Epithelium

• Four basic types of tissues

  • Epithelium (90% of tumors)
  • Connective tissue
  • Muscular tissue
  • Nervous tissue

  
• Types of tissues

  • Epithelial – lining and covering
  • Connective – support
  • Muscle – movement
  • Nervous – control
• Haematoxylin and eosin stain (H+& E-)

  Histological staining technique: Tissue colour differentiation
• Haematoxylin and eosin stain (H+& E-) - H

  • Acts as a basic dye,
  • Positively charged – cationic +,
  • Reacts with negatively charged basophilic (basic loving) cell components (nucleic acids DNA and RNA in the nucleus),
  • The result - blue stain
• Haematoxylin and eosin stain (H+& E-)- E

  • Is an acidic dye,
  • Negatively charged - anionic -,
  • React with positively charged acidophilic (acid loving) components in the tissue (amino groups in proteins in the cytoplasm or mitochondria),
  • The result - pink stain
• Epithelial Tissue

  • Covers a body surface or lines a body cavity (viscera and blood vessels)
  • Forms most glands (secretory cells of glands)

  
• Functions of epithelium

  • Protection
  • Absorption, secretion, and diffusion
  • Filtration
  • Forms slippery surfaces (mucus secretion)
  • Sensation
• Epithelium distinguishing features

  • Always sit on a basement membrane
  • Come in a variety of configurations: classified on the basis of the shape of the surface cells and whether one (simple) or more (stratified) layers of cells are stacked upon each other.
  • These cells are always attached to their neighbors by desmosomes, tight junctions, and gap junctions.
• Special/main Characteristics of Epitheliapart 1

  • Cellularity: cells are in close contact with each other with little or no intercellular space between them
  • 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 from basal layer

  
• Cell Surface Specializations

  • Apical domain specializations: Microvilli, Stereocilia, Cilia, Flagellum
  • Lateral domain specializations: Cell-to-cell junctions: Occluding: tight junctions, Anchoring: Zonulae adherentes, Gap Jns- communicating jns
  • Basal domain specializations: Basement membrane, Cell-to-extracellular matrix (ECM) junctions- infoldings: Anchoring

  
• Gap Junctions

  • Permit direct passages of signaling molecules between the cells (e.g. epithelia engaged in fluid and electrolyte transport),
  • Direct communication by diffusion (maintaining organ homeostasis),
  • Formed by a family of connexin proteins which form connexon channels (12 subunits form one single gap junction),
  • Conformational changes
• Classifications 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 (plate or "scale" like), Cuboidal – cells are as wide as tall, as in cubes, Columnar – cells are taller than they are wide, like columns

  
• Naming Epithelia

  • Naming the epithelia includes both the layers (first) and the shape of the cells (second), i.e. stratified cuboidal epithelium,
  • The name may also include any accessory structures (goblet cells, cilia, keratin)
  • Special epithelial tissues (don't follow naming convention): Pseudostratified, Transitional
• Simple Squamous Epithelium

  • single layer of flat cells with disc-shaped nuclei
• Special types of Simple Squamous Epithelium

  • Endothelium (inner covering) –inner layer of blood vessel well,
  • Mesothelium (middle covering) - Lines peritoneal, pleural, and pericardial cavities, Covers visceral organs of those cavities

  
• Function of Simple Squamous Epithelium

  Passage of materials by passive diffusion and filtration, Secretes lubricating substances in serous membranes
• Locations of Simple Squamous Epithelium

  • Renal corpuscles (kidneys), Alveoli of lungs, Lining of heart, blood and lymphatic vessels, Lining of ventral body cavity (serosae/serous memb.)
• Simple Cuboidal Epithelium

  • single layer of cube-like cells with large, spherical central nuclei
• Function of Simple Cuboidal Epithelium

  secretion and absorption
• Locations of Simple Cuboidal Epithelium

  • kidney tubules, thyroid gland, pancreas, secretory portions of small glands, ovary surface, Ducts of exocrine glands or lining of thyroid follicles

  
• Simple Columnar Epithelium

  • single layer of column-shaped (rectangular) cells with oval nuclei, Some bear cilia at their apical surface, May contain goblet cells
• Function of Simple Columnar Epithelium

  Absorption; secretion of mucus, enzymes, and other substances, Ciliated type propels mucus or reproductive cells by ciliary action
• Locations of Simple Columnar Epithelium

  • Non-ciliated form - Lines digestive tract, gallbladder, ducts of some glands,
  • Ciliated form - Lines small bronchi, uterine tubes, and uterus

  
• Pseudostratified Columnar Epithelium

  • 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

  
• Function of Pseudostratified Columnar Epithelium

  secretion of mucus; propulsion of mucus by cilia
• Locations of Pseudostratified Columnar Epithelium

  • 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
  • Named according to the shape of cells at apical layer

  
• Stratified Squamous Epithelium

  • Many layers of cells – squamous in shape
  • Deeper layers of cells appear cuboidal or columnar
  • Thickest epithelial tissue – adapted for protection

  
• Keratinized Stratified Squamous Epithelium

  Surface cells are dead and full of keratin
• Non-keratinized Stratified Squamous Epithelium

  Forms moist lining of body openings
• Stratified Squamous Epithelium

  • Function: Protects underlying tissues in areas subject to abrasion
  • Keratinized type forms epidermis
  • Non-keratinized type forms lining of esophagus, mouth, and vagina

  
• Transitional Epithelium

  • Basal cells usually cuboidal or columnar
  • Superficial cells dome-shaped or squamous
• Transitional Epithelium

  • Function: Stretches and permits distension of urinary bladder
  • Lines ureters, urinary bladder and part of urethra
• Keratin is one of the fibrous structural proteins. It is the key structural material making up hair, horns, claws, hooves and the outer layer of human skin. Keratin protects epithelial cells from damage or stress.
• Epithelia have surface specializations such as numerous microvilli or cilia.
• Microvilli

  • Finger-like extensions of plasma membrane
  • Abundant in epithelia of small intestine and kidney
  • Maximize surface area for transport and absorption

  
• Cilia

  • Whip-like, highly motile extensions of apical surface membranes
  • Contains a core of nine pairs of microtubules encircling one middle pair
  • Movement of cilia is in coordinated waves
  • Function: Moving fluid and particles along epithelial surfaces
• Cilia are found in trachea, bronchi or oviduct.
• Epithelia are specialized for functions such as

  • absorption (intestine), secretion (pancreas), transport (eye & vessel endothelium), excretion (kidney), protection (against mechanical damage & dehydration), sensory reception (pain to avoid injury, taste buds, olfactory, etc.), and contraction (myoepithelium).
• Epithelial development

  • Epithelia are derived from ectoderm, endoderm, and mesoderm.
  • At the end of the second month of development, the primary tissue types have appeared and the major organs are in place.
  • In adulthood, only a few tissues regenerate, but many tissues still retain populations of stem cells.
  • With increasing age, epithelia thin, collagen decreases, and bones, muscles, and nervous tissue begin to atrophy.