Epithelial Tissue: Types, Characteristics, and Functions
Epithelial Tissue
Epithelial tissue has two main forms: lining epithelia, which covers parts of the body (including the face’s interior), and glands originating from epithelial cells.
This tissue is formed by juxtaposed cells, closely attached to each other.
Lining Epithelia
Cells in this tissue are always based on the basement membrane. Just below it is the connective tissue, which nourishes the epithelial cells because they are mostly avascular (lacking blood vessels). Nutrient exchange occurs through diffusion.
Classification
The primary classification depends on the number of layers or strata:
- Simple epithelia (monostratified): A single layer of cells.
Subdivisions of Simple Epithelia
- Simple squamous (flat): A single layer of flat cells, usually attached laterally. Height increases only where the nucleus is located.
- Simple cuboidal: Cells with similar width and height, appearing cube-like under a microscope, with a rounded central nucleus.
Surface layers: Mature cells are named based on their composition.
The epithelial basement membrane is not smooth but has finger-like projections formed by connective tissue, allowing it to reach more epithelial layers.
Types of Stratified Epithelia
- Cornified flat: Covers the outer surface (epidermis). The most superficial layer consists of flat scales filled with keratin, which waterproofs the skin. Polygonal epidermal cells contain tonofilaments and keratohyalin granules. The nucleus is lost in the last layers.
- Non-cornified flat: Found in high-humidity areas. Superficial layer cells are similar to those in earlier strata, but with a flat shape.
- Cylindrical: Surface layer cells are cylindrical, maintaining the same layer structure as other multi-layered epithelia.
- Urothelium (transitional and pseudostratified): Found in the bladder and urinary tract. Its conformation changes depending on whether it is contracted or relaxed. When retracted, it appears as a multi-layered cubic epithelium. When relaxed, it has 2-3 layers of cells, with large, flat superficial cells.
Epithelial cells have unique structures at their free edge:
- Microvilli: Involved in absorption or secretion, increasing the contact surface with the environment. A collection of microvilli is known as a streaking or brush border. They are numerous (up to 2000 per cell), cylindrical, enveloped by the plasma membrane, and contain actin.
- Stereocilia: Longer than microvilli, lack proper motion, and are believed to increase the contact surface.
- Cilia: Mobile and long, easily observed with an optical microscope. They consist of 2 central microtubules and nine peripheral pairs. Their movement contributes to the transportation of solids, liquids, or mucus.
Other differentiations (not seen in class):
- Lateral side surface differentiations: Contact points between adjacent cells, including zonula occludens (tight junctions), zonula adherens, macula adherens (desmosomes), and nexus (gap junctions).
- Basal cell surface differentiations: The lamina lucida, lamina densa, and lamina fibroreticular.
Additional Epithelial Types
- Simple cuboidal epithelium
- Transitional epithelium
- Epidermis
Multi-layered Epithelium
Epithelia formed by more than one layer or stratum. Each set of layers can be divided into:
- Basal or germ layer: A single layer of cubic cells immediately above the basement membrane. These cells have a high rate of protein synthesis and mitotic activity.
- Polyhedral or polygonal stratum: Several layers of polygon-shaped cells. Some authors call this stratum spinosum because desmosomes appear as thorns under a microscope.
- Monostratified cylindrical or columnar cells: Cells taller than they are wide, with oval nuclei located near the basement membrane. The apical portion may have microvilli or other differentiations.
- Pseudostratified epithelium: Consists of a single layer, but cells vary in size, so not all reach the apical surface. Nuclei are also at different positions.