Histology and Tissues Lecture Notes

Tissues

• Collections of specialized cells and cell products performing specific functions.

• Tissues combine to form organs (e.g., heart, liver).

• Organ systems form an organism.

• Histology is the study of tissues.

Four Categories of Tissues

• Epithelial tissue: Lines organ surfaces and cavities (external and internal).

• Connective tissue: Includes proper, loose, dense, blood, lymph, and bone.

• Muscle tissue: Includes smooth, cardiac, and skeletal muscle.

• Nervous tissue: Includes neurons and neuroglia

Epithelial Tissue

• Includes epithelia and glands.

• Epithelium: Layers of cells covering internal or external surfaces (organs, tissues, cavities).

• Glands: Structures producing fluid secretions.

Classification of Epithelial Tissues

1. Number of layers:

   • Simple epithelial tissue: One layer.

   • Stratified epithelial tissue: Two or more layers.

   • Exceptions: Pseudostratified and transitional.

2. Shape:

   • Squamous: Flat.

   • Cuboidal: Square-shaped.

   • Columnar: Tall, slender rectangles.

Intercellular Connections (Cell Junctions)

• Form bonds with other cells or extracellular materials.

  1. Gap junctions: Direct communication; small molecules pass through.

  2. Tight junctions: Create a seal between cells.

  3. Desmosomes: Provide mechanical strength, allowing epithelial cells to resist stretching and tearing.

  4. Hemidesmosomes: Anchor epithelial cells to the basement membrane (layer between epithelial cells and underlying tissue).

Classification of Epithelia

1. Based on Shape

   • Squamous: Thin and flat.

   • Cuboidal: Square-shaped.

   • Columnar: Tall, slender rectangles.

2. Based on Layers

   • Simple epithelium: Single layer of cells.

   • Stratified epithelium: Several layers of cells.

Examples of Epithelial Tissue

• Simple squamous: Found in blood vessels.

• Stratified squamous: Found in skin and mouth.

• Simple cuboidal: Found in glands and kidney tubules.

• Simple columnar: Found in microvilli of the small intestine.

• Stratified cuboidal: Found in sweat glands.

• Stratified columnar: Found in the pharynx and urethra.

• Pseudostratified: Found in the trachea and bronchi (appears stratified but is simple).

• Transitional: Found in the urinary bladder, ureters, and urethra.

Glands

• Collection of epithelial cells that produce secretions.

Types of Glands

• Endocrine glands: Release hormones into the bloodstream (e.g., thyroid).

• Exocrine glands: Discharge secretions through ducts onto epithelial surfaces (e.g., sweat, oil, and salivary glands).

• Pancreas: Exhibits both exocrine and endocrine functions.

Methods of Secretion of Glands

• Merocrine:

  1. Secretion via exocytosis (e.g., sweat and salivary glands).

• Apocrine:

  1. Part of the cell is pinched off (e.g., ceruminous glands (earwax)).

• Holocrine:

  1. The cell bursts to release its contents (e.g., sebaceous and meibomian glands).

Secretions of Exocrine Glands

• Serous glands: Involved in digestion, lubrication, and enzymatic processes; produce watery secretions.

• Mucous glands: Trap allergens and pathogens for protection and lubrication; secrete mucins.

• Mixed exocrine glands: Contain both serous and mucous cells.

Enzymes and Carbohydrates

• Enzymes typically end in "ase" (e.g., salivary amylase).

• Sugars/carbohydrates typically end in "ose" (e.g., glucose).

Connective Tissue

Components

• Specialized cells:

  1. Fibroblasts → builders of connective tissue — they make fibers and fluid that give tissues strength, structure, and support.

  2. Adipocytes

  3. Chondrocytes

  4. Osteocytes

  5. Macrophages

  6. Mast cells

• Extracellular protein fibers:

  1. Collagen fibers

  2. Elastic fibers

  3. Reticular fibers

• Fluid called ground substance:

  • Gel-like fluid, part of the extracellular matrix.

• Matrix: Consists of extracellular components (fibers and ground substance).

Functions

• Establishing a structural framework for the body (bones/cartilage).

• Transporting fluids and dissolved materials (blood and lymph).

• Protecting delicate organs (adipose tissue, bones, cartilage).

• Supporting, surrounding, and interconnecting other tissue types (adipose and macrophages).

• Storing energy reserves, especially triglycerides (adipose tissue).

• Defending the body from invading microorganisms (macrophages, mast cells).

Categories of Connective Tissue Proper

• Connective tissue proper: Connect and protect (e.g., areolar, adipose, and reticular tissue [loose], tendons, ligaments [dense]).

• Adipose tissue: Loose connective tissue; includes brown (in babies) and white (in adults) adipose tissue.

• Dense connective tissue: Tendons (muscle to bone) and ligaments (bone to bone).

• Joints = Articulations

Fluid Connective Tissues

• Function: Transport

• Examples: Blood and lymph.

Supporting Connective Tissues

• Function: Provide structural strength.

• Examples:

  • Cartilage (hyaline, elastic, fibrocartilage)

  • Bone (compact and spongy bone)

Cells of Connective Tissue

• Fibroblasts: Most abundant; produce collagen and ground substance.

• Fibrocytes: Maintain connective tissue matrix.

• Adipocytes: Store energy (fat).

• Mesenchymal cells: Stem cells of connective tissue. embryonic connective tissue stem cells:

• Melanocytes: Produce melanin (pigment).

• Macrophages: Immunity (white blood cells).

• Mast cells: Activate inflammation.

• Lymphocytes and microphages: Immunity.

Connective Tissue Fibers

• Collagen fibers: Found in tendons, ligaments, and dermis.

• Reticular fibers: Found in liver, spleen, lymph nodes, and bone marrow; form a supportive network (stroma).

• Elastic fibers: Found in skin, lungs, blood vessels, and elastic ligaments; allow stretching and recoil.

Ground Substance

• Clear, colorless, and viscous.

• Fills spaces between cells and slows pathogen movement.

• Part of the extracellular matrix.

Loose Connective Tissue

• Functions as packing material, filling spaces between organs, cushioning, and supporting epithelia.

• Types:

  1. Areolar

     • around skin, blood vessels, nerves, lungs and between muscles

  2. Adipose

     • made of adipocytes (fat cells)

  3. Reticular

     • liver, spleen, lymph nodes and bone marrow

Embryonic Connective Tissues

• Not found in adults.

• First connective tissue in embryos.

• Types:

  • Mesenchyme

  • Mucous connective tissue: Found in the umbilical cord (Wharton's jelly).

Types of Loose Connective Tissue in Adults

• Areolar tissue:

  1. Cushions/supports.

  2. Found around skin, blood vessels, nerves, lungs, and between muscles.

• Adipose tissue:

  • Made of adipocytes (fat cells).

  • Stores long-term energy.

  • Includes brown fat and white fat.

• Reticular tissue:

  • Forms a framework.

  • Found in liver, spleen, lymph nodes, and bone marrow.

White and Brown Fat

• White fat (adults):

  • Most common.

  • Stores fat and absorbs shocks.

  • Slows heat loss.

• Brown fat (babies and young kids):

  • Involved in thermogenesis (heat generation).

Dense Connective Tissue

• Dense regular: Found in tendons and ligaments.

• Dense irregular: Found in skin and periosteum (outer layer of bones).

• Elastic: Found in arteries, ligaments, and lungs.

Blood

• Whole blood consists of plasma (55%) and formed elements (45%).

Layers of Skin

• Cutaneous membrane (skin):

  • Epidermis

  • Dermis

  • Hypodermis (not a layer of skin)

Connective Tissue Functions

• Provides strength and stability, forming a stable framework.

• Maintains the position of internal organs and minimizes shock absorption.

• Provides routes for blood vessels, lymphatic vessels, and nerves.

Fasciae

• Connective tissue layers and wrappings that support and surround organs.

• Types:

  • Superficial fascia

  • Deep fascia

  • Subserous fascia

Fluid Connective Tissue Components

• Red blood cells (erythrocytes): Transport O2 and CO2.

• White blood cells (leukocytes): Fight infections.

• Platelets (thrombocytes): Function in clotting.

• Lymph: Carries waste/dead pathogens; part of the lymphatic system.

Supportive Connective Tissues

• Cartilage: Provides shock absorption and protection; found in the nose, joints, ears, and ribs.

• Chondroitin sulfates: Resist compression.

• Chondrocytes: Cells in the cartilage matrix, located in lacunae.

Muscle Tissue

Types of Muscle Tissue

• Skeletal muscle: Striated, tubular/cylindrical shape, voluntary movement.

• Cardiac muscle: Striated, branched shape, involuntary movement.

• Smooth muscle: Non-striated, spindle-shaped, and involuntary movement.

Types of Cartilage

• Hyaline cartilage (most common): Found at the ends of long bones, nose, trachea, and larynx.

• Elastic cartilage: Found in the external ear (pinna), epiglottis, and Eustachian tube.

• Fibrocartilage: Found in intervertebral disks, pubic symphysis, and meniscus.

Bone

• Bone (osseous) tissue: A supportive connective tissue.

• Calcified: Cartilage in bone becomes hard due to calcium phosphate (Ca^{2+} + P).

Tissue Membranes

• Mucous membranes: Line body cavities of the digestive, respiratory, urinary, and reproductive tracts.

• Serous membranes: Line internal cavities and cover organs (e.g., pleura, pericardium, peritoneum).

• Cutaneous membrane: The skin, including the epidermis and dermis.

• Synovial membranes: Found in joints, particularly synovial joints (freely movable joints).

Muscle Tissue (My/o or Sarc/o)

• Specialized for contraction.

• Three types:

  • Skeletal muscle: Responsible for body movement.

  • Cardiac muscle: Found only in the heart.

  • Smooth muscle: Found in the walls of hollow, contracting organs.

Skeletal Muscle Tissue

• Consists of long, thin cells called muscle fibers.

• Cells do not divide; new fibers are produced by divisions of myosatellite cells (stem cells).

• Striated voluntary muscle.

Cardiac Muscle Tissue

• No regeneration (e.g., after myocardial infarction).

• Regulated by pacemaker cells.

• Striated involuntary muscle.

• Cardiocytes (cells of the heart).

• Cells form branching networks connected at intercalated discs (only found in cardiac muscle).

Smooth Muscle Tissue

• Cells are small and spindle-shaped.

• Can divide and regenerate.

• Non-striated involuntary muscle.

Nervous Tissue

• Specialized for conducting electrical impulses.

• Concentrated in the brain and spinal cord.

Divisions of Nervous System

• Central Nervous System (CNS)

• Peripheral Nervous System (PNS)

Types of Cells in Nervous Tissue

• Neurons: Basic functional unit.

• Neuroglia: Supporting cells including:

  • Satellite cells and Schwann cells -- PNS

  • Oligodendrocytes, microglia, ependymal cells, and astrocytes (produce CSF) -- CNS