Which reagent is primarily used in the dehydration step of tissue processing?

In the context of tissue processing, the dehydration step is crucial for preparing tissue specimens for embedding in a medium such as paraffin. Ethanol is the primary reagent used for dehydration because it effectively removes water from the tissues. Water must be eliminated to allow for proper infiltration of the embedding medium, which does not mix with water. Ethanol facilitates gradual dehydration due to its ability to mix well with water and its varying concentrations.

During tissue processing, tissues are first fixed (often with formalin) to preserve cellular structures and proteins. Following fixation, tissues are subjected to graded ethanol solutions, starting from lower concentrations and progressing to 100% ethanol. This step ensures complete removal of water, which is essential for the subsequent embedding process where the tissue is infiltrated with paraffin wax.

The other choices do not serve as effective dehydrating agents in this context. Formalin is primarily a fixative, glycerol is used as a clearing agent in some processes but not for dehydration, and water is the solvent that needs to be removed during dehydration. Thus, ethanol is the quintessential reagent used for dehydrating tissues before embedding.