The Tissue Level of Organization

Epithelial Tissue

• Overview: Epithelial tissue includes epithelia (singular: epithelium) and glands.

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

  • They form the surface of the skin and line the digestive, respiratory, and reproductive tracts (passageways that communicate with the outside world).

  • Delicate epithelia line the chest cavity, fluid-filled spaces of the brain, and the inner surfaces of blood vessels and heart chambers.

  • Glands are structures that produce fluid secretions.

• Functions of epithelial tissue:

  • Provide physical protection against abrasion, dehydration, and destruction by chemical or biological agents.

  • Control permeability (e.g., ions and nutrients transport; hormones affecting epithelial transport).

  • Provide sensation (sensory nerve supply; neuroepithelia for smell, taste, sight, equilibrium, and hearing).

  • Produce specialized secretions (glandular epithelium) released onto surfaces or into surrounding interstitial fluid and blood.

• Characteristics of epithelia:

  • Polarity: apical surface (toward lumen or outside) and basal surface (attached to the basement membrane).

  • Cellularity: tightly packed cells with cell junctions.

  • Attachment: basal lamina/basement membrane to connect to underlying tissues.

  • Avascularity: lack of blood vessels within epithelial tissue.

  • Regeneration: high capacity for renewal and repair.

• Specializations of epithelial cells:

  • Move fluids over the epithelium (protection).

  • Move fluids through the epithelium (permeability control).

  • Produce secretions (glandular epithelium).

• Polarity details:

  • Apical surface: microvilli increase absorption or secretion; cilia on ciliated epithelia move fluids.

  • Basolateral surfaces: include the base where the cell attaches to underlying epithelial cells and the sides contacting neighboring cells.

Intercellular connections and Cell Junctions

• Integrity of epithelia is maintained by:

  • Intercellular connections

  • Attachment to the basement membrane

  • Epithelial maintenance and repair

• Intercellular connections provide support and communication via:

  • Cell adhesion molecules (CAMs): transmembrane proteins

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

  • Cell junctions form bonds with other cells or extracellular material

• Major cell junctions:

  • Gap junctions: allow rapid communication; 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 lumen of digestive tract.

  • Desmosomes: CAMs and proteoglycans link opposing plasma membranes; spot desmosomes tie cells together; allow bending and twisting; resist mechanical forces; hemidesmosomes attach cells to the basement membrane.

• Diagram references: junctions include tight junctions, gap junctions, desmosomes, and hemidesmosomes (illustrated in figures such as Fig. 4-3a).

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 (thin and fragile; located in protected internal areas; all cells have the same polarity; distance from nucleus to base membrane is constant across cells)

  • Stratified epithelium: several layers (located where tissues face mechanical/chemical stresses; e.g., skin, lining of mouth).

• Common classifications (examples):

  • Simple squamous epithelium

  • Stratified squamous epithelium

  • Simple cuboidal epithelium

  • Stratified cuboidal epithelium

  • Simple columnar epithelium

  • Stratified columnar epithelium

  • Simple/Stratified columnar epithelia (pseudo-stratified) and other variants as noted in tables.

Epithelia: Squamous, Cuboidal, Columnar

• Squamous epithelia:

  • Simple squamous epithelia: absorption and diffusion; e.g., mesothelium (lines body cavities); endothelium (inner lining of heart and blood vessels); alveoli of lungs.

  • Stratified squamous epithelia: provides protection against mechanical stresses; apical layers may be keratinized (keratin provides strength and water resistance) or non-keratinized (no water resistance); found in oral cavity, pharynx, esophagus, etc.

• Cuboidal epithelia:

  • Simple cuboidal epithelia: resemble hexagonal boxes; nuclei near center; provide limited protection; functions include 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.

• Columnar epithelia:

  • Simple columnar epithelia: tall cells with elongated nuclei near the basement membrane; functions: absorption and secretion; found in stomach, small intestine, large intestine.

  • Pseudostratified columnar epithelia: several cell types with varying shapes; all contact basement membrane; typically have cilia; found in nasal cavity, trachea, bronchi.

  • Stratified columnar epithelia: relatively rare; provide protection in pharynx, anus, urethra.

• Transitional epithelia: tolerate repeated cycles of stretching and recoiling without damage; appearance changes as stretching occurs; found in the urinary bladder.

• Columnar epithelia notes: superficial layer is closest to lumen in tissue sections (as seen in figures 4-6a/6b/6c).

Glandular epithelia

• Glands: collections of epithelial cells that produce secretions.

  • Endocrine glands: release hormones into bloodstream; no ducts.

  • Exocrine glands: discharge secretions through ducts onto epithelial surfaces.

• Methods of secretion (secretory mechanisms):

  • Merocrine: produced in Golgi; secretions released by exocytosis in secretory vesicles; examples: sweat glands, mucin.

  • Apocrine: produced in Golgi; secretions released by shedding cytoplasm; examples: mammary glands.

  • Holocrine: secretions released as glands cells burst; gland cells replaced by stem cells; examples: sebaceous glands.

• Glandular epithelia categorization by secretions produced:

  • Serous glands: watery secretions containing enzymes (e.g., parotid salivary gland)

  • Mucous glands: mucins hydrated to form mucus (e.g., submucosal glands of small intestine)

  • Mixed exocrine glands: both serous and mucous secretions (e.g., submandibular salivary gland)

Connective Tissue

• Components of connective tissues:

  • Specialized cells

  • Extracellular protein fibers

  • Ground substance (fluid extracellular matrix)

• Matrix: extracellular components of connective tissue (fibers + ground substance); occupies the majority of tissue volume and determines specialized function.

• Primary functions of connective tissues:

  • Establish structural framework for the body

  • Transport fluids and dissolved materials

  • Protect delicate organs

  • Support, surround, and interconnect other tissue types

  • Store energy reserves (especially triglycerides in adipose tissue)

  • Defend the body from invading microorganisms

• Categories of connective tissues:

  • Connective tissue proper: connective tissue that supports and protects

  • Fluid connective tissues: transport system (blood and lymph)

  • Supporting connective tissues: provide structural strength (cartilage and bone)

Connective Tissue Proper

• Categories:

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

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

• Cells found in connective tissue proper:

  • Fibroblasts, fibrocytes, adipocytes, mesenchymal cells, melanocytes, macrophages, mast cells, lymphocytes, microphages

• Fibers:

  • Collagen fibers: most common; long, straight, unbranched; strong and flexible; resist force in one direction; abundant in tendons and ligaments

  • Reticular fibers: form a network (stroma) that stabilizes functional cells and organ structures

  • Elastic fibers: contain elastin; branched and wavy; return to original length after stretching (e.g., elastic ligaments of the vertebrae)

• Ground substance: clear, colorless, viscous; fills spaces between cells and slows pathogen movement

• Embryonic connective tissues:

  • Mesenchyme: first connective tissue in embryos

  • Mucous connective tissue: loose embryonic tissue (wharton's jelly in umbilical cord)

• Loose connective tissues in adults:

  • Areolar tissue: least specialized; open framework; viscous ground substance; elastic fibers; holds capillary beds (e.g., subcutaneous layer)

  • Adipose tissue: contains many adipocytes; adipocytes in adults do not divide; expand to store fat; mesenchymal cells can differentiate to produce more fat cells; may be removed (liposuction)

• Two types of adipose tissue:

  • White fat: stores fat, provides cushioning and insulation

  • Brown fat: found in babies/young children; more vascularized; adipocytes with many mitochondria; lipid breakdown releases energy to warm body

• Reticular tissue: provides support via reticular fibers forming a stroma that supports organs (liver, kidney, spleen, lymph nodes, bone marrow)

• Dense connective tissues (also called collagenous tissues):

  • Dense regular: tightly packed parallel collagen fibers; tendons (muscle to bone), ligaments (bone to bone), aponeuroses (tendinous sheets)

  • Dense irregular: interwoven collagen fibers with no consistent pattern; provides strength to dermis; forms periosteum and perichondrium; capsules around organs

  • Elastic tissue: dense regular with many elastic fibers; elastic ligaments of the vertebrae

• Fasciae: connective tissue layers that support and surround organs; three layers:

  • Superficial fascia: separates skin from underlying tissues

  • Deep fascia: sheets of dense regular connective tissue

  • Subserous fascia: lies between deep fascia and serous membranes lining body cavities

Fluid Connective Tissues

• Blood:

  • Matrix is a watery plasma

  • 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

  • Returned to veins near the heart

• Formed elements and cell types (as in blood):

  • White blood cells: neutrophils, eosinophils, basophils, monocytes, lymphocytes

  • Platelets: membrane-enclosed packets of cytoplasm involved in clotting

  • Erythrocytes: red blood cells (not shown in all diagrams but implied)

Supporting Connective Tissues

Supporting Connective Tissues

• Cartilage:

  • Provides shock absorption and protection.

  • Matrix is a firm gel containing chondroitin sulfates.

  • Cells, called chondrocytes, are located in small chambers called lacunae.

  • It is avascular, meaning it lacks blood vessels.

  • Chondrocytes produce antiangiogenetic factor to discourage blood vessel formation.

  • A perichondrium separates cartilage from surrounding tissues, consisting of two layers: an outer fibrous layer and an inner cellular layer.

• Types of cartilage: $3$ types

  • Hyaline cartilage: most common; tough yet flexible; reduces friction between bones; found in synovial joints, rib tips, sternum, trachea

  • Elastic cartilage: supportive but bends easily; found in external ear and epiglottis

  • Fibrocartilage: very durable; dense collagen fibers; limits movement and prevents bone-to-bone contact; found around joints, between pubic bones, intervertebral discs

• Cartilage growth:

  • Interstitial growth: enlarges cartilage from within

  • Appositional growth: growth at outer surface of cartilage

• Bone (osseous tissue):

  • For weight support; strong and calcified (rigid due to calcium salts) with flexible collagen fibers

  • Osteocytes reside in lacunae; arranged around central canals within matrix; canaliculi allow exchange with blood

  • Periosteum covers bone surfaces (outer fibrous layer and inner cellular layer)

Tissue Membranes

• Tissues membranes are physical barriers that line or cover body surfaces and consist of an epithelium supported by connective tissue.

• Four types of membranes:

  • Mucous membranes (mucosae):

    • Line passageways that connect to the outside (like digestive, respiratory, urinary, and reproductive tracts).

    • Epithelial surfaces are kept moist to reduce friction and help with absorption and secretion.

    • The underlying connective tissue layer is called the lamina propria, which is areolar tissue.

  • Serous membranes:

    • Line cavities that do not open to the outside.

    • They are thin but strong.

    • Have two parts:

    • Parietal portion: lines the inner surface of the body cavities.

    • Visceral portion (serosa): covers the organs within those cavities.

    • Produce serous fluid to reduce friction.

    • Examples include:

    • Peritoneum: lines the peritoneal cavity and covers abdominal organs.

    • Pleura: lines the pleural cavities and covers the lungs.

    • Pericardium: lines the pericardial cavity and covers the heart.

  • Cutaneous membrane:

    • This is your skin.

    • It is thick, relatively waterproof, and usually dry.

  • Synovial membranes:

    • Line the moving spaces of synovial joint cavities.

    • Movement encourages the production of synovial fluid, which acts as a lubricant.

    • They protect the ends of bones.

    • They do not have a true epithelium.

  • ]Figures referenced: Fig. 4-18 shows the membranes and their relationships.

Muscle Tissue

• Muscle tissue is specialized for contraction and movement.

• Types ($3$ total):

  • Skeletal muscle:

    • These are large muscles responsible for body movement.

    • They consist of long muscle fibers.

    • They look striated (striped).

    • You have voluntary control over them, meaning your nervous system tells them what to do.

  • Cardiac muscle:

    • Found only in the heart.

    • Its cellsbranch and connect at special junctions called intercalated discs.

    • It is striated but works involuntarily (you don't control it).

    • Pacemaker cells keep the heart beating rhythmically.

  • Smooth muscle:

    • Made of small, spindle-shaped cells.

    • These cells can divide and regenerate.

    • It is non-striated and involuntary.

    • Found in the walls of hollow organs that contract, such as blood vessels, the urinary bladder, and parts of the respiratory, digestive, and reproductive tracts.

Nervous Tissue

• Nervous tissue is specialized for conducting electrical impulses; rapid sensing of internal and external environments; processing information and controlling responses.

• Two cell types:

  • Neurons (nerve cells): perform electrical communication

  • Neuroglia (supporting cells): repair and supply nutrients to neurons

• Parts of a neuron:

  • Cell body: nucleus and nucleolus

  • Dendrites: short branches that receive incoming signals

  • Axon (nerve fiber): long, thin extension that carries outgoing electrical signals to destination

Tissue Injuries and Repair

Tissue Injuries and Repair

• Inflammation and regeneration are two stages that maintain homeostasis after injury:

  • Inflammation (inflammatory response): tissue's first response, characterized by:

    • Signs and symptoms: swelling, redness, heat, pain.

    • Triggers: trauma or infection.

    • Cellular response: damaged cells release prostaglandins, proteins, and potassium ions; damaged connective tissue activates mast cells.

  • Regeneration: restoring normal function after cleanup; this process varies by tissue type.

• Inflammation process (continued):

  • As cells break down, lysosomes release enzymes that destroy injured cells and attack surrounding tissues (this process is called necrosis).

  • Necrotic tissue and cellular debris (known as pus) may accumulate; abscesses form when pus is trapped in an enclosed area.

  • Injury stimulates mast cells to release histamine, heparin, and prostaglandins.

  • This leads to the dilation of blood vessels, and plasma diffuses into the area, causing swelling.

  • Pain results from abnormal tissue conditions and chemical mediators.

  • Phagocytic white blood cells are activated to clean up debris.

• Regeneration specifics:

  • Epithelium and connective tissues (except cartilage) and smooth muscle regenerate well

  • Skeletal muscle, cardiac muscle, and nervous tissue regenerate poorly; damaged cardiac muscle cells are often replaced by fibrous tissue (fibrosis)

  • Regeneration often involves fibroblasts migrating into necrotic area and laying down a collagenous scar network

• Illustrative diagram: Fig. 4-21 describes inflammation and regeneration dynamics, including mast cell activation and the cardinal signs of inflammation

Aging, Regeneration, and Cancer

• Aging and tissue structure:

  • Regeneration speed and effectiveness slow with age due to slowed repair/maintenance, hormonal changes, and reduced physical activity

  • Aging-related changes include thinner epithelia, fragile connective tissues, increased bruising, brittle bones, cardiovascular disease, and potential mental deterioration

• Aging and cancer:

  • Cancer incidence increases with age; about $25\%$ of 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; approximately $40\%$ of cases are attributed to cigarette smoke