Epithelial Tissue

Overview

Epithelial tissue is one of the four primary tissue types in the human body. It forms protective barriers, lines body surfaces and cavities, and is specialized for absorption, secretion, and filtration. Understanding its structure and function is essential for studying anatomy and physiology.

Functions of Epithelial Tissue

Main Functions

• Protection: Acts as a selective barrier to protect underlying tissues from mechanical injury, pathogens, and chemical exposure.

• Secretion: Releases substances such as enzymes, hormones, and mucus onto free surfaces (e.g., glandular epithelium).

• Absorption: Takes up nutrients and other substances from the external environment or lumen into the cell (e.g., intestinal lining).

• Filtration: Provides a surface for pressure-mediated movement of fluids and solutes (e.g., kidney tubules).

Categories:

• Covering/Lining Epithelium: Forms outer layers of skin and lines internal cavities and organs.

• Glandular Epithelium: Forms glands that produce and secrete products.

Structural Characteristics of Epithelial Tissue

Key Features

• Closely-packed cells: Cells are tightly joined by specialized cell junctions, minimizing intercellular space and forming continuous sheets.

• Polarity: Epithelial cells have an apical surface (facing the body exterior or lumen) and a basal surface (attached to underlying connective tissue).

• Avascularity: Epithelial tissue lacks blood vessels and relies on diffusion from nearby vascular connective tissue for nutrients and waste removal.

• High mitotic rate: Rapid cell division allows for renewal and repair, especially in areas subject to friction or chemical damage.

• Innervation: Epithelial tissue is supplied by nerves, allowing for sensation (touch, pain) and control of glandular secretion.

Cell Junctions in Epithelial Tissue

Types of Cell Junctions

• Cell-cell junctions:

  • Tight junctions: Seal adjacent cells to prevent passage of substances between them; maintain tissue integrity.

  • Desmosomes: Provide strong adhesion between cells, resisting mechanical stress.

  • Gap junctions: Allow direct communication between cells via channels for ions and small molecules. Additional info: Gap junctions are important for coordinated activity, such as in cardiac muscle tissue.

• Cell-extracellular matrix junctions:

  • Hemidesmosomes: Anchor epithelial cells to the basement membrane, providing structural stability.

Diagram: Major Cell Junctions

The diagram (see image) illustrates the arrangement of tight junctions, desmosomes, gap junctions, and hemidesmosomes in epithelial tissue. The apical surface faces the free space or lumen, while the basal surface attaches to the basement membrane.

Polarity and Surface Specializations

Apical and Basal Surfaces

• Apical surface: Exposed to the body exterior or lumen; may have specializations such as microvilli (increase surface area for absorption), cilia (move substances along the surface), or stereocilia.

• Basal surface: Attached to underlying connective tissue via the basement membrane.

Basement Membrane

• Structure: Composed of two layers:

  • Basal lamina: Secreted by epithelial cells; contains glycoproteins and collagen.

  • Reticular lamina: Secreted by connective tissue; contains collagen fibers.

• Functions:

  • Anchors epithelium to connective tissue.

  • Acts as a diffusion barrier, regulating movement of molecules.

  • Influences cell growth and differentiation.

Surface Specializations

Microvilli and Cilia

• Microvilli: Finger-like projections that increase surface area for absorption (e.g., small intestine).

• Cilia: Hair-like structures that move substances along the epithelial surface (e.g., respiratory tract).

Note: Not all epithelial tissues have cilia; their presence depends on the tissue's function and location.

Summary Table: Key Characteristics of Epithelial Tissue

Key Terms and Definitions

• Epithelium: A sheet of cells that covers body surfaces or lines body cavities.

• Basement membrane: A thin, fibrous layer anchoring epithelium to connective tissue.

• Microvilli: Surface projections that increase absorptive area.

• Cilia: Motile extensions that move substances over the epithelial surface.

• Desmosome: Cell junction providing mechanical strength.

• Tight junction: Cell junction preventing passage of materials between cells.

• Hemidesmosome: Junction anchoring cells to the basement membrane.

Example Applications

• Intestinal epithelium: Absorbs nutrients via microvilli.

• Respiratory epithelium: Uses cilia to move mucus and trapped particles out of airways.

• Skin epithelium: Provides protection against pathogens and physical injury.

Review Questions

• What is the functional significance of closely-packed epithelial cells?

• How do avascular epithelial tissues receive nutrients?

• Which feature allows epithelial tissue to be located where nutrient absorption occurs?

• Do epithelial tissues have gap junctions?

• On which surface are cilia located? Are cilia present in all types of epithelial tissues?