Anatomy & Physiology I: Tissues Overview

Overview of Tissues

• Tissues are groups of cells organized into layers or groups with specialized functions.
• Major types of tissues: Epithelial, Connective, Muscular, Nervous.

Epithelial Tissue

• Composed of layers that cover surfaces and form glands.
• Key characteristics include:
  • Cellularity: High density of cells with strong intercellular junctions.
    • Tight junctions: Prevent leakage between cells.
    • Adherens junctions: Link cytoskeletons of adjacent cells for mechanical stability.
    • Gap junctions: Allow communication and transfer of ions and small molecules.
  • Polarity: Distinct apical (top) and basal (bottom) surfaces; uneven distribution of organelles.
  • Basement membrane attachment: Connects epithelium to underlying tissues.
  • Avascularity: Lacks blood vessels; derive nutrients through diffusion from connective tissues.
  • Innervation: Rich in nerve endings for sensation; regenerate rapidly from the basement membrane.

Functions of Epithelial Tissues

• Provides physical protection against environmental factors.
• Regulates permeability, controlling fluid loss.
• Highly sensitive, capable of forming neuroepithelial structures sensitive to stimuli such as smell and taste.
• Functions in secretion and absorption, forming secretory glands.

Specializations of Epithelial Tissue

• Surface Movements: Microvilli increase surface area for absorption; cilia aid in movement across cell surfaces.
• Secretory Functions: Goblet cells produce mucus; various epithelial forms produce hormones.

Types of Epithelial Tissue

• Classified by shape and number of layers:
  • Layers:
    • Simple: Single layer, protective in nature.
    • Stratified: Multiple layers, withstands wear and tear.
    • Pseudostratified: Appears stratified but is a single layer with varying nuclei levels.
    • Transitional: Specializes in expansion, seen in the urinary bladder.
  • Shapes:
    • Squamous: Flat cells; maximize surface area.
    • Cuboidal: Cube-shaped; suited for secretion and absorption.
    • Columnar: Taller than wide; increases secretion and absorption capabilities.

Epithelial Tissue Types

• Simple squamous: Found in low-friction areas like lungs; facilitates diffusion.
• Stratified squamous: Located in areas subjected to abrasion (skin, esophagus).
• Simple cuboidal: Common in glands and kidney tubules, involved in secretion/absorption.
• Stratified cuboidal: Rare, primarily found in gland ducts.
• Transitional: Accommodates stretching in the urinary system.
• Simple columnar: Lines digestive tract; often features microvilli and goblet cells.
• Pseudostratified: Lines respiratory and male reproductive systems; may have cilia and goblet cells.
• Stratified columnar: Rare; found in salivary glands.

Glandular Epithelium

• Specialized to produce secretions.
• Divided into two types:
  • Exocrine Glands: Secrete substances via ducts.
    • Merocrine: Secretion through exocytosis (e.g., sweat glands).
    • Apocrine: Budding secretion, loses part of the cell membrane (e.g., mammary glands).
    • Holocrine: Rupture of the cell to release contents (e.g., sebaceous glands).

Connective Tissue

• Functions include binding structures, transporting fluids, providing support and framework, energy storage, and immune responses.
• Contains cells dispersed in an extracellular matrix made of fibers and ground substance.

Types of Connective Tissue

• Connective Tissue Proper: Includes cell types such as:
  • Fibroblasts: Secrete matrix components (collagen, elastin).
  • Macrophages: Engulf pathogens; found in both fixed and free forms.
  • Adipocytes: Store fat droplets.
  • Mesenchymal cells: Stem cells that respond to injury.
  • Mast cells: Release substances that mediate inflammation.
  • Lymphocytes: Involved in immune response.

Classification of Connective Tissue

• Loose Connective Tissue: Provides cushioning and support (e.g., areolar tissue).
• Dense Connective Tissue: More fibers than ground substance; provides strength and can be irregular or regular.
• Cartilage: Composed of chondrocytes in a matrix; types include:
  • Hyaline Cartilage: Flexible, with collagen fibers, providing support (e.g., nose, joints).
  • Elastic Cartilage: Contains many elastin fibers; maintains shape (e.g., ear).
  • Fibrous Cartilage: Very tough; found in intervertebral discs.

Bone Tissue

• Made up of mineralized matrix; provides rigidity and structure.
• Functional Units: Osteons, consisting of:
  • Osteocytes: Mature bone cells in lacunae.
  • Periosteum: Surrounding tissue, involved in growth and repair.
  • Osteoblasts and Osteoclasts: Cells that build and break down bone.

Membranes

• Serve as barriers; consist of multiple tissue types.