The Tissue Level of Organization

Tissue Level of Organization

• Tissues are specialized collections of cells with specific functions; histology is the study of tissues.
• Four major tissue types: Epithelial, Connective, Muscle, and Neural.

Epithelial Tissue

• Covers surfaces exposed to the environment (skin, airways, digestive tract, glands).
• Types include:
  • Simple (one layer) and Stratified (multiple layers)
  • Shapes: Squamous (flat), Cuboidal (square), Columnar (tall), Pseudostratified, Transitional.
• Characteristics:
  • Cellularity: Tightly bound cells.
  • Polarity: Distinct apical and basal surfaces.
  • Attachment: Bound to basal lamina or basement membrane.
  • Avascularity: No blood vessels present.
  • Regeneration: High rate due to stem cells.

Classification of Epithelia

• Squamous Epithelia:

  • Simple Squamous: Thin and flat (e.g., mesothelium, endothelium).
  • Stratified Squamous: Many layers for protection (e.g., mouth, esophagus).

• Cuboidal Epithelia:

  • Simple Cuboidal: Involved in secretion/absorption (e.g., kidney tubules).
  • Stratified Cuboidal: Rare type.

• Transitional Epithelia:

  • Adapts to stretching (e.g., urinary bladder).

• Columnar Epithelia:

  • Simple Columnar: Absorption/secretion areas (e.g., intestines).
  • Pseudostratified Columnar: Appears stratified but is simple; contains cilia (e.g., respiratory tract).
  • Stratified Columnar: Rare type.

• Glandular Epithelia:

  • Endocrine glands release hormones into interstitial fluids without ducts (e.g., thyroid, pituitary).
  • Exocrine glands release substances into ducts (e.g., sweat, digestive enzymes).

Connective Tissue

• Functions: Connects epithelium, supports structure, stores energy, and transports materials.
• Basic characteristics:
  • Specialized cells scattered throughout.
  • Extracellular protein fibers.
  • Fluid extracellular ground substance.

Classification of Connective Tissues

• Connective Tissue Proper:
  • Loose CT Proper: Areolar tissue.
  • Dense CT Proper: Dense regular connective tissue.
• Other types:
  • Blood and lymph.
  • Adipose Tissue: Energy storage.
  • Bone: Structural framework.
  • Cartilage Types:
    • Hyaline: Most common, in growth plates and joint surfaces.
    • Elastic: Found in the external ear.
    • Fibrocartilage: In intervertebral discs and knee pads.

Muscle Tissue

• Specialized for contraction, producing body movement.
• Types:
  • Skeletal Muscle: Long, striated, voluntary, multinucleated; major body movements; new fibers produced by satellite cells.
  • Cardiac Muscle: Striated, involuntary, single nucleus; forms a branching network with intercalated disks; found only in the heart.
  • Smooth Muscle: Non-striated, involuntary, single nucleus; located in walls of hollow organs (e.g., blood vessels, bladder).

Neural Tissue

• Specialized for conducting electrical impulses, sensing environment, processing information, and controlling responses.
• Composed of:
  • Neurons: Nerve cells responsible for electrical communication.
  • Neuroglia: Supportive cells providing nutrition and repair.
• Neurons consist of:
  • Cell body: Contains the nucleus.
  • Dendrites: Receive incoming signals.
  • Axon: Transmits outgoing signals.

Tissue Injuries and Repair

• Inflammation is the initial response to injury, characterized by swelling, redness, heat, and pain.

• Regeneration follows once the injury clears:

  • Fibroblasts lay down collagen, forming scar tissue.
  • New cells may arise from nearby tissues or mesenchymal stem cells.

• Regenerative capacity varies:

  • Epithelia and connective tissues regenerate well.
  • Cardiac muscle and neurons have limited or no regeneration.