Tissues

Tissues are collections of specialized cells and cell products that perform specific functions. The study of tissues is known as histology. Tissues combine to form organs, such as the heart or liver.

4-1 Four Types of Tissue

• Epithelial

• Connective

• Muscle

• Nervous

Epithelial Tissue

Epithelial tissue covers exposed surfaces, lines internal passageways, and forms glands. It serves as a protective barrier and is involved in absorption, secretion, and sensation.

• Covers exposed surfaces

• Lines internal passageways

• Forms glands

Connective Tissue

Connective tissue fills internal spaces, supports other tissues, transports materials, and stores energy. It is the most diverse tissue type in the body.

• Fills internal spaces

• Supports other tissues

• Transports materials

• Stores energy

Muscle Tissue

Muscle tissue is specialized for contraction and includes skeletal, heart, and muscular walls of hollow organs.

• Specialized for contraction

• Includes skeletal, heart, and smooth muscle

Nervous Tissue

Nervous tissue carries electrical signals from one part of the body to another, enabling communication and control.

• Carries electrical signals

4-2 Epithelial Tissue

Structure and Function

• Epithelia: Layers of cells covering internal or external surfaces

• Glands: Structures that produce fluid secretions

Functions of epithelial tissue include:

1. Providing physical protection

2. Controlling permeability

3. Providing sensation

4. Producing specialized secretions

Characteristics of Epithelia

• Cellularity: Composed almost entirely of cells

• Polarity: Has an exposed apical surface and an attached basal surface

• Attachment: Bound to a basement membrane

• Avascularity: Lacks blood vessels

• Regeneration: Cells are replaced by division of stem cells

Specializations of Epithelial Cells

• Move fluids over the epithelium (protection)

• Move fluids through the epithelium (permeability)

• Produce secretions (protection and messaging)

Polarity

• Apical surface: May have microvilli (increase absorption/secretion) or cilia (move fluids)

• Basolateral surface: Attachment to underlying tissues

Integrity of Epithelia

• Maintained by intercellular connections, attachment to the basement membrane, and epithelial maintenance and repair

Intercellular Connections

• Cell adhesion molecules (CAMs): Transmembrane proteins that bind cells together

• Proteoglycans: Act as intercellular cement (e.g., hyaluronan)

• Cell junctions:

  • Gap junctions: Allow rapid communication

  • Tight junctions: Prevent passage of water and solutes

  • Desmosomes: Tie cells together

Basement Membrane

• Basal lamina: Closest to the epithelium

• Reticular lamina: Deeper portion, provides strength

Epithelial Maintenance and Repair

• Epithelial cells are replaced by continual division of stem cells near the basement membrane

4-3 Classification of Epithelia

Classification Criteria

• Shape:

  • Squamous: Thin and flat

  • Cuboidal: Square shaped

  • Columnar: Tall, slender rectangles

• Layers:

  • Simple epithelium: Single layer of cells

  • Stratified epithelium: Several layers of cells

Types of Epithelia

• Simple squamous epithelium: Absorption and diffusion; includes mesothelium (lines body cavities) and endothelium (lines heart and blood vessels)

• Stratified squamous epithelium: Protects against mechanical stresses; keratin adds strength and water resistance

• Simple cuboidal epithelium: Secretion and absorption; found in glands and kidney tubules

• Stratified cuboidal epithelium: Relatively rare; ducts of sweat glands and mammary glands

• Transitional epithelium: Tolerates repeated cycles of stretching without damage; found in urinary bladder

• Simple columnar epithelium: Absorption and secretion; found in stomach, small intestine, large intestine

• Pseudostratified columnar epithelium: Typically have cilia; found in nasal cavity, trachea, bronchi

• Stratified columnar epithelium: Relatively rare; provides protection in pharynx, anus, urethra

Glandular Epithelia

• Glands: Collections of epithelial cells that produce secretions

• Endocrine glands: Release hormones into the bloodstream; no ducts

• Exocrine glands: Produce exocrine secretions; discharge secretions through ducts onto epithelial surfaces

Gland Structure

• Unicellular glands: Single cells, e.g., goblet cells (secrete mucus)

• Multicellular glands: Composed of many cells; classified by duct structure (simple or compound) and shape (tubular or alveolar)

Methods of Secretion

• Merocrine secretion: Released by secretory vesicles (exocytosis); e.g., merocrine sweat glands

• Apocrine secretion: Released by shedding cytoplasm

• Holocrine secretion: Released by cells bursting, killing gland cells

Types of Secretions

• Serous glands: Watery secretions

• Mucous glands: Secrete mucins

• Mixed exocrine glands: Both serous and mucous

4-4 Connective Tissue

Components of Connective Tissues

• Specialized cells

• Extracellular protein fibers

• Fluid called ground substance

The matrix consists of extracellular fibers and ground substance, making up most of the tissue volume and determining its function.

Functions of Connective Tissues

• Establishing a structural framework for the body

• Transporting fluids and dissolved materials

• Protecting delicate organs

• Supporting, surrounding, and interconnecting other tissues

• Storing energy reserves (especially triglycerides)

• Defending the body from invading microorganisms

Categories of Connective Tissues

• Connective tissue proper: Connects and protects

• Fluid connective tissues: Transport (e.g., blood, lymph)

• Supporting connective tissues: Structural strength (e.g., cartilage, bone)

4-5 Connective Tissue Proper

Cells of Connective Tissue Proper

• Fibroblasts: Most abundant; secrete proteins and hyaluronan

• Fibrocytes: Maintain connective tissue fibers

• Adipocytes: Store fat

• Mesenchymal cells: Stem cells that respond to injury or infection

• Melanocytes: Synthesize and store melanin

• Macrophages: Engulf pathogens and damaged cells; can be fixed or free

• Mast cells: Stimulate inflammation; release histamine and heparin

• Lymphocytes: Migrate through the body; may develop into plasma cells (produce antibodies)

• Microphages: Phagocytic blood cells (neutrophils, eosinophils)

Connective Tissue Fibers

• Collagen fibers: Most common; strong, flexible, resist force in one direction

• Reticular fibers: Network of interwoven fibers; strong, flexible, resist force in many directions

• Elastic fibers: Contain elastin; branched and wavy, return to original length after stretching

Ground Substance

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

Loose Connective Tissues

• "Packing materials" of the body; fill spaces, cushion cells, support epithelia

• Types: Areolar, adipose, reticular

Dense Connective Tissues

• Also called collagenous tissues; contain many collagen fibers

• Types: Dense regular, dense irregular, elastic

Dense Regular Connective Tissue

• Tightly packed, parallel collagen fibers

• Examples: Tendons, ligaments, aponeuroses

Dense Irregular Connective Tissue

• Interwoven networks of collagen fibers

• Examples: Dermis, sheaths around cartilage and bone, capsules around organs

Elastic Tissue

• Made of elastic fibers; found in elastic ligaments of vertebrae

Fasciae

• Connective tissue layers and wrappings that support and surround organs

• Types: Superficial, deep, subserous

4-6 Blood and Lymph

Blood

• Contains a watery matrix called plasma

• Formed elements: Red blood cells (erythrocytes), white blood cells (leukocytes), platelets

Lymph

• Forms as interstitial fluid that enters lymphatic vessels

• Monitored by immune system

• Returned to veins near the heart

4-7 Supporting Connective Tissues

Cartilage

• Provides shock absorption and protection

• Matrix is firm, contains chondroitin sulfates

• Cells in the matrix are chondrocytes in chambers called lacunae

• Avascular; chondrocytes produce antiangiogenesis factor

• Perichondrium: Outer, fibrous layer for support and protection

Types of Cartilage

Bone (Osseous Tissue)

• Strong, calcified matrix with collagen fibers

• Cells: Osteocytes in lacunae arranged around central canals

• Periosteum covers bone surfaces

Membranes

• Physical barriers composed of epithelium and connective tissue

• Types: Mucous, serous, cutaneous, synovial

Mucous Membranes

• Line passageways open to the exterior (digestive, respiratory, urinary, reproductive tracts)

• Moistened by mucous secretions

Serous Membranes

• Line sealed internal cavities

• Produce transudate to reduce friction

• Examples: Pleura, peritoneum, pericardium

Cutaneous Membrane

• Skin; thick, waterproof, and dry

Synovial Membranes

• Line joint cavities; produce synovial fluid for lubrication

Muscle Tissue

• Skeletal muscle: Large body muscles responsible for movement

• Cardiac muscle: Found only in the heart; regulated by pacemaker cells

• Smooth muscle: Found in walls of hollow organs; nonstriated, involuntary

Nervous Tissue

• Specialized for conducting electrical impulses

• Rapidly senses internal or external environment

• Processes information and controls responses

• Contains neurons (nerve cells) and neuroglia (supporting cells)

Tissue Injury and Repair

• Tissues respond to injury to restore homeostasis

• Inflammation: Isolates injured area while damaged cells, tissue components, and dangerous microorganisms are cleaned up

• Regeneration: Epithelia and connective tissues regenerate well; cardiac and neural tissue regenerate poorly

Aging and Cancer

• Speed and efficiency of tissue repair decrease with age

• Hormonal and lifestyle changes affect tissue structure and chemical composition

• Cancer rates increase with age; most cancers are caused by chemical exposure or environmental factors