Overview And Types Of Epithelial Tissue - Kenhub

Structure

Epithelial cells are the building blocks of epithelial tissue. Like every other cell, they are surrounded by cellular membranes. Epithelial cell membranes have three regions (domains) different in structure and function; apical, lateral and basal. This feature is called membrane polarity. 

Polarity 

Plasma membrane Plasmalemma 1/2 Synonyms: Cell membrane, Membrana cellularis

Each membrane pole exhibits various features. These can include receptors and channels for transportation of substances that the epithelial cell needs to internalize or expel, or membrane specializations.

Apical poles project towards the external surface or the lumen of an organ which epithelia cover, which is why they’re also called free surfaces or free domains. This pole may also show apical membrane specializations which alter the shape of this surface. Lateral surfaces are the sites where adjacent cells interconnect, forming tightly packed contiguous cells. Basal surfaces are in contact with the underlying basement membrane. As both lateral and basal surfaces are adjusted to interact with surrounding structures, they’re often mentioned together as a basolateral surface. 

Apical specializations

Apical specializations are different types of fingerlike cytoplasmic extensions of the apical surface. They differ in their length, motility and function. There are three types of specializations;

Microvilli 1/3 Synonyms: none

• Microvilli - are microscopic protrusions of the cell membrane which mainly function to increase the apical surface area for absorption. Large quantities of microvilli are said to form a 'brush border', due to their appearance in histological preparation. The enterocytes of the small intestine has such specializations to facilitate absorption of food in the gut. 
• Stereocilia - a type of microvilli, but these are immotile and longer than microvilli. Their functions are absorption (epididymis and ductus deferens) and mechano-sensory reception (inner ear)
• Cilia - fingerlike structures present on almost every epithelial cell. There are three types of cilia; motile, non-motile (primary) and nodal. Their respective functions are; removing foreign particles from the epithelial surface, transportation of signals from the ECM to the intracellular space, and fetal development.

Junctions 

Intercellular junctions are protein complexes on the basolateral cell membranes of epithelial cells. Their functions are to establish membrane polarity, connect adjacent cells and anchor the epithelial tissue to the connective tissue underneath it. There are five main types of junctions:

• Tight junctions (occluding junctions) – a complex intercellular junction that blocks the intercellular space between adjacent cells making it impassable. 
• Adhering junctions and desmosomes – connect the cytoskeletons of adjacent cells.
• Communicating (gap) junctions – passageways between adjacent cells that allow the transfer of small molecules between adjacent cells.
• Anchoring junctions (hemidesmosomes) – a cell matrix junction that anchors the cell to the basement membrane.

Extracellular matrix

Basement membrane Membrana basalis 1/3 Synonyms: none

Epithelial cells are aligned into one or more rows, separated by thin layers of extracellular matrix. The deepest row (basal domain) produces a layer of specialized extracellular matrix called the basement membrane. This membrane demarcates the epithelial tissue from the underlying connective tissue. Remember hemidesmosomes? These and other cell-to-extracellular matrix junctions anchor epithelial cells to the underlying basement membrane.

A layer of connective tissue called the lamina propria, attaches to the basal surface of the basement membrane. Lamina propria holds the epithelial tissue glued to the deep tissues and contains blood vessels that supply the epithelial tissue.

Tissue

Let’s round this all up to understand how epithelial cells make the epithelial tissue. So, epithelial cells are tightly packed into one or more layers, held together by a variety of intercellular junctions. The deepest layer sits on a basement membrane, while the surface layer is free. Epithelial tissue is avascular but innervated. Nutrients are supplied by diffusion from blood vessels of the lamina propria. Epithelia contain stem cells in their basement membranes which enable continuous epithelial renewal. Some epithelia renew very fast (skin), while others do it at a slower pace. 

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