Tissue: The Living Fabric

Definition of Tissues

• Collection of structurally similar cells working together for a common function.

Four Main Classes of Human Tissue

1. Nervous Tissue
   • Location: Brain, spinal cord, nerves
   • Function: Control and communication
2. Muscle Tissue
   • Location: Anchored to bones, heart, walls of hollow organs
   • Function: Generate contractions to produce movement
3. Epithelial Tissue
   • Location: Skin, external surface of organs, lining closed ventral body cavities, interior of hollow organs
   • Function: Covers/lines body structures, filtration, secretion, absorption
4. Connective Tissue
   • Function: Provides support, reinforcement, and unites other tissue types

Epithelial Tissue (Epithelium)

• Types:
  1. Covering and Lining Epithelia
     • Lines interior of closed ventral cavities and hollow organs
     • Forms external surfaces of organs, and skin
  2. Glandular Epithelia
     • Forms glands, specialized in secretion (e.g., sweat glands, salivary glands)

Classification of Epithelial Tissue

• Criteria: Number of cell layers and cell shape
  • Layers:
    • Simple: Single layer
    • Stratified: Multiple layers
  • Shapes:
    • Squamous: Flattened
    • Cuboidal: Cube-like
    • Columnar: Tall, column-like

Epithelial Cell Surfaces

• Apical Surface: Free, faces external environment or lumen
• Basal Surface: Attached to basement membrane

Types of Epithelial Tissue

1. Simple Squamous Epithelium
   • Features: Single layer of thin, flattened cells
   • Function: Allows material transfer (gases, water, ions)
   • Location: Air sacs of lungs, blood vessels, kidney structures
2. Simple Cuboidal Epithelium
   • Features: Single layer of cube-like cells
   • Function: Secretion and absorption
   • Location: Kidney tubules, small glands, ovaries
3. Simple Columnar Epithelium
   • Features: Single layer of tall cells
   • Function: Absorption, secretion of mucus and enzymes
   • Location: Digestive tract, bronchi, uterus
4. Pseudostratified Columnar Epithelium
   • Features: Single layer, nuclei at different heights
   • Function: Propels mucus
   • Location: Upper respiratory tract
5. Stratified Squamous Epithelium
   • Features: Several layers, apical cells flattened
   • Function: Protection from abrasion
   • Varieties: Keratinized (e.g., skin), Non-keratinized (e.g., oral cavity)
6. Stratified Cuboidal & Columnar Epithelium
   • Features: Rare, 2 layers
   • Location: Sweat glands, pharynx
7. Transitional Epithelium
   • Features: Apical cells change shape
   • Function: Allows stretch
   • Location: Urinary bladder

Features of Epithelial Tissue

• Polarity: Apical and basal surfaces
• Cell-to-Matrix Ratio: High, densely packed cells
• Avascular: Lacks blood vessels, but innervated
• High Regeneration Rate: High mitotic activity

Glands

• Classification:
  • Endocrine: Ductless, secretes hormones into blood
  • Exocrine: Secretes into ducts, leading to body’s external environment
• Unicellular vs. Multicellular Glands:
  • Unicellular: Single cell (e.g., goblet cells)
  • Multicellular: Multiple cells (most glands)