MLT GlobeHistopathology: Fixation & fixatives – page 3
50 Histopathology MCQs on Fixation & fixatives with answers and explanations.
Formalin pigment is chemically known as:
Acidic formalin converts haemoglobin to acid formaldehyde haematin. Haematein is the oxidized haematoxylin dye and anthracotic pigment is carbon.
Sections from tissue fixed in which fixative need iodine and thiosulfate treatment before staining?
B5 contains mercuric chloride, which leaves black mercury pigment removed by iodine then sodium thiosulfate. Bouin's leaves yellow picric acid removed by alcohol.
Which is NOT a goal of fixation?
Fixation preserves tissue and enhances optical differentiation, but formalin masks epitopes; unmasking is done later by antigen retrieval.
A tissue pigment is removed by a saturated alcoholic picric acid solution. It is most likely:
Alcoholic picric acid removes formalin (acid haematin) pigment. Mercury pigment needs iodine then thiosulfate, and melanin needs an oxidizer.
A lymph node is received with suspected metastatic melanoma. Fixation must be compatible chiefly with which diagnostic method?
Confirming melanoma relies on IHC markers such as S100, SOX10, HMB-45 and Melan-A, which work well on formalin-fixed tissue.
Which fixative is traditionally chosen for nervous tissue when Nissl substance is to be demonstrated?
Carnoy's alcohol-based mixture preserves nucleic acids well, giving good Nissl (RNA) staining; picric and dichromate fixatives give poorer results.
Which fixative traditionally gave the crispest nuclear detail in lymph node sections?
Mercuric-chloride-containing B-5 gave sharp chromatin in lymphoid tissue; it is now largely replaced for safety reasons.
Which fixative acts by coagulating proteins?
Chromic acid precipitates proteins; formaldehyde, osmium tetroxide and dichromate are non-coagulant fixatives.
Zenker's and Helly's fluids share mercuric chloride and potassium dichromate. Which component is in Zenker's but replaced by formalin in Helly's?
Zenker's contains acetic acid, making it good for nuclei; Helly's substitutes formalin, favouring cytoplasmic granules and bone marrow. Both leave mercury pigment.
A renal biopsy fixed for EM in buffered glutaraldehyde is later processed for light microscopy. Which staining problem is most likely?
Glutaraldehyde leaves free aldehyde groups that react with Schiff reagent, giving non-specific PAS positivity. Morphology is otherwise very well preserved.