ch4 a

Chapter 4: Histology

Overview

  • Tissues: Groups of cells similar in structure and function.

  • Four main types of tissues:

    • Epithelial

    • Connective

    • Muscle

    • Nerve

Epithelial Tissue

  • General Characteristics:

    • Cellularity: Composed almost entirely of cells; minimal extracellular matrix.

    • Special Contacts: Continuous sheets held together by tight junctions and desmosomes.

    • Polarity: Has an apical surface (exposed) and a basal surface (attached to the basement membrane).

  • Support & Supply:

    • Supported by connective tissue (reticular and basal lamina).

    • Avascular but innervated: No blood vessels, but supplied with nerve fibers.

    • Regenerative: Rapidly replaces lost cells through cell division.

Classification of Epithelia

  • By Number of Layers:

    • Simple: Single layer of cells.

    • Stratified: Multiple layers of cells.

  • By Cell Shape:

    • Squamous: Flat, tile-like cells.

    • Cuboidal: Cube-shaped cells.

    • Columnar: Tall cells that are taller than wide.

Specific Types of Epithelia

Simple Squamous Epithelium

  • Description: Single layer of flattened cells with disc-shaped nuclei.

  • Function: Primarily for diffusion and filtration; reduces friction in cardiovascular and lymphatic systems.

  • Location: Found in kidney glomeruli, endothelial lining of heart and blood vessels, alveoli, etc.

Simple Cuboidal Epithelium

  • Description: Single layer of cube-like cells with large, spherical nuclei.

  • Function: Involved in secretion and absorption.

  • Location: Present in kidney tubules and glandular ducts.

Simple Columnar Epithelium

  • Description: Single layer of tall cells with oval nuclei; may contain cilia and goblet cells.

  • Function: Absorption and secretion; cilia help move substances.

  • Location: Lines digestive tract, gallbladder, bronchi, and uterus.

Pseudostratified Columnar Epithelium

  • Description: Appears stratified but is a single layer with varying cell heights.

  • Function: Secretion and propulsion of mucus; ciliated version lines respiratory tract.

  • Location: Found in the trachea and ducts of large glands.

Stratified Squamous Epithelium

  • Description: Thick membrane of several layers of flat cells.

  • Function: Protects underlying areas from abrasion.

  • Location: Forms the epidermis of skin, lining of esophagus, mouth, and vagina.

Stratified Cuboidal Epithelium

  • Description: Rare; typically two layers thick.

  • Function: Secretion and absorption.

  • Location: Found in some sweat and mammary glands.

Stratified Columnar Epithelium

  • Description: Limited distribution; usually two layers thick.

  • Function: Secretion and absorption.

  • Location: Found in the pharynx and male urethra.

Transitional Epithelium

  • Description: Several layers; basal cells are cuboidal, surface cells dome-shaped.

  • Function: Stretches to permit urinary bladder distension.

  • Location: Lines the urinary bladder, ureters, and part of the urethra.

Glandular Epithelium

  • Definition: A gland is one or more cells that secrete an aqueous fluid; considered epithelial tissue.

  • Types:

    • Endocrine Glands: Ductless, secrete hormones directly into the bloodstream.

    • Exocrine Glands: Secrete products onto body surfaces or into cavities (e.g., sweat, salivary glands).

  • Classification of Exocrine Glands:

    • Duct Type:

      • Simple (unbranched)

      • Compound (branched)

    • Secretory Unit Structure:

      • Tubular (no swelling at the end)

      • Alveolar (swelling at the end)

      • Tubuloalveolar (both types present).

Important Cell Types

  • Goblet Cells: A unicellular exocrine gland producing mucus.

  • Photomicrographs: Visual examples included for better understanding of each epithelium type.