Epithelial Tissues Explained

Types of Tissues

• Tissue refers to a group of similar cells performing specific activities together.
• The four main types are epithelial, connective, muscular, and nervous tissues.

Epithelial Tissues

• Epithelial cells cover or line body surfaces, cavities, and tubes.
• This tissue consists of closely packed cells, lacking intervening intracellular substance.

General Characteristics of Epithelial Tissues

• Epithelial tissues cover and line cavity walls and form boundaries.
• Cell junctions bind adjacent cells together for physical strength and metabolite exchange.

Special Characteristics of Epithelial Tissues

• Cells are closely attached.
• Exhibit polarity, with apical (superior) and basal (inferior) surfaces interacting with the outside and underlying connective tissue, respectively.
• Epithelial tissues are avascular but innervated.
• Supported by connective tissues, with cells attached to a basement membrane.
• They have the ability to regenerate quickly.

Functions of Epithelial Tissues

• They provide protection against mechanical injury, pathogens, and fluid loss.
• They participate in secretion, releasing substances like enzymes, hormones, mucus and sweat.
• Epithelial tissues facilitate absorption of nutrients and respiratory gases.

Classification of Epithelial Tissues

• Classification is based on the number of cell layers (simple or compound), and the shape of the cells (squamous, cuboidal, or columnar).

Simple Epithelia

• Simple epithelia consist of a single layer of cells and are found at interfaces involved in selective diffusion, absorption, and secretion.
• They offer little protection against mechanical abrasion.
• Cell shapes range from flattened to tall.
• Flattened cells are suited for diffusion.
• Columnar cells, present in highly active epithelial tissues, accommodate appropriate organelles.

Simple Squamous Epithelium

• This epithelium consists of a single layer of thin, flattened, irregularly shaped cells, resembling a fried egg.
• It is easily damaged.
• This epithelium forms a continuous surface known as pavement epithelium.
• Found commonly at sites of filtration, diffusion, and osmosis and covers surfaces.
• Locations include air sacs of the lungs, walls of capillaries, and linings of blood and lymph vessels.
• Functions include limiting membrane, fluid transport, lubrication, gaseous exchange, and reducing friction.

Simple Cuboidal Epithelium

• Characterized by a single layer of cube-shaped cells with centrally located nuclei.
• This functions in secretion, absorption, and surface barriers.
• Locations include surface of ovaries, lining of kidney tubules, and linings of ducts of certain glands.

Simple Columnar Epithelium

• Defined as a single layer of cells that are longer than they are wide, with elongated nuclei near the basement membrane.
• Can be ciliated or non-ciliated.
• Some have microvilli.
• Goblet cells secrete mucous.
• Locations include the linings of the uterus, stomach, and intestines.
• Functions include transportation, absorption, secretion, and protection.

Pseudostratified Columnar Epithelium

• This type appears stratified but aren't, with nuclei at two or more levels, though not all cells touch basement membrane.
• This is ciliated and has goblet cells.
• The functions are protection, secretion, and movement of mucus.
• Found in the linings of respiratory systems.

Stratified Epithelia

• Consist of two or more layers of cells.
• Aid in protecting regions of wear and tear.
• The nature and degree of stratification is related to physical stresses to which the surface is exposed.
• They are poorly suited for absorption because of their thickness.
• Classification is based on the appearance of the top layer of cells.
• Stratified squamous epithelium refers to the top layer of squamous cells.
• Stratified cuboidal epithelium has a top layer of cuboidal cells.

Stratified Squamous Epithelium

• Consists of multiple layers of flattened cells.
• Regenerates rapidly.
• Old cells slough off and are replaced by new cells.
• The main function is protection.
• Locations include the outer layer of skin, lining of the oral cavity and throat, the lining of the vagina, and the lining of the anal canal.

Stratified Cuboidal Epithelium

• Characterized by two or three layers of cuboidal cells and functions in protection.
• Locations include the linings of large ducts of mammary glands, sweat glands, salivary glands, and pancreas.

Stratified Columnar Epithelium

• It has a top layer consisting of columnar cells and primarily functions in protection.
• Found in the vas deferens, part of the male urethra, and a part of the pharynx.

Transitional Epithelia

• This tissue can stretch and exhibits distensibility (stretching).
• In a non-distended state, there are four to five cell layers thick.
• It exists in a stretched state of one to three layers (but the number of layers remains constant).
• The appearance is due to flattening of intermediate and surface layers.
• Its basal cells are cuboidal, with polygonal intermediate cells.
• Surface cells are large, rounded, and may contain two nuclei.
• Functions include protection and stretchability.
• It is located in the inner lining of the urinary bladder, as well as the lining of the ureters and part of the urethra.