Anatomy and Physiology - Histology and Tissues
Introduction to Tissues and Microscopy
The Significance of Tissue Study:
Specialization of individual body cells allows each type to perform specific functions necessary to maintain homeostasis.
Tissues: Defined as groups of cells that are similar in structure and perform a common or related function.
Histology: The scientific study of tissues.
Clinical Importance: Understanding tissue types allows healthcare professionals to monitor potential damage, such as bedsores (decubitus ulcers), in patients.
The Four Basic Tissue Types:
Epithelial tissue
Connective tissue
Muscle tissue
Nervous tissue
Microscopy Preparation Steps:
Fixed: Tissues must be preserved with a solvent.
Sectioned: Tissues are cut into slices thin enough to transmit light or electrons.
Stained: Dyes or salts are used to enhance contrast.
Artifacts: Minor distortions in the sample that detract from its appearance in the living state.
Light Microscopy (LM): Uses colored dyes.
Electron Microscopy (EM): Uses heavy metal salts.
Transmission Electron Microscopy (TEM): Displays a sectional view.
Scanning Electron Microscopy (SEM): Displays a surface view.
Epithelial Tissue Characteristics
Definition: Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities.
Main Forms:
Covering and Lining Epithelium: Forms the outer layer of skin; lines open cavities of the urogenital, digestive, and respiratory systems; and covers walls and organs of the ventral body cavity.
Glandular Epithelium: Forms the glands of the body (e.g., salivary glands).
Primary Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.
Distinguishing Characteristics:
Polarity: Cells have a top and bottom.
Apical Surface: The upper free side exposed to the surface or cavity. Most are smooth, but some feature microvilli (fingerlike projections).
Basal Surface: The lower attached side facing inward toward the body. It attaches to the basal lamina, an adhesive sheet holding epithelium to underlying cells.
Specialized Contacts: Cells fit closely to form continuous sheets.
Lateral contacts include tight junctions and desmosomes.
Connective Tissue Support: All epithelial sheets are supported by connective tissue.
Reticular Lamina: Deep to the basal lamina, consisting of a network of collagen fibers.
Basement Membrane: Composed of the basal and reticular lamina; it reinforces the sheet, resists stretching/tearing, and defines the epithelial boundary.
Avascular but Innervated: Contains no blood vessels; cells are nourished by diffusion from underlying connective tissues. However, they are supplied by nerve fibers.
Regeneration: High capacity for repair. Triggered by a loss of apical-basal polarity or broken lateral contacts. Requires adequate nutrients and cell division.
Clinical Correlation—Homeostatic Imbalance :
Cancerous epithelial cells are not contained by the basement membrane. They penetrate this boundary and invade underlying tissues, leading to the spread of cancer (metastasis).
Classification of Epithelial Tissue
Naming Convention: All epithelia have two names.
First Name (Layers):
Simple: Single cell layer; used where a thin barrier is desirable for diffusion.
Stratified: Two or more cell layers; found in high-abrasion areas.
Second Name (Shape):
Squamous: Flattened and scale-like.
Cuboidal: Box-like or cube.
Columnar: Tall and column-like.
Note: In stratified epithelia, the tissue is named based on the shape of cells in the apical layer.
Specific Types of Epithelia
Simple Squamous Epithelium:
Description: Simple layer of flattened cells with disc-shaped nuclei and sparse cytoplasm.
Function: Allows materials to pass by diffusion and filtration; secretes lubricating substances.
Location: Kidney glomeruli, air sacs of lungs, lining of the heart, blood vessels, lymphatic vessels, and serosae.
Endothelium: Specialized simple squamous lining lymphatic vessels, blood vessels, and the heart.
Mesothelium: Specialized simple squamous found in serous membranes of the ventral body cavity.
Simple Cuboidal Epithelium:
Description: Single layer of cube-like cells with large, spherical central nuclei.
Function: Secretion and absorption.
Location: Kidney tubules, ducts/secretory portions of small glands, and ovary surfaces.
Simple Columnar Epithelium:
Description: Single layer of tall cells with round to oval nuclei. May contain microvilli, cilia, or goblet cells (mucus-secreting unicellular glands).
Function: Absorption and secretion of mucus and enzymes. Ciliated types propel mucus or reproductive cells.
Location: Nonciliated types line the digestive tract (stomach to rectum), gallbladder, and excretory ducts. Ciliated variety lines small bronchi and uterine tubes.
Pseudostratified Columnar Epithelium:
Description: Single layer of cells differing in heights; nuclei are seen at different levels, giving a false (pseudo) impression of stratification.
Function: Secretion and propulsion of mucus via ciliary action.
Location: Ciliated variety lines the trachea and upper respiratory tract; nonciliated type is in sperm-carrying ducts.
Stratified Squamous Epithelium:
Description: Thick membrane; basal cells are cuboidal/columnar and mitotically active; surface cells are flattened (squamous).
Function: Protects underlying tissues in areas subjected to abrasion.
Types:
Keratinized: Surface cells are dead and full of keratin (e.g., epidermis of skin).
Nonkeratinized: Moist linings (e.g., esophagus, mouth, vagina).
Transitional Epithelium:
Description: Resembles both stratified squamous and cuboidal. Basal cells are cuboidal/columnar; surface cells are dome-shaped depending on organ stretch.
Function: Stretches readily to permit urine distension.
Location: Lines the ureters, bladder, and part of the urethra.
Rare Epithelia:
Stratified Cuboidal: Rare; found in some sweat and mammary glands; typically two layers thick.
Stratified Columnar: Limited distribution; found in transitions between epithelia, pharynx, male urethra, and glandular ducts.
Glandular Epithelia
Gland: One or more cells producing a secretion (aqueous fluid).
Classification by Site of Release:
Endocrine Glands: Ductless glands. They produce hormones (messenger chemicals) via exocytosis directly into interstitial fluid, which then enters the blood or lymph to reach specific target organs.
Exocrine Glands: Secrete products into ducts that open onto body surfaces (skin) or into body cavities. Examples include mucous, sweat, oil, and salivary glands.
Unicellular Exocrine Glands:
Primary examples: Mucous cells and goblet cells.
Found in the epithelial linings of intestinal and respiratory tracts.
Produce mucin: A sugar-protein that dissolves in water to form mucus.
Multicellular Exocrine Glands:
Composed of a duct and a secretory unit, supported by connective tissue (capsules/lobes).
Structural Classification:
Simple: Unbranched duct.
Compound: Branched duct.
Tubular: Secretory cells form tubes.
Alveolar: Secretory cells form sacs.
Tubuloalveolar: Contains both tubes and sacs.
Modes of Secretion:
Merocrine: Products secreted by exocytosis (e.g., sweat, pancreas, salivary glands). Most common.
Holocrine: Products accumulate until the entire cell ruptures (e.g., sebaceous/oil glands).
Apocrine: Products accumulate at the apex, which pinches off. Occurs in some animals; mammary glands are a contested example in humans but are often classified as merocrine.
Connective Tissue (CT) Fundamentals
General Characteristics: Most abundant and widely distributed tissue.
Major Functions: Binding and support, protection, insulation, fuel storage, and substance transport (blood).
Primary Classes:
Connective tissue proper
Cartilage
Bone
Blood
Commonality Factors:
Extracellular Matrix (ECM): Nonliving portion (ground substance and fibers). Suspends living cells, allowing the tissue to bear weight and withstand tension.
Common Origin: All arise from mesenchyme (embryonic tissue).
Structural Elements:
Ground Substance: Unstructured gel-like material between cells. Composed of interstitial fluid, cell adhesion proteins ("glue"), and proteoglycans (protein core + glycosaminoglycans/GAGs like chondroitin sulfate and hyaluronic acid).
Fibers:
Collagen Fibers: Strongest, most abundant; provide high tensile strength.
Elastic Fibers: Long, thin elastin fibers for stretch and recoil.
Reticular Fibers: Short, fine, highly branched collagenous fibers; form networks that offer "give."
Living Cells:
-blast cells: Immature, actively secreting form (fibroblasts in CT proper, chondroblasts in cartilage, osteoblasts in bone, hematopoietic stem cells in blood).
-cyte cells: Mature form that maintains the matrix.
Additional CT Cell Types:
Adipocytes: Fat cells for energy storage.
Leukocytes: White blood cells (neutrophils, eosinophils, lymphocytes) for injury response.
Mast Cells: Detect foreign microbes and initiate inflammation. Contain heparin (anticoagulant), histamine (increases capillary leakiness), and proteases.
Macrophages: Phagocytic cells that break down dead cells and foreign material.
Subclasses of Connective Tissue Proper
Loose Connective Tissues:
Areolar: Most widely distributed; universal packing material. Contains fibroblasts, macrophages, and fat cells. High ground substance capacity acts as a water reservoir.
Adipose:
White fat: Nutrient storage, shock absorption, insulation. Adipocytes have scanty matrix.
Brown fat: Used to heat the bloodstream instead of producing ATP.
Reticular: Fibers are thin reticular fibers; cells are reticular cells. Forms a stroma (internal framework) to support blood cells in lymph nodes, spleen, and bone marrow.
Dense Connective Tissues:
Dense Regular: Closely packed parallel collagen bundles; high tensile strength. Poorly vascularized. Found in tendons and ligaments.
Dense Irregular: Thicker, irregularly arranged collagen bundles. Resists tension from many directions. Found in the dermis, joint capsules, and organ coverings.
Elastic: Contains high amounts of elastic fibers. Found in walls of large arteries and ligaments connecting vertebrae.
Cartilage, Bone, and Blood
Cartilage:
Characteristics: Tough, flexible, lacks nerve fibers, and is avascular. Matrix is water. Chondrocytes reside in lacunae. Nutrients provided by the perichondrium.
Hyaline: Blue-white "gristle." Most abundant. Tips of long bones, nose, trachea, larynx, ribs.
Elastic: More elastic fibers. Found in ears and epiglottis.
Fibrocartilage: Strongest; between hyaline and dense regular. Intervertebral discs and knees.
Clinical Correlation—Homeostatic Imbalance : Cartilage heals slowly due to avascularity. Aging can lead to calcification or ossification, causing chondrocyte death.
Bone (Osseous Tissue):
Description: Hard tissue with inorganic calcium salts and more collagen than cartilage. Structural units are osteons. Richly vascularized.
Functions: Support, protection, fat storage, and hematopoiesis (blood cell synthesis).
Blood:
Description: Fluid matrix (blood plasma) with red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. Fibers are soluble proteins that appear during clotting.
Function: Transport of nutrients, wastes, and gases.
Muscle and Nervous Tissues
Muscle Tissue: Highly vascularized; contains myofilaments (actin and myosin).
Skeletal Muscle: Voluntary, striated (banded), multinucleate cells (muscle fibers). Attached to bones.
Cardiac Muscle: Involuntary, striated, uninucleate. Branching cells join at intercalated discs. Found only in the heart wall.
Smooth Muscle: Involuntary, non-striated, spindle-shaped cells with one nucleus. Found in walls of hollow organs (digestive tract, urinary tract).
Nervous Tissue: Main component of the nervous system.
Neurons: Specialized cells that respond to stimuli (via dendrites) and transmit electrical signals (via axons).
Neuroglia (Glial Cells): Nonexcitable cells that support, insulate, and protect neurons.
Covering and Lining Membranes
Cutaneous Membrane: The skin. Consists of keratinized stratified squamous epithelium (epidermis) and dermis. It is a dry membrane.
Mucous Membranes (Mucosae):
Line body cavities that open to the exterior (digestive, respiratory, urogenital).
Moist membranes bathed in secretions. Epithelial sheet lies over the lamina propria (loose CT).
Serous Membranes (Serosae):
Moist membranes in closed ventral cavities. Simple squamous epithelium (mesothelium) on areolar CT.
Parietal Layer: Lines internal body walls.
Visceral Layer: Covers internal organs.
Serous Fluid: Slippery fluid between layers.
Locations: Pleurae (lungs), Pericardium (heart), Peritoneum (abdominopelvic).
Tissue Repair
Regeneration vs. Fibrosis:
Regeneration: Replace destroyed tissue with same kind; function is restored.
Fibrosis: Replace with connective (scar) tissue; function is lost.
The Three Steps of Repair:
Inflammation: Chemicals cause blood vessel dilation and increased permeability. Clotting occurs.
Organization: Blood clot is replaced by granulation tissue (capillary-enriched). Fibroblasts bridge the gap with collagen. Debris is phagocytized.
Regeneration and Fibrosis: Scab detaches; epithelium thickens to resemble adjacent tissue. Results in regenerated epithelium with underlying scar tissue.
Regenerative Capacity:
Extremely Well: Epithelia, bone, areolar CT, dense irregular CT, blood-forming tissue.
Moderate: Smooth muscle, dense regular CT.
Virtually None: Cardiac muscle, brain/spinal cord nervous tissue.
Clinical Correlation—Homeostatic Imbalance : Scar tissue (adhesions) in muscular organs can reduce volume capacity, block movement, or impair nerve/muscle signals, potentially leading to organ failure.
Developmental Aspects
Primary Germ Layers:
Ectoderm: Becomes nervous tissue and some epithelia.
Mesoderm: Becomes muscle, connective tissue, and some epithelia.
Endoderm: Becomes epithelial linings of the digestive and respiratory tracts.
Aging Tissues: Epithelia thin and repair becomes less efficient. Bone, muscle, and nervous tissues atrophy. Increased DNA mutations elevate cancer risk.