Histology Study Notes

Chapter 3: Histology Learning Objectives

  • L.O. 3.1: Name the four different categories of tissues found in the human body.

  • L.O. 3.2: Describe the key characteristics of each tissue as they relate to the overall function.

  • L.O. 3.3: Demonstrate how differences in the tissue types enable them to carry out specific functions within the human body.

  • L.O. 3.4: Compare the functions of the epithelium with regard to the following: cells layers, cell shapes, free cell surfaces, and connections between cells.

  • L.O. 3.5: Organize histological images of the different tissue types into categories with the same function.

  • L.O. 3.6: Compare the functional differences between connective tissue types and summarize why they can be either liquid (blood), semi-solid (fat/cartilage), or very rigid (bone).

Tissues: The Living Fabric

  • Individual body cells are specialized; each type performs specific functions that maintain homeostasis.

  • Tissues (derived from the French "tissu" meaning woven) are groups of cells that are similar in structure and perform a common or related function.

  • Histology is the study of tissues.

Four Major Tissue Types

  1. Epithelial Tissue

    • Function: Covers surfaces.

  2. Connective Tissue

    • Function: Supports.

  3. Muscle Tissue

    • Function: Contracts/moves.

  4. Nervous Tissue

    • Function: Coordinates and communicates.

Epithelial Tissues

Definitions and Functions

  • Epithelial tissue, or epithelium (plural: epithelia), consists of sheets of cells that cover or line body surfaces or cavities.

  • Main Functions Include:

    • Protection

    • Absorption

    • Filtration

    • Excretion

    • Secretion

    • Sensory Reception

  • Two Forms of Epithelium:

    1. Covering and Lining Epithelia: Covers and lines cavities, walls, and surfaces. Examples: skin, mucus membranes.

    2. Glandular Epithelia: Forms glands, both exocrine (e.g., salivary glands) and endocrine (e.g., thyroid gland).

Characteristics of Epithelial Tissues

  1. Polarity:

    • Epithelial cells exhibit polarity with an apical surface (free side) and a basolateral surface (attached side).

    • The apical surface can have microvilli, increasing absorption, while the basal surface attaches to the basal lamina, a foundation structure.

  2. Specialized Contacts:

    • Epithelial cells fit closely together, often forming continuous sheets.

    • Lateral contacts include tight junctions and desmosomes, which bind adjacent epithelial cells together.

  3. Support by Connective Tissue:

    • Epithelial sheets are always supported by connected tissue via the basement membrane, which consists of basal lamina and reticular lamina, reinforcing the epithelial structure.

  4. Avascularity:

    • Epithelia do not contain blood vessels; they receive nourishment via diffusion from underlying connective tissues and are innervated (have nerve fibers).

  5. Regeneration:

    • Epithelial cells have high regenerative capacities due to their rapid turnover, which is stimulated by loss of polarity and broken cell connections.

General Classification of Epithelial Tissue

  • SIMPLE: Single layer of cells.

  • STRATIFIED: Two or more layers of cells.

  • PSEUDOSTRATIFIED: Single layer that appears stratified due to varied cell heights.

  • SQUAMOUS: Flat, scale-like cells.

  • CUBOIDAL: Box-like cells.

  • COLUMNAR: Tall, rectangle-shaped cells.

Types of Epithelia

  1. Simple Squamous Epithelium

    • Description: Single layer of flat, disc-like cells; functions in filtration and diffusion.

    • Location: Air sacs of lungs, glomeruli, blood vessels, and cavity linings.

  2. Simple Cuboidal Epithelium

    • Description: Single layer of cube-like cells with large nuclei; functions in secretion and absorption.

    • Location: Ducts and glands, kidney tubules, ovaries, thyroid.

  3. Simple Columnar Epithelium

    • Description: Single layer of tall, rectangular cells with microvilli/cilia.

    • Function: Absorption (digestive tract) and secretion (mucous and reproductive).

    • Location: Lining of the digestive and respiratory tracts.

  4. Pseudostratified Columnar Epithelium

    • Description: Single layer of columnar cells of different heights, nuclei appear at different heights; functions in secretion and propulsion.

    • Location: Male sperm ducts, respiratory tract, large glands.

  5. Stratified Squamous Epithelium

    • Description: Multiple layers of squamous cells; surface cells may be dead/keratinized.

    • Function: Protection against abrasion.

    • Location: Skin, esophagus, vagina, mouth lining.

  6. Stratified Cuboidal/Columnar Epithelium

    • Description: Two or more layers of cuboidal/columnar cells; functions in secretion.

    • Location: Male sperm ducts, mammary glands, pharynx, male urethra.

  7. Transitional Epithelium

    • Description: Appears stratified squamous or cuboidal; permits stretch to allow for urinary storage.

    • Location: Lining of urinary tracts (ureters, bladder, urethra).

Classification of Glandular Epithelium

  • ENDOCRINE GLANDS: Secrete hormones directly into capillaries.

  • EXOCRINE GLANDS: Secrete products into ducts. Examples include:

    • Simple Tubular: Unbranched ducts (e.g., intestinal glands).

    • Compound Tubular: Branched glands (e.g., mucous glands in the mouth).

    • Simple Alveolar: Not present in adults (e.g., sebaceous glands).

    • Compound Alveolar: Mammary glands.

    • Compound Tubuloalveolar: Salivary glands.

Functional Classification of Exocrine Glandular Epithelium

  1. MEROCRINE: Products secreted directly via exocytosis (e.g., salivary glands).

  2. APOCRINE: Products secreted by pinching off the apical membrane (e.g., mammary glands).

  3. HOLOCRINE: Products secreted by total cell rupture (e.g., sebaceous glands).

Connective Tissues

Characteristics of Connective Tissues

  • Protection: Protects tissues and organs.

  • Structural Support: Provides support for muscle attachments for movement.

  • Energy Storage and Insulation: e.g., adipose tissue provides energy reserves.

  • Transporting Substances: e.g., blood transports nutrients and gases throughout the body.

Cells and Fibers in Connective Tissues

Cell Types

  1. Osteoblasts/Cytes: Cells of bone.

  2. Fibroblasts/Cytes: Cells of connective tissue proper.

  3. Chondroblasts/Cytes: Cells of cartilage.

  4. Adipocytes: Fat cells.

  5. Blood Cells: Cells involved in immunity.

Fiber Types

  • Collagen Fibers: Most abundant fiber type; resist tension by assembling into fibrils and thick, rope-like bundles.

  • Elastin Fibers: Provide flexibility; can stretch and recoil.

  • Reticulin Fibers: Short, thin collagenous fibers that create a mesh-like network for movement.

Types of Loose Connective Tissue

  1. Areolar Tissue

    • Description: Loosely-packed fibers, fibroblasts, and immune cells; cushions organs.

    • Location: Supporting under epithelial layers and surrounding organs.

  2. Adipose Tissue

    • Description: Matrix with packed adipocytes; insulates and protects organs.

    • Location: Around major organs and within the subcutaneous layer of the skin.

  3. Reticular Tissue

    • Description: Meshwork of reticular fibers; supports tissue and immune cells.

    • Location: Lymph nodes, bone marrow, splenic pulp.

Types of Dense Connective Tissue

  1. Dense Regular Tissue

    • Description: Fibroblasts within regularly ordered collagen fibers; resists pulling stress.

    • Location: Attaches muscles to bone (tendons) and bone to bone (ligaments).

  2. Dense Irregular Tissue

    • Description: Irregularly arranged collagen fibers; resists stresses in multiple directions.

    • Location: Dermis, joint capsules, underlying epithelial linings.

  3. Elastic Tissue

    • Description: Dense regular connective tissue with elastin fibers arranged for stretch and recoil.

    • Location: Walls of arteries and bronchial tubes.

Types of Cartilage Connective Tissue

  1. Hyaline Cartilage

    • Description: Flexible gel matrix; cushions and supports organs.

    • Location: Nose, trachea, ribs, ends of long bones.

  2. Elastic Cartilage

    • Description: Similar to hyaline, but with elastin fibers for greater flexibility.

    • Location: Outer ear, epiglottis.

  3. Fibrocartilage

    • Description: Firmer matrix with organized collagen fibers; resists compressive force.

    • Location: Intervertebral discs, menisci, pubic symphysis.

Special Types of Connective Tissue

  1. Blood

    • Description: Composed of erythrocytes, leukocytes, and platelets in plasma matrix; transports oxygen, carbon dioxide, and nutrients.

    • Location: Within vessels (arteries, veins, capillaries).

  2. Bone

    • Description: Calcified matrix with osteocytes; highly vascular; functions in calcium storage.

    • Location: Skeleton.

Muscle Tissue

  • Highly Vascularized: Supplies necessary nutrients for contraction.

  • Function: Responsible for most types of movement.

  • Myofilaments: Composed of actin and myosin, which are responsible for muscle contractions.

  • Three Types of Muscle Tissue:

    1. Skeletal Muscle

    • Description: Elongated myotubes formed from fused cells; multinucleated with visible striations.

    • Location: Attached to bones; allows for voluntary movement.

    1. Cardiac Muscle

    • Description: Branched myotubes with intercalated discs; multinucleated with striations visible.

    • Location: Walls of the heart; ensures blood flow throughout body.

    1. Smooth Muscle

    • Description: Spindle-shaped cells with central nucleus, arranged in sheets; contracts to provide force and accommodate stretch.

    • Location: Walls of hollow organs, such as the digestive tract.

Nervous Tissue

  • Main component of the nervous system, regulating and controlling body functions.

Specialized Cells

  1. Neurons

    • Function: Receive sensory information and transmit motor impulses; form synapses.

  2. Glial Cells

    • Function: Provide support, protection, and nourishment to neurons.

    • Location: Brain, spinal cord, cranial and peripheral nerves.

Tissue Injury and Repair

  • Inflammation: White blood cells and clotting factors enter the damaged site; clotting factors form a blood clot (scab).

  • Regeneration: Fibroblasts lay down temporary extracellular matrix (ECM) and new capillaries grow; epithelium forms a new tissue layer.

  • Fibrosis: Scab falls off, and scar formation may become visible depending on the severity of damage.

Regenerative Capacity of Different Tissues

  • Extremely Well Regenerating Tissues:

    • Epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissues.

  • Moderate Regenerating Capacity:

    • Smooth muscle and dense regular connective tissue.

  • Minimal Regenerative Capacity:

    • Cardiac muscle and nervous tissue (brain and spinal cord).

    • New research shows potentially increased regenerative capabilities in these tissues in the future.

Developmental Aspects of Tissues

  • Tissue functions optimally through youth and middle age with proper diet and circulation; minimizes infections and wounds.

  • As aging occurs, tissues exhibit:

    • Thinning epithelia, increased tearing.

    • Less efficient tissue repair.

    • Atrophy of bone, muscle, and nervous tissues.

    • Increased DNA mutations raise cancer risks.

Apoptosis

  • Definition: Programmed cell death of damaged or unnecessary cells.

    • Controlled by the caspase cascade.

    • Extrinsic Pathway: Stimulated by external events.

    • Intrinsic Pathway: Stimulated internally.

Stem Cells and Regenerative Capacity

  • Stem Cells:

    • Have unlimited mitotic division capacity.

    • Must self-renew (maintain their own number) and differentiate into daughter cells with specialized properties.

    • Form intermediate populations that further increase the production of mature cells.