Human Epithelial Tissues Overview

Epithelial Tissues Overview

• Epithelial tissues cover every internal and external body surface.
• Functions include acting as barriers, lining organs and cavities, providing protection, immune defense, secretion, transport, and sensation.
• Composed of tightly packed cells linked by tight junctions and desmosomes, leading to avascularity and nutrient diffusion from deeper tissues.

Types of Epithelia

Nonkeratinized Stratified Squamous Epithelium

• Contains living cells in the apical layers that retain their nuclei.
• Provides protection in areas experiencing mechanical stress while maintaining moisture.
• Found in the mouth, throat, esophagus, anus, and vagina.

Stratified Cuboidal Epithelium

• Rare in humans, consisting of two cell layers.
• Primarily lines ducts of sweat glands.

Stratified Columnar Epithelium

• Also uncommon in humans with only a few layers, featuring a columnar apical layer and a cuboidal basal layer.
• Located in male urethra, cornea of the eye, and ducts of certain glands like salivary glands.

Transitional Epithelium

• Unique to the urinary system.
• Characterized by cuboidal basal layers and dome-shaped apical layers when relaxed; allows stretching.

Simple Squamous Epithelium

• A thin single layer of cells that facilitates rapid diffusion of gases and fluids.
• Found in lung air sacs, segments of kidney tubules, and blood vessel linings.

Simple Cuboidal Epithelium

• Single layer of cube-shaped cells with a large central nucleus suited for diffusion.
• Found in renal tubules, respiratory passages, and various gland ducts including the thyroid.

Simple Columnar Epithelium

• Rectangular-shaped cells with nuclei located at the basal end, often with microvilli or cilia.
• Commonly found in the small intestine, uterine tubes, and segments of the respiratory tract.

Pseudostratified Columnar Epithelium

• Appears layered due to varying nucleus heights but is a single layer with basal membranes in contact with the basement membrane.
• Located in segments of the respiratory tract and nasal cavity, typically ciliated.

Keratinized Stratified Squamous Epithelium

• Contains dead apical layers filled with keratin, making the tissue resistant to friction.
• Found in the outer layers of skin.

Transport Mechanisms in Simple Epithelia

• Paracellular transport: Involves substances leaking between cells, limited by tight junctions.
• Transcellular transport: Substance enters a cell via plasma membrane, diffuses across the cytosol, and exits through the opposite membrane.

Glandular Epithelia

• Glands are epithelial structures that synthesize and secrete products.
• Classified by shape or secretion mechanisms; products released through basal lamina (ECM from epithelial cells) and reticular lamina (from underlying connective tissue).
• Basal lamina consists of collagen fibers and ground substance, while reticular lamina consists of reticular fibers and ground substance.

Cell Junctions

• Tight junctions: Hold cells closely, preventing macromolecule movement; found in blood vessel cells.
• Desmosomes: Linking proteins that allow fluid passage and improve tissue strength under mechanical stress; common in skin epithelia.
• Gap junctions: Protein channels forming small pores allowing small substances to pass and facilitating cell communication, especially in cardiac muscle cells.