Histology

• Tissues


  Cells work together in functionally related groups called tissues
• Types of tissues


  • Epithelial – lining and covering
  • Connective – support
  • Muscle – movement
  • Nervous – control
• Epithelial Tissue


  • Covers a body surface or lines a body cavity
  • Forms most glands
  • Functions: Protection, Absorption, secretion, and ion transport, Filtration, Forms slippery surfaces
• Special Characteristics of Epithelia


  • Cellularity: cells are in close contact with each other with little or no intercellular space
  • Specialized contacts: may have junctions for both attachment and communication
  • Polarity: epithelial tissues always have an apical and basal surface
  • Support by connective tissue: at the basal surface, both the epithelial tissue and the connective tissue contribute to the basement membrane
  • Avascular: nutrients must diffuse
  • Innervated
  • Regeneration: epithelial tissues have a high capacity for regeneration
• Lateral Surface Features


  • Adhesion proteins link plasma membranes of adjacent cells
  • Contours of adjacent cell membranes
  • Special cell junctions: Tight Junctions, Adherens Junctions, Desmosomes
• Basal Feature: The Basal Lamina


  • Noncellular supporting sheet between the epithelium and the connective tissue deep to it
  • Consists of proteins secreted by the epithelial cells
  • Functions: Acts as a selective filter, determining which molecules from capillaries enter the epithelium, Acts as scaffolding along which regenerating epithelial cells can migrate
  • Basal lamina and reticular layers of the underlying connective tissue deep to it form the basement membrane
• Classifications & Naming of Epithelia

  • First name of tissue indicates number of layers: Simple – one layer of cells, Stratified – more than one layer of cells
  • Last name of tissue describes shape of cells: Squamous – cells wider than tall, Cuboidal – cells are as wide as tall, Columnar – cells are taller than they are wide
• Naming Epithelia


  • Naming the epithelia includes both the layers (first) and the shape of the cells (second)
  • The name may also include any accessory structures: Goblet cells, Cilia, Keratin
  • Special epithelial tissues (don't follow naming convention): Psuedostratified, Transitional
• Simple Squamous Epithelium


  • Description: single layer of flat cells with disc-shaped nuclei
  • Functions: Passage of materials by passive diffusion and filtration, Secretes lubricating substances in serosae
  • Locations: Renal corpuscles, Alveoli of lungs, Lining of heart, blood and lymphatic vessels, Lining of ventral body cavity (serosae)
• Simple Cuboidal Epithelium


  • Description: single layer of cube-like cells with large, spherical central nuclei
  • Functions: secretion and absorption
  • Locations: kidney tubules, secretory portions of small glands, ovary & thyroid follicles
• Simple Columnar Epithelium


  • Description: single layer of column-shaped (rectangular) cells with oval nuclei, Some bear cilia at their apical surface, May contain goblet cells
  • Functions: Absorption; secretion of mucus, enzymes, and other substances
  • Locations: Non-ciliated form - Lines digestive tract, gallbladder, ducts of some glands, Ciliated form - Lines small bronchi, uterine tubes, uterus
• Pseudostratified Columnar Epithelium


  • Description: All cells originate at basement membrane, Only tall cells reach the apical surface, May contain goblet cells and bear cilia, Nuclei lie at varying heights within cells, Gives false impression of stratification
  • Functions: secretion of mucus; propulsion of mucus by cilia
  • Locations: Non-ciliated type - Ducts of male reproductive tubes, Ducts of large glands, Ciliated variety - Lines trachea and most of upper respiratory tract
• Stratified Epithelia


  • Contain two or more layers of cells, Regenerate from below, Major role is protection, Are named according to the shape of cells at apical layer
• Stratified Squamous Epithelium


  • Description: Many layers of cells – squamous in shape, Deeper layers of cells appear cuboidal or columnar, Thickest epithelial tissue – adapted for protection
  • Specific types: Keratinized – contain the protective protein keratin, Surface cells are dead and full of keratin, Non-keratinized – forms moist lining of body openings
  • Functions: Protects underlying tissues in areas subject to abrasion
  • Locations: Keratinized – forms epidermis, Non-keratinized – forms lining of esophagus, mouth, and vagina
• Transitional Epithelium

  • Description: Basal cells usually cuboidal or columnar, Superficial cells dome-shaped or squamous
  • Function: stretches and permits distension of urinary bladder
  • Location: Lines ureters, urinary bladder and part of urethra
• Glandular Epithelium


  • Ducts carry products of exocrine glands to epithelial surface
  • Include: Mucus-secreting glands, Sweat and oil glands, Salivary glands, Liver and pancreas, Mammary glands
  • May be: unicellular or multicellular
• Unicellular Exocrine Glands (The Goblet Cell)

  • Goblet cells produce mucin, Mucin + water = mucus, Protects and lubricates many internal body surfaces
• Multicellular Exocrine Glands


  • Classified by structure (branching & shape) of duct, Can also be classified by mode or type of secretion: Merocrine secretion – secretory vesicles released via exocytosis, Apocrine secretion – apical portion of the cell is lost, cytoplasm + secretory product, Holocrine secretion – entire cell is destroyed during secretion
• Types of secretions from exocrine glands

  • Serous – mostly water but also contains some enzymes, Ex. parotid glands, pancreas
  • Mucous – mucus secretions, Ex. sublingual glands, goblet cells
  • Mixes – serous & mucus combined, Ex. submandibular gland
• Connective Tissue


  • Most diverse and abundant tissue
  • Main classes: Connective tissue proper, Blood – Fluid connective tissue, Cartilage, Bone tissue
  • Components: Cells (varies according to tissue), Matrix, Protein fibers (varies according to tissue), Ground substance (varies according to tissue)
  • Common embryonic origin – mesenchyme
  • Supporting connective tissues
• Cells found in connective tissue proper

  • Fibroblasts
  • Macrophages, lymphocytes (antibody producing cells)
  • Adipocytes (fat cells)
  • Mast cells
  • Stem cells
• Fibers in connective tissue proper

  • Collagen – very strong & abundant, long & straight
  • Elastic – branching fibers with a wavy appearance (when relaxed)
  • Reticular – form a network of fibers that form a supportive framework in soft organs (i.e. Spleen & liver)
• Ground substance


  Along with fibers, fills the extracellular space, Helps determine functionality of tissue
• Classifications of Connective Tissue Proper

  • Loose Connective Tissue: Areolar, Reticular, Adipose
  • Dense Connective Tissue: Regular, Irregular, Elastic
• Areolar Connective Tissue


  • Gel-like matrix with all three fiber types (collagen, reticular, elastic) for support, Ground substance is made up by glycoproteins also made and secreted by the fibroblasts, Cells – fibroblasts, macrophages, mast cells, white blood cells, adipocytes, Highly vascular tissue
  • Functions: Wraps and cushions organs, Holds and conveys tissue fluid, Important role in inflammation
  • Locations: Widely distributed under epithelia, Packages organs, Surrounds capillaries
• Adipose Tissue


  • Closely packed adipocytes with nucleus pushed to one side by fat droplet
  • Functions: Provides reserve food fuel, Insulates against heat loss, Supports and protects organs
  • Locations: Under skin, Around kidneys, Behind eyeballs, within abdomen and in breasts
• Reticular Connective Tissue


  • Description – network of reticular fibers in loose ground substance
  • Function – form a soft, internal skeleton (stroma) – supports other cell types
  • Location – lymphoid organs: Lymph nodes, bone marrow, and spleen
• Dense Irregular Connective Tissue

  • Description: Primarily irregularly arranged collagen fibers, Some elastic fibers and fibroblasts
  • Functions: Withstands tension, Provides structural strength
  • Locations: Dermis of skin, Submucosa of digestive tract, Fibrous capsules of joints and organs
• Dense Regular Connective Tissue

  • Description: Primarily parallel collagen fibers, Fibroblasts and some elastic fibers, Poorly vascularized
  • Functions: Attaches muscle to bone, Attaches bone to bone, Withstands great stress in one direction
  • Locations: Tendons and ligaments, Aponeuroses, Fascia around muscles
• Cartilage


  • Characteristics: Firm, flexible tissue, Contains no blood vessels or nerves, Matrix contains up to 80% water, Cell type – chondrocyte
  • Types: Hyaline, Elastic, Fibrocartilage
• Hyaline Cartilage


  • Description: Imperceptible collagen fibers (hyaline = glassy), Chodroblasts produce matrix, Chondrocytes lie in lacunae
  • Functions: Supports and reinforces, Resilient cushion, Resists repetitive stress
  • Locations: Ends of long bones, Costal cartilage of ribs, Cartilages of nose, trachea, and larynx
• Elastic Cartilage


  • Description: Similar to hyaline cartilage, More elastic fibers in matrix
  • Functions: Maintains shape of structure, Allows great flexibility
  • Location: Supports external ear, Epiglottis
• Fibrocartilage


  • Description: Matrix similar, but less firm than hyaline cartilage, Thick collagen fibers predominate
  • Functions: Tensile strength and ability to absorb compressive shock
  • Locations: Intervertebral discs, Pubic symphysis
• Bone Tissue


  • Functions: Supports and protects organs, Provides levers and attachment site for muscles, Stores calcium and other minerals, Stores fat, Marrow is site for blood cell formation
  • Location: Bones
• Blood Tissue


  • Description: red and white blood cells in a fluid matrix
  • Functions: transport of respiratory gases, nutrients, and wastes
  • Locations: within blood vessels
  • Characteristics: An atypical connective tissue, Consists of cells surrounded by fluid matrix
• Covering and Lining Membranes

  • Combine epithelial tissues and connective tissues, Cover broad areas within body, Consist of epithelial sheet plus underlying connective tissue
• Types of Membranes


  • Cutaneous membrane – skin
  • Mucous membrane – Lines hollow organs that open to surface of body, An epithelial sheet underlain with layer of lamina propria
  • Serous membrane – slippery membranes, Simple squamous epithelium lying on areolar connective tissue, Line closed cavities: Pleural, peritoneal, and pericardial cavities
  • Synovial membranes – lining joint cavities, Loose connective (areolar) + simple squamous epithelium, Secretes fluid (synovial fluid) which lubricates, protects & cushions joint structures
• Skeletal Muscle Tissue


  • Characteristics: Long, cylindrical cells, Multinucleate, Obvious striations
  • Functions: Voluntary movement, Manipulation of environment, Facial expression
  • Location: Skeletal muscles attached to bones (occasionally to skin)
• Cardiac Muscle Tissue


  • Function: Contracts to propel blood into circulatory system
  • Characteristics: Branching cells, Uni-nucleate, Intercalated discs
  • Location: Occurs in walls of heart
• Smooth Muscle Tissue


  • Characteristics: Spindle-shaped cells with central nuclei, Arranged closely to form sheets, No striations
  • Function: Propels substances along internal passageways, Involuntary control
  • Location: Mostly walls of hollow organs