Epithelial Tissue

EPITHELIA Overview

  • Dr. Emma Bailey, School of Life Sciences, Thomson Building, Room 344, Contact via email: Emma.Bailey@glasgow.ac.uk

Intended Learning Outcomes (ILOs)

  • Categorise the morphological features of the 4 main classes of tissue.

  • Compare characteristics of simple & stratified epithelial tissues.

Tissues Within the Body

  • Each specialized tissue/cell has distinct functions.

  • Structure & function are closely related, allowing predictions based on one another.

  • Understanding structure aids in understanding function.

Tissue Types

  • 4 types of tissues:

    • Connective Tissues: Supports and packages various tissues.

    • Nervous Tissue: Composed of nerve and glial cells.

    • Muscle: Generates force for movement.

    • Epithelia: Sheets of cells covering body surfaces.

Epithelium Characteristics

  • Basic Cell Type: Tight cohesive sheets that cover and protect surfaces.

  • Lines internal cavities & vessels, forming glandular structures and barriers.

  • In combination with nervous tissue, constructs special sense organs.

General Characteristics of Epithelial Tissue

  • Highly cellular with tight junctions to bind cells together and attach to a basement membrane.

  • Avascular, meaning it lacks blood vessels.

  • Capable of regeneration, vital for tissues experiencing wear and tear.

  • Can arise from all three germinal layers: ectoderm, endoderm, mesoderm.

Structural Polarity of Epithelial Tissue

  • Distinct Cell Surfaces: Polarity

    • Apical Surface: Free surface not attached to other cells, often lining lumens of ducts/cavities.

    • Lateral Surface: Attached to neighboring epithelial cells.

    • Basal Surface: Attached to the basement membrane, linking epithelia to underlying tissues.

    • Each surface may feature distinct specialisations.

Apical Surface Specialisations

  • Microvilli: Projections increasing surface area, often seen in internal passages (e.g., intestines).

  • Stereocilia: Longer, non-motile projections found in the male reproductive system and inner ear, function for sensory perception.

  • Cilia: Motile, hair-like structures that move substances along, seen in respiratory epithelium and fallopian tubes; smoking impairs cilia activity.

Basolateral Specialisations

  • Epithelial cells are cohesive and closely apposed.

  • Junctions at the basolateral surface maintain polarized state, linking cells and facilitating information/metabolite exchange.

Junction Types

  • Occluding/Tight Junctions: Prevent leakage between cells, found in the apical region.

  • Anchoring Junctions: Mechanically attach cells; abundant in stress-prone tissues like skin and cardiac muscle.

  • Communicating/Gap Junctions: Allow chemical/electrical signal passage between adjacent cells.

Functions of Epithelial Cells

  • Protection: Safeguarding underlying structures.

  • Selective Permeability: Restricting and allowing substance passage (varies by epithelium type).

  • Secretion: Releasing substances onto epithelial surface or into the bloodstream.

  • Sensory Reception: Large sensory supply to relay environmental information.

Classification of Epithelia

  • Based on:

    • Number of Layers:

      • Simple: Single layer, ideal for absorption/secretion.

      • Stratified: Two or more layers, better for protection.

      • Pseudostratified: Appears stratified but is one layer with variously shaped cells.

    • Shapes of Cells:

      • Squamous: Flat-shaped.

      • Cuboidal: Cube-shaped.

      • Columnar: Tall, cylindrical.

      • Transitional: Flexible, accommodating stretch.

  • Named by combining the number of layers with the shape of cells (e.g., Simple Squamous, Stratified Cuboidal).

Types of Simple Epithelium

  • Simple Squamous:

    • Structure: Single layer of flat cells.

    • Function: Diffusion and filtration; minimal protection.

    • Location: Blood vessels, heart linings, alveoli.

  • Simple Cuboidal:

    • Structure: Single layer of cube-shaped cells.

    • Function: Secretion and absorption; transport of mucus particles.

    • Location: Kidney tubules, glands.

  • Simple Columnar:

    • Structure: Tall, narrow cells, some with microvilli/cilia.

    • Function: Absorption and secretion.

    • Location: Glands, bronchioles, stomach, intestines.

Smoking and Epithelial Health

  • Smoking leads to epithelium destruction, causing metaplasia in bronchial epithelium.

Types of Stratified Epithelium

  • Stratified Squamous:

    • Structure: Multiple layers; can be keratinized or non-keratinized.

    • Function: Protection against abrasion and infection.

    • Location: Skin (keratinized), mouth, larynx (non-keratinized).

  • Stratified Cuboidal:

    • Structure: Multiple layers of cube-shaped cells.

    • Function: Secretion and protection.

    • Location: Rare, found in sweat glands, ovarian follicles.

  • Stratified Columnar:

    • Structure: Multiple layers with tall cells on top of bulbous cells.

    • Function: Secretion and protection.

    • Location: Mammary gland ducts, larynx, male urethra.

Pseudostratified Columnar Epithelium

  • Structure: Appears stratified; all cells reach the basement membrane but not the free surface.

  • Function: Synthesizes and secretes mucus; moves mucus over the surface.

  • Location: Lining of respiratory pathways.

Transitional Epithelium

  • Unique because it accommodates stretching and recoil.

  • Located in bladder and ureters; variable cell shape depending on stretch.

  • Maintains protective functions against urine's caustic effects.

Summary of Epithelial Tissues

  • Organism > Organ System > Organ > Tissue > Cell > Chemical Levels of Organization.

  • Major tissue types: Connective, Muscle, Nervous, and Epithelial.

  • Characteristics, functions of epithelium, and types of epithelial cells classified by layers and shapes.

Recommended Reading

  • Seeleys - Chapter 4

  • Tortora - Chapter 4