Tissues and Epithelial Cells

Epithelial Tissue Characteristics

  • Definition: Epithelial tissue is composed of one or more layers of cells that line body cavities or cover body surfaces.

  • Characteristics:

    • Cells are tightly glued together with minimal extracellular matrix present.

    • All substances that either enter or exit the body must pass through an epithelial surface.

Cellularity and Polarity of Epithelial Tissue

  • Cell Arrangement:

    • Epithelial cells typically exhibit a hexagonal shape.

    • Form continuous sheets with little or no extracellular matrix present between cells.

  • Polarity:

    • Epithelial cells have different surfaces:

    • Apical Surface: The top surface exposed to the body exterior or internal cavity.

    • Basal Surface: The bottom surface attached to the basement membrane.

Characteristics of Epithelial Cells

  • Avascular Nature: Epithelial tissues lack blood vessels and depend on diffusion from underlying connective tissues for nutrition.

  • Nerve Innervation: Highly innervated by nerve fibers that detect environmental changes.

  • Regeneration: Epithelial cells can regenerate via cell division if adequately nourished.

Functions of Epithelial Tissue

  • Physical Protection: Serves as a barrier to protect both internal and external surfaces.

  • Selective Permeability: Controls substance entry and exit through the epithelium.

  • Secretion: Some epithelial cells function in secretion of various substances.

  • Sensory Reception: Contains nerve fibers to sense changes in the external environment.

Classifying Epithelial Tissue

  • Each class of epithelium has a three-word structural designation.

  • Special Classes:

    • Simple Epithelium

    • Stratified Epithelium

Classification by Layer and Shape of Epithelial Tissue

  • Layer Classifications:

    • Simple Epithelia (One layer)

    • Stratified Epithelia (Multiple layers)

  • Shape Classifications:

    • Squamous Cells: Flattened and scale-like.

    • Cuboidal Cells: About as tall as they are wide.

    • Columnar Cells: Taller than they are wide, potentially ciliated or non-ciliated.

Types of Simple Epithelial Tissue

  • Simple Squamous Epithelium:

    • Structure: Single layer of flat cells with sparse cytoplasm.

    • Functions: Diffusion and filtration due to thinness.

    • Special Types:

      • Endothelium: Lines blood vessels.

      • Mesothelium: Lines body cavities.

  • Simple Cuboidal Epithelium:

    • Structure: Single layer of cube-shaped cells.

    • Functions: Secretion and absorption.

    • Locations: Kidney tubules.

  • Simple Columnar Epithelium:

    • Structure: Single layer of tall cells; may be ciliated or non-ciliated.

    • Functions: Absorption and secretion.

    • Special Feature: Contains goblet cells for mucus secretion.

      • Ciliated: Aids in movement of substances along the surface (e.g., in the uterine tubes to move ova).

      • Non-ciliated: Primarily involved in absorption and secretion, often found in the digestive tract.

  • Pseudostratified Columnar Epithelium:

    • Structure: Appears stratified but is a single layer with varying heights; has goblet cells and cilia (in the ciliated form).

    • Functions: Protection and mucus movement.

    • Locations: Respiratory tract (ciliated form).

Types of Stratified Epithelial Tissue

  • Stratified Squamous Epithelium:

    • Structure: Several layers with surface cells flattened.

    • Functions: Protects against abrasion.

    • Types:

      • Keratinized: Contains keratin in outer cells, providing waterproofing and protection; found in the skin.

      • Non-Keratinized: Retains moisture and provides protection in moist environments; found in oral cavity and esophagus.

  • Stratified Cuboidal Epithelium:

    • Structure: Two or more layers of cuboidal cells.

    • Functions: Protection and secretion.

    • Locations: Sweat glands.

  • Stratified Columnar Epithelium:

    • Structure: Multiple layers with columnar apical cells.

    • Functions: Protection and secretion.

    • Locations: Male urethra.

  • Transitional Epithelium:

    • Structure: Varies based on stretch; ranges from polyhedral to flattened cells.

    • Function: Accommodates distension in urinary bladder.

    • Locations: Urinary bladder, ureters, part of urethra.

Glands and Secretion Types

  • Definition of Glands: Organs primarily composed of epithelial tissue specialized for secretion.

  • Types of Glands:

    • Endocrine Glands:

      • Ductless: Secrete hormones directly into body fluids (often blood).

      • Secrete substances like: amino acids, peptides, proteins, and steroids.

    • Exocrine Glands:

      • Secrete products onto body surfaces or into cavities via ducts.

Unicellular and Multicellular Exocrine Glands

  • Unicellular Exocrine Glands:

    • Contain no ducts; primarily goblet cells.

    • Located near epithelial surfaces.

  • Multicellular Exocrine Glands:

    • Composed of many cells that work together to produce secretions.

    • Contains: acinus (secretory unit) and ducts leading to epithelial surface.

Anatomical Classification of Exocrine Glands

  • Structural Classification: Based on duct complexity.

    • Simple Glands: Unbranched ducts.

    • Compound Glands: Branched ducts.

  • Shape Classification:

    • Tubular: Glands with tube-shaped secretory portions.

    • Acinar: Glands with sac-like secretory portions.

    • Tubuloacinar: Combination of tubular and acinar shapes.

Method of Secretion in Exocrine Glands

  • Merocrine Glands:

    • Secrete by exocytosis, packaged into vesicles.

    • Examples: Pancreas, salivary glands.

  • Apocrine Glands:

    • Part of the cell membrane pinches off, carrying secretory product (e.g., lipid droplets).

    • Examples: Mammary glands.

  • Holocrine Glands:

    • Entire cell ruptures to release secretions, with cell debris included.

    • Examples: Sebaceous (oil) glands of the skin.