Customers ask us for the same organ in two forms: fresh, shipped chilled the day it's collected, or frozen. Neither is better across the board. It depends on what you're going to measure and how soon you can get to it.
What freezing does
Snap-freezing stops metabolism within seconds, so a frozen sample reflects the state of the tissue at the moment it went into the freezer. Proteins, DNA and RNA survive well and the sample can sit for months at −80 °C; what you lose is structure. Ice crystals rupture membranes, so a thawed sample releases its enzymes into the extract and no longer behaves like intact cells or an intact organ. Slow freezing does more damage than fast freezing, and a second freeze-thaw does more than the first.
When you need fresh
If the protocol depends on living cells or intact membranes, fresh tissue is the only option. That covers primary cell isolation, organ slices, mitochondrial preparations, membrane-bound enzyme assays, and any perfusion or contraction study. Fresh tissue is also the cleanest starting point for enzymes that don't tolerate freezing, and for histology where ice artifact would confuse the read.
The cost is logistics, since fresh tissue has to be collected to order, kept cold without freezing, and used within a day or two of harvest. That's workable when the supplier can collect on a schedule you set and ship overnight.
When frozen is better
For most biochemical work, frozen is the practical answer. Homogenates for enzyme kinetics, Western blots, nucleic-acid extraction, antigen preparation and acetone-powder production don't care about cell integrity, and frozen tissue lets you order once, store the material, and pull samples as the project needs them. Because every sample comes from the same lot, frozen tissue also gives you better run-to-run consistency than a series of fresh collections would.
How we supply both
We harvest rabbit tissue in house, so most items are listed both ways. Fresh rabbit muscle is collected to order and shipped the same day on wet ice; frozen rabbit muscle ships on dry ice from stock. Many organs, brain and liver among them, are available by age of animal so you can match the tissue to your model.
Whichever you order, thaw frozen tissue once, on ice, and use fresh tissue when it arrives. If you're not sure which form suits your assay, describe the experiment to us and we'll tell you what other labs running it have chosen.