Specimen Collection, Handling, and Fixation
Why pre-analytical control matters
Most errors that ruin a section begin before the microtome is touched. Candidates must know the sequence of events that preserves morphology and antigenicity: rapid excision, immediate immersion in fixative at an adequate volume ratio, and prompt transport. The standard rule of thumb is a fixative-to-tissue volume ratio of at least 10:1, with 10% neutral buffered formalin (NBF) as the default routine fixative because it cross-links proteins while buffering prevents formalin pigment and pH drift.
Timing and thickness
Fixation rate depends on penetration distance, not area. Formalin penetrates roughly 1 mm per hour at room temperature, so a 5 mm thick slice needs about 2.5 hours to be adequately penetrated; a whole organ placed intact will be fixed on the outside and autolyzed inside. Ischemia time and cold ischemia are the concepts behind questions about delayed fixation artifact (nuclear detail loss, cytoplasmic streaming).
Special specimens
Know the exceptions: bone and dense fibrous tissue may require decalcification or stronger fixation; renal biopsy for immunofluorescence needs a separate fresh or transport-medium specimen; cytology and hematology specimens have their own preservatives; frozen-section specimens must not be fixed.
Traps
Questions love the near-miss: too little formalin, non-buffered formalin, over-fixation of small biopsies, or confusing 10% formalin with 10% NBF (the latter contains phosphate buffer). Another trap is assuming longer fixation always improves quality; over-fixation can mask antigens and harden tissue, causing chatter.
Worked rule
If a specimen is 8 mm thick, estimate penetration of 1 mm/hour from each surface: roughly 4 hours to reach the center. For a 3 mm biopsy, 1.5 hours suffices; longer is acceptable but not required.