Tissue Level of Organization

Tissue Level of Organization

Epithelial Tissue Learning Outcomes

• Identify the four types of tissues in the body and describe their roles.
• Describe three microscopy techniques.
• Describe epithelial tissues, including cell shape, layers, and functions.
• Discuss the types and functions of intercellular connections between epithelial cells.
• Describe the structure and function of squamous epithelium.
• Describe the structure, function, and locations of cuboidal and transitional epithelia.
• Describe the structure, function, and locations of columnar epithelia.
• Compare the three different methods of exocrine secretion by glandular epithelia.
• Explain how multicellular exocrine glands are classified by their structure.

Four Types of Tissue

• Body organization review:
  • Atoms → Molecules → Cells → Tissues
  • The chemical level can only be seen using special imaging techniques.
  • Cellular level details are best seen with an electron microscope.
• Trillions of cells in the body.
• There are about 200 different types of cells.
• Cells working together form tissues.
• The study of tissues is called histology.
• Four basic types of tissues:
  1. Epithelial
  2. Connective
  3. Muscle
  4. Neural

Tissue Types Functions

• Epithelial Tissue:
  • Covers exposed surfaces.
  • Lines internal passageways and chambers.
  • Forms secretory glands.
• Connective Tissue:
  • Fills internal spaces.
  • Provides structural support.
  • Stores energy.
• Muscle Tissue:
  • Contracts to produce movement.
  • Includes skeletal muscle, cardiac muscle, and smooth muscle.
• Nervous Tissue:
  • Conducts electrical impulses.
  • Carries information.

Microscopy

• Microscopy (the use of microscopes) began about 400 years ago.
  • Early magnification levels were 10–20 times actual size.
• Simple microscope: uses only one lens.
• Compound microscope: uses >1 lens.
• Electron microscope:
  • Can magnify over 1 million times.
• The amount of fine detail (resolution) of an image varies with magnification and type of microscope used.

Types of Microscopes

Compound Light Microscope

• Detects visible light through a thin section of tissue.
• Two lenses magnify the specimen:
  1. Objective lens (located on the revolving nosepiece)
  2. Ocular lens (located in the eyepiece)
• Total magnification is calculated by multiplying the two lens powers (objective × ocular).
• Resolution:
  • Ability to distinguish between two separate points.
  • The wavelength of light limits the resolution on a light microscope to about 200 nm (0.2 µm).

Transmission Electron Microscope (TEM)

• "Transmits" electrons through the specimen.
• Uses magnets to direct a beam of electrons through the surface of a very thin object onto a photographic plate.
• The wavelength of the electron beam is 0.00001 of white light.
• Maximum resolution is 0.2 nm (0.0002 µm).

Scanning Electron Microscope (SEM)

• Uses electrons, but not by sending them through a specimen.
• The specimen is coated with an electron-dense material.
• Electron beams are focused on the specimen.
• The reflection of electrons bouncing off the object produces a three-dimensional image of the surface.
• Can view surface features only.
• The maximum resolution is about 10 nm (0.01 µm).

Magnification Notation Examples

• LM × 400: Light micrograph magnified 400 times.
• TEM × 3000: Transmission electron micrograph magnified 3000 times.
• SEM × 15,846: Scanning electron micrograph magnified 15,846 times.

A&P Lab Microscopy Tips

• Light microscopes are most often used.
• Begin with the lowest magnification objective lens over the specimen.
• When in focus on that power, carefully rotate objective lenses with greater magnification into place.
• Compare images from the textbook to the microscope image at the same magnification.

Epithelial Tissue

• Covers surfaces, lines cavities, and forms secretory glands.
• Divisions of epithelial tissue:
  1. Epithelia
     • Avascular layers.
     • Cover exposed surfaces.
     • Line internal cavities and passageways.
     • Often contain secretory or gland cells scattered among other cell types.
  2. Glands
     • Derived from epithelia.
     • Predominantly secretory cells.
     • Two types:
       1. Exocrine glands: Secrete onto external surfaces or into ducts.
       2. Endocrine glands: Secrete hormones into interstitial fluid, and hormones are then distributed by the bloodstream.

Functions of Epithelial Tissue

• Provide physical protection:
  • Protect surfaces from abrasion, dehydration, or destruction by chemical or biological agents.
• Control permeability:
  • Most epithelia are capable of selective absorption or secretion.
  • The epithelial barrier can be modified in response to stimuli (Example: calluses).
• Provide sensation:
  • Specialized epithelial cells detect changes in the environment (e.g., touch receptors).
  • Neuroepithelium: Sensory epithelium found in special sense organs.
• Produce specialized secretions:
  • Glandular epithelial cells produce secretions.

Features of Epithelial Tissue

• Surfaces:
  • Apical surface: Faces the exterior of the body or an internal space.
  • Base: Attached to underlying tissues.
  • Basolateral surface: Includes the base and sides (lateral surfaces) attached to neighboring cells.
• Polarity:
  • Refers to structural differences between exposed and attached surfaces.
• Apical surface features:
  • If lining a tube, the apical surface is exposed to the space inside the tube, called the lumen.
  • Microvilli are found on this surface in the digestive, urinary, and reproductive tracts.
  • Cilia are found on this surface in parts of the respiratory and reproductive tracts.
• Epithelial cells also contain membranous organelles comparable to other cell types.

Epithelial Cell Shapes

• Epithelial cells have three basic shapes (viewed perpendicular to the exposed surface):
  1. Squamous: Thin and flat.
  2. Cuboidal: Cube-shaped (like little boxes).
  3. Columnar: Taller than they are wide (slender rectangles).

Epithelial Cell Layers

• Single layer: Simple epithelium.
• Several layers of cells: Stratified epithelium.
  • Found in areas that need protection from abrasion or chemical stress.
  • Examples: the surface of the skin, the lining of the mouth.

Epithelial Attachments

• Extensive attachments between adjacent cells and adjacent tissues.
• To function as a barrier, must have an intact, complete lining.
• Must be able to replace damaged or lost cells.
• Epithelia lack blood vessels (avascular).
  • Requires attachment to underlying connective tissue for nourishment from blood vessels there.

Intercellular Connections

• Types of intercellular connections:
  1. Hemidesmosomes:
     • Attach deepest epithelial cells to the basement membrane.
       1. Basal lamina
          • Contains glycoproteins and fine protein filaments.
          • Produced by the basal surface of the epithelium.
       2. Reticular lamina
          • Contains bundles of coarse protein fibers.
          • Gives strength and restricts diffusion.
  2. Tight (occluding) junctions:
     • Interlocking membrane proteins bind adjacent plasma membranes together.
     • Prevent the passage of water and solutes between cells.
     • Isolate basolateral surfaces and deeper tissues from contents in the lumen.
     • Found in the intestinal tract.
  3. Adhesion belts:
     • Continuous band of membrane proteins.
     • Strengthens the apical region of cells.
     • Reinforces tight junctions.
     • Dense proteins are attached to microfilaments of the terminal web (part of the cytoskeleton).
     • Belts encircle cells and bind to adjacent cells.
  4. Gap junctions:
     • Held together by interlocking transmembrane proteins (connexons).
     • Assist in chemical communication to help coordinate functions such as secretion or beating cilia.
     • Also found in cardiac muscle and smooth muscle tissue to coordinate contraction.
  5. Desmosomes:
     • Provide firm attachments by interlocking adjacent cells’ cytoskeletons.
     • Opposing plasma membranes are locked together by cell adhesion molecules (CAMs).
     • A thin layer of proteoglycans may also bond (contain polysaccharide, notably hyaluronic acid).
     • Very strong; resist stretching and twisting.
     • Found in superficial layers of the skin.

Squamous Epithelium

• Thin, flat, irregularly shaped cells (like jigsaw puzzle pieces).
  • Viewed from above, cells look like fried eggs.
  • In sectional view, a disc-shaped nucleus is found in the thickest part of the cell.
• May be a single layer (simple) or multiple layers (stratified).

Simple Squamous Epithelium

• The most delicate epithelium (one layer thick).
• Functions include absorption, diffusion, and reduction of friction.
• Found in protected regions such as the peritoneum, capillary walls, inside the eye, and lung alveoli.
• Certain locations have special names:
  • Mesothelium: Lines ventral body cavities.
  • Endothelium: Lines the heart and blood vessels.

Stratified Squamous Epithelium

• Located where severe mechanical or chemical stresses exist.
• Many layers of cells.
• The superficial layer is flattened.
• Forms the surface of the skin and lines the mouth, throat, esophagus, rectum, anus, and vagina.
• Two types:
  1. Keratinized:
     • Superficial layers are packed with keratin.
     • Tough and water-resistant.
     • Resists both mechanical stress and dehydration.
     • Found on the surface of the skin and in hair and nails.
  2. Nonkeratinized:
     • Resists abrasion but can dry out.
     • Found lining the oral cavity, pharynx, esophagus, anus, and vagina.

Cuboidal Epithelium

• Cells resemble hexagonal boxes.
  • In sectional view, cells appear square.
• Spherical nucleus near the center of each cell.

Simple Cuboidal Epithelium

• Functions in secretion and absorption.
• Lines exocrine glands and ducts.
• Lines parts of kidney tubules and the thyroid gland.

Stratified Cuboidal Epithelium

• Rare tissue.
• Found in the ducts of sweat glands and mammary glands.

Transitional Epithelium

• An unusual stratified epithelium that can stretch and recoil without damage.
  • The transitional name is because it changes appearance.
• Found only in the urinary system (urinary bladder, ureters, urine-collecting chambers of kidneys).
• Changes in appearance:
  • Relaxed (e.g., empty bladder): Superficial cells are cuboidal.
  • Stretched (e.g., full bladder): Superficial cells are flattened.

Columnar Epithelium

• In sectional view, cells appear rectangular.
• Cells are taller and more slender than cuboidal.
• Elongated nuclei in a band close to the basement membrane.
• Types:
  1. Simple columnar epithelium
  2. Pseudostratified columnar epithelium
  3. Stratified columnar epithelium

Simple Columnar Epithelium

• Found where absorption or secretion takes place.
  • Lines the stomach, intestine, gallbladder, uterine tubes, and kidney ducts.
• May have microvilli (for absorption) or cilia (for movement) on the apical surface.

Pseudostratified Columnar Epithelium

• Cells of varying shapes and functions.
• The distance between nuclei varies, giving the appearance of layering or being stratified.
  • Each cell contacts the basement membrane.
• Cells usually have cilia.
• Lines nasal cavities, trachea, larger airways in lungs, and portions of the male reproductive tract.

Stratified Columnar Epithelium

• A rare tissue type.
• Two or more layers of cells.
  • Superficial layer of columnar cells.
• Found lining large ducts such as those of salivary glands or the pancreas.

Glandular Epithelia

• Collections of epithelial cells (or derived structures) that produce secretions.
• Can be scattered cells or complex organs.
• Categorized into two types:
  1. Endocrine glands: Release secretions into interstitial fluid.
  2. Exocrine glands: Release secretions into ducts onto the epithelial surface.

Exocrine Gland Secretion

• Three types of exocrine gland secretion:
  1. Merocrine (meros, part) secretion:
     • The product is released from secretory vesicles by exocytosis.
     • The most common mode of secretion.
     • Example: salivary gland secretion.
     • Mucin: A merocrine secretion that mixes with water to form mucus.
  2. Apocrine (apo, off) secretion:
     • Apical cytoplasm is packed with secretory vesicles.
     • The cell releases cytoplasm as well as the secretory product.
     • Example: mammary gland secretion (involves a combination of merocrine and apocrine secretions).
  3. Holocrine (holos, entire) secretion:
     • Destroys gland cell.
     • The entire cell bursts, releasing secretions and killing the cell.
     • Destroyed cells are replaced by stem cell division.
     • Example: sebaceous glands.

Multicellular Exocrine Gland Classification

• Based on duct structure:
  • Simple: A single duct that does not divide.
  • Compound: A duct divides one or more times.
• Based on the shape of the secretory area:
  • Tubular: Glandular cells form tubes.
  • Alveolar or acinar: Glandular cells form sacs.
  • Tubuloalveolar: Glandular cells form both tubes and sacs.

Unicellular Exocrine Gland Classification

• Mucous (goblet) cells:
  • Only unicellular exocrine glands.
  • Independent, scattered secretory cells in epithelium.
  • Secrete mucin.

Connective Tissue Learning Outcomes

• Describe the general structure of connective tissue.
• Describe the structure, function, and locations of areolar tissue, adipose tissue, and reticular tissue.
• Describe the structure, function, and locations of dense connective tissues and fluid connective tissues.
• Describe the structure, function, and locations of cartilage.
• Describe the structure and function of bone.
• Describe the arrangements of epithelial and connective tissues in the four types of tissue membranes, and describe the structures and locations of the three types of fasciae.

Connective Tissue Overview

• Varies widely in appearance and function.
  • Found throughout the body, but never exposed to the surface.
• Ranges from highly vascular to avascular.
• Many contain sensory receptors that detect pain, pressure, temperature, and other stimuli.

Connective Tissue Components

• Three basic components shared by connective tissues:
  1. Specialized cells
  2. Extracellular protein fibers
  3. Fluid called ground substance
• Matrix:
  • Extracellular fibers and ground substance.
  • Surrounds the cells.
  • Accounts for the majority of connective tissue volume.
  • Fewer cells and more extracellular material compared to epithelial tissue.

Connective Tissue Subdivisions

• Three subdivisions of connective tissue:
  1. Connective tissue proper:
     • Contains many types of cells.
     • Extracellular fibers in a syrupy ground substance.
     • Loose (fibers create a loose, open framework)
     • Dense (fibers are densely packed)
  2. Fluid connective tissue:
     • Distinctive group of cells.
     • Watery matrix.
     • Blood (within the cardiovascular system)
     • Lymph (within the lymphatic system)
  3. Supporting connective tissue:
     • Less diverse cell population.
     • More densely packed matrix.
     • Cartilage (solid, rubbery matrix)
     • Bone (solid, crystalline matrix)

Loose Connective Tissues

• Support other tissue types.
• Connective tissue proper components:
  • Extracellular protein fibers:
    • Reticular fibers (strong and form a branching network)
    • Collagen fibers (thick, very strong)
    • Elastic fibers (slender, very stretchy)
  • Ground substance:
    • Clear and colorless
    • Viscous (syrupy) due to the presence of proteoglycans and glycoproteins
• Two classes of cells:
  1. Fixed (stationary; involved with maintenance, repair, and energy storage):
     • Melanocytes (synthesize melanin pigment)
     • Fixed macrophages (engulf cell debris and pathogens)
     • Mast cells (stimulate inflammation and mobilize defenses)
     • Fibroblasts (synthesize extracellular fibers)
     • Adipocytes (store lipid reserves)
     • Fibrocytes (differentiate from fibroblasts and maintain extracellular fibers)
  2. Wandering (move throughout tissue; function in defense and repair):
     • Plasma cells (immune cells producing antibodies)
     • Free macrophages (engulf debris and pathogens)
     • Mesenchymal cells (stem cells that aid in tissue repair)
     • Neutrophils and eosinophils (phagocytic blood cells)
     • Lymphocytes (immune system cells)

Loose Connective Tissue Types

• Three types of loose connective tissue:
  1. Areolar tissue:
     • The most common connective tissue proper.
     • Packing material of the body.
     • Has all connective tissue proper cell types.
  2. Adipose tissue:
     • Found deep to the skin in various areas of the body.
     • Forms a layer of padding around the eyes and kidneys.
     • Cells (adipocytes) account for most of the tissue volume.
  3. Reticular tissue:
     • Found in the liver, kidney, spleen, lymph nodes, and bone marrow.
     • Provides support and resists distortion.
     • Many reticular fibers form a network (stroma).

Dense Connective Tissues

• Dominated by extracellular fibers.
• Three types of dense connective tissues:
  1. Dense regular connective tissue:
     • Found in cords (tendons, ligaments) or sheets.
     • Collagen arranged in parallel bundles.
  2. Dense irregular connective tissue:
     • Fibers arranged in a meshwork (no consistent pattern) to resist tension in many directions.
     • Found covering visceral organs; in superficial layers of bones, cartilages, and peripheral nerves; in the dermis of the skin.
  3. Elastic tissue:
     • More elastic fibers than collagen.
     • Is springy and resilient.
     • Found between vertebrae, in walls of large blood vessels, and erectile tissues of the penis.

Fluid Connective Tissues

• Have an aqueous matrix.
• A fluid matrix with many suspended proteins.
• Usually contains no insoluble fibers.
• Two types of liquid connective tissue:
  1. Blood
  2. Lymph

Blood Components

• Watery matrix called plasma.
• Formed elements suspended in plasma:
  • Red blood cells (transport oxygen)
  • White blood cells (bodily defense):
    • Monocytes (large phagocytes)
    • Lymphocytes (uncommon in the blood)
    • Eosinophils/neutrophils (small phagocytes)
    • Basophils (promote inflammation)
  • Platelets (involved in the clotting response)

Lymph

• Watery matrix called lymph located in lymphatic vessels.
• Collected from interstitial fluid.
• The majority of cells are lymphocytes.
• Returned to blood at large veins near the heart.
• Functions to maintain solute levels, blood volume, and alert the immune system of infection.

Extracellular Fluid Circulation

• Contractions of the heart move blood through blood vessels.
  • Arteries (away from the heart)
  • Capillaries (smallest vessels; sites of exchange)
  • Veins (toward the heart)
• Water and solutes are exchanged between plasma and interstitial fluid.
• Lymphatic vessels collect excess interstitial fluid.
• Lymphatic vessels return lymph to large veins near the heart.

Cartilage

• Provides a flexible support for body structures.
• The matrix is a firm gel containing chondroitin sulfates (chondros, cartilage), polysaccharide derivatives.
  • Form complexes with proteins forming proteoglycans.
• Only one type of cell (chondrocyte).
  • Found in small chambers called lacunae (lacus, lake).
• Avascular.

Cartilage Types

• Three types of cartilage:
  1. Hyaline cartilage:
     • Found between ribs and sternum, covering bones in mobile joints, part of the nasal septum, and supporting respiratory passageways.
     • Provides stiff but flexible support.
     • Reduces friction.
  2. Elastic cartilage:
     • Distorts without damage and returns to its original shape.
     • Found in the external ear and smaller internal structures.
  3. Fibrocartilage:
     • Found in the knee joint, between pubic bones, and in intervertebral discs.
     • Durable and tough.
     • Resists compression, prevents bone-to-bone contact, and limits relative movement.

Cartilage Properties

• Set apart from surrounding tissues by the perichondrium (peri-, around).
  • Two layers of perichondrium:
    1. Outer layer of dense irregular connective tissue:
       • Mechanical support, protection, and attachment.
    2. Inner cellular layer:
       • Where cartilage growth and maintenance occur.
• Blood vessels in the perichondrium provide oxygen and nutrients to chondrocytes.

Cartilage Growth

• Two types of cartilage growth:
  1. Appositional growth (at cartilage surface):
     • Chondroblasts (immature chondrocytes) divide in the cellular layer of the perichondrium.
     • Chondroblasts secrete new matrix.
     • Once surrounded by the matrix, chondroblasts mature into chondrocytes.
  2. Interstitial growth (within cartilage):
     • Chondrocytes divide within a lacuna.
     • Daughter cells secrete additional matrix and move apart.
• Both types of cartilage growth occur during development.
• Normally, there is no growth and repair in adults.
• With slight damage or with hormonal stimulation, some appositional growth is possible.

Bone

• Provides a strong framework for the body.
• Osseous (os, bone) tissue (bone tissue):
  • Connective tissue with a solid, crystalline matrix.
    • Small volume of ground substance.
    • \frac{2}{3} of the matrix is calcium salts (provide strength):
      • Mostly calcium phosphate
      • Some calcium carbonate
    • Many collagen fibers (provide flexibility).
  • Strong, somewhat flexible, and resistant to shattering.

Bone Structure

• Typical long bone structure:
  • Hollow with two types of bone:
    1. Compact bone:
       • Outer layer of bone
    2. Spongy bone:
       • Lines internal cavity
       • Finer network

Compact Bone Structure

• The matrix is organized in concentric layers.
  • Organized into functional units (osteons):
    • The central canal contains blood vessels in the center
    • Cells (osteocytes) are located between layers
    • Canaliculi (little canals) connect osteocytes
• A superficial layer of solid, calcified bone prevents interstitial growth.
• Surrounded by the periosteum:
  • Two layers:
    1. The outer fibrous layer allows the attachment of ligaments
    2. The inner cellular layer allows appositional growth and repair

Tissue Membranes

• Are physical barriers, and fasciae support and surround organs.
• Overview of Membranes:
  • Line or cover body surfaces
  • Typically consist of epithelium supported by connective tissue
  • Four types in the body:
    1. Mucous membranes
    2. Serous membranes
    3. Cutaneous membrane
    4. Synovial membranes

Membrane Types

• Four types of membranes:
  1. Mucous membranes
     • Line passageways open to the exterior of the body
       • Digestive, respiratory, reproductive, and urinary tracts
     • Must be kept moist to facilitate movement, absorption, or secretion
     • Lubricated by mucus or bodily fluids
     • Supported by areolar connective tissue (lamina propria)
  2. Serous membranes (serosae)
     • Composed of mesothelium supported by areolar connective tissue
     • Delicate and never connected to the exterior
     • A watery serous fluid coats the surface
     • Three line subdivisions of the ventral body cavity
       • Pleura (pleural cavity and lungs)
       • Pericardium (pericardial cavity and heart)
       • Peritoneum (peritoneal cavity and visceral organs)
  3. Cutaneous membrane
     • Covers the surface of the body (skin)
     • Composed of:
       • Stratified squamous epithelium
       • Layer of areolar tissue
       • Underlying dense irregular connective tissue
     • Relatively thick, waterproof, and usually dry
  4. Synovial membrane
     • Lines freely movable joint cavities
     • Lubricates the joint cavity with synovial fluid
     • Provides oxygen and nutrients to cartilage cells
     • Not true epithelium
       • Develops within connective tissue
       • Lacks a basement membrane
       • Contains gaps between cells (up to 1 mm)
       • Synovial fluid and capillaries continuously exchange fluid and solutes

Fasciae

• Support and surround organs.
• Three types of layers:
  1. Superficial fascia
     • Under the skin
     • Consists of areolar and adipose tissue
  2. Deep fascia
     • Continuous with capsules, ligaments, and other connective tissue structures
     • Consists of dense irregular connective tissue
     • Forms a strong, fibrous internal framework
  3. Subserous fascia
     • Between serous membranes and deep fascia
     • Consists entirely of areolar tissue

Muscle Tissue

• Outweighs nervous tissue by 25:1.
• Tissue types by weight:
  • Muscle tissue is 50 percent of total body weight
    • The most of any major tissue type
  • Nervous tissue is 2 percent of total body weight
    • The least of any major tissue type
  • Connective tissue (45 percent) and epithelial tissue (3 percent) provide an interwoven framework for the body.

Muscle Tissue Types

• Skeletal muscle tissue
  • Moves the body
• Cardiac muscle tissue
  • Moves blood within the heart and through blood vessels
• Smooth muscle tissue
  • Moves fluids and solids along the digestive tract
  • Regulates the diameter of small arteries, among other functions

Muscle Tissue Functions

• Specialized for contraction to cause movement
  • Movement of the body
  • Movement of blood around the cardiovascular system
  • Movement of materials along the digestive tract
• Three types of muscles:
  1. Skeletal muscle tissue
  2. Cardiac muscle tissue
  3. Smooth muscle tissue

Skeletal Muscle Tissue

• Found in skeletal muscle
• Elongated, cylindrical, banded (striated) cells with multiple nuclei (multinucleate)
• Functions
  • Move and stabilize the skeleton
  • Guard entrances and exits to the digestive, respiratory, and urinary tracts
  • Generate heat
  • Protect internal organs

Cardiac Muscle Tissue

• Found only in the heart
• Cells (“cardiocytes”) are short, branched, and usually have a single nucleus
  • Interconnected with special junctions (intercalated discs) that help synchronize cardiocyte contractions
• Functions to move blood and maintain blood pressure

Smooth Muscle Tissue

• Found throughout the body (skin, blood vessel walls, many organs of various systems)
• Cells are short, spindle-shaped, nonstriated, and have a single nucleus
• Functions
  • Move food, urine, and reproductive secretions
  • Control the diameter of respiratory passageways and blood vessels

Nervous Tissue

• Specialized for communication.
• Specialized for the conduction of electrical impulses
• 98 percent is found in the brain and spinal cord
• Two basic types of cells
  1. Neurons (neuros, nerve)
  2. Neuroglia or glial cells (glia, glue)
     • Various supporting cells

Neurons

• Transfer information around the body and perform information processing
• Vary in size and shape
  • The longest cells in the body are neurons (up to 1 meter)

Neuron Structure

• Dendrites (dendron, tree)

  • Receive information

• Axon

  • Conducts information to other cells
  • Also called nerve fibers

• Cell body

  • Contains a large nucleus and other organelles
  • Cell control center and site of information processing
  • Most lack centrioles and cannot divide

• Neuroglia

  • Several different structural types with associated functions

• CLINICAL MODULE: The response to tissue injury involves inflammation and regeneration
  Tissues respond in a coordinated way to restore homeostasis
  Two restoration processes

• Inflammation

• Regeneration

Responses to Tissue Injury

• Injury occurs
  • The body is exposed to pathogens and toxins and chemicals from injured cells
  • The body activates defense mechanisms including:
    • Mast cell activation
      • Mast cells release histamine and other chemicals
      • Stimulates inflammation

Inflammation

• Produces indications of injury
  1. Swelling
  2. Redness
  3. Warmth
  4. Pain
• May result from injury or from infection (presence of pathogens within the tissue)
• Occurs in connective tissue, so may occur anywhere in the body (as all organs contain connective tissue)
• Dilates (enlarges) blood vessels
• Increases blood vessel permeability
• Increased blood flow to the area causes:
  • Swelling in the area
  • Increased local temperature
  • Increased delivery of oxygen and nutrients
  • Increased removal of toxins and wastes
• Stimulates increased phagocytosis in tissues
• The sensation of pain is from abnormal conditions and chemicals released by mast cells
• The cleanup process (removing toxins and wastes) usually eliminates inflammatory stimuli in hours to days
  Regeneration
• Occurs after damaged tissue has stabilized
• Fibroblasts produce collagen fibers to stabilize the injury site
• Produces a dense, collagenous framework called scar tissue
• Scar tissue is usually remodeled, and normal tissue conditions are restored

Tissue Regeneration

• Each tissue has a different ability to regenerate
  • Epithelial, connective (except cartilage), and smooth muscle regenerate well
  • Other muscle types and neural tissue regenerate poorly, if at all
• In tissues that regenerate poorly, scar tissue replaces tissues that do not regenerate
  • Fibrosis is the permanent replacement of normal tissue by scar tissue