Types Of Tissue: Structure And Function - Kenhub

Epithelial tissue

Epithelial tissue is a highly cellular tissue that overlies body surfaces, lines cavities, and forms glands. In addition, specialized epithelial cells function as receptors for special senses (smell, taste, hearing, and vision). Epithelial cells are numerous, exist in close apposition to each other, and form specialized junctions to create a barrier between connective tissues and free surfaces. Free surfaces of the body include the outer surface of internal organs, lining of body cavities, exterior surface of the body, tubes and ducts. The extracellular matrix of epithelial tissue is minimal and lacks additional structures. Although epithelial tissue is avascular, it is innervated.

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Cell surfaces

The cells of epithelial tissue have three types of surfaces differentiated by their location and functional specializations: basal, apical, and lateral.

Basal surface

The basal surface is nearest to the basement membrane. The basement membrane itself creates a thin barrier between connective tissues and the most basal layer of epithelial cells. Specialized junctions called hemidesmosomes secure the epithelial cells on the basement membrane.

Apical surface

Microvilli 1/3 Synonyms: none

The apical surface of an epithelial cell is nearest to the lumen or free space. Apical cell surfaces may display specialized extensions. Microvilli are small processes projecting from the apical surface to increase surface area. They are heavily involved in diffusion in the proximal convoluted tubule of the nephron and in the lumen of the small intestines.

Cilia are small processes found in the respiratory tract and female reproductive tract. Their complex structure facilitates movement that brushes small structures through the lumen of either the trachea or Fallopian tubes. Stereocilia are similar to cilia in size and shape, however they are immotile and more frequently found in the epithelium of the male reproductive tract, specifically in the ductus deferens and the epididymis.

Lateral surfaces

The lateral surfaces of epithelial cells are located between adjacent cells. The most notable lateral surface structures are junctions. Adhering junctions link the cytoskeleton of neighboring cells to produce strength in the tissue. Desmosomes can be thought of as spot-welding for epithelial tissues. They are usually located deep to adhering junctions and are found in locations subject to stresses. For example in the stratified epithelium of the skin.

Tight junctions form a solid barrier to prevent movement of molecules between adjacent epithelial cells. Tight junctions are found in the simple columnar epithelium of the gut tube to regulate absorption of nutrients. Finally, gap junctions perform the opposite function. Gap junctions allow small molecules and structures to pass freely between cells. For example, gap junctions in cardiac muscle tissue allow for coordinated contraction of the heart.

Summary of epithelial tissue surfaces and characteristics

Characteristics	Highly cellular, function as receptors, form a barrier, minimal extracellular matrix, avascular, innervated,
Basal surface	Basement membrane, hemidesmosomes
Apical surface	Microvilli, cilia, stereocilia
Lateral surface	Adhering junctions, desmosomes, tight junctions, gap junctions

Tissue structure

Two major characteristics of epithelial tissue divide it into subclasses: the shape of the cells and the presence of layers.

Shape of cells:

• Squamous – cells are flattened, can be keratinized or nonkeratinized, involved in protection and diffusion, found in capillary walls and skin
• Cuboidal – cells are cube-shaped, can be found forming tubes in the nephrons of the kidney, involved in secretion and absorption
• Columnar – cells are rectangular, cilia are often present, involved in absorption, secretion, protection, and lubrication, form the inner lining of the gut tube

Simple columnar epithelium of endometrium 1/4 Synonyms: none

Layers:

• Simple – one layer of cells
• Stratified – two or more layers of cells
• Pseudostratified – simple epithelia that appear to be stratified when viewed in cross-section though they are only one layer of cells

Specialized epithelial tissue

• Urothelium (transitional epithelium) – distends tissues of urinary tract
• Keratinized stratified squamous epithelium – makes up the epidermis of skin
• Nonkeratinized stratified squamous epithelium – found in regions subject to abrasion, for example oral mucosa and vaginal lining
• Pseudostratified ciliated columnar epithelium – lines the inner surface of the trachea
• Endothelium - lines the inner surface of blood vessels
• Ependymal cells - present in the nervous system

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