Why does biological tissue shrink in ethanol?

Question

Moving aldehyde-fixed biological tissue (in my case, 200um thickness mouse brain slices) from water to ethanol shrinks them considerably, even when transitioning slowly. I know that ethanol moves across membranes faster than water, so I would expect the cells to swell, which is the opposite of the observed effect.

Anyone know why cells shrink in ethanol?

Answer 1

All solvents create something known as a solvation shell - a region of highly organized solvent around a solute. Due to the permanent dipole on water, it creates a large and highly organized hydration shell up to 5 molecules thick around the polar surfaces of biomolecules. Ethanol in contrast, does not form such highly organized hyration shells. In fact, for most large biomolecules, 96% ethanol is a poor solvent and precipitates the molecules from solution (e.g. DNA, most proteins). The reduced hydration shell essentially reduces the space between molecules, hence those molecules interact more strongly with each other (coagulate) and cause the tissue to shrink.

Answer 2

Ethanol dehydrates tissue, for the reasons explained in previous answer. If this is a desirable outcome, it's best to do it in series. I used to use 50%, 75% and then 95% ethanol to dehydrate my brain slices (that were anywhere between 100-400 microns, depending on if they were fresh or cultured for awhile first). Doing so in a series makes the changes less disturbing, and better maintains original tissue organization. Hope that helps!

Answer 3

I can think of 2 good reasons:

1. water in the tissue does it's thing and goes to the lower osmotic pressure (outside of the preserved cells).
2. ethanol is an excellent organic solvent, so it will dissolve some things that won't normally dissolve in water.