Paraffin embedding is a fundamental technique in histology that allows for the preservation and preparation of tissues for microscopic analysis. This process involves a series of meticulously designed steps to maintain the structural and molecular integrity of the samples. From initial fixation to final embedding, each stage is crucial to obtaining high-quality sections for detailed tissue studies in biomedical research, clinical diagnostics, and other scientific applications. Below, we present an optimized protocol for paraffin embedding, designed to ensure the best results for your samples.
1. Sample Preparation
Use a stainless steel scalpel (blade #24) or cryostat to trim the samples.
Optimal size: 0.5 cm³ cubes (up to 1 cm³ for porous tissues like lung or liver).
2. Fixation Chemical
Choose the appropriate fixative based on the tissue type:
Buffered Formalin: Immerse in a 10:1 ratio (fixative:tissue) at 4 °C with gentle orbital shaking (50 rpm) for 24–48 hours.
Bouin: Immerse in a 10:1 ratio at room temperature for 12–18 hours.
FAA (Formaldehyde-Acetic Acid-Ethanol): Immerse under vacuum at 4 °C for 24 hours (ideal for plant tissues).
3. Washing Post-Fixation
Buffered Formalin: Wash in distilled water (3 changes of 10 minutes each).
Bouin: Wash in 70% ethanol (3–4 changes of 30 minutes) until the yellow color of picric acid is eliminated.
FAA: Wash with 50% ethanol (2 changes of 15 minutes each).
4. Gradual Dehydration
Immerse samples in an ethanol series with the following concentrations and times:
- 70% ethanol: 4 washes of 15 minutes each.
- 80%, 90%, and 96% ethanol: 4 washes of 10 minutes each.
- 100% ethanol: 2 washes of 15 minutes each.
5. Diafanization in Xylene
- Xylene I: Immerse for 4 × 5 minutes with gentle agitation.
- Xylene II: Immerse for 4 × 5 minutes until tissue refractive index ≈1.5 is achieved.
6. Infiltration with Paraffin
In an oven at 60 °C:
- Paraffin/xylene mixture (1:1): Immerse for 60 minutes.
- Pure paraffin I: Immerse for 60 minutes.
- Pure paraffin II: Immerse under vacuum (500 mmHg) for 60 minutes (recommended for dense tissues).
7. Blocked and Orientation
7.1 Embedding Technique:
- Preheat metal molds to 60 °C.
- Cover the sample with approximately 2 cm³ of melted paraffin.
- Use preheated forceps to position cutting axes (transverse or sagittal).
7.2 Cooling Cycle:
- Phase I: Cool at room temperature (~22 °C) for 30 minutes.
- Phase II: Cool at least 24 hours at 4 °C before sectioning with a microtome.