Analysis of Human Tissue Paraffin Block Preparation Process and Key Points of Pathological Quality Control

Fixation and Sampling: Fundamental Steps Affecting Paraffin Block Quality

The preparation of human tissue paraffin blocks typically begins with standardized sampling. After excision, tissue should be fixed as soon as possible to slow autolysis and putrefaction. The commonly used fixative is neutral buffered formalin, and the volume of fixative is generally recommended to be 5 to 10 times the tissue volume to ensure adequate penetration. The sampling thickness should not be too thick, with a common requirement of 3 to 5 millimeters; excessive thickness can easily lead to inadequate central fixation. Fixation time should be adjusted according to tissue type and size. Insufficient fixation can affect section integrity, while excessive fixation may reduce the sensitivity of subsequent immunohistochemistry or molecular testing.

Common Requirements for Dehydration, Clearing, and Wax Infiltration

After fixation, tissue must undergo graded ethanol dehydration, xylene clearing, and paraffin infiltration. Incomplete dehydration leaves residual water in the tissue, resulting in soft depressions or honeycomb-like structures within the paraffin block and affecting section continuity. Generally, a gradual transition from low-concentration ethanol to absolute ethanol is used, with the time for each step varying according to tissue size and the operating program of the automated tissue processor. The role of the clearing agent is to replace ethanol and create conditions for paraffin penetration. The wax infiltration temperature is usually slightly above the melting point of paraffin, avoiding excessively high temperatures that can make tissue brittle or damage antigens.

The volume of fixative should be sufficient to avoid tissue compression

The concentration and replacement cycle of dehydration reagents should be regularly verified

Infiltration paraffin should be kept clean to avoid contamination with impurities

Quality Control for Embedding and Pre-Sectioning Handling

During embedding, the embedding orientation should be adjusted according to tissue morphology to ensure that the target section plane is flat and convenient for subsequent observation. After the paraffin block cools, it should be inspected for cracks, bubbles, or exposed tissue; if obvious defects are present, re-embedding should be performed. Before sectioning, the paraffin block may be appropriately trimmed so that the tissue surface is fully exposed. The common section thickness is 3 to 5 micrometers, and thinner sections may be used for special tests. The water bath temperature, section floating time, and baking temperature all need to be controlled to reduce wrinkles, bubbles, and section detachment.

Effects of Storage and Environment on Long-Term Preservation of Paraffin Blocks

Human tissue paraffin blocks should be stored in a cool, dry, and light-protected environment, and numbering and traceability records should be established. Temperature fluctuations and excessive humidity may cause paraffin block deformation, mold growth, or label detachment. It is recommended to regularly check storage conditions, and for important samples, double-person review and information backup may be considered. Standardized preservation not only facilitates pathological review but also preserves traceable original materials for subsequent scientific research testing. Through full-process quality control, human tissue paraffin blocks can serve diagnosis and research work in a stable state.