Semithin

Semithin refers to a thickness category used in microscopy, describing resin-embedded tissue sections that are thicker than ultrathin sections but thin enough to be examined with light microscopy. In electron microscopy workflows, semithin sections provide a rapid, low-magnification view of tissue architecture and serve as a guide for selecting areas to image at higher resolution with transmission electron microscopy (TEM).

Typical semithin section thickness ranges from about 0.5 to 2 micrometers, with common practice clustering around 0.5–1 μm. The sections are produced with an ultramicrotome using glass or diamond knives and are then mounted on glass slides or suitable substrates for staining. The most common stain is toluidine blue (often with ruthenium or basic fuchsin counterstains), which highlights nuclei, cytoplasm, myelin, and connective tissue, facilitating quick assessment of tissue integrity and structure.

Use and purpose in research and diagnostics include evaluating tissue preservation, orientation, and general morphology before committing to ultrathin sectioning for TEM. Semithin sections can reveal cellular boundaries, organelles, and pathology at a level sufficient for rapid screening and accurate placement of regions of interest for subsequent ultrathin sectioning and high-resolution imaging.

Variations in thickness and staining protocols reflect differences in laboratory practice and the specific tissue type being examined. While predominantly associated with electron microscopy workflows, the term semithin may appear in broader histology contexts to denote mid-thickness sections intended for light microscopy.