Anatomy & Physiology - Tissues

Tissue Level of Organization

Types of Tissue

• The human body contains trillions of cells.
• There are approximately 200 different types of cells.
• Cells working together form tissues.
• Histology is the study of tissues.
• Four basic types of tissues:
  • Epithelial
  • Connective
  • Muscle
  • Neural

Epithelial Tissue

• Covers surfaces, lines cavities, and forms secretory glands.
• Divisions:
  • Epithelia
    • Avascular layers.
    • Cover exposed surfaces.
    • Line internal cavities and passageways.
    • Often contain secretory or gland cells scattered among other cell types.
  • Glands
    • Derived from epithelia.
    • Predominantly secretory cells.
    • Two types:
      • Exocrine glands: Secrete onto external surfaces or into ducts.
      • Endocrine glands: Secrete hormones into interstitial fluid, 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.
• Provide sensation
  • Specialized epithelial cells detect changes in the environment (e.g., touch receptors).
• 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.

Epithelial Cell Shapes

• Viewed perpendicular to the exposed surface:
  • Squamous
    • Thin and flat.
  • Cuboidal
    • Cube-shaped, like little boxes.
  • Columnar
    • Taller than they are wide, slender rectangles.

Epithelial Cell Layers

• Single layer
  • Simple epithelium.
• Several layers of cells
  • Stratified epithelium.
  • Found in areas needing protection from abrasion or chemical stress.
    • Examples: surface of skin, 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:
  • Hemidesmosomes
    • Attach deepest epithelial cells to the basement membrane.
      • Basal lamina
        • Contains glycoproteins and fine protein filaments.
        • Produced by the basal surface of the epithelium.
      • Reticular lamina
        • Contains bundles of coarse protein fibers.
        • Gives strength and restricts diffusion.
  • Tight (occluding) junctions
    • Interlocking membrane proteins bind adjacent plasma membranes together.
    • Prevent passage of water and solutes between cells.
    • Isolate basolateral surfaces and deeper tissues from contents in lumen.
    • Found in the intestinal tract.
  • Adhesion belts
    • Continuous band of membrane proteins.
    • Strengthens the apical region of cells.
      • Reinforces tight junctions.
    • Belts encircle cells and bind to adjacent cells.
  • Gap junctions
    • Held together by interlocking transmembrane proteins (connexons).
    • Assist chemical communication to help coordinate functions such as secretion or beating cilia.
    • Also found in cardiac muscle and smooth muscle tissue to coordinate contraction.
  • Desmosomes
    • Provide firm attachments by interlocking adjacent cells’ cytoskeletons.
    • Opposing plasma membranes locked together by cell adhesion molecules (CAMs).
    • Very strong; resist stretching and twisting.
    • Found in superficial layers of skin.

Squamous Epithelium

• Thin, flat, irregularly shaped cells (like jigsaw puzzle pieces).
• May be a single layer (simple) or multiple layers (stratified).

Simple Squamous Epithelium

• Most delicate epithelium (one layer thick).
• Functions include absorption, diffusion, reduction of friction.
• Found in protected regions such as peritoneum, capillary walls, inside the eye, lung alveoli.

Stratified Squamous Epithelium

• Located where severe mechanical or chemical stresses exist.
• Many layers of cells.
• The superficial layer is flattened.
• Forms surface of skin and lines mouth, throat, esophagus, rectum, anus, vagina.

Types of Stratified Squamous Epithelium

• Keratinized
  • Superficial layers 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.
• Nonkeratinized
  • Resists abrasion but can dry out.
  • Found lining the oral cavity, pharynx, esophagus, anus, vagina.

Cuboidal and Transitional Epithelium

• Simple Cuboidal Epithelium
  • Functions in secretion and absorption.
  • Lines exocrine glands and ducts.
  • Lines parts of kidney tubules and thyroid gland.
• Stratified Cuboidal Epithelium
  • Rare tissue.
  • Found in ducts of sweat glands and mammary glands.
• Transitional Epithelium
  • Unusual stratified epithelium that can stretch and recoil without damage.
    • The name transitional because its appearance changes.
  • Found only in the urinary system (urinary bladder, ureters, urine-collecting chambers of kidneys).
  • Changes in appearance
    • Relaxed (e.g., empty bladder)—superficial cells cuboidal.
    • Stretched (e.g., full bladder)—superficial cells flattened.

Columnar Epithelium

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

Simple Columnar Epithelium

• Found where absorption or secretion takes place.
• Lines stomach, intestine, gallbladder, uterine tubes, 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 basement membrane.
• Cells usually have cilia.
• Lines nasal cavities, trachea, larger airways in lungs, portions of the male reproductive tract.

Stratified Columnar Epithelium

• 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 pancreas.

Glandular Epithelia

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

Exocrine Gland Secretion Types

• Merocrine secretion
  • Product released from secretory vesicles by exocytosis.
  • Most common mode of secretion.
  • Example: salivary gland secretion.
  • Mucin
    • Merocrine secretion that mixes with water to form mucus.
• Apocrine secretion
  • Apical cytoplasm packed with secretory vesicles.
  • Cell releases cytoplasm as well as secretory product.
  • Example: mammary gland secretion (involves a combination of merocrine and apocrine secretions).
• Holocrine 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
    • The duct divides one or more times.

Unicellular Exocrine Gland Classification

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

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.

Basic Components of Connective Tissues

• Specialized cells
• Extracellular protein fibers
• 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

Subdivisions of Connective Tissue

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

Connective Tissue Proper Components

• Extracellular protein fibers
  • Reticular fibers
    • Strong and form 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

Classes of Cells in Connective Tissue Proper

• Fixed
  • Stationary; involved with maintenance, repair, energy storage
    • Melanocytes
      • Synthesize melanin pigment
    • Fixed macrophage
      • Engulfs 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
• 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 tissue repair
    • Neutrophils and eosinophils
      • Phagocytic blood cells
    • Lymphocytes
      • Immune system cells

Types of Loose Connective Tissue

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

Dense Connective Tissues

• Most volume occupied by extracellular fibers
  • Dense regular connective tissue
    • Found in cords (tendons, ligaments) or sheets
    • Collagen arranged in parallel bundles
  • 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 dermis of skin
  • Elastic tissue
    • More elastic fibers than collagen
    • Is springy and resilient
    • Found between vertebrae, in walls of large blood vessels, erectile tissues of the penis

Fluid Connective Tissue

• Fluid matrix with many suspended proteins
• Usually contains no insoluble fibers
• Two types of liquid connective tissue
  • Blood
  • 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 blood)
    • Eosinophils/neutrophils (small phagocytes)
    • Basophils (promote inflammation)
  • Platelets (involved in 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

Cartilage Overview

• The matrix is a firm gel containing chondroitin sulfates, polysaccharide derivatives
  • Form complexes with proteins forming proteoglycans
• Only one type of cell (chondrocyte)
  • Found in small chambers called lacunae
• Avascular

Types of Cartilage

• Hyaline cartilage
  • Found between ribs and sternum, covering bones in mobile joints, part of the nasal septum, supporting respiratory passageways
  • Provides stiff but flexible support
  • Reduces friction
• Elastic cartilage
  • Distorts without damage and returns to its original shape
  • Found in the external ear and smaller internal structures
• 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

Osseous Tissue (Bone)

• Connective tissue with solid, crystalline matrix
  • Small volume of ground substance
  • 2/3 of matrix is calcium salts (provide strength)
    • Mostly calcium phosphate
    • Some calcium carbonate
  • Many collagen fibers (provide flexibility)
• Strong, somewhat flexible, resistant to shattering

Bone Structure

• Hollow with two types of bone
  1. Compact bone Outer layer of bone
  2. Spongy bone Lines internal cavity, a finer network

Cartilage vs. Bone

Tissue Membranes Overview

• Line or cover body surfaces
• Typically consist of epithelium supported by connective tissue
• Four types in the body
  • Mucous membranes
  • Serous membranes
  • Cutaneous membrane
  • Synovial membranes

Types of Membranes

• Mucous membranes
  • Line passageways open to the exterior of the body
    • Digestive, respiratory, reproductive, 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)
• Serous membranes (serosae)
  • Composed of mesothelium supported by areolar connective tissue
  • Delicate and never connected to the exterior
  • Watery serous fluid coats surface
  • Three line subdivisions of ventral body cavity
    • Pleura (pleural cavity and lungs)
    • Pericardium (pericardial cavity and heart)
    • Peritoneum (peritoneal cavity and visceral organs)
• Cutaneous membrane
  • Covers surface of body (skin)
  • Composed of:
    • Stratified squamous epithelium
    • Layer of areolar tissue
    • Underlying dense irregular connective tissue
  • Relatively thick, waterproof, and usually dry
• Synovial membrane
  • Lines freely movable joint cavities
  • Lubricates joint cavity with synovial fluid
  • Provides oxygen and nutrients to cartilage cells
  • Not true epithelium
    • Develops within connective tissue
    • Lacks basement membrane
    • Contains gaps between cells (up to 1 mm)
    • Synovial fluid and capillaries continuously exchange fluid and solutes

Types of Muscle Tissue

• 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

Functions of Muscle Tissue

• Skeletal muscle tissue
  • Found attached to bones of the skeleton
  • Elongated, cylindrical, banded (striated) cells with multiple nuclei (multinucleate)
  • Functions
    • Move and stabilizes skeleton
    • Guard entrances and exits to digestive, respiratory, 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, have a single nucleus
  • Functions
    • Move food, urine, and reproductive secretions
    • Control diameter of respiratory passageways and blood vessels

Nervous Tissue

• Specialized for conduction of electrical impulses
• 98 percent found in brain and spinal cord
• Two basic types of cells
  • Neurons (nerve)
  • Neuroglia or glial cells (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
  • Receive information
• Axon
  • Conducts information to other cells
  • Also called nerve fibers
• Cell body
  • Contains 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

Tissue Injury Response

• Tissues respond in a coordinated way to restore homeostasis
  • Two restoration processes
    • Inflammation
    • Regeneration

Injury Occurs

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

Inflammation

• Produces indications of injury
  • Swelling
  • Redness
  • Warmth (heat)
  • 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)

Inflammation (Continued)

• 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
• Sensation of pain from abnormal conditions and chemicals released by mast cells

Regeneration

• Occurs after damaged tissue has stabilized
• Fibroblasts produce collagen fibers to stabilize the injury site
• Produces dense, collagenous framework called scar tissue
• Scar tissue is usually remodeled and normal tissue conditions are restored

Tissue Regeneration Abilities

• 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