Histology

Histology

1. MAKING SLIDES

   • Fixation - preserve tissue e.g. formaldehyde
   • Embedding - infiltrate with small molecules that cross link to form hard matrix to withstand thin sectioning
     • Paraffin wax - first remove water by dehydration in series of graded alcohol then immersed in wax at 60 degrees
   • Sectioning - microtome to make thin enough sample to transmit light ~7um
   • Staining
     • Alcian blue
       • Stains mucus blue/ green
     • Eosin
       • Stains acidophilic components e.g. mitochondria/ collagen/ secretory granules
     • Haematoxylin
       • Stains basophilic components - nuclei/ ribosomes/ DNA
     • Van Gieson
       • Stains collagen red
       • Smooth muscles yellow
     • Osmium tetroxide
       • Stains lipids black
   • Samples may be distorted or contain artefacts

2. USING LIGHT MICROSCOPES

   • Vary light intensity to suit the objective and slide using
   • Condenser focus light beam onto slide
   • Start by examining with naked eye, low power then move higher

3. NERVES

   • Ulnar nerves carry motor fibres to forearm muscles and hand
   • Sciatic nerve carries motor fibres to leg skeletal muscle
     • Larger than ulnar
   • Splanchnic nerve carries sympathetic fibres to gut
   • Each contains sensory fibres also
   • Epineurium = lovely organised connective tissue
   • Each ulnar nerve branch formed separate fascicle dussouned by perineurium (dense connective tissue)
   • Endoneurium - extends in from perineurium to form network round each axon
   • Myelin - lipid, surrounded by
     • Noes of Ranvier between
       • Visualise by staining black with osmium tetroxide. Soften by immersion in glycerol, tease fibres apart and look under microscopes
     • Increases membrane resistance to increase speed
     • Schwann cells in periphery
     • Oligodendrocytes in CNS

4. STRIATED MUSCLE

   • Muscle fibre
     • Multinucleate (syncytium) structure, peripheral nuclei
     • Sarcolemma = membrane
     • Myofibrils = cylindric collections of sarcomere (contractile unit)
       • A band = dark, thick myosin. Anisotropic as alter polarised light in 2 planes
       • I band = thin actin, isotropic (same properties in all directions)
   • Red
     • Rich in myoglobin, mitochondria, capillaries
     • Aerobic exercise
     • Use labelling of succinate dehydrogenase to distinguish (present in TCA so red more)
   • White
     • Less myoglobin/ mitochondria/ poorer blood supply, much glycogen
     • Anaerobic

5. NEUROMUSCULAR JUNCTION

   • Signal from motor neurone to cause action in muscle
   • Ca2+ influx causes t-SNARE and v-SNARE interaction for exocytosis of ACh
     • Acetylcholinesterase removes signal
   • End plate potential - graded/ local, triggers AP
   • Motor unit = all fibres innervated by one motor neurone
     • Smooth contraction by asynchronous recruitment

6. SMOOTH MUSCLE

   • CARDIAC MUSCLE
     • Striated -intercalated discs - junctional complex where cells joined end on = darker
     • Nuclei in centre= bad staining
     • Purkinje fibres = high glycogen to rapidly conduct AV depolarisation to ventricles
   • GUT WALL
     • Cytoplasm has specific actin and myosin
       • - intermediate actin attached to dense bodies in cell via anchoring protein
     • Inner circular layer, outer longitudinal layer (pink)
     • Cells linked by gap junction (syncytium)

7. SMALL INTESTINE

   • Inner-most = mucosa.
     • Luminal epithelium (secrete mucus) and connective tissue
     • Folded into villi - increase SA for absorption
   • Muscular mucosae
     • Separate mucosa from submucosa connective tissue
   • Sub mucosa
     • Blood vessels and nerve fibres from enteric system
   • Smooth muscle
     • Pink, enteric neurones between
   • Serosa
     • Epitheluum and connective tissue, secretes lubricating fluid adn extends to form mesentery (suspends gut in body)

8. KIDNEY

   • Renal corpuscle of glomerulus and Bowman's capsule
   • PCT leaves BC- brush border of microvilli
   • Papilla
     • Medulla tapers down from cortex. Collecting ducts from medulla -> calyx
     • Calyx = interior kidney region that drains to ureter
   • Tubules between collecting ducts = LOH or vasa recta (if blood stained)

9. LUNG

   • Larger region= alveolar ducts, smaller = alveoli
   • Alveolar septum between adjacent alveoli.
     • Covered by respiratory endothelium
   • Blood vessel network between alveoli (veins often shorter distance)i
   • Bronchi = cartilage in walls - pink
   • Bronchiole = smaller, no cartillage, smooth muscle layer (Asthma if contract pathologically)

10. SKIN

    • Covered by keratin layer - cells die as move up towards surface
      • Keratinized stratified squamous epithelium = inert, impervious layer
    • Dermis = deep, dense connective tissue
    • Hyperdermis = loose connective tissue with fat and part of sweat gland
    • Eccrine sweat glands through epidermis
    • Pacinian corpuscule - mechanoreceptor that detects change in pressure and vibration

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