**Formalin-Fixed, Paraffin-Embedded (FFPE)** tissue represents the fundamental standard for preserving clinical biopsy and surgical tissue specimens for pathological diagnosis and long-term archival storage worldwide, ensuring cellular morphology and critical structural components are retained indefinitely. The process involves treating the tissue with **formalin** (a formaldehyde solution) to chemically cross-link proteins, followed by dehydration and embedding the fixed tissue in molten **paraffin wax**, which hardens into a block. This robust preparation method makes the tissue suitable for subsequent slicing into extremely thin sections for microscopic examination and advanced molecular analysis.
The two-step process of fixation and embedding is deliberate. **Fixation with formalin** prevents tissue degradation by stopping all biological activity and preserving cellular structure. The chemical action of formaldehyde creates cross-links between molecules, primarily proteins, which strengthens the tissue and maintains its integrity. **Paraffin embedding** then provides a rigid external matrix, allowing the pathologist or technician to cut ultra-thin sections, typically 3 to 5 micrometers thick, using an instrument called a **microtome**. These sections can then be mounted onto glass slides and stained—most commonly with Hematoxylin and Eosin (H&E)—for routine pathological review and diagnosis.