Lecture notes
Lecture notes Year 1 MBChB: Introduction to Medical Sciences (IMS)
Concise lecture notes from the cells and tissues strand of the IMS module taught in the first year of the MBChB course at the University of Leeds!
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CELLS AND TISSUESINTRODUCTION TO HISTOLOGY
Histology: study of tissues usually at the microscopic/sub-microscopic level
Fixation
• Stop decay
o Intrinsic – autolytic enzymes
o Extrinsic – microbial, trauma
• Preserve morphology
• Formation of x-links within tissue, either by denaturing proteins or by the formation of covalent bonds within the
tissue
Formalin
• Forms covalent bonds between proteins
• Good tissue penetration
• Not good for cytoplasmic structure or nucleic acids
Alternatives of formalin
• Glutaraldehyde
o For electron microscopy
o Poor tissue penetration
• Ethanol
o For nucleic acids
o Poor for morphology
• Freezing
o Good for nucleic acids, proteins etc.
o Poor for morphology
o Refrigeration required
Wax impregnation
• Gives tissue rigidity
o Enables sectioning
o Protects tissue
• Wax is not water/formalin, or alcohol soluble
• It is soluble in other hydrocarbons e.g. benzene (which are also insoluble in water)
Staining
• Highlight structure of interest in a cell
• Haematoxylin: stains acidic structures blue or purple, e.g. DNA in nuclei
• Eosin: acidic dye that stains basic structures pink, e.g. proteins (cytoplasm)
• Haematoxylin and eosin account for 95% of staining
Special stains
• Mucin: Alcian Blue, PAS (stains basement membrane, glycogen, mucopolysaccharieds and mucins); diastase digests
glycogen
• Connective tissue: Masson’s trichrome (haematoxylin, acid fuschin, methyl blue)
o Collagen = blue
o Muscle/cytoplasm/RBC = red
o Nuclei = black
• Fat: Oil red ‘O’
• Misc: Perl’s Prussian Blue, ZN stain, others
Immunohistochemistry
• Used to identify a specific molecule of interest (antigen (Ag)) in a cell
• The antibodies (Ab) to this Ag are generated by exposing an animal e.g. rabbit, to this antigen
• This Ab is applied and binds to a tissue section
• A second, labelled anti-rabbit Ab is applied
, RAS-RAF pathway
RAS-GTP (activated RAS) → BRAF → MEK → ERK →normal cell proliferation and survival
EPITHELIA
Epithelium – layer of cells that cover the body surfaces (internal, external) or line body cavities
Main features
• Derived from germ cell layers – endoderm, mesoderm, ectoderm (nervous system derived from)
• Line all body surfaces apart from articular cartilage, tooth enamel, anterior iris
• Sit on a layer of connective tissue – basal lamina (part of the basal membrance)
• Stuck closely together – intra-cellular junctions/complexes
• Polarity – apical/basal i.e. top/bottom
• Avascular –no direct blood supply, rely on diffusion
• Regeneration – rapid-turn-over
Main functions
1. Absorption (e.g. stomach / small / large intestine)
2. Surface movement (e.g. cilia in airways / fallopian tube)
3. Secretion of substances (e.g. glands such as sweat, pancreas)
4. Gas exchange (e.g. lungs)
5. Surface lubrication (e.g. mesothelial linings – pleural / pericardial / peritoneal)
6. Sensation
7. Protection
Cell-cell adhesion complexes
• Without them, cell would fall apart
• TJs: occludin/claudin seals to protein movement/paracellular diffusion; apical
• AJs: transmembrane proteins connect across cell cytoskeletons, below TJs
• GJs: small channels, allow intercellular ion/small molecule exchange
• DSs: Transmembrane proteins connect to others (linked to intermediate
filaments) form adjacent cells
• HDs: provide attachment to underlying basal lamina
• CAMs are critical for epithelium integrity, adherence to underlying basal lamina and intrinsic function
Naming/classification of epithelia
• Number of cells, shape, specialisations
• 1 layer – simple
o Squamous
o Cuboidal
o Columnar
o Pseudo-stratified – columnar cells of different heights, looks like multiple layers; upper airways,
cilia/goblet cells
• Multi-layered – stratified
o Squamous – skin, oesophagus, oral cavity
o Cuboidal
o Columnar
o Transitional – changes shape (columnar and flat),
distention; bladder/urinary tract
Specialisation
• Cilia
• Secretory – mucus for protection and lubrication
• Microvilli – increase SA for absorption
• Keratinisation – forms a protective layer
Epithelium for protection
• Prevents dehydration, chemical, mechanical etc. damage
• Covering of inter/outer surfaces
• Multi-layered for strength – i.e. stratified
• Replicative to replace sloughed / damaged cells
• Tight seals between cells
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