What causes tissue to appear opaque?

Tissue appears opaque primarily due to insufficient clearing during the histopathological preparation process. The clearing step involves the removal of dehydrating agents like alcohol and replacing them with a substance (often xylene or a similar clearing agent) that allows the embedding medium, typically paraffin, to infiltrate the tissue properly. If this clearing is not thorough, remnants of alcohol can remain in the tissue, altering the refractive index and preventing light from passing through. This lack of transparency results in an opaque appearance.

In contrast, excessive dehydration tends to cause brittleness and can lead to tissue damage, but it does not directly lead to opacity. Inadequate fixation may result in poor preservation of cellular structures and can affect stain uptake, yet it does not necessarily cause opacity either. Overembedding can lead to difficulties in cutting or visualizing the sections but is not a primary factor influencing transparency. Therefore, insufficient clearing is the key reason for tissue opaqueness in histopathological specimens.