Which impregnating agent is rarely used except when dehydration is to be avoided?

The correct answer is gelatin, as it is specifically known for its unique property of being used as an impregnating agent when preservation of the tissue's water content is essential. Gelatin allows for the embedding of specimens without the dehydration processes that typically accompany other embedding methods. This makes it especially valuable in situations where maintaining the integrity of cellular morphology and fine details is crucial.

In histopathology, dehydration is a standard step in preparing tissues for embedding in more common media like paraffin, where water is removed to allow for proper infiltration of the embedding medium. However, there are instances where dehydration might compromise the tissue’s structural features, making gelatin a more suitable choice.

Other agents like celloidin, carbowax, and paraffin are generally used in contexts where dehydration is part of the standard procedure. Celloidin functions as an embedding medium but requires dehydration of the specimen before embedding, while paraffin is a solid embedding agent designed to infiltrate dehydrated tissue. Carbowax, while it can allow for some moisture retention, is still not utilized primarily to avoid dehydration in the same way gelatin is. Thus, gelatin stands out as the impregnating agent specifically chosen to minimize dehydration effects.