Epithelial Tissues and Cell Junctions

Overview of Epithelial Tissue

  • First basic tissue type of the human body discussed in the lecture.

  • Goals of the lecture:

    • Understand epithelial tissues.

    • Discuss the basement membrane.

    • Describe different types of epithelial tissue.

    • Discuss structure and function.

    • Introduction to glands (time permitting).

Definition of Epithelial Tissue

  • Epithelial tissue: Tissues that cover body surfaces and line hollow organs and cavities.

    • Examples of locations:

    • Skin: Interface with external environment.

    • Hollow organs: Lining in the mouth, intestines, and other cavities.

    • Functions:

    • Protective barriers against external abrasions and contamination.

    • Secretion of substances.

    • Absorption of nutrients.

    • Filtration roles.

Structure of Epithelial Tissue

  • Arranged in continuous sheets: Can be single (simple) or multiple layers (stratified).

    • Cell connections are critical for maintaining the integrity and function of the epithelial layers.

Cell Junctions

  • Cell junctions: Structures that connect adjacent epithelial cells, preventing them from sliding apart while allowing them to coordinate activities.

    • Types of junctions:

    1. Tight Junctions:

      • Located near the apical surface of epithelial cells.

      • Prevent leakage of molecules between cells, acting like a seal.

      • Proteins involved: Claudins and occludins.

      • Found in areas like the stomach, intestines, and bladder.

    2. Adherence Junctions:

      • Also called belt junctions due to their shape.

      • Provide strength and mechanical stability by anchoring cells together.

      • Key proteins: Cadherin (spans the gap) and catenin (links cadherin to cytoskeleton).

    3. Desmosomes:

      • Appear as buttons connecting adjacent cells, offering strong adhesion.

      • Proteins involved: Cadherin linking to desmoplakin, which binds to keratin filaments within cells.

    4. Gap Junctions:

      • Specialized for communication between cells, allowing small molecules and ions to pass through.

      • Proteins involved: Connexins form connexons (small tunnels) that connect two cells.

    5. Hemidesmosomes:

      • Anchor cells to the basement membrane.

      • Proteins involved: Integrin connects keratin filaments in the cell to laminin in the membrane.

Basement Membrane

  • A critical structure between epithelial layers and underlying connective tissue.

    • Composed of two parts:

    1. Basal Lamina: Produced by epithelial cells, contains glycoproteins and collagen.

    2. Reticular Lamina: Secreted by fibroblasts in connective tissue.

    • Functions include:

    • Support for epithelium.

    • Nutrient and waste exchange between connective tissue and epithelium.

    • Important in the healing process post-injury.

Types of Epithelial Tissue

  • Arrangements of epithelial cells:

    1. Simple (single layer): Good for secretion, absorption, and filtration.

    2. Stratified (multiple layers): Primarily protective.

    3. Pseudostratified: Appears layered but is actually a single layer; all cells connect to the basement membrane.

  • Shapes of epithelial cells:

    1. Squamous: Flat, thin cells (good for diffusion).

    2. Cuboidal: Equal height and width, often involved in secretion.

    3. Columnar: Taller than wide, involved in secretion and absorption.

    4. Transitional: Changes shape, allowing expansion (found in the bladder).

Specific Types of Epithelial Tissue Examples

  • Simple Squamous Epithelium:

    • Appearance: Fried egg or cobblestone-like.

    • Special names: Mesothelium (lining abdominal cavities), Endothelium (lining blood vessels and the heart).

  • Simple Cuboidal Epithelium:

    • Function: Secretion and absorption; found in ducts (e.g., pancreas).

  • Simple Columnar Epithelium:

    • Non-ciliated: Has microvilli for absorption (e.g., gallbladder).

    • Ciliated: Contains cilia for movement (e.g., respiratory epithelium).

  • Stratified Squamous Epithelium:

    • Provides protection with multiple layers; can be keratinized (skin) or non-keratinized (oral cavity).

  • Pseudostratified Columnar Epithelium:

    • Appears layered due to varying cell height but is a single layer; ciliated version common in the respiratory tract.

Glandular Epithelium

  • Glands are epithelial tissues that secrete substances:

    • Endocrine glands: Secrete hormones directly into the blood (e.g., thyroid).

    • Exocrine glands: Secrete into ducts (e.g., sweat, salivary glands).

  • Goblet cells: Single-cell exocrine glands that secrete mucus.

Gland Structure Classifications

  • Duct Structure:

    • Simple (single duct) vs. Compound (branched duct).

  • Secretory Part:

    • Tubular or acinar (rounded).

Conclusion

  • Summarized understanding of epithelial tissue: cell types, junctions, and roles.

  • Discussion on connective tissue to follow in the next lecture.