The Tissue Level of Organization
The Tissue Level of Organization
Introduction to Tissues
Tissues: Collections of cells and cell products performing specific, limited functions.
Possess discrete structural and functional properties.
Tissues combine to form organs (e.g., heart, liver).
Organs are grouped into organ systems.
Four Types of Tissue
1. Epithelial Tissue
Covers exposed surfaces.
Lines internal passageways and chambers.
Forms glands.
2. Connective Tissue
Fills internal spaces.
Provides structural support.
Transports materials.
Stores energy.
3. Muscle Tissue
Specialized for contraction.
Includes skeletal muscle, heart muscle, and muscle in the walls of hollow organs.
4. Neural Tissue
Carries electrical signals from one part of the body to another.
Epithelial Tissue
Epithelia: Layers of cells covering internal or external surfaces.
Form the surface of the skin.
Line digestive, respiratory, and reproductive tracts – all passageways communicating with the outside world.
Delicate epithelia line the chest cavity, fluid-filled spaces of the brain, inner surfaces of blood vessels, and chambers of the heart.
Glands: Structures that produce secretions.
Characteristics of Epithelia
Cellularity: Composed almost entirely of cells bound closely together by cell junctions.
Polarity: Possess distinct apical (exposed surface) and basal (attached surface) surfaces.
Attachment: Base is bound to a basement membrane (or basal lamina).
Avascularity: Lack blood vessels; obtain nutrients by diffusion from underlying connective tissue.
Regeneration: Cells are continuously replaced through stem cell division.
Functions of Epithelial Tissue
Provide Physical Protection: Against abrasion, dehydration, and destruction by chemical or biological agents.
Control Permeability: Regulates the movement of substances into or out of the body; can be affected by hormones (e.g., transport of ions and nutrients).
Provide Sensation: Highly innervated; sensitive to stimulation. Neuroepithelia are specialized epithelial cells that provide sensations of smell, taste, sight, equilibrium, and hearing.
Produce Specialized Secretions (Glandular Epithelium): Secretions are either released onto the epithelial surface or into interstitial fluid and blood.
Specialization of Epithelial Cells
Polarity defines two distinct regions:
Apical surfaces: Exposed to the external environment or internal passageways.
Microvilli: Small, finger-like extensions that increase surface area for absorption or secretion.
Cilia (ciliated epithelium): Hair-like projections that move fluid (e.g., mucus) across the surface.
Basolateral surfaces: Include both the base (where the cell attaches to the basement membrane) and the sides (where the cell contacts its neighbors).
Classification of Epithelia
Singular = Epithelium; Plural = Epithelia.
Based on Shape
Squamous epithelia: Thin and flat, resembling fried eggs.
Cuboidal epithelia: Square-shaped, like hexagonal boxes in a view, with spherical nuclei near the center.
Columnar epithelia: Tall, slender rectangles, with elongated nuclei crowded near the basement membrane.
Based on Layers
Simple epithelium: A single layer of cells. All cells have the same polarity, so the distance from the nucleus to the basement membrane does not change. These tissues are thin, fragile, and located in protected areas for absorption, secretion, and diffusion.
Stratified epithelium: Several layers of cells. Generally located in areas exposed to mechanical or chemical stresses (e.g., skin, lining of the mouth), providing greater protection.
Specific Epithelial Types
Squamous Epithelia
Simple Squamous Epithelium:
Functions: Absorption, diffusion, reduces friction, controls vessel permeability, performs secretion.
Locations: Mesothelia lining ventral body cavities (pleura, peritoneum, pericardium); endothelia lining heart and blood vessels; portions of kidney tubules; inner lining of cornea; alveoli of lungs.
Mesothelium: Simple squamous epithelium lining body cavities.
Endothelium: Simple squamous epithelium lining the heart and blood vessels.
Stratified Squamous Epithelium:
Functions: Provides physical protection against abrasion, pathogens, and chemical attack.
Locations: Surface of skin, lining of mouth, throat, esophagus, rectum, anus, and vagina.
Cells form a series of layers, with superficial cells typically flattened.
Keratinized: Apical layers of epithelial cells packed with keratin protein, which adds strength and water resistance (e.g., skin surface).
Non-keratinized: Resists abrasion but can dry out (e.g., oral cavity, pharynx, esophagus).
Cuboidal Epithelia
Simple Cuboidal Epithelium:
Functions: Limited protection, secretion, and absorption.
Locations: Glands, ducts, portions of kidney tubules, thyroid gland.
Cells resemble hexagonal boxes with spherical nuclei near the center.
Stratified Cuboidal Epithelia:
Functions: Protection, secretion, absorption.
Locations: Relatively rare, found in sweat ducts and mammary ducts.
Transitional Epithelium
Functions: Permits expansion and recoil after stretching without damage; appearance changes as stretching occurs.
Locations: Situated in regions of the urinary system (e.g., urinary bladder, renal pelvis, ureters).
Columnar Epithelia
Simple Columnar Epithelium:
Functions: Protection, secretion, absorption.
Locations: Lining of stomach, intestine, gallbladder, uterine tubes, and collecting ducts of kidneys.
Taller, slender cells with elongated nuclei crowded near the basement membrane.
Pseudostratified Columnar Epithelium:
Functions: Protection, secretion, move mucus with cilia.
Locations: Lining of nasal cavity, trachea, and bronchi; portions of the male reproductive tract.
Appears stratified due to nuclei at varying levels, but all cells maintain contact with the basement membrane.
Stratified Columnar Epithelium:
Functions: Protection.
Locations: Relatively rare, found in small areas of the pharynx, epiglottis, anus, mammary glands, salivary gland ducts, and urethra.
Glandular Epithelia
Endocrine Glands:
Examples: Thyroid, pituitary glands.
Release hormones directly into interstitial fluid.
Lack ducts (ductless glands).
Exocrine Glands:
Produce secretions onto epithelial surfaces.
Secretions are delivered through ducts.
Modes of Secretion
Merocrine Secretion:
Produced in the Golgi apparatus.
Released by vesicles via exocytosis.
Examples: Sweat glands, mucin (forms mucus when mixed with water).
Apocrine Secretion:
Produced in the Golgi apparatus.
Released by shedding cytoplasm, containing secretory product.
Examples: Mammary glands.
Holocrine Secretion:
Released by cells bursting, which kills the gland cells.
Gland cells are then replaced by stem cells.
Examples: Sebaceous glands (oil glands).
Types of Exocrine Gland Secretions
Serous Glands:
Produce watery secretions containing enzymes.
Example: Parotid salivary glands.
Mucous Glands:
Secrete mucins, which hydrate to form mucus.
Example: Submucosal glands of the small intestine.
Mixed Exocrine Glands:
Produce both serous and mucous secretions.
Example: Submandibular salivary glands.
Connective Tissue
Characteristics of Connective Tissue
Consists of specialized cells.
Contains solid extracellular protein fibers.
Possesses fluid extracellular ground substance.
The extracellular fibers and ground substance together form the matrix.
Functions of Connective Tissue
Establishing a structural framework for the body.
Transporting fluids and dissolved materials.
Protecting delicate organs.
Supporting, surrounding, and interconnecting other tissue types.
Storing energy reserves, especially in the form of triglycerides (fat).
Defending the body from invading microorganisms.
Classification of Connective Tissues
Classified based on physical properties into three main types:
Connective tissue proper: Connects and protects (e.g., loose and dense types).
Fluid connective tissues: Transports (e.g., blood and lymph).
Supporting connective tissues: Provides structural strength (e.g., cartilage and bone).
Connective Tissue Proper
Loose connective tissue: More ground substance, fewer fibers (e.g., fat/adipose tissue).
Dense connective tissue: More fibers, less ground substance (e.g., tendons).
Connective Tissue Proper Cell Populations
Fibroblasts:
Most abundant cell type in connective tissue proper.
Secretes proteins (fibers) and hyaluronan (cellular cement).
Hyaluronan, proteins, and extracellular fluid interact to form proteoglycan, which makes the ground substance viscous.
Fibrocytes:
Second most abundant cell type.
Spindle-shaped cells that maintain the fibers of connective tissue proper.
Adipocytes:
Fat cells, each storing a single, large fat droplet.
Mesenchymal Cells:
Connective tissue stem cells that respond to injury or infection.
Can differentiate into fibroblasts, macrophages, and other connective tissue cells.
Macrophages:
Large, amoeba-like cells of the immune system.
Phagocytize pathogens and damaged cells.
Fixed macrophages: Stay in specific tissues.
Free macrophages: Migrate throughout the body.
Mast Cells:
Stimulate inflammation after injury or infection.
Release histamine (vasodilator) and heparin (anticoagulant).
Basophils are leukocytes (white blood cells) that also contain histamine and heparin, enhancing the inflammatory process.
Lymphocytes:
Specialized immune cells primarily found in the lymphatic system.
Can develop into plasma cells (plasmocytes), which produce antibodies.
Microphages:
Phagocytic blood cells.
Respond to signals from macrophages and mast cells.
Examples: Neutrophils and eosinophils.
Melanocytes:
Synthesize and store the brown pigment melanin.
Connective Tissue Fibers
Collagen Fibers:
Most common fibers in connective tissue proper.
Long, straight, and unbranched.
Strong and flexible, resisting force in one direction.
Examples: Tendons (muscle to bone) and ligaments (bone to bone).
Reticular Fibers:
Network of interwoven fibers (stroma). (Stroma is the supporting framework)
Strong and flexible, resisting force in many directions.
Stabilize functional cells (parenchyma) and structures.
Examples: Sheaths around organs (liver, spleen, lymph nodes).
Elastic Fibers:
Contain the protein elastin.
Branched and wavy structure.
Return to original length after stretching.
Example: Elastic ligaments of vertebrae.
Ground Substance
Clear, colorless, and viscous fluid.
Fills spaces between cells and slows pathogen movement.
Embryonic Connective Tissues
Not found in adults.
Mesenchyme (embryonic stem cells):
The first connective tissue to appear in embryos.
Mucous connective tissue (Wharton’s Jelly):
Loose embryonic connective tissue found in many parts of the embryo, including the umbilical cord.
Loose Connective Tissues (Packing Materials)
Areolar Tissue:
Least specialized type.
Open framework with viscous ground substance that absorbs shocks.
Contains elastic fibers.
Holds blood vessels and capillary beds.
Example: Under the skin (subcutaneous layer).
Adipose Tissue:
Contains many adipocytes (fat cells).
White Fat:
Most common type.
Stores fat, absorbs shocks, slows heat loss (insulation).
Adipocytes in adults do not divide; they expand to store fat and shrink as fat is released.
Brown Fat:
More vascularized than white fat.
Adipocytes have many mitochondria.
When stimulated by the nervous system, fat breakdown accelerates, releasing energy and absorbing energy from surrounding tissues (for thermogenesis).
Mesenchymal cells can divide and differentiate to produce more fat cells when more storage is needed.
Reticular Tissue:
Provides support through a complex, three-dimensional network of reticular fibers (stroma).
Supports functional cells (parenchyma).
Found in reticular organs: Spleen, liver, lymph nodes, and bone marrow.
Dense Connective Tissues
Connective tissues proper, tightly packed with a high number of collagen or elastic fibers.
Dense Regular Connective Tissue:
Tightly packed, parallel collagen fibers.
Tendons: Attach muscles to bones.
Ligaments: Connect bone to bone and stabilize organs.
Aponeuroses: Attach in sheets to large, flat muscles.
Dense Irregular Connective Tissue:
Interwoven networks of collagen fibers.
Provides strength to resist forces applied from many directions.
Helps prevent overexpansion of organs (e.g., urinary bladder).
Locations: Layered in the skin (dermis), around cartilages (perichondrium), around bones (periosteum), and forms capsules around some organs (e.g., liver, kidneys).
Elastic Tissue:
Made predominantly of elastic fibers.
Example: Elastic ligaments of spinal vertebrae (ligamentum flavum, ligamentum nuchae), ligaments supporting the penis, ligaments supporting transitional epithelia, blood vessel walls.
Fluid Connective Tissues
Blood:
Watery matrix called plasma with dissolved proteins.
Transports specific cell types called formed elements.
Formed elements of blood:
Red blood cells (erythrocytes): Account for roughly half the volume of whole blood; transport oxygen (and, to a lesser degree, carbon dioxide).
White blood cells (leukocytes): Help defend the body from infection and disease.
Eosinophils and Neutrophils: Phagocytes.
Basophils: Promote inflammation, similar to mast cells.
Lymphocytes: Uncommon in blood but dominant in lymph; involved in specific immune responses.
Monocytes: Phagocytes, similar to free macrophages.
Platelets: Membrane-enclosed packets of cytoplasm that function in blood clotting.
Lymph:
Extracellular fluid collected from interstitial space.
Monitored by the immune system.
Transported by the lymphatic (lymphoid) system.
Returned to the venous system.
Fluid Tissue Transport Systems:
Cardiovascular system: Uses blood (arteries, capillaries, veins).
Lymphatic (lymphoid) system: Uses lymph (lymphatic vessels).
Supporting Connective Tissues
Support soft tissues and body weight.
Cartilage:
Matrix: A firm gel containing chondroitin sulfates (polysaccharide derivative).
Chondrocytes: Cartilage cells that occupy small chambers called lacunae.
Structure: Avascular (no blood vessels). Chondrocytes produce antiangiogenesis factor, which inhibits blood vessel formation.
Perichondrium: Fibrous layer (for strength) and inner cellular layer (for growth and maintenance) that surrounds most cartilage, setting it apart from surrounding tissues.
Strength: Limited; bends easily but hard to break.
Nutrient delivery: By diffusion through matrix.
Growth: Interstitial and appositional.
Repair capabilities: Limited.
Types of Cartilage:
Hyaline Cartilage: Stiff, flexible support; reduces friction between bones. Matrix has collagen fibers, making it flexible. Found in synovial joints, rib tips, sternum, trachea, and forming part of the nasal septum.
Elastic Cartilage: Supportive but bends easily. Contains numerous elastic fibers. Found in the external ear (auricle), epiglottis, auditory canal, and cuneiform cartilages of the larynx.
Fibrocartilage (Fibrous Cartilage): Has little ground substance; matrix dominated by densely interwoven collagen fibers. Limits movement, prevents bone-to-bone contact. Found in pads within knee joints (menisci), between pubic bones of the pelvis (pubic symphysis), and intervertebral discs.
Bone (Osseous Tissue):
Matrix: Calcified (made rigid by calcium salt deposits, primarily insoluble crystals of calcium phosphate and calcium carbonate). Resists shattering due to flexible collagen fibers.
Bone Cells (Osteocytes): Arranged around central canals within the matrix; occupy lacunae.
Canaliculi: Small channels through the matrix that connect osteocytes to blood vessels for nutrient access.
Periosteum: Covers bone surfaces; consists of an outer fibrous layer and an inner cellular layer.
Strength: Strong; resists distortion until breaking point.
Vascularity: Extensive.
Oxygen demands: High.
Nutrient delivery: By diffusion through cytoplasm and fluid in canaliculi.
Growth: Appositional only.
Repair capabilities: Extensive.
Membranes
Definition: Physical barriers that line or cover portions of the body.
Consist of an epithelium supported by underlying connective tissue.
Four Types of Membranes
Mucous Membranes (Mucosae):
Line passageways that have external connections (e.g., digestive, respiratory, urinary, and reproductive tracts).
Epithelial surfaces must be kept moist to reduce friction and facilitate absorption and excretion.
Lamina propria: The areolar tissue component supporting the epithelium.
Serous Membranes:
Line cavities not open to the outside of the body.
Are thin but strong.
Produce transudate (fluid) to reduce friction between parietal and visceral layers.
Have a parietal portion covering the cavity wall and a visceral portion (serosa) covering the organs within the cavity.
Three Serous Membranes:
Pleura: Lines pleural cavities, covers the lungs.
Peritoneum: Lines the peritoneal cavity, covers abdominal organs.
Pericardium: Lines the pericardial cavity, covers the heart.
Cutaneous Membrane:
Is the skin, covering the outer surface of the body.
Thick, waterproof, and dry.
Synovial Membranes:
Line moving, articulating joint cavities.
Produce synovial fluid (a lubricant).
Protect the ends of bones within joints.
Lack a true epithelium (often described as epithelium-like layer of macrophages and fibroblasts).
Muscle Tissue
Specialized for contraction, producing all body movement.
Three Types of Muscle Tissue:
Skeletal Muscle Tissue:
Large body muscles responsible for movement.
Cells are long, cylindrical, striated, and multinucleate.
Functions: Moves or stabilizes the position of the skeleton, guards entrances and exits to digestive, respiratory, and urinary tracts, generates heat, protects internal organs.
Voluntary control.
Cardiac Muscle Tissue:
Found only in the heart.
Cells are short, branched, and striated, usually with a single nucleus.
Cells are interconnected by intercalated discs.
Functions: Circulates blood, maintains blood (hydrostatic) pressure.
Involuntary control.
Smooth Muscle Tissue:
Found in the walls of hollow, contracting organs (e.g., blood vessels, urinary bladder; respiratory, digestive, and reproductive tracts).
Cells are short, spindle-shaped, and nonstriated, with a single, central nucleus.
Functions: Moves food, urine, and reproductive tract secretions; controls the diameter of respiratory passageways; regulates the diameter of blood vessels.
Involuntary control.
Neural Tissue
Also called nervous or nerve tissue.
Specialized for conducting electrical impulses.
Rapidly senses internal or external environments, processes information, and controls responses.
Concentrated in the central nervous system (CNS), which includes the brain and spinal cord.
Two Types of Neural Cells
Neurons:
Nerve cells that perform electrical communication.
Cell body (soma): Contains the nucleus and nucleolus, responsible for controlling nerve cell activity.
Dendrites: Short branches extending from the cell body that receive incoming signals from other neurons.
Axon (nerve fiber): A long, thin extension of the cell body that carries outgoing electrical signals (action potentials) to their destination (other cells).
Neuroglia (supporting cells):
Maintain the physical structure of neural tissue.
Repair tissue framework after injury.
Perform phagocytosis.
Regulate the composition of the interstitial fluid surrounding neurons.
Provide nutrients to neurons.
Tissue Injuries and Repair
Tissues respond to injuries to maintain homeostasis.
Cells restore homeostasis through two main processes: Inflammation and Regeneration.
Inflammation = Inflammatory Response
Definition: The tissue's first response to injury.
Signs and symptoms: Swelling, redness, heat, pain.
Triggered by: Trauma (physical injury) or infection (presence of harmful pathogens).
The Process of Inflammation:
Chemical signals released: Damaged cells release chemical signals into the surrounding interstitial fluid (e.g., prostaglandins, proteins, potassium ions).
Tissue destruction (Necrosis): As cells break down, lysosomes release enzymes that destroy the injured cell and attack surrounding tissues. This tissue destruction is called necrosis.
Accumulation of debris: Necrotic tissues and cellular debris accumulate in the wound; if pus is trapped in an enclosed area, it forms an abscess.
Mast cell activation: Injury stimulates mast cells to release potent chemicals: histamine, heparin, and prostaglandins.
Vascular changes: These chemicals cause dilation of blood vessels, increasing blood circulation in the area (causing warmth and redness). This increased circulation brings more nutrients and oxygen to the area and removes wastes.
Fluid leakage and pain: Plasma diffuses into the area, causing swelling (edema) and pain (due to pressure on nerve endings and released chemicals).
Phagocytosis: Phagocytic white blood cells (e.g., microphages, free macrophages) clean up the area by engulfing tissue debris and pathogens.
Regeneration
Definition: The repair that occurs after the damaged tissue has been stabilized and the inflammation has subsided.
The Process of Regeneration:
Scar tissue formation: Fibrocytes move into the necrotic area and lay down collagen fibers to bind the area together, forming scar tissue. Over time, scar tissue is usually