The Tissue Level of Organization

Introduction to Tissues

Tissues are groups of specialized cells and cell products that work together to perform specific functions. In the human body, tissues combine to form organs, which in turn interact within organ systems to maintain homeostasis and support life.

• Tissues: Specialized cells and cell products assembled to perform specific functions.

• Four primary tissue types:

  • Epithelial tissue

  • Connective tissue

  • Muscle tissue

  • Nervous tissue

• Histology: The study of tissues.

• Tissues assemble to form organs (e.g., heart, liver), which perform complex functions.

Overview of Tissue Types

Each tissue type has distinct roles in the body, contributing to the structure and function of organs and organ systems.

• Epithelial Tissue: Covers exposed surfaces, lines internal passageways and chambers, and forms glands.

• Connective Tissue: Fills internal spaces, provides structural support, stores energy.

• Muscle Tissue: Contracts to produce movement.

• Nervous Tissue: Propagates electrical impulses, carries information.

Epithelial Tissue

General Features and Functions

Epithelial tissue consists of layers of cells that cover internal or external surfaces and form glands. It serves as a protective barrier, controls permeability, and provides sensation.

• Epithelium: Layers of cells covering surfaces (e.g., skin, digestive tract, respiratory tract, blood vessels, heart).

• Glands: Collections of epithelial cells that produce fluid secretions.

Characteristics of Epithelia

Epithelial tissues have several key structural and functional characteristics.

• Polarity: Epithelia have distinct "top" (apical) and "bottom" (basolateral) surfaces.

  • Apical surface: Exposed to the environment or internal space; may have microvilli (increase surface area for absorption/secretion) or cilia (move fluids).

  • Basolateral surface: Attached to underlying tissues.

• Cellularity: Cells are closely packed with minimal extracellular material; extensive intercellular connections.

  • Cell adhesion molecules (CAMs): Transmembrane proteins that help cells stick to each other and their surroundings.

  • Proteoglycans: Act as intercellular cement.

  • Cell junctions: Specialized areas of plasma membranes, including:

    • Gap junctions: Allow free passage of ions and small molecules between cells.

    • Tight junctions: Form a barrier to prevent diffusion of fluids and solutes.

    • Desmosomes: Provide strong adhesion between cells.

Classification of Epithelia

Epithelia are classified based on cell shape and number of layers.

• Cell Shape:

  • Squamous: Thin and flat cells.

  • Cuboidal: Square-shaped cells.

  • Columnar: Tall, slender, rectangular cells.

• Number of Layers:

  • Simple epithelium: Single layer of cells.

  • Stratified epithelium: Multiple layers of cells.

Connective Tissue

General Features and Functions

Connective tissues are diverse and provide structural support, protection, and energy storage. They consist of specialized cells, extracellular protein fibers, and ground substance (fluid).

• Matrix: Combination of extracellular protein fibers and ground substance; forms most of the tissue volume.

Categories of Connective Tissue

• Connective tissue proper:

  • Loose connective tissue: More ground substance, fewer fibers (e.g., adipose tissue).

  • Dense connective tissue: More fibers (especially collagen), less ground substance (e.g., tendons).

• Fluid connective tissues: Blood and lymph.

• Supporting connective tissues: Cartilage and bone.

Specialized Cells of Connective Tissue Proper

• Fibroblasts: Secrete ground substance and proteins.

• Fibrocytes: Maintain connective tissue fibers.

• Adipocytes: Fat cells; store energy.

• Mesenchymal cells: Stem cells for repair and regeneration.

• Melanocytes: Synthesize and store melanin pigment.

• Macrophages: Large phagocytic immune cells.

• Mast cells: Release histamine (inflammation) and heparin (anticoagulant).

• Lymphocytes: Immune system cells.

• Microphages: Small phagocytic immune cells.

Connective Tissue Fibers

• Collagen fibers: Strong, flexible, resist force in one direction.

• Reticular fibers: Thinner, form networks, resist forces in many directions.

• Elastic fibers: Provide flexibility.

Types of Loose Connective Tissue

• Areolar tissue: Least specialized, open framework, viscous ground substance, elastic fibers.

• Adipose tissue: Contains adipocytes; stores fat, can be white or brown fat.

• Reticular tissue: Rich in reticular fibers; supports organ structure.

Types of Dense Connective Tissue

• Dense regular: Tightly packed, parallel collagen fibers (tendons, ligaments, aponeuroses).

• Dense irregular: Interwoven collagen fibers, strength and support.

• Elastic tissue: Mainly elastic fibers, flexibility with support.

Fluid Connective Tissues

• Blood: Watery matrix (plasma) with suspended proteins; formed elements include red blood cells, white blood cells, and platelets.

• Lymph: Forms from interstitial fluid entering lymphatic vessels; returns to veins near the heart.

Supporting Connective Tissues

• Cartilage: Firm gel matrix with chondroitin sulfates; chondrocytes in lacunae; avascular.

• Bone: Matrix of rigid calcium salts (2/3) and collagen fibers (1/3); osteocytes in lacunae; canaliculi for material exchange; covered by periosteum.

Types of Cartilage

Cartilage Growth

• Interstitial growth: Enlarges cartilage from within.

• Appositional growth: Enlarges cartilage at the outer surface.

Membranes

Types and Functions

Membranes line or cover body surfaces, providing physical barriers. They consist of an epithelium supported by connective tissue.

• Mucous membranes: Line passageways with external connections; moist to reduce friction and facilitate absorption/secretion (digestive, respiratory, urinary, reproductive tracts).

• Serous membranes: Line cavities not open to the outside; consist of parietal and visceral portions separated by serous fluid.

• Cutaneous membrane: Skin; covers the body, relatively waterproof and dry.

• Synovial membranes: Line joint cavities; lack true epithelium, produce synovial fluid for lubrication and nutrient delivery.

Muscle Tissue

Types and Functions

Muscle tissue is specialized for contraction and movement. There are three types, each with unique characteristics.

• Skeletal muscle: Responsible for body movement; cells are large, multinucleate, striated, voluntary.

• Cardiac muscle: Found only in the heart; cells are short, branched, striated, single nucleate, involuntary, connected by intercalated discs, limited repair ability.

• Smooth muscle: Lines organs, vessels, tracts; cells are short, spindle-shaped, nonstriated, single nucleate, involuntary, can divide and regenerate.

Nervous Tissue

Structure and Function

Nervous tissue is specialized for conducting electrical impulses and is found primarily in the brain and spinal cord. It consists of two main cell types.

• Neurons: Nerve cells; transmit electrical signals.

  • Cell body: Contains the nucleus.

  • Dendrites: Receive incoming signals.

  • Axon: Carries outgoing signals.

• Neuroglia: Supporting cells; provide structural support, protection, and nutrition for neurons.

Tissue Injury and Repair

Inflammation and Regeneration

When tissues are injured, a series of responses occur to restore normal function.

• Inflammation: Mast cells release chemicals (e.g., histamine, prostaglandins) that stimulate inflammation, producing redness, heat, pain, and sometimes loss of function.

• Infection: Inflammation may result from pathogens (e.g., bacteria) in the tissue.

• Necrosis: Enzymes from immune cells destroy injured cells and pathogens, resulting in tissue death and pus formation.

• Regeneration: Restores tissue to normal function; varies among tissue types.

  • Epithelia, connective tissues (except cartilage), and smooth muscle regenerate well.

  • Skeletal muscle, cardiac muscle, and nervous tissue regenerate poorly.

  • Damaged cardiac muscle cells are replaced by fibrous tissue (fibrosis).