Microanatomy - Epithelia

EPITHELIUM

  • Dr Matthew Valentine BVMS MRCVS PhD Dipl ACVP

Lecture Overview

  1. Definitions, functions, and relevance of epithelium

  2. Characteristics of epithelium

  3. Types and classification of epithelia

Definitions

  • Cells, Tissues, Organs, Systems:

    • Hierarchical organization: Cells form tissues, tissues form organs, and organs compose systems.

  • Tissues: Aggregation of cells and extracellular substances with a common purpose.

  • Basic Types of Tissue: Contains four primary tissues:

    • Epithelial tissue

    • Connective tissue

    • Muscular tissue

    • Nervous tissue

Types of Epithelium

  1. Lining (Surface) Epithelium:

    • Forms sheets covering body surfaces or lining hollow organs.

  2. Glandular Epithelium:

    • Specialized for synthesis, storage, and release of products.

  3. Special Epithelium:

    • Contains sensory nerve endings, found in skin, ears, tongue; modified neurons present in the nose and eyes.

Functions of Epithelial Tissues

  • Protection:

    • Epidermis protects against mechanical abrasion, chemicals, microbes, and water loss.

  • Friction Reduction:

    • Smooth endothelial cells line circulatory systems, minimizing friction against blood vessels.

  • Cleaning:

    • Ciliated respiratory epithelium helps remove dust and foreign particles from airways.

  • Diffusion:

    • Endothelium facilitates diffusion of gases, liquids, and nutrients.

  • Absorption:

    • Epithelial cells in the small intestine absorb nutrients from digested food.

  • Secretion:

    • Epithelial tissues secrete various substances like enzymes, hormones, and lubricants.

  • Sensation:

    • Specialized epithelial cells transmit sensations from sensory organs.

Relevance

  • Fundamental understanding of tissue organization and functioning is necessary for:

    • Systemic histology, as many organs and glands have epithelial components.

    • Understanding terminology related to tumors/neoplasia.

Characteristics of Epithelium

  • Supported by connective tissue.

  • Avascular (no blood vessels).

  • Presence of a basement membrane.

  • Cohesive epithelial cells.

  • Polarized structure of epithelial cells.

Epithelial Cells Structure

  • Apical Surface: Free surface, contacting the external environment.

  • Lateral Surfaces: Adjacent to neighboring epithelial cells.

  • Basal Surface: Attached to the basement membrane.

Classification of Lining Epithelia

  1. Simple: One layer of cells.

  2. Stratified: Multiple layers of cells.

  3. Types include:

    • Squamous

    • Cuboidal

    • Columnar

    • Pseudostratified

    • Transitional/Urothelium

    • Non-keratinized and keratinized variants.

Simple Squamous Epithelium

  • Composed of flat, elongated cells with a centrally located round to oval nucleus.

  • Common Locations:

    • Body cavities (mesothelium), alveolar walls in lungs, inner lining of blood vessels (endothelium).

Stratified Squamous Epithelium

  • Consists of multiple layers of cells, with the superficial cells determining the name.

  • Types:

    • Keratinized (cornified)

    • Non-keratinized

Simple Columnar Epithelium

  • Tall, narrow cells with an ovoid nucleus near the cell base.

  • Common Locations:

    • Lining of stomach and intestines, gall bladder.

Pseudostratified Epithelium

  • Appears stratified but is composed of a single layer of irregularly shaped cells.

  • Characteristics:

    • All cells touch the basement membrane but not all reach the apical surface.

    • Can be ciliated, found in nasal cavity and upper respiratory tract.

Transitional Epithelium (Urothelium)

  • Lines urinary passages such as the bladder.

  • Structural Adaptation:

    • Superficial cells have a specialized plasma membrane that provides an osmotic barrier.

Epithelial Repair

  • Constant loss and replacement of epithelial cells facilitated by stem cells.

  • Location of Stem Cells:

    • In the basal cell layer for different epithelial types/functions.

Exfoliative Cytology

-Process: Harvesting and microscopic examination of cells shed from surface tissues.

  • Applications:

    • Skin cytology, urine analysis, vaginal smears, fine needle biopsy.

Classification of Glandular Epithelia

  • Based on:

    • Number of cells (unicellular or multicellular).

    • Shape of secretory units (adenomere).

    • Type of product secreted.

Unicellular Glands

  • Found in epithelial linings of intestines and respiratory tracts.

  • Example: Goblet cells characterized by mucinogen granules in their structure.

Multicellular Glands

  • Modified epithelial structures:

    • Types:

      • Tubular: Either straight or coiled (e.g., sweat glands).

      • Acinar: Pie-shaped with a small lumen (e.g., pancreas).

      • Alveolar: Larger luminal spaces (e.g., mammary glands).

Classification According to Product

  1. Serous: Watery product, e.g., sweat.

  2. Mucous: Viscous secretion, e.g., tracheal mucus.

  3. Mixed: Produces both mucus and serous secretions, e.g., saliva.

  4. Sebaceous: Oily secretion known as sebum.