Tissue: The Living Fabric

# Human Anatomy & Physiology - Chapter 04: Tissue: The Living Fabric

Why This Matters

  • Understanding types of tissues allows for monitoring potential tissue damage in patients, such as bedsores.

Tissue: The Living Fabric

  • Individual body cells are specialized:

    • Each type performs specific functions that maintain homeostasis.

  • Tissues:

    • Groups of cells similar in structure that perform a common or related function.

  • Histology:

    • The study of tissues.

  • Four basic tissue types:

    1. Epithelial

    2. Connective

    3. Muscle

    4. Nervous tissue

Tissue Samples for Microscopy

  • To view tissues under a microscope:

    • Fixed: Tissue is preserved with solvent.

    • Sectioned: Cut into thin slices to transmit light or electrons.

    • Stained: Enhances contrast; however, artifacts may distort the appearance.

    • Light microscopy uses colored dyes.

    • Electron microscopy uses heavy metal salts.

      • Transmission Electron Microscopy (TEM) shows a section of the tissue.

      • Scanning Electron Microscopy (SEM) shows the surface of the tissue.

Epithelial Tissue

  • Epithelial tissue (epithelium)

    • A sheet of cells covering body surfaces or lining body cavities.

    • Two main forms:

    1. Covering and lining epithelium:

      • Outer layer of skin, lines open cavities, and covers walls and organs of the ventral body cavity.

      • Examples: Skin, lining of urogenital, digestive, and respiratory systems.

    2. Glandular epithelium:

      • Forms glands of the body.

      • Example: Salivary glands.

  • Main functions:

    • Protection

    • Absorption

    • Filtration

    • Excretion

    • Secretion

    • Sensory reception

Special Characteristics of Epithelial Tissues

  • Five distinguishing characteristics:

    1. Polarity

    2. Specialized contacts

    3. Supported by connective tissue

    4. Avascular but innervated

    5. Regeneration

Polarity

  • Cells have polarity (top and bottom).

    • Apical surface: Upper free side, exposed to surface or cavity (most apical surfaces are smooth, but some have microvilli).

    • Basal surface: Lower attached side, facing inwards toward the body; adheres to the basal lamina, holding the basal surface of epithelial cells to underlying cells.

    • Both surfaces differ in structure and function.

Specialized Contacts

  • Epithelial tissues fit closely together, forming continuous sheets.

  • Specialized contact points bind adjacent epithelial cells together:

    • Lateral contacts include:

    • Tight junctions: Prevent movement between cells.

    • Desmosomes: Provide mechanical strength.

Supported by Connective Tissue

  • All epithelial sheets are supported by connective tissue.

  • Reticular lamina:

    • Deep to the basal lamina.

    • Consists of a network of collagen fibers.

  • Basement membrane:

    • Composed of basal and reticular lamina, reinforces epithelial sheet, resists stretching and tearing, and defines epithelial boundary.

Clinical - Homeostatic Imbalance 4.1

  • Cancerous epithelial cells can breach the basement membrane boundary, invading underlying tissues and causing the spread of cancer.

Avascular but Innervated

  • No blood vessels are found in epithelial tissue; cells are nourished by diffusion from underlying connective tissues.

  • Epithelia are supplied by nerve fibers.

Regeneration

  • Epithelial cells have high regenerative capacities, stimulated by loss of polarity and broken cell contacts.

  • Cells subjected to friction or harmful substances must be replaced, requiring adequate nutrients and cell division.

Classification of Epithelial Tissue

  • Naming: All epithelial tissues have two names.

    • The first name indicates the number of cell layers:

    1. Simple epithelia: Single cell layer, desirable where thin barriers are needed.

    2. Stratified epithelia: Two or more cell layers, found in high-abrasion areas (e.g., lining of mouth).

    • The second name indicates the shape of cells:

    1. Squamous: Flattened and scale-like.

    2. Cuboidal: Box-like, cube-shaped.

    3. Columnar: Tall, column-like.

    • In stratified epithelia, shape can vary; the cell shape is named according to the shape in the apical layer.

Examples of Epithelial Types

  1. Simple Squamous Epithelium:

    • Structure: Single layer of flattened cells with disc-shaped central nuclei and sparse cytoplasm.

    • Function: Allows materials to pass via diffusion and filtration; secretes lubricating substances in serosae.

    • Location: Kidney glomeruli; air sacs of the lungs; lining of heart and blood vessels; serosae.

  2. Simple Cuboidal Epithelium:

    • Structure: Single layer of cube-like cells with large, spherical central nuclei.

    • Function: Involved in secretion and absorption.

    • Location: Kidney tubules; ducts and secretory portions of small glands; ovary surface.

  3. Simple Columnar Epithelium:

    • Structure: Single layer of tall, closely packed cells; some have microvilli and goblet cells.

    • Function: Absorption and secretion of mucus, enzymes, and other substances; ciliated types propel mucus.

    • Location: Digestive tract (stomach to rectum), gallbladder, ducts of some glands, bronchi, and uterine tubes.

  4. Pseudostratified Columnar Epithelium:

    • Structure: Cells vary in height; appears stratified but is actually simple.

    • Function: Secretes mucus and propels it via ciliary action.

    • Location: Upper respiratory tract; ducts of large glands; tubules in testes.

  5. Stratified Squamous Epithelium:

    • Structure: Thick epithelium with several cell layers; surface cells are squamous.

    • Function: Protects underlying tissues from abrasion.

    • Location: Moist linings of esophagus, mouth, and vagina; keratinized variety forms epidermis of skin.

  6. Stratified Cuboidal Epithelium:

    • Structure: Rare, typically two cell layers thick.

    • Location: Some sweat and mammary glands.

  7. Stratified Columnar Epithelium:

    • Structure: Limited distribution; apical layer is columnar.

    • Location: Pharynx, male urethra, and some glandular ducts.

  8. Transitional Epithelium:

    • Structure: Resembles both stratified squamous and cuboidal; surface cells dome-shaped.

    • Function: Allows stretch and permits storage of urine.

    • Location: Lines ureters, bladder, and part of urethra; changes shape when organ stretches.

Glandular Epithelia

  • Gland: One or more cells that produce and secrete an aqueous fluid called a secretion.

  • Classified by:

    • Site of product release:

    • Endocrine: Ductless, produce hormones that are secreted into the bloodstream.

    • Exocrine: Secrete products onto body surfaces or into cavities (more numerous).

    • Number of cells forming the gland:

    • Unicellular (e.g., goblet cells)

    • Multicellular (e.g., salivary glands)

Endocrine Glands

  • Ductless glands.

  • Secretions released into interstitial fluid picked up by the circulatory system.

  • Secrete hormones that travel to target organs.

Exocrine Glands

  • Secretions released onto body surfaces or into cavities via ducts.

  • Examples: Mucous, sweat, oil, and salivary glands.

  • Can be:

    • Unicellular or multicellular.

Unicellular Exocrine Glands

  • The only important unicellular glands are mucous cells and goblet cells.

  • Produce mucin, which dissolves in water to form mucus, providing a slimy protective layer.

Multicellular Exocrine Glands

  • Composed of a duct and a secretory unit.

  • Surrounded by supportive connective tissue that supplies blood and nerve fibers.

  • Classified by structure and mode of secretion:

    • Structure:

    • Simple: Unbranched ducts.

    • Compound: Branched ducts.

    • Secretory units: Tubular or alveolar (sac).

Modes of Secretion

  • Merocrine: Produce secretions by exocytosis; do not alter secretory cells (e.g., sweat glands).

  • Holocrine: Accumulate products and rupture (e.g., sebaceous glands).

  • Apocrine: Accumulate products and apex ruptures; repairs damage (e.g., mammary glands).

Connective Tissue

  • Connective tissue: The most abundant and widely distributed primary tissue.

  • Major functions:

    • Binding and support

    • Protecting

    • Insulating

    • Storing reserve fuel

    • Transporting substances within the body (e.g., blood).

  • Four main classes:

    1. Connective tissue proper

    2. Cartilage

    3. Bone

    4. Blood

Common Characteristics of Connective Tissue

  • Extracellular matrix: Nonliving portion of connective tissues where cells are suspended; supports cells to bear weight and withstand tension.

  • Common origin: All connective tissues arise from mesenchyme (embryonic tissue).

Structural Elements of Connective Tissue

  • Main components:

    • Ground substance

    • Fibers

    • Cells

  • Ground substance and fibers together make up the extracellular matrix, with composition varying in different connective tissue types.

Connective Tissue Ground Substance

  • Ground substance: Unstructured gel-like material filling spaces between cells, allowing for solute diffusion.

  • Components:

    • Interstitial fluid

    • Cell adhesion proteins

    • Proteoglycans: large polysaccharides attached to a protein core (e.g., chondroitin sulfate, hyaluronic acid).

Connective Tissue Fibers

  • Fibers provide support:

    • Collagen fibers: Strongest and most abundant; provides high tensile strength.

    • Elastic fibers: Allow stretching and recoiling.

    • Reticular fibers: Form networks that offer more “give”.

Connective Tissue Cells

  • Cell types:

    • Immature (-blast): Actively secrete ground substance and fibers.

    • Mature (-cyte): Maintain the health of the matrix; can revert to blast for repair.

  • Examples of cell transformations:

    • Fibroblasts become fibrocytes.

    • Chondroblasts become chondrocytes.

    • Osteoblasts become osteocytes.

    • Hematopoietic stem cells: Immature form of blood cells.

Other Structural Elements of Connective Tissue

  • Adipocytes: Store energy as fat.

  • Leukocytes (WBCs): Respond to tissue injury. - Types include neutrophils, eosinophils, and lymphocytes.

  • Mast cells: Initiate local inflammatory responses and contain secretory granules (e.g., heparin and histamine).

  • Macrophages: Break down foreign materials and dead cells.

Overview of Types of Connective Tissue

  • Four main classes:

    1. Connective tissue proper

    2. Cartilage

    3. Bone

    4. Blood

Connective Tissue Proper

  • Consists of all connective tissues except bone, cartilage, and blood.

  • Two subclasses:

    1. Loose connective tissues:

    • Areolar: Most widely distributed; supports and binds tissues. Contains fibroblasts, macrophages, and fat cells.

    • Adipose: Specialized for nutrient storage; cushions and insulates.

    • Reticular: Forms stroma to support blood cells in lymph nodes, spleen, and bone marrow.

    1. Dense connective tissues:

    • Dense regular: Provides high tensile strength and support.

    • Dense irregular: Forms sheets; provides strength under tension from multiple directions.

    • Elastic: Allows elasticity in some ligaments and arterial walls.

Connective Tissues: Cartilage

  • Matrix secreted by chondroblasts and maintained by chondrocytes found in lacunae.

  • Three types of cartilage:

    1. Hyaline cartilage: Most abundant; provides support and flexibility. Found in tips of long bones, nose, trachea, and ribs.

    2. Elastic cartilage: Contains more elastic fibers; found in ears and epiglottis.

    3. Fibrocartilage: Provides tensile strength; located in intervertebral discs and knee.

Connective Tissues: Bone

  • Supports and protects body structures; stores fat and synthesizes blood cells in cavities.

  • Has more collagen compared to cartilage and includes inorganic calcium salts.

  • Osteoblasts produce matrix; osteocytes maintain matrix.

  • Richly vascularized.

Connective Tissues: Blood

  • Atypical connective tissue; consists of blood cells in fluid matrix (blood plasma).

  • Functions in the transport of nutrients, waste, and gases.

Muscle Tissue

  • Muscle tissues are highly vascularized and responsible for movement.

  • Muscle cells have myofilaments made of actin and myosin proteins that bring about contraction.

  • Three types of muscle tissues:

    1. Skeletal muscle

    2. Cardiac muscle

    3. Smooth muscle

Skeletal Muscle

  • Attached to bones and causes movement.

  • Also called voluntary muscle due to conscious control.

  • Contains multiple nuclei and appears striated.

Cardiac Muscle

  • Found only in walls of the heart; involuntary muscle.

  • Striated with one nucleus per cell; cells branch with intercalated discs connecting them.

Smooth Muscle

  • Mainly found in walls of hollow organs (except the heart); involuntary muscle.

  • Non-striated and spindle-shaped with one nucleus per cell.

  • Propels substances or objects along internal passageways.

Nervous Tissue

  • Main component of the nervous system (brain, spinal cord, nerves).

  • Regulates and controls body functions.

  • Composed of two types of specialized cells:

    • Neurons: Respond to stimuli and transmit electrical signals.

    • Supporting cells (glial cells): Support, insulate, and protect neurons.

Membranes

  • Membranes: Organs composed of more than one type of tissue covering and lining structures.

  • Types of covering and lining membranes:

    1. Cutaneous membranes

    2. Mucous membranes

    3. Serous membranes

Cutaneous Membranes

  • Another name for skin.

  • Composed of keratinized stratified squamous epithelium attached to connective tissue (dermis).

  • Unique as it is a dry membrane.

Mucous Membranes

  • Line body cavities open to the exterior (e.g., digestive, respiratory, urogenital tracts).

  • Moist membranes bathed by secretions or urine.

  • Epithelial layer over loose connective tissue (lamina propria).

  • May or may not secrete mucus.

Serous Membranes

  • Moist membranes lining closed body cavities.

  • Constructed from simple squamous epithelium resting on loose connective tissue.

    • Parietal serosae: Line body cavity walls.

    • Visceral serosae: Cover internal organs.

  • A cavity between layers contains slippery serous fluid.

Tissue Repair

  • When barriers are compromised, inflammatory and immune responses are activated for tissue repair.

  • Repair begins rapidly and is primarily due to the inflammatory response.

Steps in Tissue Repair

  1. Inflammation: Sets the stage; involves dilation of blood vessels and increased permeability, accompanied by blood clotting.

  2. Organization: Restores blood supply; blood clot replaced with granulation tissue; fibroblasts produce collagen fibers; epithelium regenerates.

  3. Regeneration and fibrosis: Scab detaches; fibrous tissue matures; resulting in thickening epithelium and visible or invisible scars.

Regenerative Capacity of Different Tissues

  • High regenerative capacity: Epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissues.

  • Moderate regenerative capacity: Smooth muscle, dense regular connective tissue.

  • Minimal regenerative capacity: Cardiac muscle, nervous tissue of the brain, spinal cord.

  • Clinical observation: Scar tissue in organs (especially bladder and heart) can impair function, cause volume loss, block material movement, and may interfere with muscle contraction.

Developmental Aspects of Tissues

  • Primary germ layers: Ectoderm, mesoderm, endoderm; specialized to form primary tissues.

  • Tissues function well through youth and middle age with adequate nutrition and circulation.

  • Aging effects on tissues:

    • Epithelia thin, making them prone to damage.

    • Tissue repair decreases efficiency.

    • Bone, muscle, and nervous tissues may undergo atrophy.

    • Increase in DNA mutations correlates with heightened cancer risk.

Embryonic Germ Layers and Their Derived Tissues

  • Ectoderm: Gives rise to nerve tissue.

  • Mesoderm: Gives rise to muscle and connective tissues.

  • Endoderm: Gives rise to epithelial tissues. Inner lining of digestive system