Corpus: Epithelium

The term epithelium refers to the boundary or covering tissue originating from all three cotyledons, which lines the inner and outer surfaces of the body and separates them from their surroundings. It is one of the four basic tissues.

Despite the great variety of different epithelia, the following similarities exist:

• Polarity: aligned construction and function of the cells
• Cell contacts: Connection between the cells
• Basement membrane: Anchoring of the cells to their support
• Intermediate filaments made of keratin

Epithelia do not contain blood vessels and are nourished by the underlying connective tissue by diffusion through the basement membrane. As a result, the layer thickness of the epithelia is limited to a few cell layers. The epithelium of the stria vascularis is an exception.

Polarity[Bearbeiten]

A distinction is made according to the orientation of the epithelium:

• an apical side, which faces outwards or towards the lumen
• a basal side, which faces the basement membrane or the neighbouring tissue

This polarity exists not only in the epithelium as an overall structure, but also at the level of the individual epithelial cells, whose apical and basal cell poles can also show differences.

Cell contacts[Bearbeiten]

The cohesion and communication of the epithelial cells with each other is ensured by four groups of cell junctions that connect the cell membranes of neighbouring cells:

• Zonula occludens (Tight junction)
• Zonula adhaerens (adhaerens junction, girdle desmosome)
• Desmosomes
• Gap junction

The unit consisting of the zonula occludens, zonula adhaerens and desmosome is also known as the epithelial junction complex. Cellular adhesive contacts to the basement membrane exist via so-called hemidesmosomes.

Basal membrane[Bearbeiten]

The basement membrane is a thin but complex extracellular matrix of fibrils and fibres that mediates the anchoring of the epithelium to the subepithelial connective tissue. Ultrastructurally, the basement membrane is further subdivided into:

• Lamina basalis (basal lamina)
• Lamina fibroreticularis

The subepithelial connective tissue under the mucosa of hollow organs such as the respiratory, digestive or urogenital tract is known as the lamina propria.

Apart from the basement membrane, epithelia contain hardly any intercellular substance, as the cells form a closed bandage.

Intermediate filaments[Bearbeiten]

Immunohistochemically, epithelial tissue can be differentiated from other basic tissues by the detection of keratins as intermediate filaments.

Depending on localisation and predominant function, two basic types of epithelia can be distinguished:

• Surface epithelia: covering of the outer and inner body surfaces (barrier function)
• Glandular epithelia: production of secretions of all kinds (secretory function)

This classification is a simplification, as glandular epithelia also fulfil a barrier function.

To further differentiate between the numerous forms of epithelium, three characteristics of epithelial tissue are highlighted: the number of cell layers, the shape of the cells and the degree of keratinisation.

...according to the number of cell layers[Bearbeiten]

• single-layered
• multi-layered
• multi-layered (several layers of cells, all of which are in contact with the basement membrane)
• Transitional epithelium (urothelium)

...according to the shape of the cells[Bearbeiten]

• flat
• isoprismatic (prismatic) or cubic
• highly prismatic or often incorrectly called "cylindrical

...according to degree of keratinisation[Bearbeiten]

• keratinised
• unkeratinised

Epithelium that has cilia at the apical cell pole is called ciliated epithelium.

The exact tasks of the epithelium depend on the respective organ that lines the epithelium. Overarching tasks include:

• Protection against mechanical damage
• Prevention of the penetration of microorganisms
• Diffusion barrier:
  • essential task of all surface epithelia
  • Important: protection against water loss through evaporation
  • Arachnoid neurothelium as blood-cerebrospinal fluid barrier of the outer cerebrospinal fluid space
• Perception of stimuli
• Resorption
• Secretion, excretion (glandular epithelium)
• Transport along the epithelial surface: ciliated epithelium, e.g. in bronchi, uterus or fallopian tubes
• Surface enlargement, e.g. covering cells of the urothelium

All epithelia of the human body can be categorised according to the above classification criteria:

• monolayer squamous epithelium
  • Serous membranes
  • Endothelium of the heart, blood and lymph vessels (cave: not really epithelial tissue!)
  • posterior corneal epithelium
• Single-layered isoprismatic epithelium
  • renal tubules and collecting ducts
  • Glandular ducts
  • Pigment epithelium and anterior lens epithelium in the eye
  • Amniotic epithelium
  • Choroid plexus
• Single-layered highly prismatic epithelium (cylindrical epithelium)
  • Stomach, small and large intestine
  • Gall bladder
  • Tuba uterina, uterus
• Double-row epithelium
  • Ducts of the major salivary glands of the oral cavity
  • Nasolacrimal duct
  • Epididymal duct
  • Ductus deferens
• Multi-row highly prismatic epithelium
  • Nasal cavity (except regio olfactoria)
  • Epipharynx
  • Larynx (except vocal plica)
  • Trachea and bronchial tree
  • Male urethra
  • Tuba auditiva
• Transitional epithelium (urothelium)
  • renal pelvis
  • ureter
  • Urinary bladder
  • Initial part of the urethra
• Multilayered uncornified squamous epithelium
  • Anterior corneal epithelium
  • Plica vocalis
  • Oral cavity
  • Gingiva, inner marginal epithelium (surrounds the neck of the tooth in a ring)
  • Pharynx (except epipharynx)
  • oesophagus
  • anus
  • Vagina
  • End part of the female urethra
• Multilayered keratinised squamous epithelium
  • Epidermis
  • Nasal vestibule
  • Meatus acusticus externus
  • Gingiva, oral epithelium (outer marginal epithelium) (parakeratinised)
  • Papillae filiformes
• Multilayered highly prismatic epithelium
  • Ducts of the sweat glands
  • Corpus ciliare
  • Large excretory ducts of the salivary glands
  • Fornix conjunctivae

Epithelial tissue can originate from all three cotyledons. The mesothelium and the endothelium develop from the mesoderm.

• Lüllmann-Rauch: Taschenlehrbuch Histologie, 5th edition, 2015, Thieme-Verlag