The Tissue Level of Organization: Structure and Function of Human Tissues

The Tissue Level of Organization

An Introduction to Tissues

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

Four Types of Tissue

Overview of Tissue Types

The human body contains four major types of tissues, each with distinct roles:

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

• Connective Tissue: Fills internal spaces, supports other tissues, transports materials, and stores energy.

• Muscle Tissue: Specialized for contraction, found in skeletal muscles, the heart, and walls of hollow organs.

• Nervous Tissue: Carries electrical signals throughout the body.

Epithelial Tissue

Structure and Function

Epithelial tissue includes layers of cells covering internal or external surfaces (epithelia) and glands that produce secretions. Its main functions are:

• Providing physical protection

• Controlling permeability

• Providing sensation

• Producing specialized secretions

Characteristics of Epithelia

• Polarity: Distinct apical (top) and basal (bottom) surfaces

• Cellularity: Cells are bound closely together by cell junctions

• Attachment: Bound to a basement membrane

• Avascularity: Lacks blood vessels

• Regeneration: High capacity for renewal

Specializations of Epithelial Cells

• Move fluids over the epithelium (protection)

• Move fluids through the epithelium (permeability)

• Produce secretions (protection and messaging)

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

Maintaining Epithelial Integrity

• Intercellular connections (cell junctions)

• Attachment to the basement membrane

• Continuous epithelial maintenance and repair

Types of Cell Junctions

• Gap Junctions: Allow rapid communication and passage of ions/small molecules; important in heart muscle coordination.

• Tight Junctions: Prevent passage of water and solutes; maintain compartmentalization (e.g., digestive tract).

• Desmosomes: Tie cells together, allow bending/twisting; hemidesmosomes attach cells to the basement membrane.

Classification of Epithelia

Epithelia are classified by cell shape and number of layers:

• Shapes: Squamous (thin, flat), Cuboidal (square), Columnar (tall, slender)

• Layers: Simple (single layer), Stratified (multiple layers)

	Simple	Stratified
Squamous	Simple squamous epithelium	Stratified squamous epithelium
Cuboidal	Simple cuboidal epithelium	Stratified cuboidal epithelium
Columnar	Simple columnar epithelium	Stratified columnar epithelium

Examples of Epithelia

• Simple Squamous: Absorption/diffusion (e.g., lining of body cavities, blood vessels)

• Stratified Squamous: Protection against abrasion (e.g., skin, mouth, esophagus)

• Simple Cuboidal: Secretion/absorption (e.g., kidney tubules, glands)

• Stratified Cuboidal: Rare, found in ducts of sweat/mammary glands

• Transitional: Stretches, found in urinary bladder

• Simple Columnar: Absorption/secretion (e.g., digestive tract)

• Pseudostratified Columnar: Ciliated, found in respiratory tract

• Stratified Columnar: Rare, protection in pharynx, anus, urethra

Glandular Epithelia

• Endocrine Glands: Release hormones into the bloodstream (no ducts)

• Exocrine Glands: Discharge secretions onto epithelial surfaces through ducts

Gland Structure

• Unicellular: Goblet cells (secrete mucin)

• Multicellular: Classified by duct structure (simple/compound) and shape (tubular/alveolar)

Methods of Secretion

• Merocrine: Released by exocytosis (e.g., sweat glands)

• Apocrine: Released by shedding cytoplasm (e.g., mammary glands)

• Holocrine: Released by cell bursting (e.g., sebaceous glands)

Connective Tissue

Structure and Function

Connective tissues consist of specialized cells, extracellular protein fibers, and ground substance. The matrix (fibers + ground substance) makes up most of the tissue volume and determines its function.

• Establish structural framework

• Transport fluids and dissolved materials

• Protect delicate organs

• Support, surround, and interconnect tissues

• Store energy (triglycerides)

• Defend against microorganisms

Categories of Connective Tissue

• Connective tissue proper (connect and protect)

• Fluid connective tissues (transport)

• Supporting connective tissues (structural strength)

Connective Tissue Proper

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

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

Cells of Connective Tissue Proper

• Fibroblasts (produce fibers/ground substance)

• Fibrocytes (maintain fibers)

• Adipocytes (store fat)

• Mesenchymal cells (stem cells)

• Melanocytes (produce melanin)

• Macrophages (phagocytosis)

• Mast cells (inflammation)

• Lymphocytes (immune response)

• Microphages (phagocytic blood cells)

Connective Tissue Fibers

• Collagen fibers: Strong, resist force in one direction (tendons, ligaments)

• Reticular fibers: Network, resist forces in many directions (sheaths around organs)

• Elastic fibers: Stretch and return to original length (elastic ligaments)

Ground Substance

• Clear, colorless, viscous

• Fills spaces between cells, slows pathogen movement

Loose Connective Tissues

• Areolar tissue: Least specialized, open framework, holds capillary beds

• Adipose tissue: Fat storage, insulation, energy reserve

• Reticular tissue: Supportive framework for organs

Dense Connective Tissues

• Dense regular: Parallel collagen fibers (tendons, ligaments)

• Dense irregular: Interwoven fibers (dermis, organ capsules)

• Elastic: Elastic fibers (vertebral ligaments)

Fasciae

• Superficial fascia: Separates skin from underlying tissues

• Deep fascia: Dense regular connective tissue, forms strong, fibrous framework

• Subserous fascia: Between deep fascia and serous membranes

Fluid Connective Tissues: Blood and Lymph

Blood

• Watery matrix: plasma

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

Lymph

• Forms as interstitial fluid enters lymphatic vessels

• Monitored by immune system

• Returned to veins near the heart

Supporting Connective Tissues: Cartilage and Bone

Cartilage

• Matrix: firm gel with chondroitin sulfates

• Cells: chondrocytes in lacunae

• Avascular, covered by perichondrium

Types of Cartilage

• Hyaline: Tough, flexible, reduces friction (joints, ribs, sternum, trachea)

• Elastic: Supportive, bends easily (external ear, epiglottis)

• Fibrocartilage: Durable, limits movement, prevents bone-to-bone contact (joints, vertebrae)

Cartilage Growth

• Interstitial growth: From within

• Appositional growth: At surface

Bone (Osseous Tissue)

• Matrix: calcified with calcium salts, flexible collagen fibers

• Cells: osteocytes in lacunae, arranged around central canals

• Covered by periosteum (fibrous and cellular layers)

Tissue Membranes

Types of Tissue Membranes

• Mucous membranes: Line passageways with external connections (digestive, respiratory, urinary, reproductive tracts)

• Serous membranes: Line cavities not open to outside; secrete serous fluid (peritoneum, pleura, pericardium)

• Cutaneous membrane: Skin; thick, waterproof, dry

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

Muscle Tissue

Types of Muscle Tissue

• Skeletal muscle: Large, striated, voluntary, responsible for body movement

• Cardiac muscle: Striated, involuntary, found only in heart, connected by intercalated discs

• Smooth muscle: Non-striated, involuntary, found in walls of hollow organs

Nervous Tissue

Structure and Function

• Specialized for conducting electrical impulses

• Located in brain and spinal cord

• Two main cell types: neurons (conduct impulses) and neuroglia (supporting cells)

Parts of a Neuron

• Cell body: Contains nucleus

• Dendrites: Receive signals

• Axon: Sends signals

Tissue Injuries and Repair

Response to Injury

• Inflammation: Triggered by trauma or infection; involves release of prostaglandins, proteins, and potassium ions

• Regeneration: Restores normal function; varies among tissue types

Epithelia, most connective tissues, and smooth muscle regenerate well; skeletal muscle, cardiac muscle, and nervous tissue regenerate poorly.

Aging, Regeneration, and Cancer

Effects of Aging

• Decreased speed and effectiveness of tissue regeneration

• Thinner epithelia, fragile connective tissues, increased bruising, brittle bones, cardiovascular disease, mental deterioration

• Cancer incidence increases with age; most cancers are due to environmental factors