Histology and Tissue Types

CH3 Histology (tissues)

• Collagen Installation Analogy:
  • Illustrates the formation of scar tissue.
  • Proper collagen alignment is crucial for tissue repair; haphazard arrangement results in scar tissue.

Embryology: Formation of Tissues

• Fertilization:
  • Produces a single cell called a zygote, containing 46 chromosomes.
• Cleavage:
  • Cell divisions that form a blastocyst (a hollow ball of cells) in about a week.
• Blastocyst Structure:
  • Trophoblast: Outer layer that forms the placenta for nourishing the embryo.
  • Inner cell mass: Develops into the actual embryo.
• Germ Layers (Late Week 4):
  • Ectoderm: Forms epidermis, nervous tissue, sense organs, pituitary gland, adrenal medulla, and enamel of teeth.
  • Mesoderm: Forms dermis, epithelial lining of vessels, muscle tissue, connective tissue, adrenal cortex, heart, kidneys, internal reproductive organs, and spleen.
  • Endoderm: Forms epithelial lining of respiratory, GI, urinary, and reproductive tracts, liver, gallbladder, pancreas, palatine tonsils, thyroid gland, parathyroid glands, and thymus.

Four Primary Tissue Types

• Epithelial:
  • Covers exposed surfaces and lines internal passageways.
  • Produces glandular secretions.
• Connective:
  • Fills internal spaces and provides structural support.
  • Stores energy.
• Muscular:
  • Contracts to produce movement.
  • Includes skeletal, cardiac, and smooth muscle.
• Neural:
  • Conducts electrical impulses and carries information.

Epithelial Tissue Characteristics

• No intercellular space.
• Polarity:
  • Exposed apical surface.
  • Attached basal surface.
  • Apical/basal surfaces are structurally and functionally different.
• Avascularity:
  • No blood vessels.
• Sheets and layers:
  • Cells arranged in sheets, single or multiple layers thick.
• Regeneration:
  • Surface cells regenerated via stem cells.

Functions of Epithelial Tissue

• Physical Protection:
  • From abrasion, dehydration, and destruction.
• Controls Permeability.
• Sensory Cells:
  • General (touch).
  • Neuroepithelia (sense organs).
• Produces Secretions:
  • Unicellular glands.
  • Glandular epithelia.

Apical Surface Structures

• Microvilli:
  • Short and motile.
  • Increase surface area.
  • Line urinary and digestive tracts.
• Stereocilia:
  • Long and rigid.
  • Found in the inner ear and male reproductive tract.
• Cilia:
  • Long and motile.
  • Move substances over the apical surface.
  • Line the respiratory tract.

Maintaining Epithelium

• Intercellular Connections:
  • Give epithelium strength and stability.
  • Hold cells together and prevent passage of chemicals, water, and pathogens.
  • Involve gap junctions, CAMs, tight junctions, hemidesmosomes, and proteoglycans.
  • Bind cells to the basal lamina.
• Epithelial Maintenance & Renewal:
  • Replaced frequently due to exposure/use, disruptive enzymes, toxic chemicals, pathogens, and mechanical abrasion.
  • Replaced through continual division of stem cells near the basal lamina.

Types of Layering

• Simple Epithelium:
  • One layer of cells.
  • Found in protected internal compartments of the body.
• Stratified Epithelium:
  • Two or more layers of cells.
  • Found in areas of mechanical or chemical stresses.

General Shapes of Epithelial Cells

• Squamous: Flat, like "pancakes".
• Cuboidal: Shaped like cubes.
• Columnar: Longer than wide, like columns.

Squamous Epithelium

• Thin, delicate layer.
• Reduces friction.
• Involved in absorption and secretion.
• Lines body cavities, the heart, blood vessels, and alveoli.

Epithelial Tissue - Cellularity

• Cells are bound closely together.
• No intercellular space.
• Lining of orifices and surface of the skin acts as a physical barrier.
• Some surfaces are keratinized, while mucosal linings are nonkeratinized.

Simple Cuboidal Epithelium

• Locations:
  • Glands, ducts, portions of kidney tubules, and thyroid gland.
• Functions:
  • Limited protection, secretion, and absorption.

Lumens

• Center of duct/tube.
• Found in ducts of sweat glands and mammary glands (modified sweat glands).
• Involved in secretion and absorption.

Columnar Epithelium

• Highly polarized.
• High secretion and absorption rates.
• Found in the stomach, intestine, gallbladder, uterine tubes, and renal collecting ducts.
• Most have microvilli or cilia.

Stratified Columnar Epithelium

• Locations:
  • Small areas of the pharynx, epiglottis, anus, mammary glands, salivary gland ducts, and urethra.
• Function:
  • Protection in high-activity areas.

Pseudostratified Ciliated Columnar Epithelium

• Locations:
  • Lining of the nasal cavity, trachea, and bronchi; portions of the male reproductive tract.
• Functions:
  • Protection and secretion.
• Has cilia.
• Actually simple epithelium.

Transitional Epithelium

• Locations:
  • Renal pelvis, ureters, and bladder.
• "Stretchy" for expansion and recoil.
• Mainly cuboid but includes a mix of other shapes.

Glandular Epithelium

• Classified based on:
  • Type of secretion released.
  • Structure of gland.
  • Mechanism of secretion.
• Types of glands:
  • Exocrine: Ducts to an epithelial surface; body opening or space.
  • Endocrine: Into ECF then to bloodstream or lymph; hormones.

Types of Exocrine Glands

• Serous: Watery fluid & enzymes.
• Mucous: Mucins + water = mucus.
• Mixed Unicellular: Mucins via goblet cells (mucous cell).
• Multicellular: Secretory sheets & clusters; mixed.

Glandular Tissue Structure

• Mix of duct cells and gland cells.
• Structure of ducts:
  • Simple: No branching.
  • Compound: Repeated branches.

Mechanisms of Glandular Secretion

• Eccrine Secretion:
  • Secretory product released through exocytosis.
  • Continuous and the fastest secretion rate.
• Apocrine Secretion:
  • Apical portion of the cell's cytoplasm is shed with secretory vesicles.
  • Non-continuous secretion with production in waves.
• Holocrine Secretion:
  • The entire cell bursts, releasing cytoplasmic contents.
  • Slowest production rate, typically for oily products.

Connective Tissues

• Specialized cells.
• Extracellular protein fibers.
• Ground substance.
• Functions:
  • Establishing body’s structural framework.
  • Transporting fluid & dissolved materials.
  • Protecting organs.
  • Supporting, surrounding, & connecting other tissues.
  • Storing energy.
  • Defending body from microorganisms.

Fibers of Connective Tissue Proper

• Collagen fibers: High tensile strength in tendons & ligaments.
• Reticular fibers: Structural mesh within organs.
• Elastic fibers: Protein called elastin; "rubbery" lining in vasculature.
• Ground substance: Thick syrupy consistency; a mix of proteins & carbohydrates.

Types of Connective Tissue

• Connective Tissue Proper:
  • Matrix of loose fibers & dense fibers.
• Fluid Connective Tissue:
  • Watery matrix of dissolved proteins.
• Supporting Connective Tissue:
  • Dense matrix of fibers & may have insoluble calcium salts.

Areolar Tissue

• Locations:
  • Within and deep to the dermis of skin, covered by the epithelial lining of the digestive, respiratory, and urinary tracts; between muscles; around blood vessels, nerves, and joints.
• Functions:
  • Cushions organs, provides support while permitting independent movement; phagocytic cells provide defense against pathogens.
• "Stuffing" of the body; light connection that allows movement between skin & muscle.

Adipose Tissue

• Locations:
  • Deep to the skin, especially at sides, buttocks, and breasts; padding around eyes and kidneys.
• Functions:
  • Provides padding and cushions shocks; insulates (reduces heat loss); stores energy.
• White fat: Energy storage.
• Brown fat: Thermogenesis.

Reticular Tissue

• Locations:
  • Liver, kidney, spleen, lymph nodes, bone marrow.
• Functions:
  • Provides supporting framework; "structural sponge".
• Matrix is stiffest of loose CT.

Dense Regular Connective Tissue

• Locations:
  • Between skeletal muscles and skeleton (tendons and aponeuroses); between bones or stabilizing positions of internal organs (ligaments); covering skeletal muscles; deep fasciae.
• Functions:
  • Provides firm attachment, conducts pull of muscles, reduces friction between muscles, stabilizes relative positions of bones.
• High tensile strength.

Dense Irregular Connective Tissue

• Locations:
  • Capsules of visceral organs, periostea and perichondria, nerve and muscle sheaths, dermis.
• Functions:
  • Provides strength to resist forces applied from many directions; helps prevent overexpansion of organs, such as the urinary bladder.
• Organ sheaths.

Elastic Tissue

• Locations:
  • Between vertebrae of the spinal column (ligamentum flavum and ligamentum nuchae); ligaments supporting penis; ligaments supporting transitional epithelia; in blood vessel walls.
• Functions:
  • Stabilizes positions of vertebrae and penis; cushions shocks; permits expansion and contraction of organs.
• Springy & resilient; "rubbery".

Fluid Connective Tissues

• Blood:
  • Moves via the cardiovascular system
    • Erythrocytes (red blood cells): Gas transport.
    • Leukocytes (white blood cells): Immune function.
    • Thrombocytes (platelets): Blood clotting.
    • Plasma: Matrix.
• Lymph:
  • Moves via lymphatic system.
    • Monitors for signs of infection; mainly lymphocytes (T & B cells), but some phagocytic cells.
• FCT matrix: Watery mixture of dissolved proteins.

Cartilage

• Contains chondrocytes.
• Gel matrix made of chondroitin sulfate.
• Cells reside in lacunae.
• Avascular.
• Have a fibrous perichondrium with cellular and fibrous layers.
• Maintenance.
• Connection to other structures.
• Most cartilage cannot repair severe damage.

Hyaline Cartilage

• Locations:
  • Between tips of ribs and bones of sternum; covering bone surfaces at synovial joints; supporting larynx (voice box), trachea, and bronchi; forming part of nasal septum.
• Functions:
  • Provides stiff but somewhat flexible support; reduces friction between bony surfaces.
• Tracheal rings, costal cartilage, and joints.

Elastic Cartilage

• Locations:
  • Auricle of the external ear, epiglottis, auditory canal, cuneiform cartilages of the larynx.
• Most flexible.
• Functions:
  • Provides support but tolerates distortion without damage and returns to original shape.

Fibrous Cartilage

• Locations:
  • Pads within the knee joint; between pubic bones of pelvis; intervertebral discs.
• Functions:
  • Resists compression, prevents bone-to-bone contact, limits relative movement.
• Firmest; strongest.

Bone

• Most external bone surfaces are covered with periosteum.
• Internal bone surface covered in endosteum.
• Osteocytes in lacunae.
• Lamellae (matrix): Mix of collagen fibers & calcium salts.

Comparison of Cartilage and Bone

Membranes

• Membranes = Epithelia + Connective tissue
• Sheet of epithelial cells and an underlying connective tissue.
• Types:
  • Mucous.
  • Serous.
  • Cutaneous.
  • Synovial.

Types of Membranes

• Mucous Membranes:
  • Coated with secretions of mucous glands.
  • Line most of the digestive and respiratory tracts and portions of the urinary and reproductive tracts.
• Serous Membranes:
  • Line the peritoneal, pleural, and pericardial body cavities.
• Cutaneous Membrane:
  • Skin; covers the outer surface of the body.
• Synovial Membranes:
  • Line joint cavities and produce the fluid within the joint.

Muscle Tissue

• Contracts to produce movement.
• Includes skeletal muscle, cardiac muscle, and smooth muscle.
• Cytoplasm = sarcoplasm; Cell membrane = sarcolemma.

Skeletal Muscle Tissue

• Cells are long, cylindrical, striated, and multinucleate.
• Locations:
  • Combined with connective tissues and neural tissue in skeletal muscles.
• Functions:
  • Moves or stabilizes the position of the skeleton; guards entrances and exits to the digestive, respiratory, and urinary tracts; generates heat; protects internal organs.
• Multinucleate.
• Voluntary or reflex arcs.
• Striations.
• Hypertrophy not hyperplasia via myosatellites

Cardiac Muscle Tissue

• Cells are short, branched, and striated, usually with a single nucleus; cells are interconnected by intercalated discs.
• Location:
  • Heart.
• Functions:
  • Circulates blood; maintains blood pressure.
• Myogenic; Branched; "mesh".
• Scar tissue.

Smooth Muscle Tissue

• Cells are short, spindle-shaped, and non-striated, with a single, central nucleus.
• Locations:
  • Found in the walls of blood vessels and in digestive, respiratory, urinary, and reproductive organs.
• Functions:
  • Moves food, urine, and reproductive tract secretions; controls diameter of respiratory passageways; regulates diameter of blood vessels.
• Non-striated; Can replicate.
• Reflex arcs or myogenic.

Neural Tissue

• Two Types:
  • Neuroglia: Supporting cells of neural tissue; can divide.
  • Neurons: Specialized to conduct electrical signals (action potential).
    • Longest cells in the body.
    • Incapable of cell reproduction.
    • Dendrite: Receives stimuli.
    • Axon: Transmits a signal; action potential.

Cancer Classification by Tissue of Origin