Tissues of the Body

Introduction to Tissues

Tissues are collections of specialized cells and cell products that perform specific, limited functions. The study of tissues is known as histology.

• Tissue types: Epithelial, Connective, Muscle, Nervous

• Function: Each tissue type contributes to the structure and function of organs and organ systems.

Epithelial Tissue

Epithelial Tissue Overview

Epithelial tissue includes layers of cells covering internal or external surfaces and glands that produce secretions.

• Epithelia: Layers of cells covering surfaces

• Glands: Epithelial cells specialized for secretion

Characteristics of Epithelia

Epithelial tissues have several defining characteristics that enable their protective and selective functions.

• Cells bound closely together by specialized junctions

• Free (apical) surface exposed to environment or internal space

• Attached to underlying connective tissue by a basement membrane

• Avascular: No blood vessels; nutrients diffuse from underlying tissues

• Continual replacement or regeneration of cells

Locations of Epithelia

Epithelial tissues are found throughout the body, forming protective and selective barriers.

• External surfaces: Skin

• Internal passageways: Digestive, urinary, respiratory, and reproductive tracts

• Internal cavities: Cavities around lungs and heart

• Function: Prevent friction, form selective barriers

Functions of Epithelia

Epithelial tissues perform four essential functions:

• Physical protection: Protect underlying tissues from abrasion, dehydration, and destruction

• Control permeability: Regulate movement of substances in and out

• Provide sensation: Detect changes in environment via sensory nerves

• Produce specialized secretions: Glandular epithelium

Glandular Epithelium

Gland cells are classified by where their secretions are discharged.

• Exocrine glands: Discharge secretions onto the surface of epithelium (e.g., sweat, saliva)

• Endocrine glands: Release secretions into surrounding tissue fluid and blood (e.g., hormones)

Intercellular Connections (Cell Junctions)

Cell junctions allow firm attachment to the basement membrane and adjacent epithelial cells.

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

• Proteoglycans: Bind CAMs to each other and to extracellular materials

• Three common cell junctions: Tight junctions, adhesion belts, gap junctions

Types of Cell Junctions

• Tight Junctions: Prevent passage of water and solutes between cells

• Adhesion Belts: Provide structural support and stability

• Gap Junctions: Permit diffusion of ions and small molecules between cells

Basement Membrane and Apical Surface

The apical surface is exposed to the environment and may have specialized structures such as microvilli or cilia. The basement membrane is a noncellular network of protein fibers that provides strength and acts as a barrier.

• Microvilli: Increase surface area for absorption

• Cilia: Move substances over epithelial surface

Epithelial Renewal and Repair

Epithelial tissues require continual renewal and repair, accomplished by division of specialized stem cells (germinative cells) near the basement membrane.

• High turnover rate: Especially in areas exposed to mechanical or chemical stress

Types of Epithelial Tissue

Simple Epithelia

• Simple Squamous Epithelium: Single layer of flat cells; found in areas of diffusion (e.g., alveoli of lungs)

• Simple Cuboidal Epithelium: Single layer of cube-shaped cells; found in glands and kidney tubules

• Simple Columnar Epithelium: Single layer of tall, column-shaped cells; found in digestive tract lining

Stratified Epithelia

• Stratified Squamous Epithelium: Multiple layers; protects against abrasion (e.g., skin, mouth)

• Stratified Cuboidal Epithelium: Rare; found in ducts of sweat and mammary glands

• Stratified Columnar Epithelium: Rare; found in pharynx, epiglottis, anus, urethra

Pseudostratified Columnar Epithelium

Appears layered due to nuclei at different levels, but all cells contact the basement membrane; often ciliated (e.g., respiratory tract).

Transitional Epithelium

Stratified epithelium that changes appearance with stretching; found in urinary bladder.

Connective Tissue

Overview and Functions

Connective tissue is the most diverse tissue type, providing structural support, protection, and other functions.

• Basic components: Specialized cells, extracellular protein fibers, ground substance

• Functions: Support/protection, transportation, energy storage, defense

Types of Connective Tissue

• Connective Tissue Proper: Many cell types, fibers, and ground substance (e.g., tendons, ligaments, adipose tissue)

• Fluid Connective Tissues: Blood and lymph

• Supporting Connective Tissues: Cartilage and bone

Cells of Connective Tissue Proper

• Fibroblasts: Produce fibers and ground substance

• Macrophages: Phagocytize damaged cells/pathogens

• Adipocytes: Store fat

• Mast cells: Release histamine and heparin for defense

Connective Tissue Fibers

• Collagen fibers: Strong, flexible, most common

• Elastic fibers: Branched, wavy, return to original length

• Reticular fibers: Thin, branching, form frameworks

Ground Substance

Fills spaces between cells and fibers; consistency varies from syrupy to solid.

Types of Connective Tissue Proper

• Loose connective tissue: More ground substance, fewer fibers; supports epithelia, cushions organs

• Dense connective tissue: More fibers (mostly collagen), less ground substance; resists tension, connects bones/muscles

Examples of Loose Connective Tissue

• Areolar tissue: Cushions organs, supports epithelia

• Adipose tissue: Stores energy, insulates

• Reticular tissue: Forms supportive framework in organs

Examples of Dense Connective Tissue

• Dense regular: Tendons, ligaments

• Dense irregular: Dermis of skin, organ capsules

Fluid Connective Tissues

• Blood: Contains red blood cells, white blood cells, platelets in plasma

• Lymph: Contains lymphocytes in lymph fluid

Supporting Connective Tissues

• Cartilage: Gel matrix, chondrocytes in lacunae, avascular, limited repair

• Bone (Osseous tissue): Hard matrix of calcium compounds and collagen fibers, osteocytes in lacunae

Types of Cartilage

• Hyaline cartilage: Most common, closely packed collagen fibers

• Elastic cartilage: Numerous elastic fibers, flexible

• Fibrocartilage: Dense collagen fibers, durable and tough

Tissue Membranes

Overview

Tissue membranes are physical barriers that line or cover portions of the body, consisting of an epithelium and supporting connective tissue.

Types of Tissue Membranes

• Mucous membranes: Line passageways open to exterior; kept moist by mucous

• Serous membranes: Line internal cavities; secrete serous fluid to reduce friction

• Cutaneous membrane: Skin; thick, waterproof, dry

• Synovial membranes: Line joint cavities; produce synovial fluid for lubrication

Types of Serous Membranes

• Pleura: Lines pleural cavities, covers lungs

• Peritoneum: Lines abdominal cavity, covers organs

• Pericardium: Lines pericardial cavity, covers heart

Muscle Tissue

Overview

Muscle tissue is specialized for contraction, enabling movement. It contains filaments of myosin and actin.

• Skeletal muscle: Striated, voluntary, long cells (muscle fibers)

• Cardiac muscle: Striated, involuntary, branched cells, intercalated discs

• Smooth muscle: Non-striated, involuntary, tapered cells

Nervous Tissue

Overview

Nervous tissue is specialized for transmitting electrical impulses, concentrated in the brain and spinal cord.

• Neurons: Nerve cells that communicate via electrical signals; limited repair ability

• Neuroglia: Supporting cells; provide support, nutrients, and defense

Parts of a Neuron

• Cell body: Contains nucleus

• Dendrites: Receive incoming signals

• Axon: Carries outgoing signals

• Axon terminals: Communicate with other cells

Response to Tissue Injury

Inflammation and Regeneration

Tissues respond to injury through inflammation and regeneration to restore homeostasis.

• Inflammation: First response; symptoms include swelling, heat, redness, pain

• Triggers: Trauma, infection, extreme temperatures

• Process: Mast cells release histamine and heparin, blood vessels dilate, increased blood flow, pain, removal of waste

• Regeneration: Damaged tissues replaced/repaired; fibroblasts produce scar tissue

• Regeneration ability varies: Good in epithelia, connective tissue, smooth muscle; poor in cardiac, skeletal muscle, cartilage, neural tissue

Tissue Repair and Aging

Effects of Aging

Aging reduces the body's ability to repair tissues due to slower metabolism, hormonal changes, and reduced activity. Cancer risk increases with age.

• Thinning of epithelium

• Fragile connective tissue

• Brittle bones, increased osteoporosis

• Less resilient cartilage

• Increased cardiovascular disease

• Deterioration in mental function

• Cancer rates: 25% of people in the US develop cancer; #2 cause of death

• Environmental factors: 70-80% of cancers due to chemical/environmental exposure; cigarette smoke causes 40% of these