Chapter 4. Tissues and Histology Practice Flashcards

Introduction to Histology and Tissue Types

• Definition of Tissues: Tissues are collections of specialized cells and cell products that work together to perform specific, limited functions.

• Organizational Hierarchy: Tissues combine to form organs, which then interact within organ systems.

• Histology: This is the formal scientific study of tissues.

• The Four Primary Tissue Types:

  • Epithelial Tissue: Primarily serves as a covering and lining.

  • Connective Tissue: Provides structural and metabolic support.

  • Muscle Tissue: Specialized for contraction and movement.

  • Nervous Tissue: Specialized for control and information processing via electrical impulses.

• Common Composition: All tissues are composed of three basic components:

  1. Cells.

  2. Extracellular materials.

  3. Extracellular fluid (ECF), also known as interstitial fluid.

Epithelial Tissue: General Characteristics and Functions

• Scope: Epithelial tissue includes both covering/lining epithelia and glandular epithelia.

• Epithelia (Epithelium): Refers to layers of cells that cover external surfaces or line internal passageways and chambers.

• Glands: Structures primarily composed of epithelia that produce and release fluid secretions.

• Distinguishing Characteristics:

  • Cellularity: Composed almost entirely of cells bound closely together.

  • Specialized Contacts: Cells are joined by specialized cell junctions.

  • Polarity: Epithelial cells have an exposed apical surface (facing the exterior or an internal space) and an attached basal surface.

  • Basement Membrane: The basal surface is bound to a thin, non-cellular basement membrane.

  • Avascularity: Epithelia lack blood vessels; they must obtain nutrients by diffusion or absorption from underlying tissues. However, they are innervated (contain nerves).

  • Regeneration: Damaged or lost cells at the surface are continuously replaced through stem cell divisions within the epithelium.

• Primary Functions:

  • Physical Protection: Shields exposed surfaces from abrasion, dehydration, and destruction by chemical or biological agents.

  • Secretion: Glandular cells produce specialized secretions.

  • Absorption: Regulates the intake of nutrients and materials.

  • Filtration: Acts as a selective barrier for fluids.

Cellular Polarity and Specialized Intercellular Connections

• Apical Surface Specializations:

  • Microvilli: Small projections that increase surface area for enhanced absorption.

  • Cilia: Slender extensions that move fluids across the apical surface through coordinated ciliary action.

• Basolateral Surface: This surface faces the underlying tissue and neighboring cells.

• Cell Adhesion Molecules (CAMs): Transmembrane proteins that facilitate intercellular connections, allowing cells to stick to one another and to external materials.

• Types of Cell Junctions:

  • Gap Junctions: Held together by transmembrane proteins called connexons. They allow rapid communication by permitting the passage of small molecules and ions between cells. These are critical for coordinating contractions in heart muscle.

  • Tight Junctions: Fusion of the lipid portions of two plasma membranes. They prevent the passage of water and solutes between cells, which is vital in the digestive tract to keep enzymes, acids, and wastes within the lumen.

  • Desmosomes: CAMs and proteoglycans link opposing plasma membranes together.

    • Spot Desmosomes: Connect two cells together at specific points.

    • Hemidesmosomes: Attach epithelial cells to the underlying basement membrane.

The Basement Membrane

• Structure: Epithelia are anchored to a basement membrane which provides structural strength.

• Layers:

  • Basal Lamina: The layer closest to the epithelium, produced by epithelial cells.

  • Reticular Lamina: The deeper portion of the basement membrane, produced by the underlying connective tissue.

Classification and Types of Epithelia

• Classification by Cell Shape:

  • Squamous: Thin and flat cells.

  • Cuboidal: Cube-shaped cells (height roughly equals width).

  • Columnar: Tall and slender cells (taller than they are wide).

• Classification by Number of Layers:

  • Simple: A single layer of cells.

  • Stratified: Several layers of cells.

Simple Epithelia (Filtration, Absorption, Secretion, Diffusion)

• Simple Squamous Epithelium:

  • Description: Single layer of flattened cells with disc-shaped central nuclei and sparse cytoplasm.

  • Types: Mesothelium (lines body cavities) and Endothelium (inner lining of heart and blood vessels).

  • Locations: Kidney glomeruli and corpuscles, air sacs of lungs, lining of heart, blood vessels, lymphatic vessels, and ventral body cavity serosae.

  • Function: Allows passage of materials via diffusion/filtration; secretes lubricating serous fluid.

  • Photomicrograph magnification: Alveolar walls at $400 \times$.

• Simple Cuboidal Epithelium:

  • Description: Single layer of cube-like cells with large, spherical central nuclei.

  • Locations: Kidney tubules, ducts and secretory portions of small glands, and ovary surfaces.

  • Function: Secretion and absorption.

  • Photomicrograph magnification: Kidney tubules at $400 \times$.

• Simple Columnar Epithelium:

  • Description: Single layer of tall cells with round to oval nuclei; may bear cilia or contain mucus-secreting goblet cells.

  • Locations: Nonciliated type lines most of the digestive tract (stomach to anal canal), gallbladder, and excretory ducts. Ciliated type lines small bronchi, uterine tubes, and regions of the uterus.

  • Function: Absorption; secretion of mucus and enzymes; ciliary action propels mucus or reproductive cells.

  • Photomicrograph magnification: Stomach mucosa at $1300 \times$.

• Pseudostratified Columnar Epithelium:

  • Description: Single layer of cells of differing heights; nuclei appear at different levels, giving a false impression of stratification. Often ciliated and contains goblet cells.

  • Locations: Nonciliated type in male sperm-carrying ducts and large gland ducts; ciliated variety lines the trachea and most of the upper respiratory tract.

  • Function: Secretion (particularly mucus) and propulsion of mucus by ciliary action.

  • Photomicrograph magnification: Human trachea at $400 \times$.

Stratified Epithelia (Protection from Abrasion)

• Stratified Squamous Epithelium:

  • Description: Thick membrane with multiple layers; basal cells are active in mitosis (cuboidal/columnar); surface cells are squamous. Keratinized cells are dead and filled with keratin.

  • Locations: Nonkeratinized type in esophagus, mouth, and vagina. Keratinized variety forms the epidermis (skin).

  • Function: Protects underlying tissues in areas of high abrasion.

  • Photomicrograph magnification: Esophagus lining at $300 \times$.

• Stratified Cuboidal Epithelium:

  • Description: Generally two layers of cube-like cells.

  • Locations: Largest ducts of sweat glands, mammary glands, and salivary glands.

  • Function: Protection.

  • Photomicrograph magnification: Salivary gland duct at $300 \times$.

• Stratified Columnar Epithelium:

  • Description: Several cell layers; basal cells are cuboidal, superficial cells are tall and columnar.

  • Locations: Rare; found in small amounts in the male urethra and large ducts of some glands.

  • Function: Protection and secretion.

  • Photomicrograph magnification: Male urethra at $900 \times$.

• Transitional Epithelium:

  • Description: Appearance changes based on stretch; relaxed state shows bulbous or dome-shaped surface cells; stretched state appears flattened (squamous-like).

  • Locations: Lines ureters, urinary bladder, and part of the urethra.

  • Function: Permits distension of urinary organs.

  • Photomicrograph magnification: Bladder in relaxed state at $500 \times$.

Glandular Epithelia: Endocrine and Exocrine Glands

• Endocrine Glands: Release hormones directly into the interstitial fluid and bloodstream. They do not use ducts.

• Exocrine Glands: Discharge secretions through ducts onto epithelial surfaces for local effects.

• Unicellular Exocrine Glands:

  • Goblet Cells: Dispersed individual cells that secrete mucin. Mucin combines with water to form mucus.

• Multicellular Exocrine Glands: Examples include intestinal glands, stomach glands (gastric juice), sebaceous glands (sebum/oil), mammary glands (milk), salivary glands (saliva), and sudoriferous glands (sweat).

Connective Tissue: Components and General Functions

• Diverse and Abundant: Connective tissue (CT) is the most diverse tissue type in the body.

• Three Basic Components:

  1. Specialized Cells.

  2. Extracellular Protein Fibers.

  3. Ground Substance: A fluid/gel-like material filling spaces between cells and fibers.

• Extracellular Matrix (ECM): Defined as the combination of protein fibers and ground substance. The ECM determines the physical characteristics of the CT type.

• General Functions of CT:

  • Binding and Support: Provides structural framework.

  • Protection: Shields delicate organs.

  • Insulation: Adipose tissue stores energy and maintains temperature.

  • Transportation: Blood moves fluids and dissolved materials.

Connective Tissue Fibers

• Collagen Fibers: The most common and abundant fibers in humans. They are long, straight, unbranched, strong, yet flexible. They resist force in one direction.

• Reticular Fibers: Composed of Type III collagen. They form a branched, interwoven network (stroma). They are moderately strong and flexible, resisting forces from many directions.

• Elastic Fibers: Contain the protein elastin. They are branched and wavy, with the ability to return to their original length after being stretched.

Specialized Cells of Connective Tissue

• Blasts (Immature Cells): Actively mitotic cells that secrete the ground substance and fibers for the matrix.

  • Fibroblasts: Found in CT proper; the most abundant cell type.

  • Chondroblasts: Found in cartilage.

  • Osteoblasts: Found in bone.

• Cytes (Mature Cells): Inactive cells that maintain the health of the matrix.

  • Fibrocytes: Second most abundant in CT proper; maintain fibers.

  • Chondrocytes: Mature cartilage cells.

  • Osteocytes: Mature bone cells.

• Additional CT Proper Cells:

  • Adipocytes: Fat cells; each stores a large fat droplet.

  • Mesenchymal Cells: Stem cells that differentiate into other CT cells (fibroblasts, macrophages) following injury.

  • Melanocytes: Synthesize and store the brown pigment melanin.

  • Macrophages: Phagocytic cells of the immune system. Fixed macrophages stay in place, while free macrophages migrate to infection sites.

  • Mast Cells: Stimulate inflammation after injury by releasing histamine and heparin. Involved in allergic responses.

  • Lymphocytes: Migratory immune cells; may develop into plasma cells that produce antibodies.

  • Microphages: Phagocytic white blood cells (neutrophils, basophils, eosinophils); also known as PMNL.

Classification of Connective Tissues

Common Origin: Mesenchyme

All connective tissues originate from the same embryonic tissue called mesenchyme.

Connective Tissue Proper

• Loose Connective Tissue: Contains more ground substance and fewer fibers.

  • Areolar: Gel-like matrix with all three fiber types. Functions to wrap and cushion organs; plays a role in inflammation. Located under epithelia and surrounding capillaries. (Photomicrograph at $400 \times$).

  • Adipose: Matrix like areolar but sparse. Composed of packed adipocytes with nuclei pushed to the side. Functions in energy storage, insulation, and protection. Located under skin, around kidneys/eyeballs, and in breasts. (Photomicrograph at $600 \times$).

  • Reticular: Network of reticular fibers in loose ground substance. Forms a soft internal skeleton (stroma) for lymphoid organs (lymph nodes, bone marrow, spleen). (Photomicrograph at $350 \times$).

• Dense Connective Tissue: Contains more fibers and less ground substance.

  • Dense Irregular: Irregularly arranged collagen fibers. Withstands tension from many directions. Located in the dermis and fibrous joint capsules. (Photomicrograph at $400 \times$).

  • Dense Regular: Parallel collagen fibers with great tensile strength in one direction. Forms tendons (muscle to bone), ligaments (bone to bone), and aponeuroses. (Photomicrograph at $1000 \times$).

  • Elastic: High proportion of elastic fibers. Allows recoil after stretching and maintains pulsatile blood flow in arteries. Located in the walls of the aorta and vocal cords. (Photomicrograph at $85 \times$).

Cartilage

• Characteristics: Avascular; chondrocytes reside in spaces called lacunae.

• Hyaline Cartilage: Amorphous but firm matrix with invisible collagen network. Supports, cushions, and resists compression. Found in embryonic skeleton, ends of long bones, coastal cartilages of ribs, nose, trachea, and larynx. (Photomicrograph at $300 \times$).

• Elastic Cartilage: Similar to hyaline but with more elastic fibers. Maintains shape with great flexibility. Supports the external ear (pinna) and epiglottis. (Photomicrograph at $400 \times$).

• Fibrocartilage: Matrix like hyaline but less firm; thick collagen fibers predominate. Provides tensile strength and absorbs compressive shock. Located in intervertebral discs, pubic symphysis, and knee menisci. (Photomicrograph at $200 \times$).

Osseous Tissue (Bone)

• Description: Hard, calcified matrix with collagen fibers; osteocytes in lacunae. Very well vascularized.

• Function: Support, protection, providing levers for muscles, storage of calcium/minerals/fat, and hematopoiesis (blood cell formation).

• Photomicrograph magnification: Cross-section at $70 \times$.

Blood

• Description: Red and white blood cells and platelets in a fluid matrix called plasma. Serum acts as the ground substance.

• Function: Transport of respiratory gases, nutrients, and wastes.

• Photomicrograph magnification: Blood smear at $1500 \times$.

Fasciae (Connective Tissue Layers)

• Superficial Fascia: Separates skin from underlying tissues.

• Deep Fascia: Sheets of dense connective tissue.

• Subserous Fascia: Located between deep fascia and serous membranes.

Covering and Lining Membranes

• Cutaneous Membrane: The skin; described as a dry membrane.

• Mucous Membranes: Line hollow organs open to the outside (respiratory, digestive, reproductive, urinary systems).

• Serous Membranes: Line closed ventral body cavities (pleural, pericardial, and peritoneal).

Muscle Tissue

• Skeletal Muscle:

  • Structure: Long, cylindrical, multinucleate, and striated cells (myofibers).

  • Control: Voluntary.

  • Locations: Combined with CT in skeletal muscles.

  • Functions: Movement, skeleton stabilization, protection, heat generation.

  • Photomicrograph magnification: $180 \times$.

• Cardiac Muscle:

  • Structure: Short, branched, striated cells with a single nucleus; cells connected by intercalated discs.

  • Control: Involuntary.

  • Functions: Blood circulation and blood pressure maintenance.

  • Photomicrograph magnification: $450 \times$.

• Smooth Muscle:

  • Structure: Short, spindle-shaped, non-striated cells with a single central nucleus.

  • Control: Involuntary.

  • Locations: Walls of blood vessels, digestive, respiratory, urinary, and reproductive organs.

  • Functions: Movement of materials (food, urine, secretions) and regulation of vessel/passageway diameter.

  • Photomicrograph magnification: $235 \times$.

Nervous Tissue

• Structure: Composed of neurons and neuroglia (supporting cells).

• Neurons: Specialized for conducting and transmitting electrical impulses.

• Concentration: Primarily found in the brain and spinal cord, with networks extending throughout the body.