Tissue Level of Organization

An Introduction to Tissues

  • The human body contains 200 types of cells.

  • Tissues are defined as collections of specialized cells and cell products that perform specific functions.

  • Tissues in combination form organs, such as the heart or liver.

  • The field of histology is the study of tissues.

Four Types of Tissue

  1. Epithelial

    • Covers exposed surfaces

    • Lines internal passageways

    • Forms glands

  2. Connective

    • Fills internal spaces

    • Supports other tissues

    • Transports materials

    • Stores energy

  3. Muscle

    • Specialized for contraction

    • Includes skeletal, cardiac, and smooth muscle

  4. Nervous

    • Carries electrical signals from one part of the body to another

Functions and Characteristics of Tissues

  • Epithelial tissue includes epithelia and glands:

    • Epithelia are layers of cells covering internal or external surfaces.

    • Glands are structures that produce fluid secretions.

Functions of Epithelial Tissue

  1. Provide physical protection.

  2. Control permeability.

  3. Provide sensation.

  4. Produce specialized secretions.

Characteristics of Epithelia

  • Polarity: Apical and basal surfaces.

  • Cellularity: Composed of closely packed cells with cell junctions.

  • Attachment: Basal surfaces attach to a basement membrane.

  • Avascularity: Epithelia are avascular, relying on diffusion for nutrition.

  • Regeneration: Epithelial tissues have high regenerative abilities.

Classification of Epithelia

  • Squamous epithelia:

    • Simple squamous epithelia:

    • Functions: Absorption and diffusion.

    • Examples: Mesothelium (lines body cavities), Endothelium (inner lining of heart and blood vessels).

    • Stratified squamous epithelia:

    • Functions: Protect against mechanical stresses.

    • Keratin adds strength and water resistance.

  • Cuboidal epithelia:

    • Simple cuboidal epithelia:

    • Functions: Secretion and absorption.

    • Locations: Glands, portions of kidney tubules.

    • Stratified cuboidal epithelia:

    • Functions: Protection, secretion, absorption.

    • Locations: Rare, found in ducts of sweat and mammary glands.

  • Columnar epithelia:

    • Simple columnar epithelia:

    • Functions: Absorption and secretion.

    • Locations: Stomach, small intestine, large intestine.

    • Pseudostratified columnar epithelia:

    • Functions: Typically have cilia, involved in secretion and movement of mucus.

    • Locations: Nasal cavity, trachea, bronchi.

    • Stratified columnar epithelia:

    • Functions: Protection.

    • Locations: Relatively rare, found in pharynx, anus, urethra.

  • Glandular epithelia:

    • Glands are collections of epithelial cells that produce secretions.

    • Endocrine glands: Release hormones directly into the bloodstream; lack ducts.

    • Exocrine glands: Produce secretions discharged through ducts onto epithelial surfaces.

Gland Structure

  • Unicellular glands: Goblet cells are unicellular exocrine glands found in intestines that secrete mucin (forms mucus when mixed with water).

  • Multicellular glands: Classified by:

    1. Structure of the duct (simple vs compound).

    2. Shape of the secretory portion (tubular vs alveolar).

    3. Relationship between ducts and glandular areas (branched).

Connective Tissue

Components

  1. Specialized cells.

  2. Extracellular protein fibers.

  3. Fluid called ground substance.

    • Matrix consists of extracellular components and the majority of tissue volume, determining specialized functions.

Functions of Connective Tissues

  • Establishing a structural framework for the body.

  • Transporting fluids and dissolved materials.

  • Protecting delicate organs.

  • Supporting, surrounding, and interconnecting other types of tissue.

  • Storing energy reserves, especially triglycerides.

  • Defending the body against invading microorganisms.

Categories of Connective Tissues

  1. Connective tissue proper: Connect and protect.

  2. Fluid connective tissues: Transport.

  3. Supporting connective tissues: Provide structural strength.

Connective Tissue Proper

  • Categories:

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

    • Dense connective tissue: More fibers, less ground substance (example: tendons).

Cells of Connective Tissue Proper

  • Cell types include:

    • Fibroblasts

    • Fibrocytes

    • Adipocytes

    • Mesenchymal cells

    • Melanocytes

    • Macrophages

    • Mast cells

    • Lymphocytes

    • Microphages

Connective Tissue Fibers

  1. Collagen fibers: Most common, strong, and flexible, resisting forces in one direction (found in tendons and ligaments).

  2. Reticular fibers: Form a network of strong, flexible fibers, resisting forces in multiple directions (example: sheaths around organs).

  3. Elastic fibers: Contain elastin, branched and wavy, returning to original length after stretching (example: elastic ligaments).

Ground Substance

  • Clear, colorless, and viscous; fills space between cells and slows pathogen movement.

  • Loosely arranged connective tissues serve as packing materials, cushioning cells, and supporting epithelia.

Types of Loose Connective Tissues

  1. Areolar tissue: Supports capillary beds and provides cushioning.

  2. Adipose tissue: Contains adipocytes, provides padding and insulation, stores energy.

  3. Reticular tissue: Supports functional cells of organs (found in liver, kidney, spleen, lymph nodes, bone marrow).

Dense Connective Tissues

  • Also known as collagenous tissues, characterized by high collagen fiber content:

    • Dense regular connective tissue: Tightly packed, parallel fibers found in tendons and ligaments.

    • Dense irregular connective tissue: Interwoven fibers providing strength to dermis and forming sheaths around organs.

    • Elastic tissue: Contains elastic fibers, found in spinal column ligaments.

Fasciae (Fascia)

  • Fasciae: Layers of connective tissue that support and surround organs.

  • Three layers:

    1. Superficial fascia: Separates skin from underlying tissues.

    2. Deep fascia: Dense regular connective tissue sheets.

    3. Subserous fascia: Lies between deep fascia and serous membranes lining body cavities.

Blood and Lymph

  • Fluid connective tissues include blood (watery matrix called plasma containing formed elements: red blood cells, white blood cells, platelets) and lymph (interstitial fluid entering lymphatic vessels, monitored by the immune system).

Supporting Connective Tissues

  • Cartilage: Provides shock absorption and protection; matrix consists of gel-like substance.

    • Avascular nature and two layers of perichondrium (outer fibrous and inner cellular).

    • Types include:

    1. Hyaline cartilage: Common; reduces friction in joints.

    2. Elastic cartilage: Flexible; found in external ear.

    3. Fibrocartilage: Durable; found in joints and intervertebral discs.

  • Bone (osseous tissue): Provides support, made rigid by calcium salts; contains osteocytes in lacunae with small canals connecting to blood vessels.

Tissue Membranes

  • Tissue membranes are physical barriers lining or covering body surfaces; consist of epithelium supported by connective tissue.

Four Types of Tissue Membranes

  1. Mucous membranes: Line passageways having external connections (digestive, respiratory, urinary, reproductive tracts).

  2. Serous membranes: Line cavities not connected to the outside; reduce friction.

    • Types: Peritoneum, Pleura, Pericardium.

  3. Cutaneous membrane: Skin, relatively thick and waterproof.

  4. Synovial membranes: Line joint cavities, lack true epithelium; stimulated by movement to produce synovial fluid for lubrication.

Muscle Tissue

  • Three types:

    1. Skeletal muscle: Large, striated, voluntary muscle responsible for body movement.

    2. Cardiac muscle: Striated, involuntary muscle found only in the heart, connected by intercalated discs.

    3. Smooth muscle: Nonstriated, involuntary muscle found in walls of hollow organs (e.g., blood vessels, digestive tract).

Nervous Tissue

  • Specialized for conducting electrical impulses, primarily found in the brain and spinal cord.

  • Types of cells:

    1. Neurons: Longest cells in the body, responsible for signal transmission.

    2. Neuroglia: Supporting cells that assist neurons.

Tissue Injuries and Repair

  • Regeneration capabilities vary among tissue types:

    • Epithelia and connective tissues regenerate well.

    • Skeletal muscle, cardiac muscle, and nervous tissues regenerate poorly, with cardiac muscle replaced by fibrous tissue.

Aging, Regeneration, and Cancer

  • Aging slows regeneration due to decreased repair and maintenance activities, hormonal alterations, and reduced physical activity.

  • Aging leads to structural changes, including thinner epithelia and fragile connective tissues.

  • Cancer incidence increases with age, with rates showing that twenty-five percent of U.S. population develops cancer, often induced by environmental factors, such as cigarette smoke.