Lect_02_107_2025_epithelia_handout

BioSci 107 Lecture 2: Tissues of the Body - Epithelium

Introduction

Objective: Understand the functions, classification, location, structure, and function of epithelial tissues and their associated glands (exocrine and endocrine).

Overview of Epithelial Tissue

Definition: Epithelial tissue covers body surfaces, lines hollow organs, cavities, ducts, and forms glands.

Key Characteristics:

• Selective barriers: Regulate passage of substances.

• Secretory function: Secrete substances onto surfaces.

• Protective function: Protect underlying structures from abrasion.

• Avascular but contains nerves

Cytoskeleton

• Is made up of both microfilaments (Actin) beneath the cell membrane and cytoplasm, and intermediate filaments (Keratin) for strength

Key Characteristics:

• Selective barriers: Regulate passage of substances.

• Secretory function: Secrete substances onto surfaces.

• Protective function: Protect underlying structures from abrasion.

Structure of Epithelial Tissue

Arrangement of Cells: Organized in continuous sheets as either single or multiple layers.Types of Cell Junctions:

• Tight junctions: Create barriers to solute movement.

• Adherens junction: Maintain cell-to-cell adhesion.

• Gap junctions: Allow intercellular communication.

• Desmosomes: Provide structural support.

• Hemidesmosomes: Anchor epithelial cells to the basement membrane.

Functions and Types of Cell Junctions

• Tight Junctions: Form a seal to ensure substances must pass through cells rather than between them. Composed of proteins such as claudins and occludins. Found in the digestive tract (e.g., intestines, stomach).

  • Electrically tight

• Adherens Junctions: Connects the actin cytoskeleton of adjoining cells (e.g., via cadherins). Provides resistance to mechanical stress/tension forces

  • Cadherins span the gap & catenins link the cadherins to the actin filaments

• Desmosomes: Connect intermediate filaments of adjacent cells, allowing them to withstand stretching and shearing forces. Common in skin and cardiac tissue.

  • Cadherin spans the gap & binds to Desmoplakin & links cell surface to keratin (Intermediate Filament)

• Gap Junctions: Consist of connexons that permit small molecules to pass between cells, important for cell communication.

  • 2 Hemichannels make up a gap junction

• Hemidesmosomes: Anchor epithelial cells to the basement membrane through integrin bindings to laminin.

  • Integrin binds to laminin in the BM and then links to keratin in cytoplasm

• Tight junction, adherns junction and desmosome is a “Junctional complex”

The Basement Membrane

Structure: Composed of two layers: basal lamina and reticular lamina.

• Basal Lamina: Created by secretion by the epithelial cells, contains collagen, laminin, other proteoglycans

• Reticular Lamina: Produced by underlying CT known as fibroblasts

• The basement membrane is thus found in between the epithelium and CT

• Functions: Supports overlying epithelium, provides a surface for epithelial cells to migrate during growth/wound healing

Types of Epithelia

Epithelia are classified into two main categories: Covering and Lining Epithelia and Glandular Epithelia (secretory cells)

Covering and Lining Epithelia

These epithelia serve as protective layers on body surfaces and line organs and cavities. They are classified by arrangement and cell shape:

• Arrangement:

• Simple Epithelium: A single layer of cells, facilitating processes such as diffusion, filtration, and secretion.

  • Examples:

    • Simple Squamous Epithelium: Thin and flat cells ideal for rapid diffusion, filtration, secretion (e.g., alveoli in lungs).

    • Specialised Subtypes:

      - Mesothelium which lines pericardial, pleural, peritoneal cavities

      -Endothelium which lines inside of the heart, blood and lymphatic vessels

    • Simple Cuboidal Epithelium: Cube-shaped cells often involved in secretion and absorption (e.g., kidney tubules).

    • Simple Columnar Epithelium: Tall cells that also assist in secretion and absorption (e.g., lining of the intestines).

    • 2 Subtypes:

      -Non-ciliated simple columnar epithelium important for secretion, lubrication, contains goblet cells, can have microvilli, found in ducts of glands

      - Ciliated simple columnar epithelium has goblet cells found in bronchioles, spinal cord, fallopian tubes, synchronous movement of substances

  • Stratified Epithelium: Multiple layers providing protection against abrasion, mainly in high-friction areas. Stratified epithelium doesn’t have Cilia (this course)

    • Example: Stratified Squamous Epithelium its lower layers may look cuboidal or columnar important for resistance against mechanical stress

    • 2 Subtypes:

      - Keratinised dry trauma ares (skin)

      - Non-keratinised Wet trauma areas (mouth, throat) protection from abrasion, defence from microbes

  • Pseudostratified Columnar Epithelium: Appears to be stratified due to differing cell heights but all cells are anchored to the basement membrane (e.g., respiratory tract).

  • If something looks stratified but has cilia it is Psudostratified

  • 2 Subtypes:

    -Pseudostratified ciliated columnar epithelium is in upper airways and secretes & moves mucus (has goblet cells)

  • Pseudostratified non-ciliated columnar epithelium is in larger ducts of glands for absorption and protection (no goblet cells)

  • Transitional Epithelium: Specialized to stretch and change shape, found in the urinary bladder for flexibility as it fills and empties.

Glandular Epithelia

These epithelia are primarily composed of secretory cells and are classified based on the mode of secretion:

• Exocrine glandular Glands: Secrete products onto surfaces or into ducts (e.g., sweat glands, salivary glands). Their secretions are released via ducts. Has local effects.

• Endocrine glandular Glands: Release hormones directly into the bloodstream without ducts (e.g., thyroid gland, adrenal glands). Their secretions regulate various physiological functions and maintain homeostasis. Has distant strong effects.

• Exocrine Glandular Epithelia (single or multicellular):

  -An exocrine gland is simple if it has a single duct that doesn’t divide & it is compound if the duct divides one or more times