The setting of this question is basically our world, with magical creatures hiding among us. They are from different races, and can be human-like (witches, santas, succubi...) or not (kitsune, swamp monsters, chimeras...). Most of them are part of supernatural mafias.
Those mafias have set very lucrative traffics around the world. Like regular criminal organizations they smuggle drugs and weapons, but some of them specialize in tissues of magical creatures (flesh, brains, bones, skin, etc.).
In this world every potion, artifact or spell uses something originating from a creature, so the demand is high enough for this traffic to be very profitable. The something in question doesn't have to be a whole organ, it can be a few grams of dried flesh, for example.
The quantities are somewhat limited because every group protects their own members, so most of the sellers get their source materials either from their own deads or from "unaffiliated" creatures.
Recently, more and more lab-grown tissues are sold on the black market. These are produced using modern equipment, like what's used to make in vitro meat and lab-grown human skin, and maybe a little magic.
I'd like to limit the production of in vitro "magical meat" by adding some kind of extra difficulty.
For now, I've found a few options that could work, but am not sure which one is the best.
- Need for blood
The tissues have to be "fed" with a solution containing blood (or blood plasma) taken either from the original donor or from another member of the same race.
- Magic decay
Each magical cell can only produce a limited number of equally magical cells once placed in vitro.
As a result, 1 mm2 of natural skin can only produce 1 cm2 of equally magical lab-grown skin.
- Consent is key
The seed sample has to be given willingly to be able to grow.
- Rule of proximity
In vitro cells present magical properties only if they were grown in the proximity of the original source. The closer the donor is, the better the result.
So if a lab wants to produce werewolf liver, they have to lock a werewolf inside the lab, collect a sample of his liver and keep him alive for as long as the growing process last.
- Magical support
A witch/magician/druid must be present and reciting incantations/casting spells/maintaining pentagrams during the whole process.
- Simple but boring option
Cells originating from magical beings don't reproduce as fast as human cells.
- All of the above
I could use all of these options if each race has its own rule. In this case, I'll have to find other rules to cover every race I've included in this universe (I'd rather not).
What rule or set of rules is the best to complicate the production of in vitro magical tissues ?
The limitations should reduce the production capacity of these labs enough so their products wouldn't flood the market, but not too much so they can still produce something.
Also, it's better if it sounds (at least a little) logical.
Disclaimer : Most of the scenes I've written for this setting so far are a bit violent and/or gross, so don't hesitate if you want to propose a solution that contains a bit of violence and/or grossness.