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Should tissue cassettes sit in melted paraffin or sit dry during embedding?
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Should tissue cassettes sit in melted paraffin or sit dry during embedding?


I am encountering push back in our lab when I fill the embedding units with melted paraffin in the embedding wells. The techs here like for the tissue cassettes to sit dry (no wax) while in the embedding units. I find that the tissue rolls out of the sections while cutting because of a layering effect between the tissue and the paraffin its embedded in. I have communicated this but they tell me I'm "old school". Does anyone have any thoughts or opinions on this topic?

Answer 1.

Since we have turned to embedding centers in the late 80ies we let the cassettes sit in the centers without additional paraffin. We only see such "jumping out" tissue, when the cassettes are not warmed (let the lid open) and the tissue renders too cold. As a result tissue and paraffin don't combine well enough. Maybe it is a matter of embedding technique? Too little paraffin in the mold before setting the tissue in? Cold embedding molds? Slow handling?


Answer 2.

Without seeing the blocks, that sounds more like cold molds being used, more Then, whether or not the tissues are kept in a dry, hot, well, or a wet well.


Answer 3.

I am old school and prefer them dry, lol. I agree with Thomas, that shouldn't have that affect.


Answer 4.

I'm interested but don't understand the variation and it's + effect I take my cassettes out of the processor and immediately place into the molten wax bath of the embedder (if I'm embedding immediately; if not I let the cassettes/tissues therein go cold until a later embedding). When embedding immediately, after 30 mins, I remove tissue from cassette into a heated metal mold filled with molten wax. All done quickly, to keep all wax molten If my people take too long, the wax around the tissue sets so when the tissue is placed into the molten wax, in mold, they sometimes get that effect of set wax-interface artefact....the section immediately pulls apart from the surrounding wax, on the water bath This can cause the section to fold.


Answer 5.

The bon-bon effect.  This can happen if the wax used for processing is different from the wax used for infiltration or when the tissue gets cold before embedding. Either way, because the tissue is cold - the embedding wax doesn't blend with the tissue wax, creating the hard outer wax shell.  The simplest way to avoid the bon-bon effect is to make sure the tissue is as warm as the wax it's being embedded in. Letting the cassettes go cold isn't the issue here, it's not letting the tissue warm up before embedding that is.  I call it "freezing" the tissue/cassette, since that's what the wax is doing, just to mess with people's heads.  That helps remind people that the wax gets cold quickly. Some folks may disagree with me about that. If the embedding wax is a different from the infiltration wax it's kind of the same thing.  The waxes need to co-mingle a little while so they can blend together before freezing the block. Just a few ideas...

I'm old, old school too.  I prefer the paraffin pool myself, it helps prevent the bon-bon effect.  I had a vendor rep tell me the holding bins were designed for molten paraffin. Probably what happened over the years is that if there's too much molten wax in the bin, it over flows when the basket of cassettes is put in. Ah, displacement, just like when you get into an over full bathtub, water (or wax) everywhere. Now a days I think the dry holding tank is the rage.  As long as the tissue stays warm, it probably doesn't matter too much.


Answer 6.

Why would anyone use a different wax for infiltrating and embedding? Yes....keep them specimens molten until embedded Thanks for your input, Paula


Answer 7.

Why use a different wax for embedding?  Cost. Those who don't fully understand the art & science of histology think a wax is a wax is a wax.


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