Fundamentals of Anatomy & Physiology - Chapter 4: Tissue Level of Organization

Introduction to Tissues

• Tissues are collections of specialized cells and cell products performing specific functions.

• Organs like the heart or liver are formed by tissues in combination.

• Histology is the study of tissues.

Four Types of Tissue

• The four main tissue types are:

  • Epithelial

  • Connective

  • Muscle

  • Nervous

Epithelial Tissue

• Covers exposed surfaces.

• Lines internal passageways.

• Forms glands.

Connective Tissue

• Fills internal spaces.

• Supports other tissues.

• Transports materials.

• Stores energy.

Muscle Tissue

• Specialized for contraction.

• Types include skeletal, heart, and muscular walls of hollow organs.

Nervous Tissue

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

Epithelial Tissue

• Epithelial tissue includes epithelia and glands.

  • Epithelia: Layers of cells covering internal or external surfaces.

  • Glands: Structures that produce fluid secretions.

Functions of Epithelial Tissue

• Provide physical protection.

• Control permeability.

• Provide sensation.

• Produce specialized secretions.

Characteristics of Epithelia

• Polarity (apical and basal surfaces)

• Cellularity (cell junctions)

• Attachment (basement membrane)

• Avascularity (avascular)

• Regeneration

Specializations of Epithelial Cells

• Move fluids over the epithelium (protection).

• Move fluids through the epithelium (permeability).

• Produce secretions (protection and messaging).

Polarity

• Apical surface

  • Microvilli increase absorption or secretion.

  • Cilia on a ciliated epithelium move fluids.

• Basolateral surface

Integrity of Epithelia

• Maintained by:

  • Intercellular connections

  • Attachment to the basement membrane

  • Epithelial maintenance and repair

Intercellular Connections

• Support and communication

  • Cell adhesion molecules (CAMs) are transmembrane proteins.

  • Proteoglycans act as intercellular cement.

    • Contain glycosaminoglycans such as hyaluronan (hyaluronic acid).

Cell Junctions

• Form bonds with other cells or extracellular material.

  • Gap junctions

  • Tight junctions

  • Desmosomes

Gap Junctions

• Allow rapid communication.

• Cells are held together by interlocking transmembrane proteins (connexons).

• Allow small molecules and ions to pass.

• Coordinate contractions in heart muscle.

Tight Junctions

• Between two plasma membranes.

• Adhesion belt attaches to terminal web.

• Prevent passage of water and solutes.

• Keep enzymes, acids, and wastes in the lumen of the digestive tract.

Desmosomes

• CAMs and proteoglycans link opposing plasma membranes.

  • Spot desmosomes tie cells together and allow bending and twisting.

  • Hemidesmosomes attach cells to the basement membrane.

Attachment to the Basement Membrane

• Basal lamina is closest to the epithelium.

• Reticular lamina is the deeper portion of the basement membrane and provides strength.

Epithelial Maintenance and Repair

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

Classification of Epithelia

• Based on shape:

  • Squamous: Thin and flat

  • Cuboidal: Square shaped

  • Columnar: Tall, slender rectangles

• Based on layers:

  • Simple epithelium: Single layer of cells

  • Stratified epithelium: Several layers of cells

Squamous Epithelia

• Simple squamous epithelia:

  • Absorption and diffusion

  • Mesothelium lines body cavities.

  • Endothelium forms inner lining of heart and blood vessels.

• Stratified squamous epithelia:

  • Protect against mechanical stresses.

  • Keratin adds strength and water resistance.

Cuboidal Epithelia

• Simple cuboidal epithelia:

  • Secretion and absorption

  • Found in glands and portions of kidney tubules

• Stratified cuboidal epithelia:

  • Relatively rare

  • Found in ducts of sweat glands and mammary glands

Transitional Epithelia

• Tolerate repeated cycles of stretching without damage.

• Appearance changes as stretching occurs.

• Found in the urinary bladder.

Columnar Epithelia

• Simple columnar epithelia:

  • Absorption and secretion

  • Found in the stomach, small intestine, and large intestine

• Pseudostratified columnar epithelia:

  • Typically have cilia

  • Found in the nasal cavity, trachea, and bronchi

• Stratified columnar epithelia:

  • Relatively rare

  • Provide protection in the pharynx, anus, and urethra

Glandular Epithelia

• Glands are collections of epithelial cells that produce secretions.

  • Endocrine glands release hormones that enter the bloodstream and have no ducts.

  • Exocrine glands produce exocrine secretions and discharge them through ducts onto epithelial surfaces.

Gland Structure

• Unicellular glands

• Multicellular glands

Unicellular Glands

• Goblet cells are unicellular exocrine glands in epithelia of intestines and secrete mucin, which mixes with water to form mucus.

Multicellular Exocrine Glands Classification

• Structure of the duct:

  • Simple (undivided)

  • Compound (divided)

• Shape of secretory portion of the gland:

  • Tubular (tube shaped)

  • Alveolar or acinar (blind pockets)

• Relationship between ducts and glandular areas

  • Branched (several secretory areas sharing one duct)

Methods of Secretion

• Merocrine

• Apocrine

• Holocrine

Merocrine Secretion

• Released by secretory vesicles (exocytosis).

• Example: merocrine sweat glands

Apocrine Secretion

• Released by shedding cytoplasm.

• Example: mammary glands

Holocrine Secretion

• Released by cells bursting, killing gland cells.

• Gland cells replaced by stem cells.

• Example: sebaceous glands

Types of Secretions Produced by Exocrine Glands

• Serous glands: Watery secretions

• Mucous glands: Secrete mucins

• Mixed exocrine glands: Both serous and mucous

Connective Tissue

• Components of connective tissues:

  • Specialized cells

  • Extracellular protein fibers

  • Fluid called ground substance

• Matrix consists of extracellular components of connective tissue (fibers and ground substance).

  • Majority of tissue volume

  • Determines specialized 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 types of tissue

• Storing energy reserves, especially triglycerides

• Defending the body from invading microorganisms

Categories of Connective Tissues

• Connective tissue proper:

  • Connect and protect

• Fluid connective tissues:

  • Transport

• Supporting connective tissues:

  • Structural strength

Connective Tissue Proper

• Categories of connective tissue proper:

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

  • Dense connective tissue: More fibers, less ground substance; example: tendons

Cells of Connective Tissue Proper

• Fibroblasts

• Fibrocytes

• Adipocytes

• Mesenchymal cells

• Melanocytes

• Macrophages

• Mast cells

• Lymphocytes

• Microphages

Fibroblasts

• The most abundant cell type

• Found in all types of connective tissue proper

• Secrete proteins and hyaluronan (cellular cement)

Fibrocytes

• Second most abundant cell type

• Maintain connective tissue fibers

Adipocytes

• Fat cells; each cell stores a single, large fat droplet

Mesenchymal Cells

• Stem cells that respond to injury or infection

• Differentiate into fibroblasts, macrophages, etc.

Melanocytes

• Synthesize and store the brown pigment melanin

Macrophages

• Large phagocytic cells of the immune system

• Engulf pathogens and damaged cells

• Fixed macrophages stay in tissue; free macrophages migrate

Mast Cells

• Stimulate inflammation after injury or infection; release histamine and heparin

• Basophils are leukocytes that also contain histamine and heparin

Lymphocytes

• Migrate throughout the body

• May develop into plasma cells, which produce antibodies

Microphages

• Phagocytic blood cells (neutrophils, eosinophils)

• Attracted to signals from macrophages and mast cells

Connective Tissue Fibers

• Collagen fibers

• Reticular fibers

• Elastic fibers

Collagen Fibers

• Most common fibers in connective tissue proper

• Long, straight, and unbranched

• Strong and flexible; resist force in one direction

• Abundant in tendons and ligaments

Reticular Fibers

• Form a network of interwoven fibers (stroma)

• Strong and flexible; resist forces in many directions

• Stabilize functional cells (parenchyma) and structures; example: sheaths around organs

Elastic Fibers

• Contain elastin; branched and wavy

• Return to original length after stretching; example: elastic ligaments of vertebrae

Ground Substance

• Clear, colorless, and viscous

• Fills spaces between cells and slows pathogen movement

Loose Connective Tissues

• "Packing materials"; fill spaces between organs, cushion cells, and support epithelia

Embryonic Connective Tissues

• Not found in adults

  • Mesenchyme (embryonic connective tissue) is the first connective tissue in embryos

  • Mucous connective tissue is loose embryonic connective tissue

Types of Loose Connective Tissues in Adults

• Areolar tissue

• Adipose tissue

• Reticular tissue

Areolar Tissue

• Least specialized

• Open framework; viscous ground substance; elastic fibers

• Holds capillary beds; example: under skin (subcutaneous layer)

Adipose Tissue

• Contains many adipocytes (fat cells)

  • Adipocytes in adults do not divide but expand to store fat and shrink as fats are released.

  • Mesenchymal cells divide and differentiate to produce more fat cells when more storage is needed. May be removed (temporarily) via liposuction in cosmetic surgery.

White Fat

• Most common; stores fat and absorbs shocks; slows heat loss (insulation)

Brown Fat

• Found in babies and young children; more vascularized; adipocytes have many mitochondria; breakdown of lipids releases energy and warms body

Reticular Tissue

• Provides support

• Reticular fibers form a complex, three-dimensional stroma

• Support functional cells of organs; found in the liver, kidney, spleen, lymph nodes, and bone marrow

Dense Connective Tissues

• Also called collagenous tissues; contain many collagen fibers

• Three types of dense connective tissues:

  • Dense regular

  • Dense irregular

  • Elastic

Dense Regular Connective Tissue

• Tightly packed, parallel collagen fibers

  • Tendons attach muscles to bones

  • Ligaments connect one bone to another and stabilize organs

  • Aponeuroses are tendinous sheets that attach a broad, flat muscle to another structure

Dense Irregular Connective Tissue

• Interwoven network of collagen fibers

  • Provides strength to dermis

  • Forms sheath around cartilages (perichondrium) and bones (periosteum)

  • Forms capsules around some organs (e.g., liver, kidneys, and spleen)

Elastic Tissue

• Made of elastic fibers; example: elastic ligaments of spinal vertebrae

Fasciae

• Connective tissues provide strength and stability, maintain positions of internal organs, and provide routes for blood vessels, lymphatic vessels, and nerves.

• Fasciae are connective tissue layers and wrappings that support and surround organs.

Three Layers of Fasciae

• Superficial fascia: Separates skin from underlying tissues

• Deep fascia: Sheets of dense regular connective tissue

• Subserous fascia: Lies between deep fascia and serous membranes that line body cavities

Blood and Lymph

• Fluid connective tissues include blood and lymph.

Blood

• Contains a watery matrix called plasma

• Contains cells and cell fragments, collectively known as formed elements:

  • Red blood cells (erythrocytes)

  • White blood cells (leukocytes)

  • Platelets

Lymph

• Forms as interstitial fluid that enters lymphatic vessels

• Monitored by the immune system and returned to veins near the heart

Supporting Connective Tissues

• Supporting connective tissues include cartilage and bone.

Cartilage

• Provides shock absorption and protection

• Matrix is a firm gel and contains polysaccharide derivatives called chondroitin sulfates

• Cells in the matrix are chondrocytes in chambers called lacunae

Cartilage Structure

• Avascular; chondrocytes produce antiangiogenesis factor that discourages the formation of blood vessels.

• Perichondrium: Outer, fibrous layer (for support and protection); inner, cellular layer (for growth and maintenance)

Types of Cartilage

• Hyaline cartilage

• Elastic cartilage

• Fibrocartilage

Hyaline Cartilage

• Most common type; tough and somewhat flexible

• Reduces friction between bones; found in synovial joints, rib tips, sternum, and trachea

Elastic Cartilage

• Supportive but bends easily; found in the external ear and epiglottis

Fibrocartilage

• Very durable and tough

• Limits movement and prevents bone-to-bone contact

• Found around joints, between pubic bones, and between spinal vertebrae

Cartilage Growth

• Interstitial growth: Enlarges cartilage from within

• Appositional growth: Growth at the outer surface of cartilage

Bone (Osseous Tissue)

• For weight support; calcified (made rigid by calcium salts)

• Resists shattering (flexible collagen fibers)

• Osteocytes (bone cells) lie in lacunae

  • Arranged around central canals within the matrix

  • Small channels through the matrix (canaliculi) allow for the exchange of materials with blood

• Periosteum covers bone: Fibrous (outer) and cellular (inner) layers

Tissue Membranes

• Physical barriers that line or cover body surfaces

• Consist of an epithelium supported by connective tissue

Four Types of Tissue Membranes

• Mucous membranes

• Serous membranes

• Cutaneous membrane

• Synovial membranes

Mucous Membranes (Mucosae)

• Line passageways that have external connections in the digestive, respiratory, urinary, and reproductive tracts

• Epithelial surfaces must be moist to reduce friction and facilitate absorption or secretion

• Lamina propria is areolar tissue in mucous membranes

Serous Membranes

• Line cavities that do not open to the outside; thin but strong

• Parietal portion lines the inner surface of the cavity; visceral portion (serosa) covers the organs

• Serous fluid reduces friction

Examples of Serous Membranes

• Peritoneum: Lines the peritoneal cavity and covers abdominal organs

• Pleura: Lines pleural cavities and covers lungs

• Pericardium: Lines the pericardial cavity and covers the heart

Cutaneous Membrane

• Skin that covers the body; thick, relatively waterproof, and usually dry

Synovial Membranes

• Line synovial joint cavities

• Movement stimulates the production of synovial fluid for lubrication

• Lack a true epithelium

Muscle Tissue

• Specialized for contraction

Three Types of Muscle Tissue

• Skeletal muscle: Large muscles responsible for body movement

• Cardiac muscle: Found only in the heart

• Smooth muscle: Found in the walls of hollow, contracting organs

Skeletal Muscle Tissue

• Consists of long, thin cells called muscle fibers; cells do not divide

• New fibers are produced by divisions of myosatellite cells

• Striated voluntary muscle

Cardiac Muscle Tissue

• Cells form branching networks connected at intercalated discs

• Regulated by pacemaker cells; striated involuntary muscle

Smooth Muscle Tissue

• Cells are small and spindle shaped and can divide and regenerate

• Nonstriated involuntary muscle

Nervous Tissue

• Specialized for conducting electrical impulses and concentrated in the brain and spinal cord

Types of Cells in Nervous Tissue

• Neurons

• Neuroglia (supporting cells)

Parts of a Neuron

• Cell body: Contains the nucleus and nucleolus

• Dendrites: Short branches extending from the cell body that receive incoming signals

• Axon (nerve fiber): Long, thin extension of the cell body that carries outgoing electrical signals to their destination

Neuroglia

• Maintain the physical structure of tissues, repair tissue framework after injury, perform phagocytosis, provide nutrients to neurons, and regulate the composition of the interstitial fluid surrounding neurons

Tissue Injuries and Repair

• Tissues respond to injury in two stages:

  • Inflammation (inflammatory response)

  • Regeneration to restore normal function

Inflammatory Response

• Can be triggered by trauma (physical injury) or infection (the presence of pathogens)

• Damaged cells release prostaglandins, proteins, and potassium ions

• Damaged connective tissue activates mast cells

Process of Inflammation

• Lysosomes release enzymes that destroy the injured cells and attack surrounding tissues.

• Tissue destruction is called necrosis, which begins several hours after injury

• Necrotic tissues and cellular debris (pus) accumulate in the wound; an abscess is pus trapped in an enclosed area

Regeneration

• The ability to regenerate varies among tissues.

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

  • Skeletal muscle, cardiac muscle, and nervous tissues regenerate poorly, if at all.

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

Aging, Regeneration, and Cancer

• Aging and tissue structure: Speed and effectiveness of tissue regeneration decrease with age due to slowing of repair and maintenance activities, hormonal alterations, and reduced physical activity.

Effects of Aging

• Chemical and structural tissue changes

  • Thinner epithelia

  • Fragile connective tissues

  • Increased bruising

  • Brittle bones

  • Cardiovascular disease

  • Mental deterioration

Aging and Cancer Incidence

*Cancer rates increase with age.
* Twenty-five percent of all people in the United States develop cancer.
* Cancer is the second leading cause of death in the United States.
* Most cancers are caused by chemical exposure or environmental factors; forty percent of cases are caused by cigarette smoke.