Histology Intro/Epithelial

• Tissues


  Aggregations of similar cells (and materials surrounding them) working together for a specific function
• Cells working together with same goal(s) is the reason our organs function
• Tissue Components

  • Cells
  • Extracellular matrix (ECM)
  • Fibrous proteins (fibers)
  • Ground substance
• Ground Substance


  AKA: Tissue fluid, Extracellular fluid, Interstitial fluid

  Amorphous, clear gel – varies in consistency

  Functions: Cells gain: water, gases, minerals, nutrients to cells

  Cells release: waste, hormones, other products

  Protection/shock absorption
• How cells join to form tissues

  Junctions between cells

  Tight – seals space between cells, prevents leaking (e.g., digestive juices in stomach)

  Desmosomes – allows space between cells, but resists mechanical stress, common between epithelial cells

  Gap – pores/channels connecting cells, connects cytoplasm to cytoplasm (like a tunnel), e.g., some muscle cells (allows for almost simultaneous contraction)
• 4 Primary Categories of Tissues

  • Epithelial – covers

  Connective – connects

  Nervous* – communicates

  Muscular* – contracts
• Epithelial Tissue

  • Closely adhering cells

  Lines organs, forms exocrine glands

  Lumen = cavity/open tube of organ/gland

  Functions: Protection, Secretion, Absorption, Filtration, Sensation
• Epithelial Tissue - Common Features

  • Closely packed cells

  Many tight junctions & desmosomes

  Little ECM

  Cell polarity: Apical surface = free surface (faces lumen), Basal surface = attached to basement membrane
• Epithelial Tissue - Common Features: Many cells, very little ECM. Think: brick & mortar
• Epithelial Tissue - Common Features

  • Avascular

  Innervated

  High rate of regeneration

  Why is this important in a covering or lining tissue?
• Classification: # of Layers

  1 = simple, > 1 = stratified, Looks like multiple layers, but not = pseudostratified

  Structure & function: One layer ?, Multiple layers ?
• Classification: Shape


  Squamous – flat ("scaly")

  Cuboidal – cube-shaped

  Columnar – column-shaped (taller than wide)

  If multiple layers, which layer do you look at for shape?

  Structure & function: Thin cells ?, Thick cells ?
• Simple squamous epithelia

  • VERY thin single layer of cells

  Allows for rapid diffusion of gases, nutrients

  E.g., in air sacs of lungs & blood vessels

  Secretes lubricating substances – decreases friction

  E.g., lines body cavities (e.g., pleural & pericardial cavities)
• Simple squamous epithelia by location

  Endothelium – lines inside of blood vessels & heart

  Mesothelium – lines inside of ventral body cavities
• Simple cuboidal epithelium

  • Single layer of cube-shaped cells

  Thicker barrier than simple squamous

  More space for organelles such as mitochondria – energy for active transport

  Microvilli (in some) – increase absorption

  Goblet cells (in some) – secrete mucus (protective function)

  Cilia (in some) – move mucus

  Location examples: Kidney tubules, Secretory glands (e.g., thyroid, salivary)
• Simple columnar epithelium

  • Single layer of tall cells (lots of space for organelles)

  Even thicker barrier than simple cuboidal

  Functions: Microvilli (in some), Goblet cells (in some), Cilia (in some)

  Location examples: Lower digestive tract (stomach through intestines)
• Goblet Cells


  Produce secretion that (when mixed with water) becomes mucus

  Mucus creates barrier, Traps unwanted particles (e.g., airway)

  Protects tissues from digestive enzymes & acids

  Home for microbiome in gut

  Mucus creates layer of lubricant, Helps food/waste move through GI tract
• Pseudostratified columnar epithelium

  • Single layer of tall cells that looks like multiple layers (BUT it's not)

  All cells connect to basement membrane

  Functions: Goblet cells, Cilia

  Locations: Respiratory tract (nasal cavity, trachea, bronchi)
• Stratified squamous epithelium

  • Multiple layers of flat(ish) cells

  High turnover (exfoliation/desquamation)

  Basal cells rounded, apical cells flatter

  Most widespread epithelium

  Keratinized or non-keratinized

  Functions: Resists abrasion, Resists water loss, Prevents pathogen invasion

  Locations: Epidermis of skin, Upper digestive tract (oral cavity, tongue, esophagus)
• Keratinized stratified squamous epithelium

  • Cells produce keratin (protein)

  Durable, protective

  Makes surfaces tough, resistant

  Apical layers dead, packed with keratin

  Location: Epidermis of skin
• Non-keratinized stratified squamous epithelium

  • Cells do NOT produce keratin

  Apical layers still alive

  Locations: Upper digestive tract (tongue, oral cavity, esophagus), Vaginal canal
• Stratified cuboidal epithelium

  • Multiple layers of cube-shaped cells

  Function = secretion

  Sweat, Hormones, Sperm production

  Locations: Sweat glands, Ovaries, Seminiferous tubules
• Urothelium

  AKA transitional epithelium

  Layers of rounded, dome-topped cells that can flatten (when organ stretches)

  "Umbrella cells"

  Has special proteins (uroplakins) – impermeable to urine

  In urinary tract (particularly the urinary bladder)

  Functions: Stretches to allow filling, Protects underlying tissue from osmotic damage (urine is hypertonic to intracellular fluid)