Tuesday 18 December 2018

Periodic Acid Schiff (PAS) staining

Tissue carbohydrates are of two types - Glycogen and Mucin (mucopolysaccharides).

Periodic Acid Schiff (PAS) stain:Indicates presence of glycogen in tissue

C-C bonds in carbohydrates (Eg:Glycogen) ----> Oxidised by periodic acid ----> Aldehyde ------> Schiff's reagent (Fuschin-sulphurous acid) ------> Alkyl sulphonate (magenta coloured compound).

Used in the diagnosis of:
1. Glycogen storage disorders


Liver biopsy (PAS stain) - demonstrating glycogen

2. Glycogen containing tumours - Clear cell renal cell carcinoma and Ewing sarcoma.

An important feature of glycogen: The ease of digestion with amylase; i.e Glycogen is digested by amylase; while mucin is not.
Thus, Glycogen is PASD sensitive while mucin is PASD resistant.
                    * Most commonly used amylase is ptyalin (salivary amylase).


Other structures highlighted by PAS stain (and of practical importance):
1. Basement membrane: 

  • Invasion of tumours especially squamous cell carcinoma of skin, cervix.
  • Thickening of basement membrane in diseases like DLE etc.
  • Renal biopsies: Capillary,  glomerular and tubular basement membranes

2. Fungal cell wall: 

  • PAS highlights the cell wall of living fungi (incontrast to Grocott's Methanamine Silver - GMS which highlights both living and dead fungi).

3. Renal biopsies - To differentiate the various glomerular diseases with organised deposits (in combination with Jones silver stain)

4. Neutral mucin

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Quiz 2 - Cervical Cytology part 2

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