structure: There are three cell layers (stratum basale, stratum intermedium, superficial layer) between the and the surface.
- Properties: adapts to variations in pressure/volumetric load; permeability barrier for urine and substances dissolved in urine
- Definition: Ciliated columnar epithelium ( epithelium with ) and numerous interspersed
- Properties: cleans the airways through mucus secretion and orally directed cilia motion (= )
- Distribution: throughout the respiratory tract: nasal cavity, nasopharynx, larynx, trachea, main bronchi to the terminal bronchioli
- 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.
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).
- Definition: glandular cells that are located within the surface epithelium
- Definition: single intraepithelial glandular cells with a vacuole that is characteristically filled with mucus (goblet form)
- Function: secrete mucus (mucins)
- Small and large intestines
- Nasal mucosa and respiratory tract (see )
- Secretory epithelium
- Definition: glandular cells that are located below the surface epithelium from which they originate
- The three large salivary glands: , ,
Typical extraepithelial gland structure
Since extraepithelial glands migrate during from the into the underlying , 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):
- 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.
- 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.
- 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
- 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 and diverts to the opening of the gland (rare in histological preparations)
Shapes of the terminal ends of exocrine glands
- Acinar glands
- Shape: sac-like , wide lumen
- Examples: apocrine sweat glands
- Tubular glands
- Mixed forms: tubuloacinar, tubuloalveolar
Secretory mechanisms of exocrine glands
Exocrine glands are or 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 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
Type of secretion
Depending on the type of secretion, glands can be categorized as serous, mucous, or seromucous.
- Secretion: low viscosity and rich in proteins and enzymes
- Histological characteristics
- Examples: parotid gland, pancreas
- Mucous glands
- Seromucous glands
- Definition: specialized epithelial cells that contain actin and myosin filaments, and contract when stimulated
- Function: contraction (through proteins of the smooth muscles such as actin, myosin, and desmin filaments) → expulsion of the gland secretion
- Appearance: star-shaped, branching
- Distribution: between glandular epithelium in several glands
- Definition: : a band-like structure consisting of components of the that is found between epithelial cells and
- Distribution: The basement membrane connects connective tissue with the following cell types:
Structure: The basement membrane consists of the basal lamina; (contains two layers) and the lamina fibroreticularis.
- Basal lamina (adjacent to the epithelium)
- Lamina reticularis (adjacent to the connective tissue)
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 and .